The Cover Crop Nitrogen Availability Calculator – Research and on-Farm Validation.

There are few tools to help farmers modify their nitrogen (N) fertilizer rates based on the cover crop properties. We have tested a Cover Crop N Availability Calculator for on-farm use that was adapted from the N subroutine of the CERES crop models. Cover crop N, carbohydrates, cellulose, and lignin are measured by NIRS and used with cover crop biomass, soil moisture, and temperature to predict available N.

The calculator has been used with 12 site-years to evaluate yields with and without predicted N credits. Research farms used no cover and cover crop treatments at four levels of N (0, 0.5x, 1x, and 1.5x the recommended N rate) with cowpea before fall broccoli; oats/Austrian winter peas before sweet corn; and black-seeded oats /crimson clover and black-seeded oats before grain corn. On-farm sites compared no cover at full N rate with full N rate - cover crop credit in cowpeas and sunnhemp before fall broccoli, and cover crop at full N rate with sidedress N modified by cover crop credit in oats before cotton, and four rates of poultry litter (0, 0.5x,1x, and 1.5x usual farmer rate) applied to cereal rye before soybeans. Nitrogen credits predicted by the Calculator ranged from 8 (black-seeded oats) to 159 kg ha^-1 (cowpeas) with an average credit of 53 kg ha^-1. Nitrogen immobilization was not predicted at any site.

Yield ratios (measured yield with N credit or debit/yield with full N rate) were used to determine if N management using the calculator would result in lower than expected yields. Cumulative probability plots indicated the mean yield ratio was 1.03 with 95% confidence interval of 0.85 to 1.15. Our results indicate the potential of the N Calculator as a tool to help farmers better manage N and understand the benefits of cover crop use.