Gene marker helps solve horn dilemma in Merino sheep

Australian research has shown the possibility of eliminating the horn gene in Merino sheep.

Using DNA technology, the occurrence of horned Merino rams in a flock can be reduced by 80% in one year and the horn gene can be deselected from the flock in seven years. This is the conclusion of Australia’s Co-operative Research Centre for Sheep Industry Innovation (Sheep CRC) which researched various DNA testing regimes for deselecting the gene responsible for horns in Merinos.

The modelling found that selecting out the gene in a breeding ram flock will quickly reduce the number of horned sheep and eventually lead to the removal of the horn gene from a poll flock in about 20 years. Sheep CRC chief executive James Rowe says the gene can be removed even faster if producers test both rams and ewes. “The development of horns in sheep appears to be controlled by a single gene for which there is a good DNA marker,” Rowe says. “The new genomic test for the horn gene means that, in polled flocks, we can avoid breeding from rams that are carriers of the horn gene.”

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Polled merinos
Commercial producers have expressed a preference for polled Merinos and as a result, Rowe says, the stud market is delivering a clear price differential of as much as A$200 (R1 798) in favour of polled rams. “At A$17 (R153) per DNA test, there is a clear return on investment for breeders and ram buyers wanting polled Merinos,” he says. Based on observations of 2 300 Merino progeny in its information nucleus flock, the Sheep CRC has identified three possible gene combinations which will determine whether a sheep is horned or polled.

Because the horned gene is recessive, horned animals must carry the trait from both parents. By identifying the marker genotype through DNA testing, breeders can predict whether the progeny of a ram are more or less likely to have horns. The gene marker test is predictive of the horn status of tested animals. “If you occasionally have horned sheep in your poll flock, the flock is carrying the horned gene,” Rowe says. “The frequency of the horn gene can be quite high even though you may not see horns very often.

“In fact, even if only 1% of males appear with horns, the horn gene is still present in around 10% of the flock. This means that in 10% of matings, a sire or dam could pass on the gene to its offspring and 18% of polled rams being used will be carriers of the horn gene,” says Rowe. “The new genomic test for the horn gene means that, in polled flocks, we can avoid breeding from rams that are carriers.”

“Simply testing rams before using them, and using only those rams homozygous for the polled gene, rapidly decreases the frequency of the horn gene in the flock.” The Sheep CRC modelled its results on a polled flock with 1 000 ewes and 25 rams. Each year, 10 rams and 410 ewes are replaced.

“While testing rams as well as replacement ewes reduces the frequency of the horn gene more quickly than just testing rams, the cost of testing is much higher and this should be carefully factored into breeding budgets,” Rowe says. “However, the number of horned rams produced by a flock of this type can be reduced from six per year to just one almost immediately by simply testing the rams that you introduce into the flock.”