Minimizing Genetic Defects in Sheep

New Mexico State University Extension
(Previously published on the NMSU Extension web page)

As we dive head first into the 2020 lambing season, be sure to keep a close eye on your lamb crop. I know what you may be thinking, everyone already does this and I hope that is true. Keeping lambs healthy, warm, fed, and alert are key to a successful lambing season. However, you also need to take into account and record any type of defects. As noted below, these defects may not be life threatening, but a defect in the genetics of your flock is not worth the hassle. Once a genetic defect is found, be sure to record this information in your lambing records so you can properly deal with this issue when it comes to culling the appropriate animals from your flock.

Fortunately, sheep have few inherited defects that reduce their survival or producing ability. A discussion of the major genetic defects follows.

Jaw defects
Jaw defects are present in almost all breeds of sheep and are associated with failure of the incisor teeth to properly meet the dental pad. A jaw is

undershot if the incisor teeth extend forward past the dental pad; it is overshot if the teeth hit in back of the dental pad (this condition is known as parrot mouth). Cull sheep with either of these genetic defects. If the sire and dam can be identified, remove them from the flock.

Rectal prolapse 
Rectal prolapse is a serious defect most commonly associated with the meat-type sheep. It is most common among lambs fed a high-concentrate ration. It is believed that this weakness is due to inheritance. This condition is sometimes corrected by surgery, but affected animals often continue to prolapse after surgery. Cull breeding sheep from the flock in which this occurs.

Inverted eyelids
Inverted eyelid (entropion) is widespread among most breeds of sheep. This trait is highly heritable. Inverted eyelids are a “turning in” of the margin of the eyelid. This condition causes extreme irritation, and, if left unattended, can eventually cause blindness. The condition may be noted at birth and treated at that time. One method of treating this condition is to clip a metal suture to the center of the affected eyelid. Gather enough skin under the clip in a vertical direction to hold the lid away from the eye. The clip can be left in place for several days. Mark the affected lambs and do not allow them to enter the breeding flock.

Rams with one or both testicles retained in the abdomen are cryptorchids. The condition usually is inherited as a simple recessive trait. There seems to be some association between this condition and the polled characteristic found in some fine-wool rams. Purebred breeders should make every effort to eliminate this condition.

Skin folds
Skin folds are highly heritable. They once were considered desirable in some fine-wool breeds because they provide more surface area to grow wool. This condition is no longer considered advantageous, and most purebred breeders are trying to breed smooth-bodied sheep. Excessive skin folds are positively associated with lower fertility and overall productivity. Additionally, folds are difficult to shear and are subject to insect attack.

Face covering
The amount of wool growing on the face is also highly heritable. Cull sheep with excessive amounts of wool growing below the eyes and on the lower part of the face because face wool can obscure vision. Ewes that have trouble seeing are generally not as productive as open-faced ewes.

Fleece defects
Some inherited fleece defects include the incidence of belly-type wool growing high on the side of the sheep, hairiness or hairy wool, and colored wool. Through a rigid selection and culling system, the potential for genetic defects can be minimized.

Black or pigmented fibers
Black or pigmented fibers will have a negative impact on wool quality and price of a fleece. Therefore, it is economically important to monitor for signs of pigmentation in rams, ewes, and their offspring. The heritability of pigmentation is approximately 30%. There is less of a correlation between “black” lambs and black fibers, but those lambs should still be closely evaluated at weaning. If pigmentation or spots still exist, those animals should be culled from the flock where wool production is important.