Performance responses of lambs of various ages to Italian ryegrass (Lolium multiflorum) fertilized with various levels of nitrogen

Abstract

Italian ryegrass (Lolium multiflorum cv. Midmar) is often fertilized with high levels of nitrogen (N) in order to ensure maximum dry matter (DM) yields. This practice also increases the N content of herbage. This study was conducted to determine whether differences in the nitrogen (N) content of Italian ryegrass would have an effect on the performance of South African Mutton Merino lambs. Fertilizer levels of 100, 200, 400, 600 or 800 kg N/ha were applied to the pastures in order to produce herbage containing varying concentrations of N. Nitrogen fertilization increased the total N and nitrate-N in the plant material and decreased non-structural carbohydrate concentrations. An increase in rumen ammonia-N concentration with increasing pasture N concentration was observed in twelve and four-month-old lambs. A quadratic relationship was found to exist between the N content of ryegrass and DM intake for twelve-month-old lambs, and a negative linear relationship was observed for four-month-old lambs. A quadratic relationship was observed between the N content of ryegrass and average daily gain for lambs of both age groups. The equations derived in this study illustrate the potentially negative effects of high concentrations of N and nitrate-N in Italian ryegrass on intake and growth rate of lambs.