Contents

1 Introduction

The integration of beef production in oil palm plantations is a suggested solution for meeting the increasing global demand for food and in particular animal-source protein. The 19 million ha of established oil palm worldwide provides an abundant area for cattle grazing in South East Asia, West Africa and South America (FAOSTAT 2018). There are a number of potential benefits of cattle grazing in oil palm plantations, both at macro- and enterprise-scales. At macro-scale, using established oil palm plantations for beef production would reduce the need for converting existing cropping, horticultural, or wilderness land into grazing land (Azhar et al. 2021), while the oil palm understorey becomes a valuable source of cattle feed (Fig. 1).

Fig. 1
figure 1

Cattle grazing in an oil palm plantation. Photograph by Jori Bremer.

There are also potential side-effects of cattle grazing of direct benefit to the oil-palm enterprise, by lowering the plantation’s herbicide and fertilizer requirements while diversifying and increasing farmer income. Reducing weeding requirements through cattle grazing would reduce herbicide expenses, which customarily constitute 10 to 15% of maintenance and cultivation costs (Nor et al. 2000), while also reducing worker exposure to herbicides and lowering the load of agrochemicals in the environment (Devendra 2004). Manure produced by cattle is an organic fertilizer, which depending on the cattle feed source can be a nutrient input in the farming system with the potential to increase oil palm yields (Devendra 2004; Quartermain 2004) and reduce fertilizer costs, which can amount to a third of total cultivation costs (Caliman et al. 2001). Cattle–oil palm integration furthermore diversifies activities and outputs for farmers, reducing and spreading production risks, while potentially increasing income (Devendra 2008a; Quartermain 2004). In recognition of the synergies between the crop and livestock components, cattle–oil palm integration can be an avenue for sustainable intensification through raised production per land area and increased resource use efficiency (Herrero et al. 2010; Castillo 1995; Stür 1995).

Although integrated cattle–oil palm farming systems are considered to have many benefits, research on this system is novel and analysis of the (potentially adverse) impacts of cattle grazing in oil palm plantations are lacking. Current adoption of these systems remains limited, attributed to the lack of relevant technical information and expertise on cattle management (Tohiran et al. 2017; Devendra 2008b), the lack of knowledge on long-term profitability and sustainability of integrated systems (Devendra and Leng 2011), the added complexity of farm management, and the requirement for capital for cattle (Zamri-Saad and Azhar 2015; Riswani et al. 2012; Devendra 2008a). The threat of cattle spreading diseases, such as basal stem rot (Ganoderma boninense, the most devastating disease in oil palm), is an important issue for oil palm producers and a hurdle to the adoption of cattle grazing in oil palm plantations (Utomo and Widjaja 2020). Additional and comprehensive expertise on grazed–oil palm systems would provide relevant technical knowledge supporting sustainable development of integrated cattle–oil palm farming systems.

Given the major economic and environmental significance of both oil palm and cattle production worldwide and the rising awareness of a need for sustainably produced palm oil and beef, the impacts of cattle integration in oil palm plantations are critical in determining the future of both these enterprises. Furthermore, in light of currently decreasing oil palm prices (The World Bank 2019), it is imperative for oil palm producers to find alternative income sources, cattle production being a possible avenue.

This systematic literature review of the current, global scientific literature (including grey literature) presents the known impacts of cattle integration in oil palm plantations on understorey, cattle, and oil palm productivity as well as the economic implications of cattle grazing in oil palm plantations. This review is structured with an initial description of the literature selection and current management of cattle grazing in oil palm plantations. The review then examines the published impacts of cattle grazing in oil palm plantations on understorey productivity and species composition, cattle productivity, and oil palm productivity. The economic implications of cattle grazing in oil palm plantations follows, and the review concludes by synthesizing research unknowns and highlighting future research avenues for developing productive and sustainable systems of grazed cattle integration with oil palm plantations.

2 Study selection

A systematic review of literature using Web of Science and Scopus databases was performed initially in October 2018, and updated in June 2021. The search was expanded to CAB Abstracts on account of the low number of results and to incorporate grey literature. The following search string was used: (cattle OR ruminant*) AND (“oil palm*” OR plantation*) AND (integration OR grazing OR “forage specie*” OR “soil disease*” OR “Ganoderma”). The term “oil palm” was broadened to “plantation” due to a low number of results.

Selection of final publications was restricted to those on cattle, discarding publications on other livestock. If oil palm, coconut or rubber were researched, the publication was retained since these crops were often studied collectively, grow under similar climatic and geographic conditions, and the principles of cattle integration were considered comparable (Devendra 2011). A total of twenty-three publications on silvopastoral farming under pine, poplar and eucalypt plantations were discarded. After assessment, 93 unique and relevant publications were examined (Supplementary Table S1).

On several occasions, it was necessary to introduce additional publications not included in the original systematic literature search. This was done for three reasons: firstly, to access original research data cited in a reviewed publication, secondly, to explain current circumstances or provide current statistics, and thirdly, when providing general definitions.

The 93 reviewed publications were rated based on author’s assessment of the presented information (called ‘evidence strength’) following the dichotomous decision tree presented in Fig. 2. Papers were given a high score for evidence strength if they had addressed both criteria fully: 1) presence of a clear statement of peer-review, and 2) for primary research articles (reporting new research findings) a clear and detailed methodology section allowing the experiment to be repeated, while also presenting specific and detailed results, or for secondary publications (syntheses, reports and extension bulletins offering analyses, interpretation or restatements of primary sources, including literature reviews) providing traceable references for supporting statements. This classification rates publications on a scientific value scales, for quality control and in order to give context specific information. Publications rated as ‘lower evidence strength’ are however not less important or relevant, especially since it is common for researchers to publish in industrial publications and conference proceedings in order to have more impact within the industry.

Fig. 2
figure 2

Pathways for assessing strength of evidence in reviewed publications.

2.1 Research theme categorization

The publications were classified into one or more research themes created post hoc, after examination of content. Publications were included in one or more specific research themes in accordance to their main objectives and research outcomes (Supplementary Table S1). If publications were syntheses or case studies which could fit in more than four research themes, they were classified as ‘Whole farm synthesis’ only.

Figure 3 illustrates the research themes: cattle, oil palm, understorey, soil, and their linkages, while also presenting quantity of publications within research themes and the average strength of evidence of the publications within theme, indicating whether areas were well researched or not (knowns and unknowns). Cattle productivity is an essential output for cattle grazing in oil palm plantations. Next to quantifying cattle productivity, an important part of this review was investigating understorey productivity and quality as feed, determined by cattle grazing and plantation age and management. The availability and quality of palm oil processing by-products was also studied for their potential as a feed source. The relationship between cattle grazing and oil palm productivity, especially in young plantations, was examined since the threat of depressed yields is a major deterrent from the adoption of cattle grazing in oil palm plantations. The studied effects of cattle grazing on soil properties was for its advantages and disadvantages to oil palm and understorey productivity. At the farming system’s level, the potential for increasing and diversifying farmer incomes through cattle grazing in oil palm plantations was studied for its importance as a driver for the adoption of cattle grazing in oil palm plantations.

Fig. 3
figure 3

Cattle grazing in oil palm plantations: quantity and average evidence strength over the research themes. Line thickness represents quantity of publications while line shading intensity represents strength of evidence presented in the research theme.

