Author
|
Submitted to: Meeting Proceedings
Publication Type: Research Notes Publication Acceptance Date: 1/14/2011 Publication Date: 1/21/2011 Citation: Cushman, R.A. 2011. Reproductive strategies to increase cow longevity. (New York Winter Beef Management Meeting Handout, Syracuse, January 22, 2011.) Cornell University, Department of Animal Science Teaching, Extension, Research. Mimeograph Series No. 250. Interpretive Summary: Technical Abstract: Choosing replacement beef heifers is a decision with long-term implications for profitability for the cow-calf producer. If a replacement heifer fails to wean the number of calves necessary to recoup her development costs then she incurs a net loss for the farm. To avoid such losses, it is imperative to develop technologies that aid in eliminating heifers with little chance of producing three or more calves from the pool of potential replacement heifers. Development of the reproductive tract begins before birth in heifers, and there is great variability in the size of the ovaries and the numbers of follicles that heifers have at birth. There are most likely genetic and environmental components to this variability, because nutrient intake by the dam during gestation has been demonstrated to influence the number of follicles that her daughter has at birth. As yearlings, palpation of the reproductive tract to identify heifers that have not initiated reproductive cycles and have small, under-developed reproductive tracts provides some utility, because yearling heifers with poor reproductive tract development have decreased pregnancy rates and later calving dates that result in lower weaning weights. These same heifers have a decreased pregnancy rate in their second breeding season. When reproductive tract development is evaluated using ultrasonography, there is a positive correlation between the number of antral follicles visible on the ultrasound screen and the size of the reproductive tract (P = 0.01), indicating that heifers with high numbers of follicles and larger ovaries may have greater fertility and a longer reproductive life. However, palpation based techniques to evaluate heifer development have limitations. It is nearly impossible to process the numbers of heifers in a day that would be necessary at a large commercial ranch, because these measurements require an average chute time of three minutes per heifer. The greater limitation is the age at which palpation can be performed. Ideally, the producer would like to choose replacement heifers at or before weaning, so that a gate sort could be made at weaning; however, the average technician cannot palpate a heifer before ten to eleven months of age due to the physical size of the heifer. Therefore, technologies that would allow the identification and removal of poor replacement heifers from the pool at a very young age would be beneficial. Reproductive tract development in heifers has a moderate heritability (h**2 = 0.3), and breed evaluations at the U.S. Meat Animal Research Center and elsewhere have identified differences in numbers of antral follicles among breeds (P < 0.05). These breed differences indicate that there are genes controlling the number of follicles that a heifer has in her ovaries at birth. An initial whole genome scan using the Bovine SNP50 BeadChip from Illumina identified 963 Single Nucleotide Polymorphisms with a significant association with antral follicle numbers in yearling heifers (P < 0.01). Further analyses are required to confirm and fine map these regions, but these results suggest that in the future it may be possible to use a panel of genetic markers to aid in identifying poor replacement heifers at a very young age and to remove them from the pool of potential replacement heifers for an overall improvement in reproductive efficiency of the herd. |
