The effect of plant structure on the intake of tropical pasture. III.* Influence of fertilizer nitrogen on the size of bite harvested by Jersey cows grazing Setaria anceps cv. Kazungula swards

Abstract

The effect of fertilizing Setaria anceps cv. Kazungula swards with nitrogen on the size of bite harvested by cattle was determined and the influence of sward canopy structure upon bite size was investigated in two experiments. The first experiment measured the effect of nitrogen at 0, 40, 60 and 100 kg/ha on 6-week regrowth, and the second measured the effect at 50 and 100 kg/ha on 4- and 6-week regrowths of both unfertilized swards and others which had received 350 kg nitrogen per hectare in each of the previous three years. The mean bite size of cows grazing immature (4-week) regrowths increased linearly with increasing applications of nitrogen, averaging 0.29, 0.33 and 0.37 g organic matter (OM) per bite on the 0, 50 and 100 kg/ha treatments respectively. Swards which allowed the largest bites to be prehended had the highest leaf yields and the highest leaf bulk densities. The mean bite size on 6-week regrowths was lower (0.28 g OM/bite) than on 4-week regrowths (0.33 g OM/bite). Cows selected leaf from the upper layers of the swards, and inaccessibility of leaf resulting from a high stem and inflorescence content prevented animals from taking larger bites. In both experiments high levels of nitrogen fertilizer advanced the maturity of 6-week regrowths, and the bite size of cows grazing these pastures did not increase beyond the level recorded at intermediate nitrogen levels. It was concluded that fertilizer nitrogen increases dry matter and leaf yields, particularly in the uppermost layers of the sward, allowing cows to harvest large bites of immature herbage. A higher stem and inflorescence content in heavily fertilized swards can result in inaccessibility of leaf when mature herbage is grazed. ________________ *Part II, Aust. J. Agric. Res., 24: 821 (1973).