2.2 Studied crops and regions

Oil palm was the most researched crop, followed by a combination of several crops (oil palm and/or coconut and/or rubber and/or forage species), coconut-only, and rubber-only (Fig. 4a). Study areas were strongly linked to the plantation crop’s growing region: Indonesia and Malaysia dominated reviewed publication as the two biggest oil palm producing countries, followed by countries in South East Asia, Africa, South America and Oceania, which are growing regions for oil palm, coconut and rubber (Fig. 4b).

Fig. 4
figure 4

(a) Crops studied in reviewed publications (percentage of total publications reviewed, ‘several crops’ stands for a combination of oil palm and/or coconut and/or rubber and/or forage species). (b) Regions studied in reviewed publications (percentage of total publications reviewed).

Publication numbers peaked in the 1990s’ and 2010s’, with less publication activity in intermediate years (Fig. 5). The 1990s’ peak related to the publication of several workshop proceedings (Shelton and Stür 1991a; Copland et al. 1994; Mullen and Shelton 1995), while the current increase most likely reflects renewed interest in the cattle-plantation integration owing to increasing demand for sustainable palm oil, declining palm oil prices and rising demand for animal protein.

Fig. 5
figure 5

Publishing date of reviewed literature, in increments of 5 years.

2.3 Cattle–oil palm integration in practice

Palm oil has two main production models: large estates (governmental or private) or smallholder plantations. In Indonesia, around 59% of the oil palm area is under estate management, the remainder belonging to smallholder farmers. Whereas estate average area is 6,800 ha, smallholder plantations typically measure 2 ha (Badan Pusat Statistik 2019; Stür 1995). Thirteen publications described cattle management in oil palm plantations in practice. Five of these were set on research stations, seven on oil palm estates, and one publication summarized interviews conducted on both estate and smallholder plantations.

In large estates, cattle were generally rotationally grazed, staying in one area for about 1 to 2 days, with grazing intervals of two to four months, and a stocking rate of one head of cattle every 4 to 6 ha. No additional feed was given, with the exception of some minerals and salt. Mostly local cattle breeds were present since they were adapted to the local conditions being hardier, disease resistant and of lighter weight (Nor et al. 2000). After grazing, the remaining undesirable woody understorey species were sprayed with herbicide (Nor et al. 2000; Nambiar 1991). In contrast, the majority of smallholder oil palm farmers practiced free-grazing, with cattle roaming unrestricted in search of feed (Kamil Azmi et al. 2014). Cattle either grazed day-round or during the daytime only, depending on feed availability and risk of cattle theft (Silalahi et al. 2018).

3 Determinants of understorey productivity and species composition

Feed availability is the major constraint limiting livestock production in South East Asia (Devendra 2012; Stür 1995). The main advantage of cattle–oil palm integration is therefore the use of understorey plants as cattle feed (Copland et al. 1994; Mullen and Shelton 1995). There are about 60 understorey plant species naturally occurring in oil palm plantations (Devendra 2011), of which 70–90% are palatable to ruminants (Shelton and Stür 1991b; Devendra 2011; Zamri-Saad and Azhar 2015). Understorey productivity and species composition is dependent on plantation age, plantation management, cattle grazing, and the introduction of specific understorey species, as discussed in Sections 3.1 to 3.5.

3.1 Impact of plantation age on understorey species composition and productivity

The most important factor determining understorey plant growth is sunlight and its penetration through the canopy (Wong et al. 2005; Samat and Shelton 1995). This topic received a great deal of research attention, but mostly from publications assessed as ‘weak evidence’ in this literature review (Fig. 3). Light penetration through oil palm and rubber canopies was comparable (Fig. 6a and b): shortly after planting light transmission was high, but 8 to 10 years after planting canopies closed, reducing light transmission to 10–20%, lowering understorey plant growth (Reynolds 1988; Stür 1995; Coomans and Gaullier 1985; Sophanodora and Tudsri 1991; Gaullier 1990). It is estimated understorey plants in mature oil palm plantations needed two to three times longer to regrow after defoliation compared to understorey in a non-shaded environment (Coomans and Gaullier 1985; Gaullier 1990). This phenomenon was reversed at the end of the plantations’ lifespan, when some palm trees died and fronds becomes sparser (Gaullier 1990). The studies modelling understorey biomass under rubber showed similar relationships, reflecting canopy closure and subsequent drop-off in understorey biomass (Fig. 6b and e). Light transmission in coconut plantations (Fig. 6c) did not drop below 40% (around year 5) and gradually increased to reach 80% at the end of the plantation’s lifespan (Stür and Shelton 1991), due to a taller stand and a lower frond density (Chen 1993). This higher light transmission level allows higher understorey biomass production in coconut compared to oil palm and rubber plantations. In Malaysia, understorey plant biomass in oil palm plantations was 6–10 t dry matter (DM) per hectare 3 years after planting, declining to 0.4 t DM/ha 7 to 26 years later (Chen 1991; Wan Mohamed et al. 1987) (Fig. 6d).

Fig. 6
figure 6

Measured and modelled light transmission (%) through ageing oil palm (a), rubber (b), and coconut (c) plantations (0–30 years) and associated understorey biomass (t DM/ha) for oil palm (d) and rubber (e) plantations.

The restricted timeframe of high understorey availability in ageing oil palm plantations, combined with the limited ability to introduce cattle in plantations under the age of 5 years (see Section 5.2), raises questions about cattle grazing viability and whether it is worth investing in knowledge, infrastructure and management skills. Future research should focus on pathways to optimize this window of opportunity with high understorey biomass. Although with some logistical considerations, one solution would be to take advantage of the age range of palms in estates and grazing cattle in the palm cohorts with high understorey productivity.

Understorey plant species composition and nutritional value were also affected by the decline in light transmission. Ten years after planting, around 66% of forage species recorded in young oil palm plantations had disappeared (Chen and Chee 1993, cited in Devendra, 2004). Whereas grasses and legumes dominated the species mixture in earlier years, the proportion of ferns increased under lower light conditions (Dahlan et al. 1993; Gaullier 1990; Wan Mohamed et al. 1987). Fifteen to twenty years later, this phenomenon was reversed as light penetration to the understorey increased (Gaullier 1990). The proportion of broadleaf species in ungrazed plantations has been reported to both increase and decrease with plantation age (Wong et al. 2005; Devendra 2004; Dahlan et al. 1993).

Understorey plant species adapt to shady environments by increasing light interception and decreasing their respiratory load (Reynolds 1988; Wilson and Wild 1995; Wilson and Schwenke 1995). Among others, increasing shade intensities led to higher silica content and lignification, lower cell wall digestibility, and increased moisture content, reducing nutritional value and lowering cattle dry matter intake (Reynolds 1988). This review found no publications comparing cattle productivity when fed forage grown under shade to cattle productivity when fed forage produced in a non-shaded environment. Understorey vegetation metabolizable energy (MJ/kg DM understorey vegetation) and protein content (g protein/kg DM understorey vegetation) both decreased by 5–8% over a period of 7 years (from 3 to 10 years after oil palm planting) (Wong and Chin 1998, cited in Wong, 2005), while understorey biomass (kg DM understorey/ha) decreased by 84% over this same period and is therefore the major nutritional drawback (Wong et al. 2005).

