110 Effects of sire selection on puberty and reproduction of offspring selected as replacement heifers

Abstract Sire selection is important to maintain the desired genetic capability of any cattle herd; therefore, selection of economic production traits affect profitability by increasing pre- and post-weaning gain and carcass quality potential. Using heifers developed from sires selected for growth and milk traits may have long-term impact on sustainability of the cowherd by influencing mature cow size and milk production. Our objective was to determine the impacts of sires selected for high and moderate growth and high and low milk traits on growth and reproductive performance of replacement heifers. Therefore, we selected replacement heifers sired by bulls within the upper 5th and 50th percentiles of the Angus breed for yearling weight (YW) EPD as high growth (HG) moderate growth (MG), the upper 15th percentile of the Angus breed for milk EPD as high milk (HM), and the lower 15th percentile for low milk (LM) in a 2×2 factorial design resulting in 4 treatments: HG/HM, HG/LM, MG/HM, and MG/LM. Heifers (n = 46) from appropriate AI sires that were born and developed at Oklahoma State University – Range Cow Research Center – South Range Unit were selected as replacements. They were raised on a base diet of dry, dormant tall grass prairie and moderate quality grass hay following weaning and 2 kg/d of 16% crude protein concentrate supplement. Body weights and biometrics were recorded at weaning and approximately at 1-yr of age, and blood was collected at 11 and 0 d before synchronization for progesterone analysis to determine pubertal status pre-breeding. Estrus was synchronized using the 14-d CIDR-PG with timed AI method followed by a 45-d natural service breeding period. Pregnancy checks were conducted by serum collection 35-d following AI and via rectal palpation by an experienced veterinarian 112 d after removal of the bulls. Statistical analyses were performed using the mixed procedure of SAS (SAS 9.4, SAS INC., Carey, NC). Heifers from high growth sires were 28 kg heavier at weaning (P 0.01) and 26 kg heavier as yearlings (P 0.01) than those sired by moderate growth sires, while sire milk EPD affected neither (P ≥ 0.55). High growth and high milk sired heifers had greater hip heights, frame scores and thus greater (P ≤ 0.05) estimated mature body weight (EMBW). Despite this, sire selection had no impact on percentage of heifers pubertal before breeding, AI pregnancy, or the total pregnancy rates (P 0.24) Based on these data, selection of sires for HG and HM leads to increased productivity of the offspring but gives rise to larger mature cows, which may pressure grazing resources in limited resource environments impacting the long-term sustainability of the cow-calf enterprise.