Common vetch cultivars improve yield of oat row intercropping on the Qinghai-Tibetan plateau by optimizing photosynthetic performance

Highlights

• Greater plant height and Pn are potential determinants of RUE of intercropped oat.

• Three common vetch cultivars improved radiation use of intercropped oat.

• Lanjian No. 2 and No. 3 are promising candidates for common vetch-oat intercropping.

Abstract

Sustainable development of livestock production is challenged by the shortage of forage resources on the Qinghai-Tibetan plateau (QTP). Common vetch-oat intercropping is used on the QTP and in other alpine regions to obtain substantial forage yield, but few common vetch cultivars have been evaluated in intercropping systems. A three-year field experiment in the eastern QTP was carried out to compare plant growth, photosynthetic rate (Pn), radiation use efficiency (RUE), aboveground biomass, land equivalent ratio (LER), and competition ratio of oat row intercropping with three new common vetch cultivars (Lanjian No. 1, Lanjian No. 2, Lanjian No. 3) under low-input conditions. Compared to sole cropping at seed densities of 141 and 667 plants m^–2, for common vetches and oats, respectively, alternate row intercropping with each species at half these densities significantly increased the Pn (by 15–19%) of oat, and reduced the Pn (by 12–26%) of common vetch. Moreover, row intercropping significantly increased intercepted photosynthetically active radiation and RUE (by 44%) for oat but significantly decreased these for common vetch (by 34%). Among common vetch cultivars, at both flowering and maturity stages, Lanjian No. 2 (1.21, 1.15) and Lanjian No. 3 (1.27, 1.21) had greater LER than the later-maturing Lanjian No. 1 (1.07, 1.01) when intercropped. Oat row intercropping with Lanjian No. 2 and Lanjian No. 3 increased forage production compared to sole cropping oat or common vetch on the QTP. These findings provide scientific support for common vetch-oat intercropping as a sustainable approach to increasing forage production from cropland in alpine regions.

Introduction

The Qinghai-Tibetan plateau (QTP) is important for livestock production in China and is dominated by approximately 60% alpine grassland ecosystems (Cao et al., 2018). However, biologically and economically sustainable development of livestock production is challenged by the productivity of forages and harsh environmental conditions. In recent years, over-grazing and subsequent land degradation due to increased livestock production on the QTP have reduced grassland productivity (Long et al., 2008; Harris, 2010; Zhang et al., 2019a; Dong et al., 2020). The degradation of grasslands and expanded demand for forage production have increased the pressure to cultivate more alpine forages (Zhang et al., 2013). Forage yield in alpine regions such as the QTP is limited by low air temperature, a short growing season, and an unstable climate (Zhang et al., 2015). With increased livestock production in alpine regions, it is crucial to improve the productivity of forages with agronomic practices capable of offsetting the negative effects of adverse climatic conditions to satisfy food security and sustainability. Therefore, advances in forage production are urgently needed.

Intercropping, the practice of growing two or more crop species in the same field (Brooker et al., 2015), is widely used on low-input farms (Anil, 1998; Feike et al., 2012). Compared to the average of both crops under sole cropping, intercropping has been shown to provide greater total yield (Wang et al., 2015a) and yield stability (Lithourgidis et al., 2007) while also increasing use efficiency of resources such as land, solar radiation, water, and nitrogen (Lithourgidis et al., 2011; Rahman et al., 2017; Ren et al., 2018). Intercropping can also offer greater financial stability and reduce negative environmental impacts than the average of both crops under sole cropping (Lithourgidis et al., 2011). Therefore, intercropping is considered an alternative practice for sustainable agriculture (Wang et al., 2015c). In intercropping systems, the combination of cereal and legume intercrops is preferred in many countries due to the nitrogen fixation benefits from legumes (Bedoussac et al., 2015; Hu et al., 2016).

