The amount of N available for cash crops 4 to 6 weeks after cover crop termination depends in part on the maturity of the cover crop when it is killed. Oregon State University researchers estimated a cereal rye cover crop yielded 20 pounds of plant-available N per acre when terminated during its vegetative growth stage and tied up 27 pounds of N per acre when terminated at heading. Source: D.M. Sullivan and N.D. Andrews, Oregon State University (2012): PNW 636