Nitrogen and phosphorus fertilization of rangelands affects yield, forage quality and the botanical composition

Abstract

Nitrogen fertilization of rangelands in order to increase dry matter yield results in a decrease in legume ratios in botanical composition, which reduces forage quality. The objectives of the present study therefore were to investigate whether this negative effect of N fertilization on forage quality can be compensated by additional P application and also to determine the optimum fertilizer doses in rangelands to obtain economical benefits. Therefore, 0, 60, 120, 180 kg N ha −1 and 0, 26, 52 kg P ha −1 fertilizer rates were applied each year over a period of 3 years to 12 plots within each of 4 blocks. Botanical composition of the plots was determined and classified as grass, legumes and others for each treatment group based on dry weights. Dry matter yield, crude protein concentration and crude protein yield in treatment groups for each year were determined. Consequently, averaged over the 3 years of experimental period, nitrogen fertilizer increased the dry matter yield. The dry matter yield was 1467 kg ha −1 in control plot, while it increased up to 3293 kg ha −1 in plot applied with 180 kg N ha −1 without P. Nitrogen fertilization slightly decreased the crude protein concentration in the forage dry matter from 120 g kg −1 in the non-fertilized control to 103–116 g kg −1 in the plots fertilized only with nitrogen. This effect can be explained by the observation that the nitrogen fertilization resulted in a decline of the legume proportion from 47% in the non-fertilized control to 5% with the highest N rate. The protein concentration in legume plants was always considerably higher than that in the grass and other species. Applying additional phosphorus compensated this negative effect of the nitrogen fertilization on the forage quality in terms of protein concentration. The economic optimum was found with the highest fertilizer doses providing 52 kg P ha −1 + 180 kg N ha −1 producing 4810 kg ha −1 forage dry matter with a crude protein concentration of 124 g kg −1 and legume proportion of 12%.