Degree of Herbage Selection by Grazing Cattle

Abstract

Summary An investigation was made of the degree of selective grazing exercised by growing steers. A series of 66 individual trials was conducted with six herbages, each at three to five different stages of growth. The degree of discriminate grazing was marked with almost all herbages examined. The diet selected by grazing steers was higher in crude protein, ether extract, and mineral matter and lower in crude fiber than the whole herbage available for consumption. All proximate constituents of grazed herbage were more digestible than those of clipped herbage from the same source fed to stall-confined steers. The extent to which animals selected was not related to the intake of dry matter. Digestibility of dry matter and the amounts of crude protein and ether extract decreased, and the amount of crude fiber increased, as first-growth herbage became more mature. Although aftermath herbage contained a high level of crude protein and a low level of crude fiber, it was somewhat less digestible than first-growth forage of comparable composition. The restriction of grazing to small areas containing small quantities of herbage tended to reduce the extent of selection, whereas providing large quantities of herbage increased the degree of selection. Certain characteristics of herbage, such as the kinds of plants present, stage of growth, proportions of leaves and stems, and accessibility of plant parts, may have influenced to some extent the results obtained. Preference of animals for herbage possessing certain physical qualities, or qualities affecting palatability, may have resulted in the consumption of a diet differing chemically from the whole herbage. The consistency with which animals grazed discriminately under a variety of herbage conditions suggests that certain qualities inherent in the animal may influence the degree of selection practiced. The results of this investigation demonstrate that the chemical composition of clipped herbage is an unreliable index of the chemical composition of herbage selected by grazing animals. Also, the digestibility of clipped herbage is of uncertain value and may be quite misleading in regard to estimating the value of pasture under grazing conditions.