Studies of the nutritive value of New Zealand dairy pastures

Abstract

Abstract The results are reported of a series of magnesium balance trials in which the intake and the utilisation of magnesium by milking cows was determined for pasture herbage during 32 weeks of the 1959–60 dairying season. Evidence is presented of seasonal changes in the percentage magnesium in the herbage dry matter, and these are attributed in part to within‐sward changes in the ratio of ryegrass/white clover. Seasonal changes in magnesium concentration of faeces, urine, and milk are also delineated. Under the conditions of this experiment, the mean daily magnesium intake of individual cattle was high (0.1 g Mg per kg L.W.). In common with overseas data, however. only a small fraction of the magnesium ingested appeared either in the milk or was retained. On average, 80 per cent was excreted in the faeces, 12 per cent in the urine. and only 8 per cent was secreted into the milk or retained. During most trials the cows were in a relatively high positive magnesium balance. This probably accounts for differences in the percentage distribution of Mg excreted from comparable lata previously reported overseas. There were marked between‐animal and between‐week differences in the percentage availability of the magnesium consumed, but no significant relationship between this and percentage apparent digestibility of the dry matter consumed. Estimates of magnesium requirements of dairy cows obtained in the present trials are discussed in relation to other previously reported figures. It is suggested that the relatively high intakes of “available” magnesium recorded here are responsible for an unrealistically high estimate of “productive maintenance” needs.