Effects of Herbage Allowance on Defoliation Patterns of Tallgrass Prairie

Abstract

Few studies have dealt with measuring individual plant defoliations in the context of intensive grazing management. In May, July, and August of 1987, grazing trials were conducted to quantify the effects of herbage allowance on defoliation patterns of big bluestem [Andropogon gerardii Vitman], indiangrass [Sorghastrum nutans (L.) Nash], and little bluestem [Schizachyrium scoparium (Michx.) Nash]. Herbage allowances of 10, 20,30, and 40 kg AUD-1 were replicated twice per trial. Tiller height, relative leaf area removed, and frequency of defoliation were measured every 2 days over 10-day trials. Indiangrass was the most preferred species in all trials. The rate of leaf area removal increased as herbage allowance decreased. Current guidelines which call for rest periods of 30-90 days would result in light to moderate intensity defoliation for indiangrass at all herbage allowances. The maximum percentage of tillers grazed only once per trial ranged from 20 to 98% depending on herbage allowance, species, and trial. Selectivity between species was reduced by decreasing herbage allowance but this effect was not large until herbage allowance was below 20-25 kg AUD-1 and selectivity was never completely removed. Grazing all tillers only once in a grazing period, even within a species, is unlikely in a tallgrass prairie community. Leaf area removal was moderate the first time a tiller was defoliated and severe for later defoliations. The goal of grazing any individual tiller at no greater than moderate intensity within a grazing period would be roughly equivalent to grazing any tiller no more than once. However such a goal would require many tillers to go ungrazed.