sheep

Minimizing Genetic Defects

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 from the flock breeding sheep 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.

Cryptorchidism. 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.