With renewed interest in legumes as green manures, it is important to understand their effect on in-season N uptake of following non-legume row crops. This study assessed the effect of legumes as green manures on in-season N uptake by subsequent grain sorghum [Sorghum bicolor (L.) Moench] grown in conservation tillage systems in the eastern Great Plains. Treatments were (i) red clover (Trifolium pratense L.) and hairy vetch (Vicia villosa Roth) before grain sorghum vs. continuous grain sorghum, (ii) reduced or no-tillage, and (iii) fertilizer N rates. The experiment was conducted on two adjacent sites (Parson silt loam: fine, mixed thermic Mollic Albaqualf) similar in organic matter but Site 1 higher in pH, P, and K than Site 2. In-season N uptake was often statistically greater in reduced-tillage than no-tillage systems. At both sites, red clover as a previous crop resulted in about 25% greater N uptake by sorghum vs. sorghum grown continuously with no previous legume crop. Nitrogen uptake by sorghum at the boot and soft dough growth stages responded linearly to increasing N rate, but the slope was 135 kg ha(-1) during the first year for both legumes at each site, but values for red clover remained greater than those for hairy vetch in subsequent years, especially at the higher fertility site. Grain yield tended to be maximized when N uptake at the soft dough stage exceeded 100 kg ha(-1) at Site 2 but continued to increase as N uptake increased at the higher-fertility Site 1. Utilizing legumes as green manures can increase in-season N uptake by following grain sorghum crops compared with continuous sorghum in these prairie soils.
