Effects of Forage Availability on Grazing Behavior of Heifers

Abstract

Effects of decreasing availability of crested wheatgrass (Agropyron desertorum and Agropyron cristatum) on grazing time and biting rate of Angus heifers were investigated. In the first year of the study, as standing crop decreased from 474 to 170 kg dry matter/ha, grazing time increased from 517 to 203 min/day, while biting rate increased from 56 to 64 bites/min. In the second year, as standing crop decreased from 919 to 144 kg dry matter/ha, grazing time increased from 380 to 656 min/day while biting rate increased from 37 to 50 bites/min. Grazing times were similar to those reported in the literature for pastures having much higher levels of available forage. Although grazing times may be correlated with available forage, comparison of grazing times under different pasture conditions are not meaningful unless considered along with other forage and animal factors. On crested wheatgrass pastures, maximum biting rates occurred at lower levels of available forage than reported on tropical pastures. There have been many studies on the grazing behavior of domestic livestock (Arnold and Dudzinski 1978). Most have been conducted in temperate or tropical pastures with standing crops above those normally occurring under semiarid conditions. There is little information on grazing times and biting rates for cattle grazing mature standing forage at standing crops below 1,000 kg dry matter (DM)/ ha. The feeding behavior of cattle under these conditions should be established to better understand energy expenditures and production of livestock grazing on semiarid ranges. Behavioral data are also needed to formulate theories on feeding strategies and tactics of livestock. The objectives of this study were: 1. To determine the relationship between available forage and both grazing time and biting rate of Angus heifers at low standing crops. 2. To examine how these relationships were affected by seasonal differences in the forage. The study was conducted in conjunction with a nutritional study (Nastis 1979) which also examined effects of forage quality and standing crop on forage intake. rhe entire standing crop in the present study was physically available to the heifers and was considered to be available forage. Therefore, the terms standing crop and available forage are used synonymously in this paper. All standing crops are reported on a dry matter basis. Effects of decreasing standing crop on grazing time and biting rate have been reported by other researchers. Hein (1935) qualitatively described an increase in grazing time by beef steers as standing crop decreased. Other investigators have reported quantitative increases in grazing time with decreasing forage availability (Atkeson et al. 1942, Hodgson 1933). Arnold (1960), working with sheep, reported a linear increase in foraging time from 7.0 to 10.3 h/day when forage availability decreased from 3,000 to 1,000 kg/ ha. In a later paper Arnold (1964) suggested that grazing time of sheep Authors are assistant professor, Department of Forestry and Range Management, Washington State University, Pullman 99164-6410; assistant professor, School of Forestry and Natural Environment, University of Thessaloniki, Thessaloniki, Greece; and professor, Department of Range Science, Utah State University, Logan 84322. Published with approval of the Director, Utah Agricultural Experiment Station, as Journal Paper 2950. Manuscript accepted September 24, 1984. decreased at forage availabilities below 560 kg/ ha. Cattle grazing time decreased at availabilities below 3,360 kg/ ha. Hafez (1975) summarized studies of biting rate in cattle; rates ranged from 50 to 80 bites/min. Chacon and Stobbs (1976) reported that the biting rate of cattle increased from 56 to 62.4 bites/ min and 51.4 to 59.4 bites/ min as available forage decreased from 7,200 to 1,500 kg/ ha and 3,900 to 2,400 kg/ ha, respectively. Because grazing time and biting rate are readily measurable variables, changes in these variables with decreasing forage availability might be used to make range management decisions such as when to move livestock from pasture. Before such applications are possible the relationships between forage availability, grazing time, and biting rate at low, standing crops must be accurately