Nitrogen Contributions from Late-Summer Planted Cover Crops to Winter Wheat in a Conservation Tillage Cropping System.

In the southeastern USA, planting late-summer cover crops after maize (Zea mays L.) may allow producers to integrate summer cover crops into cropping systems without sacrificing cash crop production. The objective of this study was to determine N contributions from sunn hemp (Crotalaria juncea L.) and cowpea (Vigna unguiculata L.) to wheat (Triticum aestivum L.) in a maize–cover crop–wheat sequence. Preliminary results (2016-2017) from Citra and Jay, FL are presented. A split-plot design was used with main plots of sunn hemp, cowpea, or fallow, and subplots of mineral N application on wheat (0, 34, 67, 101 kg N ha-1). Cover crops were drilled into maize residue in late-August, spray-terminated in late-October, and wheat was drilled one and four weeks after cover crop termination in Citra and Jay, respectively. Erratic precipitation delayed planting in Jay. Cowpea and sunn hemp accumulated 82-100 kg N ha-1 and 78-128 kg N ha-1, respectively. In Citra, soil inorganic N (0-15 cm depth) at wheat planting was higher in plots preceded by sunn hemp compared to cowpea and fallow, but cover crop effects on soil inorganic N were not observed in Jay. Normalized difference vegetation index (NDVI) readings on wheat that received no mineral N in Citra were generally higher where sunn hemp was grown compared to fallow through Feekes 5. Wheat preceded by sunn hemp accumulated 16-39% more biomass than wheat planted after cowpea and fallow in Citra, but similar effects were not observed in Jay. Cowpea and sunn hemp contributed 10 kg N ha-1 and 15 kg N ha-1, respectively, to wheat in Citra, but cover crop N contributions were not detected in Jay. These results suggest that N contributions from summer legume cover crops to wheat in North Florida are modest and may be lost entirely if wheat planting is delayed.