131. Effect of Sire Yearling Weight and Milk EPD on Yearling Weight and Hip Height in Commercial Angus Replacement Heifers Reared in a Minimal Input System

Abstract Development of replacement heifers is a key determinant of the long-term success and sustainability of beef cow-calf operations. While genetic selection for growth traits is well studied in feedlot and high-input systems, less is known about how these traits affect heifers in minimal input systems. This study evaluated the effects of sire expected progeny differences (EPDs) for growth (YW EPD) and milk (Milk EPD) on yearling weight and hip height in 334 replacement beef heifers. Heifers were sired by Angus bulls with high (HG; top 10th percentile, 72.1 ± 0.4 kg) and moderate (MG; below the 50th percentile, 42.3 ± 0.4 kg) YW EPD. Sire milk EPDs were high (HM; top 20th percentile, 14.8 ± 0.2 kg) and low (LM; bottom 20th percentile, 7.1 ± 0.2 kg). Heifers were weaned at approximately 205 days, backgrounded for 60 days, then grazed native tallgrass prairie or bermudagrass through one year of age, receiving a 30% protein supplement at the average rate of 1.5 kg/day from November through March. Contemporary groups (CG) were assigned within year and season. Data were analyzed using a mixed model with growth and milk classification, their interaction, and contemporary group as fixed effects, and sire as a random effect. Additional models used YW and Milk EPDs as continuous predictors in a regression framework, with season and year as fixed effects, and sire as a random effect. HG-sired heifers were 13.8 kg heavier (337.9 and 324.1 ± 3 kg; P = 0.0035) and 1.5 cm taller (120.7 vs 119.2 ± 0.42 cm; P = 0.018) than MG-sired heifers at one year of age. Sire milk EPD classification alone did not affect yearling weights or hip heights, but the interaction of Growth*Milk influenced yearling weight, due to 16 kg greater BW in HM heifers within the HG classification and no difference in BW due to sire milk EPD in LG classification (P = 0.039). Regression analysis on EPDs showed that with each one unit increase in YW EPD, yearling weight increased by 0.25 kg (P = 0.0001) and hip height increased by 0.026 cm (P = 0.0035). The current study observed a 13.8 kg difference in YW, yet the YW EPD difference was 30 kg based on HG and MG sire classifications. Under a minimal input system consistent with extensive beef cattle operations in Oklahoma, yearling weight and hip height were increased with selection for increased growth. However, expression of genetic potential for yearling weight was restricted to ∼ 46% of sire EPD differences.