Understanding genetics in sheep breeding

Improving the quality of your sheep flock increases its value and efficiency. One of the best ways to do this is by managing your flock's genetics.

Understanding sheep genetics helps you:

• increase productivity and profitability
• develop a breeding objective for your flock
• select rams from a stud that help you reach this objective.

How well sheep perform is due to their genetic makeup and environment. As each lamb possesses the genes of its parents, you can ensure they are more productive, marketable and better breeders themselves by selecting breeding sheep with the best genetic makeup.

Heritability traits

Heritability estimates how much of the superiority of the parents is, on average, passed onto their progeny.

Estimates of heritability tell the breeder how effective selection is likely to be. The higher the rate of heritability, the greater chance that the characteristics you are looking to encourage in your sheep will be passed from parent to offspring.

Traits with a high heritability also have a high repeatability.

High heritability

• Fibre diameter
• Clean fleece weight

Moderate heritability

• Staple strength
• Lean meat yield
• Eye muscle depth
• Fat depth
• Weight
• Worm egg count

Low heritability

• Birth weight
• Number of lambs born/weaned

Repeatability

Repeatability is a measure of the tendency of animals to maintain their ranking over time. It describes the accuracy with which early records of an animal's performance in a particular trait can predict its lifetime performance.

Repeatable traits in adult sheep:

• greasy and clean fleece weight
• yield
• fibre diameter
• staple length
• body weight.

It's almost as efficient to select sheep for production based on the earliest adult records as on the average of 2 records. The slight loss in efficiency as sheep get older is compensated by a higher gain each year because of early selection and the early disposal of surplus sheep.

Repeatability of reproduction traits is low. This suggests that culling based on a ewe's first performance may not produce much genetic gain. However there is great variation in reproductive performance, which enables more progress than the heritability and repeatability of reproduction traits initially suggest.

A weaning record is a good predictor of lifetime performance for traits other than reproduction traits.

Use repeatability to assess which sheep to cull and which to keep, rather than which are the most suitable for breeding. When making selections to improve reproduction, cull ewes that have failed to conceive or rear a lamb twice.

Genetic correlations

Selecting for a particular genetic trait may result in changes in other traits. This is because they are controlled by the same genes or by different genes that are linked.

Genetic correlations are either:

• positive: both traits improve
• negative: one trait shows improvement while the other deteriorates.

Look for genetic correlations to maximise the positive traits and minimise the negative one.

For example, there is a positive genetic correlation between clean fleece weight and fibre diameter. If selection was based on clean fleece weight alone, fibre diameter would tend to increase gradually.

There is a positive genetic correlation between clean fleece weight and staple length. Selecting for clean fleece weight will result not only in increased fleece weight but also increased staple length.

Similarly, a negative correlation between wrinkle score and clean fleece weight means that the selection of plainer sheep results in increased clean fleece weight.