Cattle Update: Improving Cowherd Reproduction Via Genetics

Cattle Update: Improving Cowherd Reproduction Via Genetics   A beef cow's job is not an easy one. She is expected to conceive at slightly over one year of age to calve by the time she is two and rebreed shortly after that while weaning a healthy, viable calf. Furthermore, we demand that she consistently repeats this cycle for the rest of her life - one stumble and, in the words of California's terminating governor, hasta la vista, baby!   To be sure, producers are best served when the cow successfully performs her task for many years, as the longer her productive life the more profitable she is to the enterprise. Is there anything that can be done to help her out? Certainly, there are environmental factors we can manage that will give her a leg up. For example, by providing adequate nutrition, a proper vaccination regimen and mating her to easy-calving sires (particularly when she is young) we increase the odds of her success. While a cow's environment has a substantial impact on her reproductive performance, her genetic makeup can too. This paper explores the genetics of female reproduction and offers suggestions on how to improve the reproductive performance of your cowherd via genetics.   Crossbreeding: The obvious place to start a discussion about the genetics of female reproduction is with the factor that far and away has the greatest affect on it - crossbreeding. It has long been recognized that crossbreeding enhances virtually all aspects of reproductive performance. Studies too numerous to list here have established the reproductive superiority of crossbred over straightbred cows.   In one of an abundance of studies with similar findings, scientists at the Meat Animal Research Center (MARC) concluded that two-breed rotational cross cows produced 20% more calves over their lifetime than straightbreds due to the favorable impact of heterosis on dam fertility/longevity and calf survivability brought about by the improved calving and mothering ability of the dam (Cundiff et al., 1992). Furthermore, they estimated that when mated to a bull of another breed the two-breed cross cows would wean 36% more weight over their lifespan than straightbred cows raising straightbred calves. The dramatic increase is attributable to the positive influence of heterosis on reproduction and production in the dam and well as increased growth and survivability in their calves.   Given the overwhelming evidence of the crossbred cow's reproductive supremacy and the fact that reproduction is a major piece of the profitability puzzle (by most accounts exceeding all other functions by a wide margin in relative importance), it is difficult to conceive of a situation where a commercial enterprise would not benefit financially from a crossbred cowherd.   Am I implying that selecting animals within a breed for reproductive performance is not a worthwhile endeavor? No! Reproductive progress can be made via selection (which I will address later); however, it would take years of intense selection within a breed to yield the kind of improvement that can be achieved in one fell swoop by simply crossbreeding.   Therefore, crossbreeding makes a logical cornerstone in any effort to enhance cowherd reproductive performance. With crossbreeding as the foundation, the selection of superior animals of multiple breeds as inputs to the crossbreeding system can be considered a supplemental means of further boosting reproductive function; however, identifying reproductively superior animals has its challenges, as I will explain.   Indirect Selection: Because the assessment of a cow's reproductive performance is generally determined later in her life, it seems logical to look for early indicators to hasten the process. For example, it is a commonly-held belief that females with a propensity toward fatness will excel reproductively.   Though research has shown that increased fatness, to a point, is strongly and favorably associated with reproductive performance on a phenotypic scale, the few attempts to assess the relationship on a genetic level show an unfavorable, though weak, relationship. Using data from the Red Angus Association of America (RAAA), researchers at Colorado State University (CSU; Beckman et al., 2006) derived genetic correlations ranging from -.12 to -.22 between body condition at various ages and stayability (by industry convention, the probability of a cow remaining in the herd through 6 years of age). At the American Simmental Association (ASA), we have found a correlation of -.19 between an animal's genetic propensity for backfat in the feedlot and their inherent stayability. We (ASA) have also calculated a -.11 genetic correlation between backfat and heifer pregnancy (the likelihood of a heifer being pregnant at the end of the breeding season) using RAAA data.   Admittedly, these unfavorable correlations between fatness and reproduction may seem illogical. We have all seen a higher proportion of thin cows open at pregnancy test time. Keep in mind, however, that the aforementioned correlations are genetic. The relationships we actually observe, i.e., phenotypic correlations, are influenced by a combination of underlying environmental and genetic relationships. There is little question that females within a herd lucky enough to experience an environment for increased body condition (e.g., extra energy intake) are likely to have better reproductive performance than their herd mates. Furthermore, this strong and positive environmental relationship between fat and reproduction apparently overwhelms what appears to be a slightly negative genetic relationship - yielding the strong, favorable phenotypic relationship we typically observe.   Frankly, there is not enough evidence about the genetic relationship between fatness and reproductive function to make recommendations based on it at this time; however, though it may fly in the face of conventional wisdom, it appears that selecting "easy-fleshing" genotypes will not gain us ground reproductively.   Scrotal circumference has been considered as a predictor of female reproductive performance. Though the preponderance of evidence indicates a strong to moderately favorable relationship between scrotal circumference and age at puberty in related females, research is less clear on the relationship between scrotal circumference and subsequent measures of reproduction. In a study based on a large population involving several breeds at the MARC, Martinez-Velazquez et al. (2003) found a slightly unfavorable (.15) relationship between scrotal circumference and age at first calving and no relationship between scrotal circumference and 1st pregnancy, 1st calving and 1st weaning rates. Their conclusion was that selection on scrotal circumference would not be effective in improving female reproduction. These findings are in agreement with some studies and contradicted by others. For those interested, Martinez Velazquez et al. (2003) provides and excellent literature review on the subject. Given the conflicting evidence, it may not be advisable to base selection decisions on scrotal circumference with the intent of enhancing maternal reproduction.   As for other traits that may be related to reproductive function, Rogers et al. (2004) found that increased levels of milk EPD increased the risk of females being culled. This finding is consistent with ASA data showing an unfavorable (-.15) genetic correlation between milk and stayability. Other ASA genetic correlations of note: -.26, .40, and -.19 between stayability and mature weight, maternal calving ease and marbling, respectively. Based on these findings, we would expect females that are inherently lower milking, smaller mature sized, easier calving and less marbled to stay in the herd longer; however, none of these relationships are strong enough to make a sizable impact on stayability by selecting for them. Furthermore, other than mature weight because of its strong relationship to early growth, determining the genetic level of a young heifer for these traits by simply observing them (which is what most commercial producers are limited to) is not possible. Therefore, a different tack will be required if we wish to improve reproductive performance via selection. Namely, select for it directly - which, as I will point out, is not a trivial task.   Next week, Part 2 of this article begins with a focus on Direct Selection.     Source: Wade Shafer, ASA Director of Performance Programs   http://www.cattlenetwork.com/agrilabs_content.asp?contentid=210241