The genetic diversity available within each sheep breed gives us tremendous opportunity to make genetic improvement. That snapshot of the genetic diversity currently available is extremely valuable since it provides a benchmark for comparing the consequences of selection over time in individual breeds. To establish that starting point, we used both pedigree- and molecular-based information to assess genetic diversity in the four breeds involved in Sheep GEMS.
For each of the breeds we evaluated—Katahdin, Polypay, Rambouillet, and Suffolk—we found substantial genetic diversity. That coincides with low inbreeding levels. So, what do we mean by inbreeding? Inbreeding arises from the mating of relatives, which leads to an increased chance that a lamb inherits identical copies of an allele from both its sire and dam. Such an increase in homozygosity is not necessarily bad. In fact, it is almost inevitable in a selection program where we retain rams and ewes with more favorable genotypes (or packages of alleles) for breeding. However, inbreeding does come with risks. Often, deleterious alleles are recessive. When that is the case, with inbreeding they will appear together in a homozygous state more often. That results in expression of their deleterious effects. Such negative consequences of inbreeding are most seen in fitness traits like health and reproductive success.
A nationwide survey conducted in 2011 by the USDA APHIS estimated that 14% of U.S. ewes (402,000 head) are culled each year. Of ewes culled, 56% are removed for old age. Culling for age is a good thing as these ewes have been productive for years, but we have limited resources and need to make room for younger, genetically superior females. The flip side is that 44% of cull ewes leave the flock early, which is not a good thing. The most common reasons for culling ewes prematurely are failure to lamb (7.7%), teeth problems (7.6%), hard-bag syndrome (7.1%), and mastitis (6.7%). To put that into perspective, over 55,000 U.S. ewes are culled each year due to udder diseases alone.
Hard-bag syndrome is a common sign of Ovine Progressive Pneumonia virus infection, and there are excellent resources available to reduce this disease in your flock through management (https://oppsociety.org) and selective breeding (see Sheep GEMS news brief #5; https://www.sheepgeneticsusa.org/sheep-gems). However, mastitis is typically due to a bacterial infection and far less research has been focused on ways to mitigate it in sheep. Most of us are familiar with clinical mastitis, which most often occurs near lambing and can be visually diagnosed. Though clinical mastitis is economically important, it has a relatively low incidence in most flocks (< 5%). In contrast, ewes with subclinical mastitis present no observable signs of disease.
The only way to diagnose subclinical mastitis is to collect a milk sample and screen it for bacteria or quantify its concentration of white blood cells (i.e., somatic cells). Past research in dairy species suggests the rate of subclinical mastitis is much higher than clinical mastitis and it was hypothesized this would hold for meat and wool ewes. Across multiple experiments, we have collected over 2,500 milk samples from eight breeds in four research flocks. Results have been shockingly consistent despite diverse breed types and management systems—approximately 25% of sampled ewes were visually healthy but had milk somatic cell count values indicative of subclinical mastitis. Furthermore, these sub clinically infected ewes weaned, on average, 13 to 35 lb. less total weight of lamb than healthy ewes, equating to a loss in market lamb revenue of $24 to $63 per ewe per year.
Despite the economic impact that subclinical mastitis poses, milking ewes to diagnose it is extremely labor intensive and not practical for most flocks. So how can we reduce the incidence of this “hidden disease”? Like any trait we want to alter, it begins with understanding the genetic and non-genetic (i.e., environmental) factors at play. One of the major goals of the Sheep GEMS project is to identify easily measured “indicator traits” that may be strongly genetically related to udder health and ewe productivity. Some preliminary results from our efforts will be the topic of the next Sheep GEMS update.
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. The USDA is an equal opportunity provider and employer. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the USDA.