Effect of grazing system, stocking rate and season of use on herbage intake and grazing behaviour of stocker cattle grazing alfalfa-grass pastures

Abstract

An experiment was conducted to determine the effects of grazing system (continuous and rotational), stocking rate (light, 1.1 steers ha−1; heavy, 2.2 steers ha−1) and season of use on forage intake and grazing behaviour of stocker cattle grazing an approximately 70% alfalfa (Medicago sativa L.), 25% meadow bromegrass (Bromus biebersteinii Roem & Schult.) and 5% Russian wildrye (Psathyrostachys juncea (Fisch.) Nevski) pasture. To determine organic matter intake, grazed herbage was collected with esophageal fistulated cattle and analyzed for in vitro digestible organic matter while fecal output was determined using chromic oxide. These variables were used to calculate organic matter intake. Daily herbage consumption (g OM kg BW−0.75 d−1 and kg OM d−1) did not differ (P > 0.05) for either grazing system or stocking rate from 1991 to 1993, with the exception of greater (P < 0.05) intakes at light compared with heavy stocking rates (10.9 vs. 8.4 kg OM d−1) late in the 1991 season. Grazing time was usually lower (8.9 vs. 10.3 h (3-yr mean); P < 0.05) in lightly than in heavily stocked pastures. As available herbage increased, cattle spent less time grazing (y = 12.46 − 0.00103 x; r = 0.48, RSD = 2.04, P < 0.001; where y = grazing time and x = herbage mass). Biting rates ranged from 28 to 32 bites min−1, regardless of year, season, grazing system or stocking rate. Rate of intake (g OM kg−1BW−0.75 h−1) did not differ (P > 0.05) for either grazing system, although it tended to be greater (P < 0.10) at light than at heavy stocking rates. As animals in lightly stocked pastures spent less time grazing, herbage was consumed at an increased rate, which was reflected in increased average daily gain (y = 0.28 + 0.067 x; r = 0.86, RSD = 0.14, P < 0.001; where y = daily gain and x = intake rate). Our results suggest that daily herbage consumption was not affected by grazing system or stocking rate; however, at lower stocking rates, grazing time declined and intake rate tended to increase. Key words: Alfalfa, cattle, ingestive behaviour, pasture