U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska
Date of this Version
2009
Citation
Published in Proceedings, Western Section, American Society of Animal Science (2009) 60: 81-84.
Abstract
Body condition score (BCS) is used as a management tool to predict reproduction of young beef cows. The objective was to determine the effects of BCS at calving on pregnancy rates, days to first estrus (DTFE), nutrient status (assessed by blood metabolites), and calf BW change in 315, 2- and 3-yr-old cows grazing native range over 5 yr at the Corona Range and Livestock Research Center, NM. Palpable BCS (1 to 9) were determined by experienced technicians prior to calving. Cows were assigned to 1 of 4 BCS groups: Thin (T; BCS = 3.5 to 4.25), Thin-Moderate (TM; BCS = 4.5), Moderate- Fat (MF; BCS = 4.75 to 5.25), or Fat (F; BCS = 5.5 to 7.0). Postpartum supplementation was terminated each year when cows reached BW nadir. Cows were weighed weekly and serum was collected 2×/wk for progesterone analysis to estimate DTFE. Year effects were also evaluated, with years identified as either above (AA) or below (BA) average rainfall. Data were analyzed as a 4 × 2 factorial. A calving BCS × rainfall interaction occurred for DTFE (P = 0.01). In AA years, all BCS groups achieved DTFE within 86 d postpartum with F cows cycling the earliest at d 68 postpartum. In contrast, during BA years, cows with a greater BCS (MF and F) took up to 61 d longer postpartum to reach DTFE compared to cows in a thinner BCS (T and TM). Pregnancy rates did not differ between BCS (P = 0.45; 90, 95, 90, 90 for T, TM, MF, and F, respectively). Calf weights at birth (P = 0.19), branding (P = 0.27), weaning (205-d weight; P = 0.51) were not affected by cow BCS at calving. Results suggest that BCS interacts with rainfall and may not be a consistent indicator of reproductive performance in young beef cows.