The Impact of Added Nitrogen to First and Second Year Corn after Alfalfa.

Corn following alfalfa is a common crop rotation in the Intermountain area of Utah and southern Idaho. This is due, in part, to the availability of residual nitrogen following this legume crop that can be credited to the fertilizer requirement of corn. The Utah Fertilizer Guide, published in 1989, has long recommended a credit to a crop following alfalfa of 112 kg/ha (100 lbs N/acre). Recent research in this region indicates that the N credit from alfalfa may be significantly higher than currently recommended. Therefore, on-farm experiments were conducted to test whether different nitrogen application rates affected corn silage yield in first- and second-year corn following alfalfa. Four replications of four different rates of nitrogen were applied (0, 56, 112, and 224 kg N/ha; or 0, 50, 100, and 200 lbs N/acre). The data from 14 site-years of first-year corn showed no statistical difference between application rates. We also had data from 8 site-years of second-year corn, where only one instance of a small statistically significant yield difference between 0 and 224 kg N/ha (0 and 200 lbs N/acre) was found. Yield under the other two rates was not statistically different. It appears that the N credit to silage corn from a preceding alfalfa crop can be significantly increased, and may result in no need for fertilizer N in the first year following alfalfa. A better understanding of the response of first- and second-year silage corn following alfalfa will improve nitrogen recommendations, protect against environmental degradation caused by excess N, reduce farm costs, and improve best management practices.