Bunchgrass basal area affects selection of plants by cattle.

Abstract

Cattle are selective foragers in response to several plant attributes. We tested hypotheses that caespitose plants of various basal areas were equally susceptible to herbivory and were defoliated with equal intensity by cattle. Five-hundred crested wheatgrass (Agropryon desertorum (Fischer ex Link) Schultes) plants, distributed among 10 basal area classes, were monitored for frequency and level of utilization after approximately 74% of all plants in pastures were grazed by cattle. Plants with < 25-cm2 basal area were less likely, and plants between 65 and 105-cm2 more likely to be grazed than other classes. Ninety-one percent of the 65 to 85-cm2 basal area plants were defoliated, while only 48% of those < 2 .5 cm2 were grazed. The fact that mid-size plants occurred least often but were defoliated most often lends further credence to the selective grazing hypothesis. Less frequent use by cattle of plants < 2 5-cm2 basal area may enhance chances of seedling establishment and survival of smaller established plants or remnants of deteriorating tufts. Among grazed plants, however, smaller plants endured higher utilization by weight than the overall population. Utilization was equal among other size classes. Because forage yield per unit of basal area declined as plant size increased, cattle probably forage most efficiently by selecting bite-size plants. Researchers using single plants, tiller, or leaves as experimental units should note that varying sized tufts are not equally likely to be defoliated, and plants less than 25-cm2 basal area may receive greater than average levels of utilization from free-ranging cattle under light to moderate utilization levels. These aspects of livestock grazing behavior and research objectives should be considered in selection of experimental units.