Productivity and water use in intensified forage soybean-wheat cropping systems of the US southern Great Plains
Introduction
Winter wheat (Triticum aestivum L.) is a key agricultural crop in the US southern Great Plains (SGP) (Redmon et al., 1992). Large portions of the SGP are planted to wheat annually, within the broad precipitation range (400–900 mm yr−1) that exists in the region (Patrignani et al., 2014; Baath et al., 2018). For example, about 6.0 million ha were recently planted to wheat across the states of Kansas, Oklahoma, and Texas (USDA-NASS, 2020). It is a flexible crop capable of providing grain, baled straw, hay, grazing - or combinations of these products - within a single growing season (Redmon et al., 1992; Fieser et al., 2006; Edwards et al., 2011). This flexibility in available products makes wheat an important income source for producers. Decisions regarding management, crop use and generated products depends on the market values and production costs of wheat hay, grain, and livestock gain that are encountered annually (Redmon et al., 1992; Peel, 2003; Decker et al., 2009; Edwards et al., 2011).
Crop rotations applied to winter wheat in recent decades in the SGP have become highly simplified. Wheat is largely grown in continuous rotations of growth during September through Early-June, separated by periods of summer fallow to conserve soil moisture for the next wheat crop (Redmon et al., 1992; Edwards et al., 2011; Patrignani et al., 2014). Moisture conservation by summer fallowing is considered an important wheat management in the SGP. It is a technique applied to minimize the potential risk of crop failure, since wheat production largely occurs under rainfed conditions, and the amounts and timing of precipitation received are variable and unpredictable (Schneider and Garbrecht, 2003). For example, a recent study in west-central Oklahoma (Patrignani et al., 2014) reported ∼73 % of all precipitation events during 01 October to 15 June (1994–2011) generated ≤ 10 mm moisture. Similar patterns were noted during summers (Schneider and Garbrecht, 2003; Oklahoma Climatological Survey, 2019).
While continuous wheat-summer fallow monocultures have been functional in production of grain and livestock gain in the US SGP (Peel et al., 2003; Fieser et al., 2006; Edwards et al., 2011), there are issues pertaining to long-term sustainability and agroecosystem function. Among the issues are low precipitation use efficiencies (PUE), generated by loss of available soil moisture to evaporation during fallow periods, instead of use in crop growth (Farahani et al., 1998). Summer fallow of wheat in the SGP generally includes tillage operations for residue incorporation and weed control, resulting in erosion from limited ground cover (Kelley and Sweeney, 2010). Such operations also result in reductions in amounts of organic N and C in soils (Peoples et al., 2009; Kelley and Sweeney, 2010). While capable of producing multiple crops in single growing seasons, wheat monocultures only provide limited ranges of marketable commodities to generate on-farm income (Redmon et al., 1992; Decker et al., 2009; Edwards et al., 2011). As such, wheat monocultures have low resilience to changes in growing conditions that affect production, and the market forces that drive the prices paid for crops (Lin, 2011). There have been recent considerations of replacing summer fallow of continuous wheat systems in the SGP with crops, to improve efficiencies in water use, provide ground cover, and diversify income streams (Farahani et al., 1998; Decker et al., 2009). Among the potential replacements are annual legumes (Rao and Northup, 2009b, c). Soybean (Glycine max.) is one of the most widely grown summer legumes across the world. It is noteworthy that soybean was initially introduced to the United States in the 19th century as a forage crop (Probst and Judd, 1973). Eventually, the land area planted to soybean as forage was surpassed by its use as a grain crop in 1941, due to increased oil and meal demands. However, researchers have shown a renewed interest in adopting soybean as a high-quality forage in several regions of the United States over the past two decades (Sheaffer et al., 2001; Rao and Northup, 2005, 2009a; Foster et al., 2009; Nielsen, 2011). In the SGP, soybean could generate forage yields of 1.1-5.4 Mg ha−1, with crude protein concentrations and in vitro digestible dry matter values ranging between 150−190 g kg−1 and 740−790 g kg−1, respectively (Baath et al., 2018).
Double-cropping soybean and wheat is successful in the US Midwest and southeastern regions, where greater amounts of precipitation occur (Sheaffer et al., 1992; Farno et al., 2002). An extension of double-cropping soybean in continuous wheat systems in the SGP might provide producers with a wider range of services than traditional wheat-summer fallow systems (Decker et al., 2009). However, the feasibility of such intensified systems under the more water-limited conditions of SGP is uncertain (MacKown et al., 2007). Roughly half of the annual precipitation received in the SGP (Schneider and Garbrecht, 2003; Oklahoma Climatological Survey, 2020) occurs during the summer (May-September). As such, the over-extraction of soil water by summer crops might affect the performance of the main cash crop of the regions, i.e. winter wheat (Rao and Northup, 2008, 2009b). Previous research has shown losses of roughly 31 % and 18 % in wheat grain and forage yields, respectively, with 132 mm less soil water in soybean-wheat rotations compared to fallow-wheat rotations in western Kansas (Aiken et al., 2013).
