REPRODUCTION TRAITS AND CALF PERFORMANCE OF FIRST CALF HEIFERS SIRED BY BULLS WITH HIGH MARBLING OR HIGH RETAIL PRODUCT EPD

A group of mature Angus cows from the Bair Ranch near Martinsdale, MT were artificially inseminated from 1999 through 2001 to Angus bulls with either high EPD for marbling (MB) or high EPD for retail product (RP). Heifer calves from these matings were retained as replacements in the herd. These females were artificially inseminated to a single sire that was also used as the clean-up sire. This study was initiated to evaluate the impact of different selection criteria on reproduction within a herd of western range cattle. Total pregnancy (AI and natural service) of heifers as yearlings (BRED), re- breeding pregnancy rate of 2-yr-old dams (REBRED), pre-breeding body weight (COWWT), and weaning weight of first calves (WWT) was analyzed. Reproductive traits were analyzed by categorical modeling with the model including year and sire type. Cow and calf weights were analyzed by general linear modeling. The model for COWWT included year and type (MB or RP), and WWT included calf age as a covariate, year, sex, and dam type. No differences in COWWT due to type were found (P=0.2). Year was significant (P<0.001) with those heifers born in 2001 being heavier than the other years. Significant calf age effects were found for WWT (P<0.01), but type and sex were not significant sources of variation (P>0.3). Each day of age of calves increased WWT by 0.75 kg. Statistical analysis showed calves weaned by dams born in 2001 were not as heavy as calves weaned in other years, though overall year affect was not significant. Year was a significant source of variation for BRED and REBRED (P<0.05), but type was not. Probabilities for non-pregnant heifers were high in 2001 (32%) compared with other years (15%). Conception to AI was at approximately 35% in 2001 heifers compared with 50% in other years. However, this trend was reversed in REBRED with 2001 born heifers averaging 5% non- pregnant compared with 20% for the other years.