The multiple impacts of the environment (climate, rainfall, soil properties) on understorey productivity and species compositions was not addressed in the literature examined. This information is available in the context of oil palm monoculture and intercropping, and publications describing specific understorey species which can be used in the context of integrated cattle–oil palm farming systems. For example, Germer (2003) found that solar radiation below the oil palm canopy, soil type and soil carbon content and effective cation exchange capacity had the greatest impact on understorey species composition.

3.2 Impact of plantation management on understorey species composition and productivity

Pruned oil palm fronds stacked in rows, cleared pathways and weeded circles around oil palm trunks reduce the available area for understorey growth to 60% of the total plantation area (Coomans and Gaullier 1985; Reynolds 1988; Singh et al. 2014). Theoretically, to increase the understorey available area, pathways could be left uncleared, weeded circle area could be reduced and oil palm fronds could be removed. However, removing pruned fronds requires labor and removes organic matter from the plantation, leaving pathways uncleared might reduce harvesting efficiency, while reducing weeded circle size might reduce oil palm yield though competition and reduced efficiency of fruit ripeness assessment (Corley and Tinker 2008). The type of weed management adopted (e.g. selective or non-selective weeding) and fertilization rate will further alter understorey species composition and biomass (Wan Mohamed et al. 1987), although no information on this topic was found in the systematic literature search. No information on competition between oil palm and understorey and associated oil palm and understorey productivity data, over the plantation’s lifespan, was found in this systematic search but is returned when omitting the search term “cattle OR ruminant*.” For example, in an oil palm monoculture no ring weeding reduced oil palm growth by 73% compared to standard ring weeding, 30 months after oil palm planting (Ojuederie et al. 1983).

Oil palm planting geometry and density control light transmission through the canopy and consequently govern understorey plant productivity (Reynolds 1988). Conventionally, oil palms are planted in a triangular configuration, with nine meters between trees (Corley and Tinker 2008). An alternative planting pattern, the “double-row avenue,” adopts standard oil palm planting density to maintain oil palm yield, while altering planting geometry (two rows of trees are followed by a wide alley allowing higher light transmission) to enhance understorey plant productivity (Kamil Azmi et al. 2014). Through a modelling study (Samat and Shelton 1995) predicted that light transmission 10 years after planting in a conventional rubber plantation was under 30%, compared to 58% with the double-row avenue system. This increased light transmission 10 years after planting in the double-row avenue system led to a predicted understorey yield more than three times higher than the predicted understorey yield in conventionally planted rubber, but was also accompanied by a reduction in tree growth rate, trees reaching maturity about 2 years after the conventionally planted rubber trees (Samat and Shelton 1995). There is no information to date on how prevalent the double-row avenue planting method or other alternative planting geometries are for oil palm. Studies should be done on potential understorey production gains and associated oil palm yield impacts.

As seen in Fig. 3, the research theme of plantation management impact on understorey productivity and species composition has received little research in the context of cattle grazing. Information on the understorey and crop yield impacts of weeding strategies, planting geometries and fertilization rates are available when omitting the research term “cattle OR ruminant*.” For example, information is available on the establishment and maintenance of legume and native covers from Corley and Tinker (2008). Germer (2003) found understorey species composition to differ between harvesting paths, palm circles and inter-rows. Future research should explore cattle productivity impacts of alternative oil palm management strategies. The trade-offs between oil palm yield, understorey productivity and associated cattle productivity can then be quantified, to determine what type of management is most advantageous at a systems level.

3.3 Impact of cattle grazing on understorey species composition and productivity

Stocking rates need to be matched to understorey forage growth and quality to achieve adequate cattle feed intake while avoiding overgrazing (Dwatmadji et al. 2015). Overgrazing is when cattle vegetation consumption exceeds the vegetation’s ability to recover, leading to a depletion of vegetation ground cover followed by increased vulnerability of soil to water and wind erosion (Tajuddin and Chong 1995; Salendu and Elly 2013; Tohiran et al. 2017; Chen 1993). Grazing pressure in ageing plantations should therefore be reduced to match the decline in understorey plant biomass to avoid overgrazing (Fig. 6d and e) (Rombaut 1974; Yusoff 1992). Whereas the carrying capacity of a 3-year-old oil palm plantation was of three steers per hectare, at 7 years of age the carrying capacity dropped by 90% to 0.3 steers per hectare (Devendra 2011; Wong et al. 2005).

Cattle preferentially graze the most palatable species, conferring competitive advantage to the less-palatable species. Another aspect of overgrazing is therefore the disappearance of desirable palatable vegetation species and a proliferation of the less-palatable species (Stür 1995; Nambiar 1991; Devendra 2011). Preferential grazing of succulent soft weeds under Malaysian oil palm led to the proliferation, and thus need for manual eradication, of woody weeds, while creeping foxglove (Asystacia), a weed difficult to eradicate in non-grazed plantations, was controlled by the cattle (Nambiar 1991). Next to palatability, understorey species’ tolerance to grazing is crucial; while some plant species persist and spread when grazed, others cannot tolerate defoliation and disappear, especially if they are attractive to cattle through high palatability (Rika et al. 1995; Wong and Stür 1995). In a grazing experiment in Bali, calliandra (Calliandra calothyrsus) was intolerant to increasing grazing pressure, while pinto peanut (Arachis pintoi cv Amarillo), even though grazed, persisted and spread (Rika et al. 1995).

In practice, several different grazing management techniques can be adopted to manage understorey species composition and utilization. The first technique keeps grazing pressure very low to minimize competitive disadvantage on the palatable species (Horne et al. 1994; Castillo 1995). The second technique maintains a high grazing pressure through heavy, short-term grazing to avoid selective grazing pressure on palatable species (Mullen 1995). The third management option adopts a medium grazing pressure leading to some competitive disadvantage, followed-up with manual or chemical removal of unpalatable species after grazing. In practice, large oil palm estates rotationally grazed cattle at high intensity so that both palatable and less-palatable species were consumed, and grazing intervals were kept large enough to allow the understorey vegetation to recover (Nambiar 1991; Gopinathan 1998). Estates also typically removed remaining undesirable understorey species after grazing (usually woody growth) to avoid their proliferation (Nambiar 1991; Gopinathan 1998). Cattle management in estates is therefore a hybrid of the second and third techniques described. In smallholder plantations cattle typically roam freely, leading to uneven grazing pressures (Kamil Azmi et al. 2014) and conferring less control over understorey species composition and utilization, potentially leading to overgrazing and the spread of undesirable understorey species.

Cattle grazing determines understorey species composition as well as productivity. When setting cattle grazing management, consideration needs to be given to understorey plant productivity and quality, as well plant species palatability and tolerance to grazing. Although the impact of cattle grazing on understorey productivity and composition was a research theme with a high number of publications and average evidence strength (Fig. 3), most oil palm producers lack experience in cattle and pasture management, a major reason for lack of adoption of integrated cattle–oil palm farming systems (Tajuddin and Chong 1995). Future studies should investigate and develop tools for assessing understorey vegetation quantity and quality and determining appropriate grazing management to ensure effective weeding and avoid overgrazing. There is currently a lack of information on the interaction between ageing oil palm plantations and cattle grazing on understorey species composition and productivity, showing the need for research over the plantation’s lifecycle. Particular attention should be given to research on smallholder farming systems to develop sustainable cattle grazing adjusted to land, labor and capital availability.