Past research has shown that summer crops with short growing seasons could provide better soil moisture conditions to subsequent wheat than summer crops with longer growing seasons (Lyon et al., 2007; Rao and Northup, 2009b). Likewise, soybeans harvested early as forage could provide adequate amounts of high-quality forage (Rao and Northup, 2005) while reducing water use and effects on wheat yields, compared to later harvests (Rao and Northup, 2008). However, the amount of forage produced by soybean during growing seasons was found to differ among maturity groups (MGs; Rao et al., 2005).
The noted variability that exists in weather of the SGP, and the potential effects of double-cropping on soil resources and wheat production, indicates the need for evaluations of the interactions that exist between growing conditions and cropping systems, to provide useful information on double-cropped agroecosystems in the SGP. It is also important to evaluate the performance of various soybean maturity groups, their harvest management (early vs. late), and their influence on residual soil water used by the subsequent winter wheat crop.
The Decision Support System for Agrotechnology Transfer-Cropping System Model (DSSAT-CSM) is a valuable tool for determining management strategies or cropping systems that may offer economic stability for agricultural production (Jones et al., 2003). Especially for regions such as SGP, where agro-climatic conditions are highly erratic, and crop management decisions are often challenging, DSSAT-CSM can provide critical solutions to the researchable questions, reducing time and resources required to investigate alternative approaches through the long-term, multi-locational research (Soltani and Hoogenboom, 2007). It can stretch information obtained from short term research on strategies of crop management over long-term historical weather records, generate robust appraisals of different cropping systems, and provide guidance on the most-effective strategies and systems to implement, to achieve intensified cropping systems that are sustainable in ecologic and agronomic terms (Boote et al., 2010; Araya et al., 2017).
The primary goal of this work was to utilize DSSAT-CSM to evaluate productivity and water use of different water-limited forage soybean-wheat cropping systems in response to weather scenarios common in the SGP. Our individual objectives were to: (1) calibrate and validate DSSAT-CSM for wheat and three forage soybean cultivars with different maturity groups; (2) compare and assess yield and water use efficiency (WUE) of forage soybean influenced by maturity groups and harvest dates, and (3) their effects on yields and WUE of wheat and overall PUE of wheat systems for SGP using long term historical weather data (1994–2018).
Section snippets
Experimental setup and treatments
The experimental site was situated at the USDA-ARS Grazinglands Research Laboratory, near El Reno, Oklahoma, with the geographical location of 35.57 °N and 98.03 °W and an elevation of 414 m above mean sea level. The soil at the experimental site is classified as Brewer silty clay loam (fine, mixed, superactive, thermic Udertic Argiustolls), with a slope of 0–1 %, pH of 6.6, moderately well drained, and rarely flooded (USDA-NRCS, 1999). Information on soil texture of the experimental site is
Model calibration
A close match between simulated and observed days to flowering and pod initiation was observed for all three soybean types, which was indicated by low d values (<10 %) (Table 3). Likewise, the simulated and observed final pod yields were in good agreement (d <10 %) for Donegal MGV. However, the model slightly over-predicted pod yield for both Derry MGVI and Tyrone MGVII, but can still be considered in good agreements (d <20 %). Biomass yields on all five dates were well predicted by the model,
Discussion
Summer precipitation is a major determinant regulating the productivity of wheat-based cropping systems in the SGP region, hence the use of fallow periods to conserve soil moisture to support wheat production. The decision to adopt a continuous double crop of forage soybean and winter wheat to intensify production by agroecosystems remains questionable. Much of this uncertainty is based on the performance of production systems within limited, short-term studies, and occurrence of high levels of
Conclusion
Simulations by DSSAT-CSM with longer-term datasets suggest that mid-maturity group cultivars of forage soybean could serve as a functional choice for the SGP conditions, by generating adequate yields within a shorter time period than late-maturing types. The optimal soybean harvest for forage could be achieved around 90 DAP to obtain sufficient yield and nutritive value, higher WUEB, and improved yield trade-offs with winter wheat. Regardless of soybean type and harvest date, double-cropped
Acknowledgments
This study was partly supported by a research grant (Project No. 2018-09025 and 2019-68012-29888) through the USDA-NIFA's Agriculture and Food Research Initiative (AFRI). The authors wish to recognize ARS technicians Delmar Shantz, Kory Bollinger, and Jeff Weik for their assistance in managing the experiment, data collection, and analyzing samples. Mention of trademarks, proprietary products, or vendors does not constitute guarantee or warranty of products by USDA and does not imply its
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