THE PHYSIOLOGICAL BASIS OF GRAZING MANAGEMENT *

Grazing management should be firmly based on knowledge derived from studies of both animal‐ and plant‐physiology. It is essential to understand both the nutritional needs of the animal and the effect of harvesting procedures on the growth rate of the plant. Animal performance, however, is not simply related to the mean nutritive value of the sward on offer; selective grazing can result in large differences between the latter and the value of the diet actually consumed. Thus it is possible to obtain similar animal growth rules on very different pastures, where grazing pressure is very low, or to obtain different animal performances on pastures which differ little in mean digestibility, but where the herbage is more, or less, ‘available’. It is therefore necessary to understand the relationship between the amounts of herbage on offer, removed and left behind in grazing, and the nutrient intake of the animal. To use biological criteria, such as the quantity of herbage present, in operating grazing methods it is essential that they are expressed in terms which are relevant theoretically and measurable in practice. The results of experimental comparisons of managements can not necessarily be used to judge the usefulness of physiological concepts, since the assumption that such concepts are embodied in the managements is often unjustified. There is, for example, little evidence that continuous and rotational grazing, under conditions of ‘correct’ stocking, result in different frequencies of defoliation of the individual plant units involved.