David L. Morris: Vet Column 6-27-11
Cattlemen recognize that getting a live calf on the ground is the basis for most all other performance traits. For seedstock suppliers, buyer inquiries concerning other economic traits beyond reproduction have become increasingly more frequent. No matter the genetic traits, is it possible that selecting for one compromises selection for another? What about marbling score and reproductive traits in beef cattle?
Investigators from the Department of Animal Science at Colorado State University recently published the results of reviewing data from the Red Angus Association of America to determine if antagonisms existed from selection for increased fertility and carcass merit. Parameters used included intramuscular fat percentage, marbling score, scrotal circumference, and heifer pregnancy.
With regard to intramuscular fat percentage, differences have been reported by sex within a breed. In Simmental cattle, heifers had greater intramuscular fat percentage (3.40 percent) than bulls (2.68 percent). Similarly, in Angus cattle, heifers also had greater intramuscular fat percentage scores (4.46 percent) than bulls (3.73 percent). Recent data also confirm that the average intramuscular fat percentage for Red Angus cattle of both sexes is not different than that of Angus cattle.
Relative to marbling score, data from this study indicate that Red Angus cattle have an average marbling score of 5.42. Compared to previously published data indicate that Red Angus marbling scores are higher than Simmental cattle (5.01, 2003) and lower than Angus cattle (6.02, 2008).
In this study, the average Red Angus scrotal circumference measurement was 35.2 cm and was not different from a previous study in 2008 (35.5 cm). Heifer pregnancy rate in this study (80 percent) was similar to studies previously reported in 1999 (78 percent) and 2000 (89.2 percent).
From this recent study, the heritability estimate for intramuscular fat percentage was 0.29; for marbling score, 0.35; for scrotal circumference, 0.32; and for heifer pregnancy rate, 0.17.
Genetic correlations among the four traits for reproductive performance and carcass merit were greatest between intramuscular fat percentage and heifer pregnancy rate (0.13). Although this value would likely have little effect in a national cattle evaluation, it is comparable to previously published data that indicate a potential biological component to intramuscular fat deposition and heifer pregnancy rate.
In the past, additive genetic relationships of scrotal circumference with age at puberty have been reported. Genetic relationships between age at puberty and heifer pregnancy rates, however, have yielded conflicting results. In this study, the genetic correlation estimate of heifer pregnancy rate with scrotal circumference was near zero (0.05). This may possibly be explained by the use of genetic selection and management emphasis on reducing age before breeding in heifer and bull development programs. There may not be enough time to demonstrate animal differences such as might be the case with cattle that reach puberty later or that measurement accuracies of heifer pregnancy rate may need to be refined.
In summary, concomitant selection for increased fertility and increased marbling in Red Angus cattle is not antagonistic. This study is a great example of discerning fact from opinion.