3.4 Impact of cattle grazing on understorey weeding requirements

Cattle grazing of understorey plants can reduce or eliminate the need to manually or chemically control weeds, decreasing weeding expenses (Seresinhe et al. 2012; Yusoff 1992). Table 1 shows savings in weeding costs related to grazing pressure: there was no clear association between grazing pressure (ranging from 73 to 5869 head.day/ha/year) and reduction in weeding costs, which might originate from disparities in weeding management adopted, cattle management and local conditions.

Table 1 Reduction in weeding cost through the integration of cattle grazing in plantations. N.A. not available.

In large oil palm estates, weeding costs decreased by 20–70% after the introduction of cattle grazing compared to before cattle integration (Nambiar 1991). Weeding costs were reduced since the undesirable species were more conspicuous after grazing, thereby increasing weeding efficiency (Gopinathan 1998; Nambiar 1991), and as a result of extended weeding intervals (Nor et al. 2000). Mohd Azid (2008) found 15–40% reduction in weeding costs across several estates after cattle integration. This range was explained by differences in grazing management adopted, even though no explicit clarification was given. Weed control before- and after-cattle grazing (Table 1) was compared without the presence of control plots, therefore requiring careful interpretation: the reduction in weeding costs may be explained by other factors such as understorey productivity reduction under ageing oil palm plantations.

In a survey of 45 Malaysian oil palm plantations, systematic cattle integration in oil palm estates led to a decrease in weeding costs compared to oil palm monocultures (systematic integration defined by Tohiran et al. (2017) as well-coordinated grazing into small areas of less than 5 ha for short durations, with one herbicide application per year) (Tohiran et al. 2017). Non-systematically grazed plantations (defined by Tohiran et al. (2017) as a free-grazing system with ad hoc grazing decisions and sprayed with herbicide more frequently), however, had higher chemical weeding costs than oil palm monocultures. Although grazing management might partly explain the differences in weeding costs, the plantations in this study (Tohiran et al. 2017) had different weed management strategies where cattle grazed systematically and non-systematically.

Cattle grazing under coconut was associated with a reduced weeding frequency from four rounds a year to one round a year by Seresinhe et al. (2012), while cattle grazing under a rubber plantation was associated with complete ending of weeding tasks in a first treatment and occurrence of spot-spraying in a second treatment (Senanayake 1996). In both cases, weeding management was determined at the beginning of the experiment and no information was available on how this decision was made and whether the impact of cattle grazing on the understorey was taken into account. In addition, the grazing pressure of 5869 head.day/ha/year adopted by Seresinhe et al. (2012) does not reflect the usual management of cattle under oil palm: in this scenario cattle were tethered to coconut palms and additional feed was given so that the cattle were not solely reliant on the understorey for feed. Different magnitudes in weeding cost reductions are therefore expected for cattle reliant on the understorey as the main feed source.

Unexpectedly, there were no articles on the impact of different grazing management strategies on weeding savings (e.g. use of high or low stocking rates). Information on grazing management and weeding costs available in the general literature on pasture grazing could be adapted to the oil palm environment. Future research should look at how to manage cattle to optimize weed control and cattle productivity simultaneously, while ensuring long-term sustainability of understorey utilization under the declining light environment of ageing plantations.

An additional advantage of cattle grazing as a weed control measure was that cattle could still graze in the oil palm plantation during the monsoon season, when chemical weeding was not possible (Nor et al. 2000). It would be interesting to explore whether the inability to chemically eradicate weeds in the monsoon season is of relevance to other plantations and whether the additional weeding the cattle grazing has an impact on oil palm yield through reduced competition by the understorey.

Although the topic of cattle grazing impact on the understorey and associated weeding requirements was one of the most prevalent in the study (Fig. 3), important research gaps remain for smallholder plantations or farms with uncontrolled grazing. Free-grazing cattle might unevenly graze the understorey, potentially leading to spread of unpalatable species, thereby requiring increased weed control measures. Since a large area under oil palm is owned by smallholder farmers (see Section 2.3), research needs to be carried out to determine feasible and practical cattle management strategies which could be implemented in these systems to reduce understorey weeding costs.

3.5 Introducing understorey plant species to increase understorey productivity and quality

Generally volunteer forage species grow naturally under oil palm. Although productivity is low, they are persistent under local management conditions (Stür and Shelton 1991). Introducing understorey plant species is an option for increasing understorey quantity and quality as feed (Agus and Widi 2018; Mudhita et al. 2016).

In a harvested plot experiment under coconut, introduced ribbed paspalum (Paspalum malacophyllum) exceeded native pasture yield by 67% (Mendra et al. 1995). In the same trial, legumes from the Arachis genus were the only persistent legumes, showing promise as a shade and heavy cutting-tolerant pasture species (Mendra et al. 1995). A trial under a 35-year-old grazed coconut plantation showed introduced cori grass (Bracharia miliiformis) and tropical kudzu (Pueraria phaseoloides) produced 20 tonnes dry matter understorey per hectare per year. Mullen and Shelton (1996) and Hutasoit et al. (2020) identified buffalo grass (Stenotaphrum secundatum) as a valuable forage species for plantation systems, due to its ease of establishment, resilience to long-term heavy grazing and shade tolerance. Many species with potential for introduction in coconut plantations were identified by Kaligis et al. (1995) and Mendra et al. (1995), based on productivity, resistance to weed invasion, shade tolerance, speed of establishment, and persistence to harvesting.

There are three main barriers to using this information in the context of grazed oil palm plantations. Firstly, three of the aforementioned experiments harvested the understorey, instead of having it grazed by cattle. Although this enabled the testing of a large amount of plant species, it did not provide information about the interaction between plant species’ palatability and tolerance to physical damage by cattle grazing (Mendra et al. 1995). Secondly, all experiments were set in coconut plantations, it is therefore unclear whether identified plant species can be established and will be productive under the lower light conditions of oil palm plantations. Thirdly, with the exception of Mullen and Shelton (1996) and Hutasoit et al. (2020), which were reviews, the maximum experimental period was 3 years, demonstrating a lack of information on the persistence of the examined plant species over the plantation’s lifecycle, which is especially important since a severe drop-off in introduced understorey productivity was observed with light transmission declining from 77 to 59%, under a 2-year experimental period in a rubber plantation aged one and a half years (Chong et al. 1995). Future assessments of introduced plant species survival and productivity should therefore be carried out under grazed oil palms, over the plantation’s life-cycle.

The impact of understorey plant species introduction on the main plantation crop’s yield should be examined since the competition for nitrogen between the plantation crop and the understorey is a major concern for the oil palm industry. This is especially of importance in the early stages of the plantation lifecycle when light availability does not yet limit understorey growth (Chen 1993). The introduction of understorey species in a young Malaysian rubber plantation was mentioned to reduce rubber tree growth due to competition (Chong et al. 1995). Watson (1964) (cited by Chen (1993)) found that Mikania cordata and woody bushed depressed early rubber tree growth, whereas leguminous creepers increased the growth rate of immature rubber.

