Editor's note: This is an in-depth explanation of the fifth of 12 Lamb Crop Best Practices. The fourth was published in the Aug. 1 edition of Agri-View.

Lamb crop is one of the most important factors affecting profitability of a sheep enterprise. Increasing the lamb crop, so long as it’s in balance with the environment and production system, should be the goal of every sheep producer.

Many factors affect lambing percentage; 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. 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.

Culling underperforming ewes will reduce the cost of maintaining the flock. Underperforming ewes consume feed, use space and require labor. They do that while producing less profit than their contemporaries – and maybe even cost money for the farm. Culling underperforming ewes is a way to help sheep production be more profitable, sustainable and viable.

But overzealous culling is discouraged because there are numerous costs associated with culling. The value of a cull ewe is considerably less than the value of the ewe lamb that’s replacing her. There are also costs associated with developing ewe lambs for breeding. Depending upon the reason for culling, it may be more economical to retain a ewe and breed her to a terminal sire. One reason is for the production of market lambs.

In a sheep enterprise it’s customary to cull about 15 percent of the flock each year. In purebred or excellent-producing flocks the rate may be even greater. According to the National Animal Health Monitoring System 14 percent of ewes were culled from the national flock in 2011, as compared to 18.3 percent in 2001.

Culling is when a ewe or ram is removed from the breeding flock. There are many reasons to cull ewes; the reasons will vary by farm or ranch. Not all flocks will have the same breeding objectives. Ewes that are profitable in some flocks may not be profitable in other flocks. Current economic conditions may weaken or strengthen culling standards.

Age reported as reason for culling

Age is usually the primary reason for culling ewes. According to the 2011 National Animal Health Monitoring System study, almost 70 percent of sheep operations cited age as the primary reason for culling ewes. Age was the reason 55.6 percent of ewes were culled in 2011. 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. On average they give birth to fewer lambs and produce less milk for their offspring, resulting in reduced pounds weaned. For those reasons it’s customary to cull ewes when they reach five or six years of age. That’s especially true in range flocks where ewes cannot receive individual attention and nutritional resources are limited. In many of those extensive operations productivity declines after five to six years of age.

Small flocks or farms with good feeding conditions may keep ewes in flocks for much longer. 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.

Health other major reason

According to the National Animal Health Monitoring System study, health issues are the other major reason why ewes are culled. Health issues are the major reason for involuntary or premature culling of ewes before they reach their productive life spans.

In the National Animal Health Monitoring System study, hard bag and mastitis were identified as primary reasons for culling ewes. Because of hard bag symptoms 7.1 percent of ewes were culled in 2011. Another 6.7 percent were culled as a result of mastitis. Hard bag, which affects both udder halves, can be caused by ovine progressive pneumonia or mastitis. Mastitis is an infection of the udder. Both conditions result in little or no milk being produced by an affected gland, causing lambs to starve or grow poorly.

Only ewes with healthy sound udders should be kept in flocks. Udders should be palpated to ensure there aren’t any lumps, hardness or fibrous material. Udder halves should be relatively equal. Both teats should be functional and of normal size because newborn lambs may have difficulty nursing over-sized teats. Ewes with long pendulous udders should be culled because lambs may have difficulty finding the teats. Such udders are also more prone to injury. Ewes that have lost all or part of their udder function should be culled.

A prolapse is when structures fall out of their normal positions. Ewes that prolapse their vaginas should be culled because they may repeat the problem in subsequent years. Their offspring shouldn’t be kept for breeding because vaginal prolapses are believed to be an inherited problem. Ewes that experience a uterine prolapse may be retained for breeding depending upon the circumstances, but most producers cull those ewes.

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. Footrot is costly to treat, especially in terms of labor. It can also be an animal-welfare issue and negatively impact productivity.

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 percent to 30 percent of the flock is usually responsible for 70 percent to 80 percent 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, marking or breeding.

  • Ewes with unidentified weight loss or ill thrift should be culled.
  • Old thin ewes that cannot maintain their body conditions should be culled.
  • Teeth or other problems may interfere with chewing. Only ewes with sound mouths should be kept. All of the ewe’s incisors should be present.
  • Ewes with genetic defects or predisposition to disease should be culled.

Consider performance when culling

Performance is another important criteria that should guide selection and culling decisions. Many of the factors already discussed account for differences in performance among ewes. Sub-clinical mastitis may be the reason that a ewe weans lambs with less-than-average weights.

It’s recommended that the breeding season be limited to two or three heat cycles, preferably only two. Mature ewes that fail to breed and maintain pregnancy should be culled. Pregnancy scanning can be used to determine which ewes are open. Pregnancy testing is especially useful for ewe lambs because open ewe lambs can be sold for greater prices than yearling ewes that fail to lamb.

Ewes that lamb late in the season may be another target for culling because ewes that lamb early in the lambing season are the most productive. If out-of-season or accelerated lambing is the goal, ewes that fail to breed out-of-season, or miss breeding opportunities, should be culled.

Lambing must be strong

While dystocia or difficult birthing is complex, research has shown that producers can reduce the incidence of dystocia by culling ewes that require assistance at lambing. Some producers will even cull lambs from assisted deliveries. Ewes that reject or harm their lambs should be culled. Ewes whose lambs are small, weak or slow to suckle should be discriminated against.

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. More-costly production systems require greater lambing percentages. Two single births in a row may be the culling standard for some sheep operations. If the single lamb is of poor quality or weight, that compounds the reason for culling.

When lamb losses are beyond the ewe’s control, such as predation or accidental death, exceptions can be made for keeping a ewe that fails to raise a lamb or fails to raise twin lambs. But if a producer makes too many excuses for a ewe, that should be a sign that the ewe is better off being put in the cull pen.

Other reasons for culling ewes remain

Various other criteria may be used to make culling decisions.

  • 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.

Susan Schoenian is a sheep and goat specialist at the University of Maryland's Western Maryland Research & Education Center.