The yield advantage with intercropping system is generally associated with greater resource use, in which solar radiation is one of the most important factors (Dutra et al., 2017). Plant biomass accumulation is related to intercepted photosynthetically active radiation (PAR) (Monteith, 1977). Canopy architecture and leaf pigments determine the percentage of intercepted PAR (Xue et al., 2016; Zhang et al., 2019b). Radiation use efficiency (RUE) represents the comprehensive response of the factors which affect photosynthesis and respiration (Gou et al., 2017). In intercropping systems, canopy structure and photosynthetic changes enable greater interception of PAR and RUE compared to sole cropping (Kermah et al., 2017). In maize (Zea mays L.)-soybean [Glycine max (L.) Merr.] intercropping, received PAR of maize in strip intercropping and single row intercropping was 1.38- and 1.27-fold greater than sole cropping, respectively, while received PAR of soybean was only 55 and 33% of that with sole cropping, respectively (Liu et al., 2018). In another study, received PAR at the top of the soybean canopy decreased by 91 and 67% in strip intercropping and single row intercropping, respectively, compared to sole cropping (Fan et al., 2018). However, RUE of intercropped soybean and maize averaged 1.5- and 1.15-fold greater than that with sole cropping, respectively (Liu et al., 2017b). In maize-wheat strip intercropping, RUE of wheat (Triticum aestivum L.) increased while that of maize decreased compared to sole cropping (Gou et al., 2017), and RUE of sole cropped maize averaged 13% greater than that of intercropping (Wang et al., 2015c).

Common vetch (Vicia sativa L.) and oat (Avena sativa L.) are the main cultivated annual forages on the QTP and are well-adapted to the harsh alpine environment (Chen et al., 2015). As annual forages, they are relatively low cost, easy to manage, and can be included in various crop rotations (Zhang et al., 2017). Both have become important forages for livestock, especially as supplementary feed during winter (Nan, 2005; Yang et al., 2010). Oat is the most widely grown annual cool-season forage cereals in the world (Andrzejewska et al., 2019). Many oat cultivars have been evaluated for forage yield in intercropping with berseem clover (Trifolium alexandrinum L.) and pea (Pisum sativum L.) (Ross et al., 2004; Baxevanos et al., 2017; Tsialtas et al., 2018). In comparison, few cultivars of common vetch have been tested in intercropping, especially in alpine regions. Common vetch-oat intercropping is widely used on the QTP and in other alpine regions to obtain greater forage yield (Zhang et al., 2015). Selection of appropriate cultivars for intercrops is important for optimizing the performance of intercropping systems (Carr et al., 2004; Gebeyehu et al., 2006). Most studies on intercropping common vetch or pea with oat mainly reported the effects of sowing rate and nitrogen fertilization on yield, forage quality, nitrogen fixation, and water use efficiency (Dhima et al., 2007; Jikai et al., 2011; Zhang et al., 2013; Luo et al., 2017; Tsialtas et al., 2018). To date, less attention has been given to the performance of intercrop cultivars. Additionally, the photosynthetic characteristics and RUE of common vetch-oat row and strip intercropping systems is unknown. Recently, three new cultivars of common vetch (Lanjian No. 1, Lanjian No. 2, and Lanjian No.3), which differ in maturity and yield, have become widely cultivated on the QTP and in other regions (Mao et al., 2012; Xu et al., 2017; Huang et al., 2019).

These cultivars were selected from four breeding lines, using genotype × environment interaction assessment to identify genotypes with high performance over a wide range of sole cropping environments (Nan et al., 2004). Genotype × cropping system interaction is most likely to occur in the understorey or dominated crop, and occurs less often in the overstorey or dominant crop (Davis and Woolley, 1993). Where the interaction is significant, it may be necessary to test and choose different cultivars for each cropping system (Oliveira Zimmermann, 1996; Santalla et al., 2001; Atuahene-Amankwa et al., 2004). However, most of the current research focuses on improving productivity of sole cropping through adjusting crop management (Zhang et al., 2017; Zhang et al., 2019c). To optimize intercropping systems, the genotype × cropping system interactions should be evaluated for their degree of influence on yield and quality (Gebeyehu et al., 2006). Such interactions have been reported in shoot and root biomass in faba bean (Vicia faba L.)-wheat intercropping, in forage yield and crude protein yield in pea-oat intercropping, and in seed and other components yield in common bean (Phaseolus vulgaris L.)-maize intercropping systems (Santalla et al., 2001; Baxevanos et al., 2017; Rediet et al., 2017; Streit et al., 2019). A similar evaluation for common vetch cultivars may provide an opportunity to improve the common vetch-oat intercropping systems.

The objectives of this study were to (i) investigate the variation of photosynthetic traits and RUE in common vetch-oat row intercropping for three common vetch cultivars and (ii) evaluate forage yield of intercrops in common vetch-oat row intercropping for three common vetch cultivars. Our results provide scientific evidence for improving productivity of intercropping systems in alpine meadows. To our knowledge, this is the first study to report genotype × intercropping effects in oat-common vetch intercropping.