There is a need for research on capital (seeds, machinery) and labor investments required to introduce understorey plant species, as well as on returns generated, in order to determine profitability of introducing understorey plant species in grazed oil palm plantations (Mullen 1995). It is important to stress that the reviewed publications did not highlight the need to examine unintended consequences (such as environmental concerns associated with invasive species) before introducing species under oil palm plantations. Through the use of the search term “cattle OR grazing”, this literature search limited results to a context of integrated cattle–oil palm farming systems. Removing this search term greatly increases the number of publications found on the topic of introduced understorey species in plantation systems (for example the 1991 ACIAR conference proceedings titled ‘Forages for Plantation Crops’ (Shelton and Stür 1991a)). In most cases, these results are however related only to forage growth, not the interaction with oil palm or cattle. The topic of factors affecting forage growth in oil palm plantations is sufficient to warrant a review of its own. This review is about the nexus of understorey–cattle–palm and so despite the large number of additional references raised this way, they don’t add much to the research questions asked by this review.

4 Determinants of cattle productivity

4.1 Cattle productivity under ageing plantations

The average weight gain of cattle in oil palm plantations was variable, ranging from 76 to 410 g/head/day (with a median of 264 g/head/day). These variable results arose under diverse conditions: countries, plantation crops, age of plantation, understorey species, cattle breed, grazing management, complicating the process of comparing findings or examining causation. Of the 15 publications classified in the cattle productivity research theme, ten were publications lacking comprehensive information on experimental settings, critical information to directly relate cattle management to productivity.

To maintain desired cattle weight gain under ageing plantations and decreasing understorey availability (both quantity and quality), Chen and Othman (1983) assessed stocking rate (number of cattle (head) per hectare) impact on individual cattle weight gain (Fig. 7). Increasing stocking rates resulted in lower individual cattle weight gains and cattle weight gains further declined under the ageing plantation, both of which can be explained by reduced understorey availability per head of cattle. There were some discrepancies, such as cattle weight gain being higher under a stocking rate of three head per hectare compared to one head per hectare, 5 years after oil palm establishment. Although cited by three separate publications included in this review (Wan Mohamed et al. 1987; Chen 1991; Yusoff 1992), the original publication containing all experimental details could not be accessed. More research is required to conclude what stocking rate is best for which plantation age in order to reach desired cattle daily weight gains, especially since Fig. 7 shows results for the five first years after establishment only, when understorey availability is still high. Declining feed availability in ageing plantations does raise questions on the long-term feasibility and profitability of integrating cattle in oil palm plantations (see Section 3.1).

Fig. 7
figure 7

Individual daily Kedah-Kelantan cattle weight gain when grazing under a Malaysian oil palm plantation, under different stocking rates and increasing plantation age, adapted from (Chen and Othman 1983, as cited by Chen (1991), Wan Mohamed et al. (1987) and Yusoff (1992)).

Limited land availability of smallholder oil palm producers (see Section 2.3) restricts their ability to sustainably feed cattle. In Malaysia for example, where the average recommended cattle stocking rate is one head of cattle per 4 ha (Ab Rahman et al. 2008), the average smallholder owns around 4 ha of oil palm (Ab Rahman et al. 2008). They can therefore theoretically not sustainably feed more than one head of cattle from understorey produced on their plantation (Kamil Azmi et al. 2014). Currently, cattle productivity in smallholder systems remains unknown and research is required to explore alternative cattle management strategies, such as collective cattle management to ensure sustainable grazing.

One option to increase cattle productivity is giving supplementary feed: feeding tree fodder, concentrates and minerals increased daily cattle weight gain by 13.7 g/head/day when grazing in a coconut plantation (Seresinhe et al. 2012). Supplementary feeding was reported to be especially valuable in case of seasonal feed shortages (Wan Mohamed et al. 1987), although no supporting evidence was provided. Research into the supplementary feeding at different plantation ages (with higher or lower understorey productivity) could be carried out in order to determine its profitability.

Cattle productivity can also be increased through the introduction of understorey species: cattle daily weight gain doubled to 330 g/head/day under the introduction of calliandra trees (Calliandra calothyrsus) in a coconut plantation, compared to weight gain with natural forages (Rika et al. 1995). Research on the introduction of forage species was detailed in earlier Section 3.5.

4.2 Cattle productivity under shade of oil palms

Oil palm is grown under tropical climate pattern and needs high air temperatures for growth (optimal temperatures are between 24ºC and 28ºC) (Corley and Tinker 2015). The threshold temperature at which cattle will suffer from heat stress is breed-dependent. For Bos Indicus cattle (including Kedah-Kelantan and Brahman breeds) it is at about 32ºC (Álvarez et al. 2020). Cattle confronted by thermal stress, will decrease feed intake, directly reducing productivity and reproductive performance. The palm trees’ canopy provides shade and reduces air and soil thermal variation (Kassio Fedrigo et al. 2018), thereby lowering cattle heat load (Stür et al. 1994). At 18–40% illumination, the ambient temperature decreased by 1–3 ºC under a mature rubber canopy, while the relative humidity index increased by 1–6% (Chen (1993), citing Wilson and Ludlow (1991)). In their experiment in Jambi, Indonesia, Putra et al. (2019) found that the temperature humidity index under oil palm led to mild to moderate cattle stress. These results were however not compared to the temperature humidity index in open areas. In the Colombian humid tropics, milk productivity of cattle with a high proportion of Bos Indicus was superior (by about 7%, or 0.3 L/head/day) under high and medium tree cover, compared to lower tree cover, explained by the advantageous microclimates formed by trees (Álvarez et al. 2020). The improved micro-climate under oil palm trees might therefore improve cattle productivity (Gutteridge and Shelton 1994), although unknown is the level of productivity improvements because of the absence of studies relating cattle productivity to improved microclimates under oil palms to date.

4.3 Oil palm by-products as feed

The first largest research theme to emerge from this review, together with the grazing impact on understorey productivity and species compositions (Fig. 3), was the use of oil palm processing by-products as feed. Two types of by-products were discerned: the first type consists of oil palm fronds and trunks available at the plantations in situ (Devendra and Leng 2011; Wong et al. 2005) and the second type consists of those products available at palm oil mills (palm kernel cake, crude palm oil, palm oil milling effluents, empty fruit bunches, and palm oil press). Palm oil milling effluents, palm kernel cake and palm press fiber alone represent 4.3 tonnes of by-products per hectare per year which can be used as cattle feed (Arief and Winugroho 2011; Hanum 2018). Research using oil palm by-products for feed supplements improved cattle growth performance (Wan Mohamed et al. 1987; Devendra 2004; Suryana and Yasin 2015), enhanced milk production (Abu Hassan and Azizan 1993, cited by Devendra, 2004), and improved carcass characteristics (Leslie et al. 2012). Palm kernel cake has received particular research attention in recognition of its high crude protein content that is important for cattle growth (Wong et al. 2005). Chemical and physical processing of the oil palm by-products can further increase their feed value, thereby improving feed intake and cattle weight gain (Rahman et al. 2010; Gomez-Vazquez et al. 2017; Nur Nazratul et al. 2019; Rusli et al. 2019). These technologies might however not be available, accessible or mastered by smallholder farmers or farming groups (Silalahi et al. 2019).

