High Rates of Nitrogen Change Composition of Shortgrass Rangeland in Southeastern Wyoming

Abstract

High rates of nitrogen applied at one time or over a 4-year period markedly changed the botanical composition of shortgrass range. Blue grama and buffalograss cover declined and western wheatgrass increased. Total yields were significantly increased, largely because of the increase of annual forbs. Thus the only desirable change in the composition resulting from the high N application was the increase of western wheatgrass. Over the 5-year period, the NO3- N accumulated in the 12- to 24-inch soil depth, whereas the grass roots were concentrated in the top foot of soil. The high rates of N increased crude protein significantly, thereby enhancing the palatability of the forage. Forbs on the high N plots were searched out and readily grazed by the sheep. Crude protein content was higher in the forbs than in the grasses in the fall. This study shows that high N rates applied either at one time or in yearly applications are neither economical nor practical because of the shift in the composition to undersirable annual forbs and slow recovery by the perennial grasses. The 150-lb N/acre rate applied once might be considered more practical than the other rates used in this study. Over the 5-year period this N rate produced 1,705 lb more total herbage than the check or 11.4 lb of herbage/lb of N. Neither the yield nor the crude protein increase was large enough to justify nitrogen fertilization of this range as an economic practice.