Sorghum–Sudangrass Responses to Nitrogen and Tillage following Polyphenol‐Containing Legumes, Alfalfa, Reed Canarygrass, and Kale

Abstract

Core Ideas Low concentrations of polyphenols in forage legumes had no impact on sorghum–sudangrass production. Sorghum–sudangrass dry matter yield and N uptake were related to herbage N yield from prior forages. Tillage enhanced sorghum–sudangrass production especially after reed canarygrass. Adjusting fertilizer N based on crop sequence and tillage practices avoids soil nitrate buildup. The collective effects of protein‐binding polyphenols (PBP), preceding forage type, tillage, and fertilizer N on soil nitrate production, N uptake, and dry matter yield (DMY) of N‐demanding crops such as sorghum–sudangrass [SS, Sorghum bicolor (L.) Moench × S. sudanese Piper] are poorly understood. These factors and response variables were evaluated in Wisconsin by growing SS for 2 yr with two harvests yr−1 using no‐tillage or conventional‐tillage and fertilizer N rates of 0, 33, 67, 112, or 220 kg N ha−1 harvest−1 following 1‐yr stands of birdsfoot trefoil (Lotus corniculatus L.) containing condensed tannins (CT), o‐quinone producing red clover (Trifolium pratense L.), alfalfa (Medicago sativa L.), reed canarygrass (Phalaris arundinacea L.), and kale (Brassica oleracea L.). Herbage PBP was associated with reduced in vitro soil NO3–N accumulation, whereas herbage nitrogen yield (NY) primarily affected in situ soil NO3–N accretion and N uptake and DMY of SS. The DMY of first‐year sorghum–sudangrass (SS1) following legumes exceeded that following kale in both tillage systems, but rotational benefits of grass equaled legumes when conventional tillage was used. To maximize net return, SS1 required 75 kg N ha−1 harvest−1 following legumes and 125 kg N ha−1 harvest−1 following nonlegumes. Depending on tillage system and harvest, SS2 required 50 to 130 kg N ha−1 harvest−1 to maximize net return. To optimize DMY and profitability of SS following short‐term forages, producers should grow legumes to maximize NY before SS production, use conventional tillage following reed canarygrass, and adjust N rates according to crop sequence and tillage practices.