For the widespread adoption of feeding by-product, they need to become an economical feed source, the most efficient process being when they are processed and sourced from the plantation property (Wong et al. 2005; Devendra 2008a). The most nutritionally valuable by-products were produced at palm oil mills (second type of by-products as described above), which might be located remotely from the plantations and are generally contracted by large-scale feeding companies. There is a high demand for palm kernel cake on the international market with over 85% of Indonesian palm kernel cake exported in 2018/2019 (McDonald and Rahmanulloh 2019). For smallholder farmers or small-scale institutions without immediate access to a palm oil mill, the use of the by-products produced at the mills was restricted by price, accessibility and availability (Silalahi et al. 2018; Utomo and Widjaja 2020). The use of the by-products by cattle farmers might be more feasible at smaller or more remote mills where the demand for by-products is lower. There is a need to determine which feed resources are both available and accessible to oil palm farmers, whether smallholders or not.

Most research on oil palm by-products as feed has been carried out in laboratories or feedlots (e.g. examination of rumen fluid characteristics or rumen fermentation Astuti et al. 2015; Komalasari et al. 2014)), showing a lack of research and development into technologies that could be scaled out in commercial and smallholder plantation settings. One possible research avenue is finding out whether simple, local and economic techniques for increasing the quality of accessible by-products exist (e.g. chopping up oil palm fronds) and what their impact is on cattle productivity.

5 Determinants of oil palm productivity

5.1 Oil palm productivity

Increases in oil palm yield ranging from 0.6–4.9 t/ha/y, or 2.8–28.9% (with a median of 2.1 t/ha/y, or 19.3%) were reported under cattle grazing in oil palm plantations (Devendra 1991, cited by Devendra, 2004; Rosli and Mohd Nasir 1997, cited by Zamri-Saad 2015; Harun and Chen 1995, cited by Wong, 2005; Nasir 1998, cited by Zamri-Saad 2015). However, all of these publications were classified as ‘weak evidence’: the original publications used could not be sourced and details regarding oil palm and cattle management were absent. There was a dearth of accessible published data on the oil palm yield impact of cattle grazing in oil palm plantations.

Under coconut palms, on average 5% more coconuts were collected under a stocking rate of five heifers per hectare combined with the establishment of a leguminous pasture (Liyanage et al. 1993). The number of coconuts per tree was found to increase by 48% compared to coconut monocultures when cattle were tethered to coconut palms and fed fodder, concentrates and minerals as well as grazing the natural forage (Seresinhe et al. 2012). Both these coconut yield increases were suggested to originate from the improvement in soil quality through the deposition of manure and the recycling of nutrients “locked-up” in the understorey (Seresinhe et al. 2012; Liyanage et al. 1993). Although this might partly explain the higher coconut yields, the eased recovery of fallen coconuts in the shorter, grazed understorey might also play a role (Stür et al. 1994). Latex yield of rubber increased by 10% through cattle grazing combined with chemical weed control compared to conventional rubber management (Senanayake 1996). This was suggested to originate from the reduction in competition between the rubber tree and the understorey plants as well as to manure deposition by cattle.

Even though cattle grazing has been shown to have a beneficial impact on coconut and rubber yield, there is only weak evidence this is the case in oil palm. The underlying reasons for the reported yield increases are currently unknown and should be explored further by examining: changes in soil properties and fertility where manure deposition occurs and its relationship to oil palm yield, as well as the grazing management impact on competition between understorey and oil palm and associated understorey and oil palm productivity. Information on competition between the understorey and the oil palm and associated productivity is available for oil palm monocultures and should be examined.

5.2 Oil palm tree damage

It was generally accepted practice that cattle should not be introduced into oil palm plantations before the palm’s growing point is above cattle reach (5 to 8 years after planting) to avoid damage to oil palm trees by frond nibbling and pushing over of trees (Coomans and Gaullier 1985; Devendra 2004; Fawzi Hj et al. 1998). This practice has been questioned since damage of oil palm basal fronds up to 30%, one and a half to 2 years after planting did not affect yield, explained by the low dry matter production of these older basal fronds situated in shaded positions (Samuel 1974; Wan Mohamed et al. 1987). Fruit bunch nibbling by cattle may occur in young plantations but is unlikely to be an issue because harvesting generally only commences 4 years after planting, when fruit bunches would be less accessible to cattle (Samuel 1974). Frond and fruit damage in young plantations can be reduced by introducing weaner cattle, as they only reach lower oil palm fronds. Alternatively, reduced stocking rates will reduce the frond damage (Wong et al. 2005; Chen 1991). Although cattle damage to oil palms is preventable, the replanting phase with palms under the age of two when cattle should not graze in the plantation presents a challenge for consistent cattle production and reduces the window of opportunity for high cattle production (Gregor 1972; Gopinathan 1998; Wan Mohamed et al. 1987).

Other than direct damage to the oil palms, cattle can also damage harvesting paths through trampling, hampering transport of harvested fruit bunches (Silalahi et al. 2018). In addition, if (electric) fencing is used to enclose the cattle, a tactic should be developed in order not to restrict machinery use for plantation upkeep and harvesting (Gaullier 1986). Since operational strategies to do so have likely developed through trial and error at oil palm plantations experienced in cattle production, those plantations would be a good choice for future studies to obtain information on integrated management of cattle and oil palm.

5.3 Soil compaction

Overgrazing by cattle may lead to soil compaction through trampling (Wong et al. 2005; Borges et al. 2019). It is generally accepted that soil compaction should be avoided as it may reduce water infiltration, restrict root penetration, and limit nutrient and water uptake, possibly reducing yields. The soil’s susceptibility to compaction is dependent on its clay and silt content as well as soil moisture (Reynolds 1988; Coomans and Gaullier 1985). Next to this inherent soil susceptibility to compaction, cattle management determines the amount of trampling and associated soil compaction. These topics have received only scant research attention in the reviewed literature (Fig. 3).

In an oil palm plantation in the Ivory Coast, soil compaction was observed around drinking troughs under a grazing pressure of 122 head.days/ha/year. Learning from this experience, grazing pressure was reduced to 73 head.days/ha/year in a plantation in Cameroon, eliminating visible problems associated with soil compaction (Gaullier 1986). No measurements were made to confirm compaction and information on climate and soils was lacking, preventing determination of cattle management as the sole factor associated with soil compaction. Heifers tethered around coconut palms in Sri Lanka significantly increased soil bulk density around these palms after 3 years, compared to control plots, but without negative yield impacts (Seresinhe et al. 2012). It is important to stress that the grazing pressure of 5869 head.day/ha/year was very high since cattle were tethered to coconut palms and supplementary feed was given. No compaction data or methodologies were presented for this research. Cattle grazing pressure as described in reviewed literature for oil palm estates (see Section 2.3), was typically significantly lower, compaction will most likely be less of an issue depending on local soils and climate. For smallholder farmers with free-roaming cattle, however, soil compaction may be of concern.

The perception that soil compaction by cattle grazing leads to depressed oil palm yields deters the adoption of cattle grazing in oil palms. However, there is currently a lack of evidence linking cattle grazing under oil palm to soil compaction, or linking soil compaction to depressed oil palm yields, suggesting both relationships are important areas for future research.

