The lamb crop is one of the most important factors affecting profitability of a sheep enterprise. Many factors affect lambing percentage, and management is a key contributor.
Culling underperforming ewes is one of 12 best-management practices that has been identified by the American sheep industry for improving lambing percentage, says Susan Schoenian, University of Maryland Extension specialist.
But unlike some of the other best-management practices — such as breeding ewe lambs or pregnancy scanning — culling underperforming ewes is something all producers can and should do, regardless of flock size or production system, Schoenian says in a news release.
In a sheep enterprise it’s customary to cull about 15% of the flock each year. In purebred or excellent-producing flocks, the rate may be even greater.
There are many reasons to cull ewes. The reasons will vary by farm or ranch. Not all flocks will have the same breeding objectives.
Age is usually the primary reason for culling ewes. According to the 2011 National Animal Health Monitoring System study, almost 70% of sheep operations cited age as the primary reason for culling ewes. The average age of culled ewes was 6.3 years in 2011, compared to 5.9 years in 2001.
Ewes tend to be most productive between the ages of three and six. After six years of age their productivity tends to decline.
Some ewes are productive well beyond six years of age. Ewes that can maintain productivity for a longer period of time should be favored in selection and culling decisions. In many instances their offspring are some of the most productive ewes in the flock. Keeping older productive ewes could be a way for some flocks to increase productivity, while simultaneously reducing replacement costs.
In the National Animal Health Monitoring System study, hard bag and mastitis were identified as primary reasons for culling ewes. Both conditions result in little or no milk being produced by an affected gland, causing lambs to starve or grow poorly.
Footrot is a bacterial infection of the hooves. It’s one of the most difficult diseases to control and eradicate from sheep farms. It has caused many sheep producers to liquidate their flocks.
Culling is one of the most powerful tools for dealing with footrot. Ewes that are chronically infected with footrot or scald, or fail to respond to treatment, should be removed from the flock. Ewes that have abnormal or excessive hoof growth should be culled. It’s possible to select for footrot-resistance in a flock.
In situations in which internal parasites are a major obstacle to profitable production, parasite-resistance should be a selection and culling criteria. Ewes that require frequent or regular deworming should be culled.
It’s possible to select for parasite resistance in sheep because 20 to 30% of the flock is usually responsible for 70 to 80% of the output of worm eggs. Parasite resistance is a moderately heritable trait.
The National Sheep Improvement Program currently provides estimated breeding values for parasite resistance in Katahdin sheep. The same can be done for other breeds once data are submitted.
There are numerous other physical problems for which ewes should be culled. Ewes should be evaluated for soundness on a yearly basis, preferably at the time of lambing, marketing or breeding.
Ewes that fail to raise a lamb should be culled. No ewe can return a profit if she fails to produce a lamb. It’s easy to identify a dry maiden ewe because she won’t have any udder development. In older ewes it’s more difficult to notice dry ewes, but they are usually in better body condition and have smaller udders.
In some production systems ewes that raise single lambs should be candidates for culling.
Other reasons for culling ewes:
- In hair-sheep flocks, failure to adequately shed may be a reason for culling.
- Wooled-sheep flocks should be culled for ewes with fleece defects or wool-quality issues.
- Temperament can be another reason for culling. Fence jumpers should be culled. Flighty ewes are more difficult to handle and can excite the entire flock. Calm ewes should be favored more than nervous ewes because their behavior has been associated with lesser lamb mortality.