Determining the value of Plasma Anti-Mullerian Hormone as a phenotypic and genetic marker for reproductive rate

Summary

There is significant potential to increase reproductive rate in Australian sheep and beef industries leading to improved productivity and economic benefits. Therefore, the ability to predict the genetic merit and/or likely reproductive rate of young replacement females would be an extremely valuable selection and monitoring tool for livestock producers. 
This project evaluated the potential value of anti-mullerian hormone (AMH) measured in plasma as a phenotypic and genetic marker for reproductive rate in sheep and beef through modelling. This involved estimating the production benefit and genetic gain for a range of correlations between AMH and reproductive rate, defined as annual number of lambs weaned (NLW) in sheep and annual cow weaning rate in cattle, the common measures used in Australian genetic evaluations for sheep and beef respectively. The outcome of this project is an informed understanding of the likely value of AMH as a predictor of reproductive rate (weaning rate) in sheep and cattle. 
This report details the predicted value of AMH for use as a phenotypic indicator and for genetic selection in Australian beef and sheep enterprises. The report provides a basis to evaluate investment in AMH based on the potential returns to industry under various scenarios. Plasma AMH levels can be readily measured in mammals, is considered to be repeatable and reported to have strong correlations to antral follicle count and size of the ovarian reserve in humans, rodents and bovine. Favourable relationships between antral follicle count and size of the ovarian reserve with some measures of fertility have been reported. However, no estimates of correlations between antral follicle count and size of the ovarian reserve with weaning rate have been reported. 
This study considered strengths of genetic correlations between plasma AMH and weaning rate from 0.00 to 0.99. Potential genetic gain in cow weaning rate for southern beef and Brahman from use of AMH as a marker was low. Specifically, at an assumed genetic correlation of 0.50, the response per year was 0.4% and 0.5% (percentage point increase) above the base scenario (described in section 3.1) for Angus and Brahman respectively. In sheep, the potential genetic gain in number of lambs weaned was low to moderate. The response per year was 1.9% higher than the base scenario (6.0% vs. 4.1%) at a genetic correlation of 0.50. The major limitation to AMH improving genetic gain is that recording of males appears to provide no predictive value for female reproduction rate. 
The use of AMH as a phenotypic marker for reproductive rate was also considered. For each system (southern beef, Brahman and a sheep enterprise) benefits for use as a current generation selection tool were assessed for correlations from 0.1 to 0.5 between AMH and weaning rate with repeatability for weaning rate of 0.1, 0.2 or 0.3 considered. Overall, potential phenotypic gain was also predicted to be low because of the low repeatability of weaning rate and low selection intensity of females in both cattle and sheep for self replacing herds/flocks. Under the scenario of phenotypic repeatability of weaning rate being 0.2 and a correlation between AMH and weaning rate of 0.3, the estimated improvement in cohort weaning was 1.3%, 2.0% and 1.8% for southern beef, Brahman and sheep respectively. 
The potential to use AMH as a screening tool to identify animals that are more likely to yield high numbers of transferrable embryos is moderate. At published estimates for dairy cattle, gains in number of transferrable embryos would be 13%. This potential increase represents a substantial improvement and is of economic value to the artificial breeding sector for beef cattle in Australia. The lower value of sheep per unit may limit the value in advanced sheep breeding enterprises. 
It is recommended that a project is developed to determine the application of using Plasma AMH as a screening tool prior to entry into an embryo production program for both southern beef and Brahman type cattle.