Sunn hemp (Crotalaria juncea L.) as a Cover Crop for Winter Wheat
Type of Degreethesis
Agronomy and Soils
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The use of cover crops in conjunction with non-inversion tillage is a popular conservation system that is incorporated into production rotations to improve profitability and sustainability of depleted soils in the Southeast. Due to fluctuating fertilizer costs and potential environmental hazards, producers should look to relinquishing their heavy dependence on synthetic fertilizers and opt to utilizing legume cover crops as a biological alternative for Nitrogen (N) fertilization. Sunn hemp (Crotalaria juncea L.) is a tropical legume capable of producing large amounts of biomass and symbiotic N in as little as 8-12 weeks during summer months, which makes it an ideal candidate to include in crop rotations where time is limited. The inability of previous sunn hemp cultivars to produce viable seed in the temperate climate of the U.S. led to limited seed supply. Extensive use of this cover crop species has been limited due to high cost and low availability of the seed. However, the latest breeding efforts of Auburn University produced the sunn hemp cultivar ‘AU Golden’, capable of producing seed under temperate climate conditions. In order to maximize profitability of a new plant species, proper management strategies must be determined through performance evaluations. The first objective of this study was to compare three sunn hemp planting dates with regard to biomass production, N accumulation, and forage quality at three locations to determine an optimum planting date. A second objective was to determine if sunn hemp would be effective in reducing N fertilizer requirements for a subsequent winter wheat (Triticum aestivum L.) crop. The final objective was to monitor the decomposition and N release of sunn hemp residue in the field at two locations between cover crop termination and wheat planting. Overall, ‘AU Golden’ biomass production averaged 7.0 Mg ha-1 in 2012 and 5.9 Mg ha-1 in 2013. Sunn hemp N content corresponded to sunn hemp biomass production, while N concentration was inversely related. Biomass and N production were 87% and 55% higher for June and July plantings when compared with May during 2012, across the three locations. Excessive precipitation and milder temperatures reduced production for the second growing season. Biomass and N production were higher at the Tennessee Valley Research and Extension Center (TVS) and Wiregrass Research and Extension Center (WGS) locations compared to the Plant Breeding Unit (PBU), with superior production seen during May and June plantings compared with July plantings. Acid Detergent Fiber (ADF) and Neutral Detergent Fiber (NDF) values for sunn hemp 25-30 days after planting (DAP) indicated that leaves would serve as suitable forage for livestock, but ADF and NDF measurements for stems were too high to be considered easily digestible. Wheat yields following sunn hemp resulted in little to no difference when compared with yields following a fallow area for the 2013 and 2014 growing seasons. Wheat yields for fallow plots were 30% higher than yields following sunn hemp plots when treated with recommended rates of urea ammonium nitrate (UAN) at the TVS location during the 2013 growing season. Little difference was observed among planting date treatments for 2014 wheat yields across three locations. Wheat yields were similar for the 2014 growing season at TVS and WGS in comparison with PBU, at which yields were 37% lower. Sunn hemp mineralization data suggests that N was released quickly, and the rapid decomposition limited the N contribution to the subsequent wheat crop. In some cases, as much as 65% of N was lost within the first two weeks of decomposition, leaving little to no available N for wheat uptake. These results indicate that further evaluation of proper management systems for this cultivar must be conducted in order to successfully introduce ‘AU Golden’ sunn hemp as an advantageous cover crop in the Southeast.