Seasonal changes in chemical composition of heavily fertilized kikuyu pasture and their potential effects on the mineral nutrition of grazing cattle

A heavily nitrogen-fertilized kikuyu pasture grown under acid soil conditions in northern New South Wales sampled at two week intervals over a period of one year. The concentrations of calcium throughout the year (0.15-0.28%) and phosphorus from late winter to early summer (0.20-0.28%) were well below the values normally considered to be required by milking cows and some classes of beef cattle. The nitrogen concentrations reached minimal values of 1.8-2.2% in winter, which appear to be adequate for milk production. From summer through to the end of winter, the ratio of K/(Ca + Mg) was much higher, and the Ca/P ratio was much lower, than those values reported to be associated with the occurrence of grass tetany in grazing cattle. Very high nitrogen concentratians in early summer (up to 5.2% N where 672 kg N ha-1 year-1 was applied) were in the range that has been associated with grass tetany in cattle. Lime application increased the nitrogen, phosphorus and calcium concentrations in the herbage throughout the year. The effect of lime in increasing nitrogen concentration in the herbage was maximal in late autumn and winter, at which time fertilizer nitrogen had little or no effect. The increased nitrogen concentration in the herbage through application of lime was associated with the development of a naturalized white clover component in the sward. Lime also increased the phosphorus concentration in the herbage, particularly from mid-winter to early summer, over which period phosphorus application per se had minimal effect. The application of lime allied with 134 kg N ha-1 year-1 maintained a high level of dry matter production as well as a more adequate and better balanced nutrient content in the kikuyu pasture throughout the year, and particularly in winter, when cattle grazing kikuyu pastures suffer a serious feed gap