Cattle Update: Improving Cowherd Reproduction Via Genetics
(Part 2 of 2) Last week in this publication we focused on the positive impact of cross-breeding and how direct selection affects cowherd reproduction. This week, let's begin with some thoughts on Direct Selection and it's impact on herd reproduction: A well-entrenched view of both commercial and seedstock producers is that the "cows left standing" after culling on the components of reproduction (e.g., pregnancy status and calf loss) are genetically superior. By extension, it is presumed that a great deal of progress in reproduction is made through rigorous culling and the retention of heifers out of dams making it to advanced ages. Though this may seem like a reasonable deduction, it is generally not the case.
Unfortunately, little genetic headway is made by simply culling cows that do not achieve reproductive thresholds. This may seem counterintuitive. Why wouldn't getting rid of the offenders improve your genetics for reproduction? The main reason lies in the fact that measures of reproduction tend to be lowly heritable (estimates typically run between 5-20%). And, with lowly heritable traits, an animal's own performance is not a good indicator of its genetic level for the trait. Therefore, many open culls may be genetically above average or even superior for reproduction. By the same token, several cows kept because they are bred will be genetically inferior for it - certainly not an outcome that will yield much improvement.
So, how do we directly select for reproduction? Because a cow's reproductive performance is expressed later in life, and even then it only provides a very cloudy picture of her genetic merit, are we relegated to making little to no selection progress for reproduction? Heck no! We can clear the clouds with reproductive EPDs.
Though EPDs always provide the best estimate of an animal's genetic merit, they are especially valuable when applied to low-heritability traits. This is because, when an animal's own record is a poor indicator of its genetic makeup, gathering information on its relatives is the only means we currently have of getting a clear picture of the animal.
You may ask yourself, "If an animal's own performance does not tell us much, what can be gained by records on its relatives?" It is not that a single relative record brings much to the mix (obviously it adds even less than the animal's own record); it is that there is strength in numbers - an animal can have many relatives with records, but only one record on itself. Through the use of EPDs we utilize information on all of an animal's relatives and, in doing so, chip away at the cloud with each record that flows in.
With a low-heritability trait expressed later in life like reproductive function, the cloud clears slowly - but it will clear. In fact, if an animal has enough progeny records, we can see its genetic merit for reproduction as clear as a bell.
Fortunately the seedstock industry now has EPDs that are, for the most part, direct measures of reproductive function: stayability (STAY) and heifer pregnancy (HP). Researchers at CSU (Snelling et al., 1995) developed the prototype for STAY, which was unveiled to the industry by the RAAA a few years later. The US breeds currently calculating STAY: Gelbvieh, Limousin, Red Angus, Salers and Simmental.
Colorado State University later developed HP based on work by Doyle et al. (2000) and implemented it for the RAAA National Cattle Evaluation in 2001. While the Angus Association of America has released a trial version, the RAAA is the only breed to publish an official HP at this point. Unfortunately, neither STAY nor HP is currently calculated on non purebred seedstock, though there are plans to do so in the future.
Though STAY and HP have potential shortcomings (e.g., seedstock breeders' culling practices are probably not in lock step with the commercial industry and breed association culling records tend to be sketchy), in my opinion they are the most effective selection tools available for improving reproductive function. What's more, based on computer simulation efforts by USDA researcher M. D. MacNeil, the economic impact of stayability when selecting a sire to keep replacement females is nearly twice that of the next closest trait, while the relative importance of heifer pregnancy is on par with the most important carcass or growth traits (personal communication) - so these reproductive EPDs certainly warrant a great deal of attention in the selection process.
Obviously, if you are a commercial producer you do not have the luxury of using STAY or HP to select replacement females; however, if you select sires with superior EPDs in these areas the reproductive function of your cowherd is likely to improve over time. Given their relationship to stayability, you may also gain some reproductive ground by selecting sires with lower milk, smaller mature size and better maternal calving ease EPDs. (In the future, these relationships will be accounted for in the computation of reproductive EPDs in the manner birth weight is incorporated in the prediction of calving ease - eliminating the benefit in considering them separately; however, they are not currently factored into the calculation of STAY or HP.)
Summary: In closing, I must reiterate that crossbreeding needs to be at the center of any effort to improve the reproductive function of your cowherd. The dramatic impact of heterosis on reproductive performance is crystal clear - no herd should be without it! Though reproductive improvement through selection is possible, it is generally limited to utilizing reproductive EPDs when selecting your herd sires. By combining crossbreeding with the selection of superior sires you will position your enterprise to excel in the most vital area of beef cattle production - cowherd reproduction.
Literature Cited:
Beckman, D. W., S. E. Speidel, B. W. Brigham, D. J. Garrick, and R. M. Enns. 2006. Genetic parameters for stayability and body condition score in beef females. Proc. West. Sect. Am. Soc. An. Sci. 57:93-95.
Cundiff, L. V., Nuiiez-Dorniguez, R., Dickerson, G. E., Gregory, K. E., and R. M. Koch. 1992. Heterosis for lifetime production in Hereford, Angus, Shorthorn, and crossbred cows. Journal of Animal Science. 70:2397-2410.
Doyle, S. P., Golden, B. L., Green, R. D., and J. S. Brinks. 2000. Additive genetic parameter estimates for heifer pregnancy and subsequent reproduction in Angus females. Journal of Animal Science. 78:2091-2098.
Martinez-Velazquez G., K. E. Gregory, G. L. Bennett and L. D. Van Vleck. 2003. Genetic relationships between scrotal circumference and female reproductive traits. Journal of Animal Science. 81:395-401.
Rogers, P. L., Gaskins, C. T., Johnson, K. A., and M. D. MacNeil. 2004. Evaluating longevity of composite beef females using survival analysis techniques. Journal of Animal Science. 82:860-866.
Snelling, W. M, Golden, B. L., and R. M. Bourdon. 1995. Within-herd genetic analyses
of stayability of beef females. Journal of Animal Science. 73:993-1001.
EDITOR's NOTE: Improving cowherd reproduction via genetics will be but one concept Dr. Shafer will focus on during the Ohio Beef Cattle Breeder Conference being hosted on the campus of The Ohio State University by Dr. Tom Turner and staff of the American Simmental Association on April 30 beginning at 3 p.m. Details follow below.
No hay comentarios.:
Publicar un comentario