357 The Influence of Supplemental Protein Levels on Beef Cattle Grazing Winter Foothill Rangelands: Cow and Subsequent Calf Performance

Abstract

Abstract Angus based bred cows [n = 150; initial shrunk body weight (BW) = 615 ± 6.61 kg; initial body condition score (BCS) = 5.2] were used in an 84-d winter grazing experiment that took place over two consecutive years. Cattle were stratified by age and body condition and, within stratum, randomly assigned to one of three protein supplementation treatments: 1) no supplement, 2) cows fed 0.20% BW· d-1 of a 30% protein supplement, and 3) cows fed 0.40% BW · d-1 of a 30% protein supplement. Protein supplement was a 30% CP canola-based pelleted supplement with 9 units protein from urea. Cows were gathered three-times weekly and provided supplement on a replicated pen basis. Performance data were analyzed as a completely randomized design with linear and quadratic contrasts used for mean separation (reps = 5; n = 7). Body weights and BCS were recorded every 28-d period throughout the study and calf BW were obtained at birth and weaning. A treatment × period interaction was detected for body weight change (P < 0.01) where a linear response (P < 0.01) where increasing protein status resulted in modest weight gains relative to the control treatment. BCS also exhibited a linear response for two of the three weight periods (P < 0.01) where level of BCS lost was least in cows with the higher supplemental protein levels. A year × period interaction was detected where cows from year two exhibited less BW change on days 28, 56, and 84 of the study (P < 0.01) and had greater BCS (P < 0.01) than those from year one during these periods. In addition, we observed a treatment × year interaction where cows from year two experienced less BW change and had greater BCS than cows from year one (P < 0.01). No treatment × year interactions were detected for birth weight, average daily gain (ADG), or adjusted 205-d weight in calves. A tendency was noted for cows offered 0.4% BW· d-1 supplement to have heavier calves at birth than their counterparts (P = 0.09) over both years. A year effect was detected where calves born in year two had increased ADG and adjusted 205-d weights compared with those from year one. A tendency (P = 0.09) for the calves in year two of the study to have heavier weights at birth compared with those from year one was also noted. In summary, increasing levels of supplemental protein resulted in linear improvements of BW and BCS of grazing beef cattle during the winter. While birth weights of calves tended to differ due to the gestational nutrition of the dam, calf growth and weaning weights were not affected.