146 - Effect of Short-term Nutritional Increase 30 Days Prior to Artificial Insemination on ADG and Reproductive Efficiency in Beef Heifer Development Systems

Abstract: The objective of this study was to determine if increasing nutrition 30d before AI would optimize overall heifer pregnancy rates. Angus crossbred heifers were used in a 3-yr study (n=100/yr) at the West Central Research and Extension Center, North Platte, NE. Heifers were blocked by BW and randomly assigned to 1 of 3 groups for the winter period (average 131±3.5d/yr): upland range (RANGE), dry lot low (DLLO), or dry lot high (DLHI). The same diet was offered to DLLO and DLHI until 30d prior to AI, where DLHI received increased nutrients. Heifers developed on RANGE grazed winter range with 0.45kg/d 30%CP cake until the treatment period where they received DLLO diet. Heifer ADG in DLLO (0.64±0.06kg) and DLHI (0.64±0.06kg) was greater (P=0.02) than RANGE (0.25±0.06kg) during the development period, but no differences were seen in ADG during the 30d treatment period between DLHI (1.12±0.13kg), DLLO (0.75±0.13kg), and RANGE (0.67±0.13kg). Heifers were synchronized with melengestrol acetate at 0.5 mg per head per d for 14d, and PGF2a given 19d later. Heifers were either bred on detected estrus or placed with bulls after 5d. All heifers were placed with bulls 10d following AI. Pre-breeding BWs were greater (P=0.0004) for DLHI (375.44±3.39kg) and DLLO (363.5±3.39kg) compared to RANGE (311.8±3.35kg.), but breeding ADG, between pre-breeding and first pregnancy diagnosis, was greater (P=0.0002) for RANGE (0.68±0.01kg) compared to DLHI (0.35±0.02kg) and DLLO (0.37±0.02kg). While pregnancy rates to AI were not different (P=0.43), final pregnancy rates tended to be greater (P=0.09) for DLHI (96%) and DLLO (95%) compared to RANGE (84%). No differences were observed in calving rate or heifers calving in the first 21d. While greater nutrition for DLHI and DLLO heifers led to higher BW, and tended to increase final pregnancy rates, overall short-term nutritional change had no detectable impact on AI conception.