Sheep GEMS News Brief 10 – February 2025
In the last Sheep GEMS news brief, we presented the economics of subclinical mastitis in meat and wool ewes. Despite the major production impact that subclinical mastitis poses, the only way to diagnose it involves collecting and testing milk from every ewe. Take it from the guy who has milked the rankest Rambouillet and most cantankerous Katahdin ewes, you don’t want to go down that road! So, we’re at a bit of an impasse; we have a disease that costs us money, but we don’t know which ewes have it. How can we reduce its incidence? It begins with understanding the genetic and environmental factors at play and identifying more easily measured “indicator traits” that may be strongly genetically related to udder health.
Past work in dairy sheep estimated that milk production, udder depth, and indicators of subclinical mastitis, like somatic cell count, are moderately heritable (20 – 35%). Perhaps unsurprisingly, milk production and udder depth are favorably genetically related. That is, selecting for deeper udders will, on average, increase milk production. However, somatic cell count is unfavorably genetically correlated with milk production and udder depth. Meaning if you only select for increased milk production or deeper udders you will, on average, increase susceptibility to mastitis in your flock. We rarely measure actual milk production in non-dairy ewes, but we’d expect the same biological relationships to apply.
Milk production has been subjectively scored (low, medium, high) shortly after lambing on U.S. Sheep Experiment Station (USSES) ewes since the 1980s. They also record a subjective udder score at weaning. We analyzed over 23,000 records from over 10,000 USSES Polypay, Rambouillet, and Targhee ewes. Our results showed that ewes with high milk production went on to wean more total weight of lamb than ewes with low or medium milk production; 21 and 8 lbs., respectively. That’s not a Nobel Prize winning discovery – of course heavy milking ewes wean heavy lambs. But we also found that ewes with high milk production had greater risk of recording an abnormal udder at weaning (“swing bag”, lopsided, damaged, etc.) than ewes with low or medium milk production; 2- and 1.6-times, respectively.
So which ewe is the most profitable? Is it the heavy milker that weans big lambs this year and has a greater likelihood of getting culled next year or the moderate milker that’s consistent for 8 years? When we shift our perspective from the single individual at a single point in time to the whole-system level, we quickly realize that maximum performance is rarely the most economical in the long run. Breeding for intermediate optimal reproductive rate or milk production is challenging, particularly when optimal performance depends on ewe age, nutritional and labor resources, and production climate. Nevertheless, our preliminary work in Sheep GEMS has found that udder depth and teat placement are moderately heritable (~25%) and most mature ewes have intermediate scores. These early results are encouraging as we continue to research genetic strategies that U.S. sheep producers can employ to breed more productive and longer-lived ewes fit for a variety of production environments.
For further information contact Tom Murphy (tom.murphy@usda.gov).
Acknowledgements. We thank U.S. sheep associations and breed organizations, the National Sheep Improvement Program, and sheep producers, for their contributions to this research. This work was supported by the Organic Agriculture Research and Extension Initiative (grant 2016-51300-25723/project accession no. 1010329), and by the Agriculture and Food Research Initiative Competitive Grant (grant 2022-67015-36073/project accession no. 1027785), from the USDA National Institute of Food and Agriculture. USDA is an equal opportunity provider, employer, and lender.
