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Exploration of anogenital distance as a novel fertility phenotype in dairy cattle
- Author / Creator
- Carrelli, Jennifer E.
Over the last 40 years, there has been an obvious decline in reproductive performance and fertility in dairy cattle, highlighting the importance of genetic improvement in this area. With more recent technological advancements, opportunity arises to identify and incorporate novel fertility traits, with the potential to complement current traits, into breeding objectives in an effort to achieve more substantial genetic gain for fertility. Recently, anogenital distance (AGD), measured as the distance from the centre of the anus to the base of the clitoris, has shown to have an inverse association with measures of fertility in dairy cows. Two studies were conducted to explore the associations between anogenital distance and measures of fertility in a larger population of dairy heifers and cows from Western Canada and the USA.
The objectives of the first study (Chapter 3) were to (1) characterize AGD in nulliparous dairy heifers, and (2) determine if an inverse relationship between AGD and fertility, previously found in lactating dairy cows, is also evident in nulliparous heifers. AGD was normally distributed, highly variable, and inversely related with measures of fertility. Heifers with short AGD required fewer services per conception (1.5 ± 0.1 vs. 1.7 ± 0.1; P < 0.01), conceived earlier (14.9 ± 0.2 vs. 15.1 ± 0.2 mo; P < 0.01), and a greater proportion of them became pregnant to first artificial insemination (AI; 58.3 ± 3.0 vs. 49.6 ± 3.1 %; P < 0.001) than their long-AGD counterparts. Moreover, heifers with long AGD had a lower relative risk for pregnancy up to 450 d of life compared with those with short AGD (hazard ratio: 0.59; P < 0.001). This study established that an inverse association between AGD and fertility exists in nulliparous heifers.
The second study (Chapter 4) aimed to validate findings that AGD is inversely related to measures of fertility in lactating Holstein cows. A secondary objective of this study was to determine the association between AGD and milk yield. AGD was normally distributed, highly variable, and inversely associated with fertility measures. Cows with short AGD had improved pregnancy to first AI (35.7 ± 2.1 vs. 31.4 ± 2.0 %; P < 0.01) and fewer days open (136.9 ± 4.3 vs. 140.9 ± 4.3 d; P = 0.05) than cows with long AGD. Regardless of parity, cows with short AGD tended to require fewer services per conception (2.3 ± 0.1 vs. 2.4 ± 0.1; P = 0.06) than their long-AGD counterparts, but cumulative pregnancy risks up to 150 and 250 DIM did not differ between AGD categories. Anogenital distance had a weak positive association (r = 0.04; P < 0.01) with 305-d mature equivalent milk yield. The results of this study confirm an inverse relationship between AGD and measures of fertility in lactating cows, validating previous findings, with no evidence of parity effects. Moreover, results indicate that the phenotypic selection for AGD will not cause a substantial decline in milk production.
Overall, results from this Master’s thesis research provide further insight into AGD and its associations with fertility on a larger scale, lending further support for AGD to become an indicator of fertility and a possible management tool in future selection programs.
- Graduation date
- Spring 2021
- Type of Item
- Master of Science
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.