Effects of Grazing Pressure 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 cattle grazing pressure on defoliation patterns of little bluestem (Schizachyrium scoparium (Michx.) Nash), big bluestem (Andropogon gerardii Vitman), and indiangrass (Sorghastrum nutans (L.) Nash). Grazing pressures of 10, 20, 30, and 40 kg Auo-1 were replicated twice per trial. Treatment pastures contained 30 marked tillers of each species. Standing crop was measured before and after grazing. Tiller height, relative leaf area removed, and frequency of defoliation were measured every 2 days over 10 day trials. The frequency and intensity of tiller defoliation was highly dependent on species and grazing pressure. Tiller height decreased more rapidly as grazing pressure increased, and leaf area removed increased as grazing pressure increased. Height and 1 eaf area removed were similar for grazing pressures of 30 and 40 kg Auo-1. Indiangrass was the most preferred species in all trials. Tillers were spread among at least three defoliation frequency classes for all species and grazing pressures. Trial 1 had the greatest proportion of undefoliated tillers regardless of species. Under most grazing pressures, indiangrass and big bluestem had more tillers defoliated 3 times in a trial. Tillers were moderately defoliated the first time and severely defoliated afterwards. Defoliating all tillers once in a rangeland community is virtually impossible to achieve without severe defoliation on some species. Planning livestock movements based on a target defoliation intensity and regulating grazing pressure to reduce the risk of severe defoliation can be useful strategies for intensive grazing management.