A large segment of the beef cattle industry is adopting expected progeny differences (EPDs) as a valuable selection tool in improving calf productivity. EPD is the difference in performance (measured in pounds, percent, inches, etc.) expected in progeny, or offspring, of individual sires. Calf weaning weight is greatly influenced by genetic potential for growth; therefore, verification of how reliable weaning weight EPDs are on commercial cow herds needs to be studied.
Calves with above-average weaning growth may stress their dams, or mothers, to the point where subsequent reproduction and pregnancy rates could be negatively affected. Therefore, information needs to be developed on the possible influence high-growth calves sired by high weaning weight EPD bulls have on the reproductive performance of beef cows, particularly in the stressful environments of Louisiana.
Maternal and reproductive performances of crossbred cows were evaluated over five years. Breed types of the cows used in this study were representative of the breeds that exist in commercial cow herds in Louisiana and the Southeast. Cows ranging in age from 4 to 10 years were impregnated by artificial insemination (AI) to Simmental sires that varied by 20.4 pounds in weaning weight EPDs. Sires included four moderate (MOD) bulls with an average weaning weight EPD of 21.4 pounds and three high (HIGH) bulls with an average weaning weight EPD of 41.8 pounds. All bulls used in this study had high accuracies (greater than 85 percent) for weaning weight EPDs, indicating that EPD values would be expected to be fairly reliable. Once randomly assigned, considering cow breed type, cow age, calving date and calf sex, cows were synchronized for estrus and inseminated.
Of the 307 cows that were synchronized and inseminated, a total of 120 AI-sired calves were born and survived to weaning. Average calf age at weaning was 229 days. Cows were weighed and body conditions were scored (on a 1 to 9 scale) and pregnancy status was determined at weaning. Cows were also weighed and body condition scored at the beginning of the breeding season. Pregnancy rates of cows were evaluated during the breeding season while nursing MOD- or HIGH-growth calves and also the subsequent breeding season after weaning MOD- or HIGH-growth calves.
There was no occurrence of calving difficulty for all 120 calvings. Calf birth date, birth weight and weaning hip height were numerically larger for the HIGH-growth calves but were not statistically different (Table 1). Calves sired by HIGHgrowth bulls were statistically heavier than calves sired by MOD-growth bulls for actual weaning weight and adjusted 205-day weaning weight (Table 1). The expected difference in weaning weights between calves from MOD and HIGH sires averaged 20.4 pounds based on weaning weight EPD information. The realized weaning weight difference between calves sired by MOD- and HIGH-growth bulls averaged 28 pounds, indicating that EPDs are fairly reliable.