Journal: J Anim Sci 1996 74: 1701-1710.
Abstract: A 4-yr experiment was conducted to determine effects of protein supplementation, age at weaning, and calf sire breed on cow and calf performance during fall grazing. Each year 48 pregnant, crossbred cows nursing steer calves (mean calving date = April 8) were assigned to a 2 x 2 x 2 factorial experiment replicated in three native range pastures. Treatment factors were: 1) no supplement (NS) or an individually fed supplement (S, 3 kg of a 34% protein supplement fed to cows every 3rd d); 2) calves weaned at the beginning (W, mid to late September) or at the end (NW, mid to late December) of the trial each year; or 3) calves sired by Hereford or Charolais bulls. Data were adjusted for cow size (initial hip height and initial and final weights and condition scores) by analyses of covariance using principal component coefficients as covariates. Change in cow weight and condition score were increased by S and W (P < .01), but these responses interacted and were not the same each year (yr x S, year x W, and year x S x W, P < .01). Forage intake was decreased (P < .1) by S and W. Total intake (forage+supplement) was not affected by S but was decreased by W (P < .1). Digestibility of OM was decreased by S (P < .01). Some carryover effects of treatments were observed the next spring in cow weight, condition score, and birth weight (NW decreased birth weight by 2 kg, P < .01), but there were no effects by the next fall on weaning weights or pregnancy rates. Milk yield decreased during the experimental period, and S maintained higher milk production in late lactation (P < .01). Calf ADG was increased by S and Charolais sires (P < .01). Efficiency (grams of output/ megacalorie of input) was not affected by sire breed but was enhanced by S (P < .01). Our conclusions are that 1) effects of feeding a 34% protein supplement to cows were to increase calf gains and improve persistency of lactation and efficiency; 2) delaying weaning decreased cow weight and condition score; 3) effects of weaning age and protein supplementation were highly dependent on forage and environmental conditions in any given year; and 4) whatever effects existed in a given year did not carry over to effects on next year's production as measured by pregnancy rates and weaning weights.