Monday, November 13, 2006

Soil Water Distribution and Water Use Efficiency of Forage and Grain Soybeans in the Southern Great Plains.

Srinivas Rao, Brian Northup, and Herman Mayeux. Grazinglands Research Laboratory, USDA-ARS, 7207 W Cheyenne St., El Reno, OK 73036-2144

Growing legumes during summer fallow periods between wheat crops in the southern Great Plains (SGP) can reduce soil erosion and add nitrogen to the soil. However, information on water use by legumes and effects on water availability for subsequent wheat crops is limited. We described soil water patterns (compared to fallow conditions) and water use efficiency of forage (n=3) and grain (n=1) soybean [Glycine max (L.)] cultivars planted no-till on silt loam soils. Inoculated seeds were planted in two replicate sets of 3 m x 20 m plots (60 cm row spacing) in June 2003, 2004, and 2005 following harvest of winter wheat. 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, and forage samples were collected when moisture measurements were taken.  Forage production, water use, and water use efficiency (WUE) of cultivars were calculated and analyzed by mixed models. Significant (P<0.01) interactions in soil water content occurred between days since planting, depth, cultivars and years. Higher values were recorded under soybean cultivars in 2003 than 2004 and 2005. Water contents under all cultivars were lower than in fallow plots. Significant variety (P=0.04) and year (P<0.01) effects were recorded in amounts of water used. Forage soybeans used 5% more water than the grain cultivar. 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 was 55% less than in 2004 and 2005 (P=0.03), and Tyrone produced 16% less biomass (P=0.07) than other cultivars. Soybeans displayed higher (P=0.08) WUE in 2004 (16.0) than 2003 (WUE=13.2) or 2005 (WUE=10.9). Incorporating soybeans into fallow periods of wheat production systems of the SGP will limit the soil water available for producing fall wheat forage.