5.4 Nutrient cycling

Manure excreted by grazing cattle is an organic fertilizer (Suryana and Yasin 2015): manure and urine contain nitrogen, phosphorus and potassium, required crop nutrients for plant growth (Fawzi Hj et al. 1998; Quartermain 2004), while organic matter is important in maintaining soil fertility and structure (Zamri-Saad and Azhar 2015; Fawzi Hj et al. 1998). Consumption of understorey by cattle and the excretion of manure and urine recycles the nutrients “locked-up” into the understorey. It is generally accepted that the over-reliance on manure as the sole source of nutrients for soil fertility, when cattle feed is produced in situ, will lead to a situation of nutrient depletion (Quartermain 2004). Nutrients are drawn from the farming system to be exported as beef and oil palm fruit, and nutrients are lost due to inefficiencies when cycling through animal-manure-soil–plant interphases (e.g. run-off, gaseous emissions). (Reynolds 1988). To avoid nutrient mining and reducing oil palm and understorey productivity, fertilizers can be applied or feed produced off-farm can be fed to cattle.

None of the reviewed publications specifically examined nutrient cycling, but data was available on the nutrient contribution of cattle manure and urine to the soil. Manure and urine from cattle tethered to coconut palms in Sri Lanka with access to supplementary feed (grazing pressure of 5,869 head.day/ha/year) added more nitrogen to the soil than standard fertilizer application, while reducing the need for phosphorus, potassium and magnesium fertilizers, 3 years after onset (Seresinhe et al. 2012). In Sri Lanka, manure from cattle tethered to coconut trees at a grazing pressure of 1,653 head.days/ha/year completely fulfilled the palm’s nitrogen requirement while partly satisfying the palm’s phosphorus and potassium requirements. A 69% reduction in inorganic fertilizer cost was achieved, while increasing the soil organic matter (Liyanage et al. 1993). These results suggest inorganic nitrogen fertilization could be substituted by manure application, while there is still a need for phosphate, potassium and magnesium fertilizers. The grazing pressures were higher in the publications examined compared to grazing pressures generally found in plantations (see Section 2.3), leading to a higher amount of manure excreted, and in both cases the cattle received supplementary feed, a source of nutrient inputs into the farming system.

Nutrient cycling within the farming system highlights the need to understand the farming system as a whole, to make informed decisions on cattle management and fertilizer application for sustainable nutrient management. Nutrient budgets could be made, or modelling studies could be carried out, in order to monitor the farm’s nutrient balance throughout the plantation’s lifecycle, for each nutrient individually. Looking at nitrogen for example, a modelling study found that after replanting the remnant crop residues are large nitrogen inputs to the soil (Pardon et al. 2017). On the one hand this means that yield response to nitrogen fertilizer was low after replanting, therefore yield response to cattle manure will likely also be limited during this timeframe. On the other hand, it was suggested cover crops could be sown early on, to capture some of the excess nitrogen and avoid nitrogen losses. It might therefore be that cattle grazing plays a more important role once nutrients become more limiting and palms become more nutrient demanding, by releasing the nutrients ‘locked up’ in the understorey. Taking into account local conditions, different scenarios of cattle management, supplementary feeding and fertilization should be explored to determine which scenarios are most profitable and sustainable (e.g. minimizing nitrogen losses).

5.5 Agroecology

Compared to oil palm monocultures, plantations with cattle grazing have several potential advantages: enhanced soil cover, reduced agrochemicals application, and increased insect and bird numbers.

Data collection over 45 oil palm plantations in Malaysia showed plantations with cattle grazing had on average 20% more soil understorey cover (Tohiran et al. 2019b): whereas herbicide application created bare soil after application, cattle grazing associated with reduced herbicide application led to short-sward throughout the year (Senanayake 1996; Tohiran et al. 2019b). Understorey soil cover protects the soil from erosion and improves both soil structure and infiltration rate (Corley and Tinker 2008), increased soil cover is therefore a major advantage of cattle grazing in oil palm plantations. Research should be carried out to evaluate the best way to manage cattle in order to reduce weeding requirements (labor and agrochemicals) while maintaining good soil cover.

The potential decreased need for agrochemicals for weed control (see Section 3.4) because of cattle grazing could also have other benefits on the environment, lessens the risk of herbicide resistance, while minimizing the hazard of operator exposure to chemicals (Mullen and Shelton 1995; Riswani et al. 2012; Senanayake 1996; Tohiran et al. 2019b).

Compared to non-grazed oil palm plantations, systematically grazed plantations (defined as well-coordinated grazing into small areas of less than 5 ha for short durations, with one herbicide application per year) promoted greater bird species richness by reducing bare ground and producing taller understorey vegetation, thus creating favorable conditions for bird habitat (Tohiran et al. 2017). More insectivorous birds were found in grazed oil palm plantations compared to non-grazed plantations as these birds were drawn by insects disturbed by cattle or insects attracted by dung (Tohiran et al. 2019a). Bird species richness and composition is anticipated to depend on cattle numbers and grazing management, determinants of manure deposition and understorey composition and biomass (Tohiran et al. 2017).

Manure presence and the grazed understorey increased insect abundance in oil palm plantations. Manure deposition enhanced dung beetle abundance up to fourfold in grazed plantations (Slade et al. 2014; Sa'roni et al. 2020). These beetles bury dung, thereby enhancing soil structure, increasing nutrient cycling, improving soil hydrological properties, increasing soil fertility and plant growth (Slade et al. 2014). The abundance of predatory insects (Reduviidae) decreased immediately after grazing but increased after a rest phase (Nazilah et al. 2020), while the abundance of predatory hymenoptera was higher in grazed plantations compared to non-grazed plantations, due to the disturbance of the understorey by grazing (Aldinas et al. 2019). There is currently no information on the potential impact of insects on oil palm yield.

In coconut plantations manure is a vector and breeding place for the rhinoceros beetle, a major coconut pest (Reynolds 1988). Comparably, oil palm farmers are concerned about dung beetles and manure acting as a vector for the fungus Ganoderma. Ganoderma leads to oil palm basal stem rot, responsible for substantial yield losses in oil palm plantations. The fungus’ spreading mechanism is still to be discovered. Since it is a major issue in the oil palm industry and an important concern limiting the adoption of cattle grazing in oil palm plantations (Sihombing et al. 2020; Utomo and Widjaja 2020), urgent research attention is required (Slade et al. 2014).

Although “agroecology” as a research theme held few publications, it is an important topic requiring more research to ensure the environmentally sustainable development of cattle grazing in oil palm plantations. Further studies could look into cattle grazing management maximizing cattle production while maintaining appropriate ground cover levels and minimizing the use of agrochemicals.

6 Economic implications

When comparing income from cattle sales to expenditures incurred for cattle acquisition and operational costs, returns on investment ranged from 4 to 112% (Table 2). The returns on investments data all originated from large estates, where cattle management required high investment costs for the installation of electric fences, pens, veterinary check-ups and presence of herders for example. Profitability depended on annual cattle prices (Nor et al. 2000), cattle feeding regimes (Seresinhe et al. 2012), cattle productivity (e.g. mortality of cattle) and cattle management in general (e.g. need for electric fencing, veterinary costs) (Gaullier 1986).

Table 2 Return on investment for cattle operations in oil palm plantations (total profit from cattle/total cost for cattle × 100).

