Increasing Lamb Survival Breeding for Cold-Tolerance

September 2006
For some years Lincoln has been working with gene marker technology to identify genes that are associated with desirable or undesirable traits in sheep. Footrot resistance is one, scrapie is another. By taking a blood sample and having it analysed for the presence of particular forms of a gene (alleles), scientists are able to predict the degree of tolerance or resistance that a particular animal will have, and be likely to pass on to its progeny.

In 1998 Rachel Forrest started a PhD looking at cold tolerance. Trials involving sheep subjected to cold conditions had identified that some lines of sheep had a better survival response to cold, and Rachels work focused on a particular gene that is a key to heat generation in response to cold stress.

She linked particular alleles of this gene to the ability of newborn lambs to survive cold weather. Her research has led to a gene-marker test that enables sheep breeders to identify animals that will produce lambs that are more likely to die from cold exposure.

The aim is to remove sheep with bad alleles from breeding programmes, as opposed to selecting for sheep with good alleles. This approach maintains genetic diversity so that the ability to make genetic gains in other traits is not put at risk by excessive culling.

The test

Blood samples collected from sheep are typed in the lab to reveal which of six alleles of the cold-tolerance gene they possess. Each animal has two alleles, one inherited from each parent. The two alleles may be the same or different, and either one of them can be passed on to progeny.

For simplicity, the alleles have been grouped according to their associated risk of cold-related mortality:

A alleles associated with an average or below average (decreased) risk of cold-related mortality

B - those tending towards an above average risk

C - those associated with a marked increased risk

All other things being equal, lambs that inherit A score alleles have a much increased likelihood of surviving a cold challenge compared with lambs inheriting C score alleles.

Breeding for cold tolerance

The approach is to identify and avoid breeding from animals with a C score. The recommended breeding strategy is:

Test existing and potential sires, as rams have greater genetic influence on flocks than ewes.

Dont use rams with good cold-tolerance gene-marker results if they have poor progeny survival rates because of other inherent faults (e.g. insufficient birth coat)

Ideally, use breeding ewes that have been sired by rams with good cold-tolerance alleles to increase the rate of genetic gain within flocks.

And dont forget:

Ewe nutrition and health are very important in ensuring that a lamb is born with adequate fat reserves and that the lamb has plentiful milk to drink. It doesnt matter how good a lambs cold-tolerance genetics are, if it is not in good condition it will not have the fat reserves required to convert into heat.

This gene test is based upon the analysis of populations of sheep, and the cold-tolerance scores reflect variation from the average in a population. In any given population there are always outliers that may produce progeny that are susceptible to cold despite having good gene marker results.

Getting your sheep tested

The tests require a drop of blood put onto an absorbent FTA card. Usually pricking the ear is the easiest and safest method. They are then returned to the Lincoln University Gene-Marker Test Laboratory.

Breeders, who select for a range of other characteristics and use the gene marker tests as one of their selection criteria, primarily use the tests. The cold-tolerance test can be combined with the footrot and scrapie tests. Costs depend on the number of animals tested and the number of tests per animal, and range from $33 to $128 per animal.