Nitrogen rate and harvesting time based on growing degree days influenced winter cereal rye morphological traits, forage yield, quality, and farm profit in poorly drained Alfisols

Winter cereal rye (Secale cereale L.) (WCR) is often double cropped with maize for silage (Zea mays L.) to increase farm forage supply and profit. Spring nitrogen (N) fertilization to WCR could influence its production and quality at different harvesting times. Therefore, two on-farm trials were conducted in the 2019–2020 and 2020–2021 growing seasons to evaluate the effect of harvesting time (late-March to end-of-April considering the growth stage) and spring N fertilization (0, 23, 47, and 71 kg N ha⁻¹) on WCR morphology, forage yield, nutrient removal, quality, and farm profit. Only two N treatments (0 and 47 kg N ha⁻¹) were evaluated for each harvesting time in trial 1. A quadratic model best explained an increase in WCR dry matter (DM) yield in response to growing degree days (GDD) accumulation (R² = 0.81). An increase in GDD linearly decreased WCR relative forage quality (RFQ). Benchmarking RFQ at 150 for dairy milk production indicates WCR should be harvested at a GDD of 543, at which WCR plant height was 31.8 cm and DM yield was 0.77 Mg ha⁻¹. This resulted in loss of profit in both study years compared to later harvesting dates at higher GDDs. Benchmarking RFQ at 125 for heifer production indicated that harvest should occur at a GDD of 668, at which the WCR was 71 cm tall, and its DM yield was 2.25 Mg ha⁻¹. Nitrogen balances were negative at the N0 treatment (0 kg N ha⁻¹), indicating a need for some N to maximize WCR yield. We found that a rate between 21 and 47 kg N ha⁻¹ maximizes yields reflecting slightly positive balances, in which the highest profits occur. Our results suggest that the harvesting time can be predicted by GDD and should be adjusted for RFQ. We conclude that less than 47 kg N ha⁻¹ N fertilizer is required for WCR production in soils with manure history and high soil organic matter (>30 g kg⁻¹).