Sidney M. DeRouen
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.
Cow body weight and body condition scores at weaning and at subsequent breeding were similar after nursing MODor HIGH-growth calves (Table 2). Cow body weights and body condition scores were numerically lower for the cows raising HIGH-growth calves but were not statistically different. It is important to note that average or adequate body condition for a mature cow is 5, and body condition scores were above this threshold regardless of type of calf raised.
Pregnancy rates were similar for cows raising MOD- or HIGH-growth calves, both while nursing calves and after weaning during the subsequent breeding season (Table 3). Numerous studies have demonstrated that for acceptable pregnancy rates to be achieved, it is important that cows have at least a moderate degree of body condition (body condition score 5 or 6) at initiation of the breeding. Thus, even though differences in weaning productivity were created through sire selection, adequate body condition was maintained. As a result, cows raising either MOD- or HIGH-growth calves had similar and highly acceptable pregnancy rates.
The mating by artificial insemination of cows to bulls with weaning weight EPDs that differed by 20 pounds resulted in actual calf weaning weight differences of 28 pounds. This indicates that EPDs are fairly reliable. The use of sires with high weaning weight EPDs did not result in increased calving difficulty, did not lower cow body condition and lastly, did not adversely affect subsequent reproductive performance of the cow herd.
Sidney M. DeRouen, Professor, Hill Farm Research Station, Homer, La.
(This article was published in the winter 2006 issue of Louisiana Agriculture.)