Early weaning in Northern Great Plains beef cattle production systems: II. Development of replacement heifers weaned at 80 or 215 d of age

Studies were conducted to evaluate effects of weaning and subsequent heifer development treatments within two herds located in the Northern Great Plains, USA. Heifer calves from predominantly Angus Hereford dams were stratified within dam age and calving date (Fort Keogh Livestock and Range Research Laboratory (LARRL), Miles City, MT, USA; n1⁄4328) and randomly assigned within study and strata to one of three weaning treatments. Heifer calves from Angus and Angus Simmental dams (n1⁄4167; Judith Gap (JG), MT, USA) were stratified within breed group by age, calving date, and AI sire. Heifer calves either remained with their dams until time of normal weaning (NW; approximately 213 d of age) or were early weaned (approximately 80 d of age) onto one of two early weaning (EW) diets. Heifer calves assigned to EW treatments received one of the following diets: (1) 17.5% CP (69% RDP and 7.53 MJ/kg NEm or (2) 17.5% CP (57% RDP and 7.69 MJ/kg NEm). At the time of normal weaning, heifers from LARRL (2005 and 2006) were further divided into two heifer development (HD) diet treatments that differed only in proportion of RDP: (1) 12.5% CP (83% RDP and 6.28 MJ/kg NEm); or (2) 12.5% CP (72% RDP and 6.28 MJ/kg NEm). Heifers from JG were fed a common heifer development diet. Heifer BW at time of normal weaning revealed that EW heifers were heavier, regardless of type of protein delivered by EW treatments (Po0.10). Heifer BW at the end of the development period was greater for EW heifers at LARRL (Po0.01) and similar for JG heifers (P1⁄40.35) regardless of weaning treatment. Heifers at LARRL that received EW treatments had a greater percentage pubertal (Po0.06) from 39 to 2 d before breeding compared to NWheifers. Pregnancy rates were not influenced by early weaning or heifer development treatments at LARRL (P40.05); however, a greater percentage of EW heifers became pregnant throughout the breeding season at JG (P1⁄40.03). These experiments demonstrate early weaning is a viable option to develop and breed heifers in extensive beef production systems in the Northern Great Plains, USA. When production may be jeopardized by environmental conditions (e.g., drought), early weaning calves will not impair a heifers opportunity to be retained as replacement females as early weaned heifers have similar or greater reproductive success than heifers that are normal weaned. Published by Elsevier B.V.