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Title: Forage and Grain Soybean Effects on Soil Water Content and Use Efficiency

item Rao, Srinivas
item Northup, Brian

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2007
Publication Date: 3/1/2008
Citation: Rao, S.C., Northup, B.K. 2008. Forage and grain soybean effects on soil water content and use efficiency. Crop Science. 48:789-793.

Interpretive Summary: Double-cropping soybeans after winter wheat is a common agricultural practice in several areas of the USA. About one-third of the soybean acreage in the southeast is double cropped with wheat. However,double cropping is not common in the southern Great Plains (SGP). Producers in the SGP rely on wheat forage for fall grazing by yearling stocker cattle, in addition to harvesting a grain crop in June. The dry conditions during summer fallow after wheat (June to August) means soil water available for growing summer forage may be limited. This study determined the water use of three forage-type and one grain-type soybean cultivars, and how they affected soil water content in a production system with continuous no-till wheat during 2003 through 2005. We found continuous double cropping of soybeans following wheat can reduce soil water available for a fall planting of wheat. Forage soybeans used 5% more water than the grain-type, and all cultivars used 2 to 2.5 times more water in a wet year than during a dry year. The dry condition in the SGP means double-cropping soybeans after wheat will be risky. Soybeans could be double cropped during years with higher levels of rainfall sufficient to allow forage production by soybean, without reducing production by the following wheat crop. A different approach would be to include wheat (September to June) and soybeans (June to October) in rotation with a winter fallow period (November to February) and spring cover crop (March to May). This combination would allow producers to conserve soil water and diversify farming operations.

Technical Abstract: Growing legumes in the southern Great Plains (SGP) during fallow periods between wheat (Triticum aestivum L.) crops can protect soil resources, add nitrogen (N) to soil, and supply high quality summer forage. The objective of this study was to determine the water use, water use efficiency and soil water depletion of forage and grain soybeans in the southern Great Plains (SPG). We determined plant available soil water (compared to fallow conditions) and water use by no-till forage (n=3) and grain (n=1) soybean [Glycine max (L.)] cultivars on silt loam soils following winter wheat harvest during the 2003, 2004, and 2005 summer growing seasons.. Soil water content was measured bi-weekly during July through September at 0-20, 20-35, 35-50 and 50-65 cm depths with a neutron meter. Forage samples were collected whenever soil water measurements were taken. Standing crop, water use, and water use efficiency (WUE) of cultivars were calculated and analyzed by mixed model techniques. Significant (P<0.01) interactions in soil water occurred between sampling date, depth, cultivars and years. More soil water was recorded under soybean cultivars in 2003 than 2004 or 2005, but were lower than in fallow plots. Significant year (P<0.05) effects were recorded for standing crop; water used, and water use efficiency. Forage soybean used 5% more water than the grain cultivar, and all cultivars used 2.0 to 2.5 times more water in 2004 and 2005 (wetter years) than 2003 (dry year). Forage production in 2003 (3034 kg ha-1) was 55% lower than in 2004 and 2005. Higher WUE was recorded in 2004 (16.0) than 2003 (WUE=13.2) or 2005 (WUE=10.9 kg ha-1 mm-1). Incorporating soybean into fallow periods following wheat production, will limit soil water for fall wheat forage in the SGP.