The effect of grazing pressure on animal performance, diet characteristics, and plant persistence of alfalfa

摘要

Alfalfa (Medicago sativa L.) has the potential to provide high quality pasture for summer grazing, but is generally thought of as a hay crop because of its poor persistence under grazing. 'Alfagraze', a new alfalfa cultivar bred for improved grazing tolerance, appears promising for use as a pasture species. It is important to determine the effect of grazing management on the persistence of Alfagraze, and the animal performance that can be expected. Alfagraze and Apollo alfalfa were grazed with beef steers at low, medium, and high grazing pressures (GP) in central Georgia for 3 yr. Increased GP resulted in decreased stands after only 2 yr continuous stocking. Alfagraze was more persistent than 'Apollo', the hay-type cultivar used in this study. After 3 yr, Alfagraze had 150% higher plant populations than Apollo. Differences in persistence between the cultivars was most evident at the high GP, where Alfagraze had significantly more plant after only 2 yr. Steer weight gain was influenced by GP and cultivar. During yr 1, increased GP resulted in reduced average daily gain (ADG). During yr 2, increased GP resulted in increased gain per acre (GPA) because of higher stocking rates. The increased persistence of Alfagraze over Apollo at the high GP resulted in more alfalfa in the available forage, and higher ADG and GPA for Alfagraze. During yr 3, ADG decreased with increased GP. Diet quality was reduced with increased GP because steers had less forage available and could not be as selective when grazing. As GP increased, steers spent more time grazing in an effort to maintain selectivity but diet quality dropped. Intake was not affected by GP. High animal gains were achieved on alfalfa, but GP influenced weight gain. Increased GP may reduce animal performance because of reduced selectivity and lower diet quality. More grazing pressure will also increase stand loss. Continuous stocking of grazed alfalfa requires the use of a grazing-tolerant alfalfa such as Alfagraze to maintain stands and productivity.