DNA test helps avoid horns of a dilemma

SuppliedThe West Australian

Effective use of DNA technology can take the guess work out of breeding towards a pure Poll Merino flock.

The science can improve chances of breeding Poll by 80 per cent in just one year and completely removing the horn gene from the flock in just seven years, according to new data from the Cooperative Research Centre for Sheep Industry Innovation (Sheep CRC).

Through the Information Nucleus Flock, the Sheep CRC has researched the impact of various DNA testing regimes for breeding out the presence of the genes responsible for horns on Merinos.

The modelling revealed that DNA testing just the rams used in a breeding program 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.

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The gene can be removed even faster if producers choose the more expensive option of testing both rams and ewes.

Sheep CRC chief executive James Rowe said the development of horns in sheep appeared to be controlled by a single gene for which there was good DNA marker.

"The new genomic test for the horn gene means that, in Poll flocks, we can avoid breeding from rams that are carriers of the horn gene," Professor Rowe said.

"Commercial producers have expressed a preference for polled Merinos and as a result, the stud market is delivering a clear price differential of as much as $200 in favour of polled rams.

At $17 a DNA test, there is a clear return on investment for breeders and ram buyers wanting polled Merinos."

Operating as part of the Federal Department of Innovation Industry Science and Research's CRC program, the Sheep CRC is a collaboration of industry, government and the commercial sector.

Based on its observations of 2300 Merino progeny in the Information Nucleus Flock, the Sheep CRC has identified three possible gene combinations which will determine whether a sheep is horned (H) or polled (P): HH, PH or PP.

Because the horned gene is recessive, horned animals must carry the trait from both parents.

By identifying through DNA testing the marker genotype - PP, PH or HH - breeders can predict whether the progeny of a ram are more or less likely to have horns when mated to ewes with different genotypes.

The gene marker test is predictive of the horn status of tested animals.

HH rams are almost always horned whereas PH rams are rarely horned and PP rams are always polled.

"If you occasionally have horned sheep in your Poll flock, the flock is carrying the horned gene," he said.

"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 per cent of males appear with horns, the horn gene is still present in around 10 per cent of the flock."

This means that in 10 per cent of matings a sire or dam could pass on a horn gene to its offspring and 18 per cent of polled rams being used will be carriers of the horn gene.

"The new genomic test for the horn gene means that, in Poll flocks, we can avoid breeding from rams that are carriers," Professor Rowe said.

Simply testing rams the before using them, and only using PP rams, rapidly decreases the frequency of the horn gene in the flock."

The Sheep CRC has modelled the impact of DNA-testing a Poll flock with 1000 ewes, using 25 rams each year.

Each year 10 of those are replaced by young rams and 410 new ewe replacements are needed.

"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 budgets," he Rowe said.

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