Phenotypic characterization of heifer development using carcass ultrasound in Angus cattle.

The utility of carcass ultrasound measurements in heifer development as indicators of reproductive performance was evaluated. Current methods of assessing heifer development include body weight (BW), nutritional status, and body condition score. However, these methods do not measure actual body composition, and the relationship between actual pre-breeding body composition measurements and heifer fertility is poorly understood. Measurements were collected on 124 purebred Angus heifers at the Iowa State University McNay Memorial and Demonstration Research Farm. One week post-weaning (day 0), heifers were randomly allocated into two dietary treatment groups stratified by age and weight. One group of heifers was developed to reach 55% of expected mature BW at the time of breeding (restricted), and the second group was developed to reach 65% of expected mature BW at breeding (non-restricted). Carcass ultrasound measurements and BW data were collected at six time points during heifer development from October 2023 to September 2024 (day 0, 55, 123, 178, 231, and 338). Carcass ultrasound measurements included ribeye area (REA), backfat thickness (FAT), rump fat thickness (RUMP), and percent intramuscular fat (IMF). A controlled internal drug release (CIDR) was used to synchronize heifer estrus cycles for timed-artificial insemination (AI), and Estrotect heat detection patches (Estrotect, Rockway, Inc., Spring Valley, WI) were used to measure estrus response. The final pregnancy diagnosis was made by a veterinarian on day 338, 12 weeks post initial AI in September 2024. Data were analyzed using R Statistical Software 4.4.1. Treatment group differences (P < 0.05) were identified at day 55 for mean measurements of weight, REA, FAT, and RUMP, and at day 178 for IMF. Estrous response rate was higher in the non-restricted group than the restricted (P < 0.05), but the overall pregnancy rates between groups was not significant (P = 0.79). Two logistic regression models were analyzed to capture pregnancy response using body composition measures. Backfat was a significant predictor of heifer pregnancy (P < 0.05). Results from this study can help beef cattle producers determine the appropriate percentage of expected mature BW for developing heifers in their operation, based on specific management practices. Additionally, findings can inspire prospective researchers to further investigate the ideal FAT levels for optimal heifer reproductive performance.