Next to the profit originating from cattle sales, four other factors may impact the profitability of cattle–oil palm integrated farming system compared to oil palm monocultures:

  • the potential reduction in weeding costs (see Section 3.4). When adding up the savings in weeding expenses to the profit from cattle sales, the return on investment for cattle grazing in a Malaysian oil palm plantation rose from 24 to 42% (Gopinathan 1998)

  • the possibility of using oil palm by-products as supplementary cattle feed (see Section 4.3)

  • the potential increase (or decrease) in plantation crop yields (see Section 5.1), and

  • the potential reduction in inorganic fertilizer requirements (see Section 5.4)

Considering expenditures and revenues for cattle, reduced fertilizer costs and absence of weeding costs under an integrated cattle-coconut farming system, net returns over a 5-year period were 18% higher in the integrated system compared to coconut monoculture (Liyanage et al. 1993). Taking into account cattle income and costs, reduced weeding and fertilizer costs, as well as increased yields under integrated cattle-coconut systems, total profits were found to be 520–570% higher compared to coconut monoculture, depending on the type of supplementary feed given to cattle (Seresinhe et al. 2012). Profit margins will differ for oil palm plantations because of coconut market maturity and the sale of copra. It was difficult to give a general estimate of the total profitability of integrated cattle–oil palm farming systems compared to monocultures, since the savings through weeding and fertilizer, increases in crop yield and use of by-products all require quantification and will be situation specific. The positive results so far however show it is a system with potential. Furthermore, the integration of cattle in plantation is a pathway for farmers to diversify income and spread production risks (Devendra 2004). It is also worth noting cattle might have non-financial, socio-cultural values, which have not been considered in examined literature.

The reviewed publications had not considered the household economic implications of integrating cattle grazing in oil palm plantations for smallholder farmers. Although dated, Moog and Faylon (1991) explained the average smallholding of 3.3 ha of coconut plantation in the Philippines raised 4290 pesos/year (about 90 USD), which was not sufficient income for the farmer to invest in cattle. Furthermore, three to 4 years were required to start receiving returns from meat or milk sales (Fawzi Hj et al. 1998; Liyanage et al. 1993). Additionally, small landholdings limit understorey feed availability and the size of the cattle herd. High investment costs, long periods for returns and small landholdings make sustainable cattle grazing less attainable for oil palm smallholders.

One of the main drivers for cattle integration in oil palm plantations in Indonesia, and other countries, is decreasing the dependency on beef imports (Meat and Livestock Australia 2019; Kamil Azmi et al. 2014). Although local beef production might offset the import charges (Devendra 2004), local markets for beef will have to be developed in order for cattle–oil palm integration to be profitable (Nor et al. 2000). Information on this topic was absent from the literature examined. Although currently most oil palm processing by-products are exported abroad, the use of these products as feed might become economic under increasing export costs.

In oil palm plantations, the primary production output is currently palm oil, with cattle production taking a secondary role and having to adapt to the oil palm environment. Oil palm canopy cover, especially as the oil palm matures, restricts understorey productivity. In view of the decreasing palm oil prices and the increasing demand for beef, it would be novel to conduct studies to explore whether reducing oil palm density or altering plantation geometry for higher understorey productivity, even at the reduction of palm oil production, could be more profitable due to higher cattle productivity.

7 Conclusion

The major attraction for integrating cattle grazing into oil palm plantations was the availability of understorey vegetation as feed. Decreasing understorey productivity under ageing plantations calls into question the long-term viability of the grazing component. To deal with the decreasing feed availability, stocking rates can be reduced or non-standard planting patterns could be adopted to increase understorey productivity over the plantation’s lifecycle. There is currently a shortage of information on cattle productivity in mature oil palm plantations, and a dearth of information on the adoption and productivity of alternative planting systems. Introduced understorey species show great promise to increase understorey productivity. Research so far has shown suitable plant species for harvested (cutting) understorey under rubber and coconut. There is a need for additional research specifically for the oil palm environment under a grazed system. Additionally, attention should be given to ensure introduced species are well suited to the light environment found under the ageing plantations.

Cattle grazing impacts both understorey productivity and species composition. There is a lack of long-term research over the plantation lifecycle which could give information on the interaction between plantation age and grazing on understorey productivity and species composition, to determine form of cattle management for sustainable grazing. Strategies on to how to integrate cattle grazing in the oil palm plantation throughout the plantation’s lifespan, especially during replanting, should be explored. Furthermore, the existent literature showed a deficiency of information or guidelines for producers on how to assess understorey quantity and quality to set sustainable grazing rates, knowledge necessary for sustainable grazing practices under oil palms.

Research is plentiful into the use of oil palm by-products as cattle feed, showing strong evidence of the beneficial impacts of feeding these on cattle productivity. Accessibility and availability of the highest quality by-products (processed at the oil palm mills) however remains an issue, especially for smallholder oil palm farmers. There is a need to, firstly, conduct more research into increasing the feed value of the by-products available on the plantation such as oil palm fronds and trunks, and secondly, to look into pathways to increasing availability and accessibility of the by-products produced at the mills.

There was no strong evidence of cattle grazing either depressing or increasing oil palm yields. Cattle damage to the oil palms is easily avoidable by either restricting grazing in young oil plantations or by reducing stocking rates to keep frond nibbling under 30%. Next to the lack of information on cattle grazing impact on yields, there is a scarcity of research on the mechanisms through which cattle grazing could impact yields, either positively or negatively. Important unknowns were the relationships between a number of variables in the cattle–oil palm system: do cattle cause soil compaction, and does soil compaction affect oil palm yields?; does the deposition of manure increase soil fertility and thereby enhance oil palm yields?; and does grazing of the understorey reduce the competition between the understorey and the oil palm, consequently increasing oil palm yields?

Reduced agrochemical application, enhanced bird diversity, and greater beetle numbers were positive outcomes for the agroecology of grazed oil palm plantations. Another benefit is increased understorey soil cover which reduces soil erosion, loss of nutrients and organic matter, increases water infiltration and improves soil structure. More research on agroecological topics is required, especially with the growing importance of sustainably produced beef and palm oil. There is a strong need to research the impact of cattle grazing on the spread of Ganoderma in oil palm plantation, since basal stem rot is responsible oil palm yield losses.

Evidence of positive financial outcomes for cattle sales when grazing in oil palm plantations were found in case studies from large-scale oil palm plantations, with cattle weight gains of 76 g/head/day to 410 g/head/day. However, comprehensive detailing of experimental conditions was lacking, restricting the formulation of definitive conclusions. The positive financial outcomes were further enhanced by additional savings from reduced weeding and fertilizing costs, although strongly influenced by site- and management-specific conditions. There was an absence of financial studies conducted in smallholder plantations, which have restricted access to land (therefore understorey availability) and oil palm by-products. Together with their current reliance on free-ranging grazing methods, it is questionable whether smallholder farms graze cattle in an environmentally sustainable and profitable way. Alternative methods of cattle management such as collective cattle management or cut and carry systems should be explored.

It is important to mention that socio-cultural impacts of cattle grazing in plantations were unknown in explored literature. These socio-cultural implications should be included in future research to promote integrated farming systems which fit within the cultural framework.

Cattle integration in oil palm plantations has great potential but further research is required to develop locally-specific productive and sustainable systems of grazed cattle integration with oil palm plantations.