Predicting Late-Season Rye Cover Crop Biomass from Early-Season Observations.

Cereal rye (Secale cereale L.) is commonly grown as a cover crop in the Eastern US due to its winter hardiness and adaptability to variable environmental conditions. Management and climate directly affect rye biomass production but these effects can be influenced significantly by background levels of soil fertility.  Scientists from USDA ARS Beltsville, MD (BARC), North Carolina State University, Pennsylvania State University, and Cornell University investigated the effects of residual Fall soil N over 6 site years. Nitrogen was applied at 0, 30, 60, 90, and 150 kg N ha^-1 to create a gradient of soil N conditions to measure rye tiller production, biomass, tissue N content, and normalized difference vegetation index (NDVI).  Data were collected at growth stages (GS) 25, 30, and 60. Rye biomass and N accumulation varied across site years and tended to increase with N application. Averaged across sites, potential biomass was 2853, 4844, and 9739 kg ha^-1 for GS25, GS30 and GS60,  respectively.  The majority of rye N accumulation occurred by GS25 but was variable across site years. Early season biomass served as the single largest source of information in the fitted regression models. Variance attributed solely to biomass constituted 38.5-65.2% of the total model variance across growth stages. Unsurprisingly, GS25 biomass alone was nearly as good of a predictor of GS60 biomass and total N (simple R² of 0.592 and 0.508, respectively) as when it was included in a more complex model (Adjusted R² of 0.526 and 0.440, respectively). The fitted models predictive abilities ranged from 34-60% depending on the GS and parameters chosen. Our results illustrate the difficulty of predicting late season rye biomass and N content based on early season measurements.