When it comes to stud rams in the sheep breeding game, size really does matter. Scrotum size that is.

Key points: Scrotum size one of dozens of physical traits measured in rams

Scrotum size one of dozens of physical traits measured in rams Researchers can now predict the later performance of sheep from an early age

Researchers can now predict the later performance of sheep from an early age High prices for sheep meat led to move towards breeding a dual-purpose animal

At 18 months of age, rams at the Gulargambone Rural Association for Sheep Selection (GRASS) merino stud near Gilgandra, in the central west of New South Wales, have their scrotums measured as an indicator of how they will perform.

Each ram sold at the GRASS stud's annual sale this month was expected to service up to 50 ewes in a four-week breeding period.

According to the stud's founder, agricultural scientist Graham Peart, any rams that do not measure up are culled.

"The main reason for buying a ram is to make sure he produces lambs for you, so unless he's got the right gear he's no use to anyone," Mr Peart said.

"The research shows the bigger the testicles, the scrotum size, then the more semen volume they produce."

Scrotum size is one of dozens of physical traits that have been measured at the GRASS stud for the past 40 years.

It all started in 1975 after a dramatic collapse in the wool industry, when Mr Peart convinced nine other farmers to pool their combined 35,000 ewes and measure them for traits such as fleece weight, fibre diameter and fertility.

The best 2 per cent were kept in a nucleus flock, which has become one of the most measured in Australia.

"If you can't measure it, you can't treasure it," Mr Peart said.

"In our system all those things have to fit together to produce a sheep that makes the most profit per hectare."

'What's an extra lamb worth?'

Stud manager Callum Moody (L) and sheep breeding consultant Allan Casey measure stud lambs ( ABC News: Sean Murphy )

GRASS is also a play on words because the sheep had to be fed on grass, not grain.

Mr Peart said the group wanted sheep that performed in the paddock, not the show ring — where stud rams were often fattened up to look good, but failed to perform where it counted.

"The 10 directors, who are good commercial farmers, say unless it's an absolute commercial winner, then you're having yourself on and they direct the policy now," Mr Peart said.

"Changing policy in the stud is very slow but about every seven or eight years we look at our index, which is all computerised.

"So [we check] what's an extra kilogram of wool worth? What's an extra kilogram of meat worth? What's an extra lamb worth? And that all goes in the computer and that tells you where to place your emphasis."

The GRASS stud also recently produced the top wool cutting ram in the national Merino Select computer program and its rams outperformed 14 other studs in a trial conducted by the Trangie research station in NSW.

The rams were joined with standard ewes to produce 50 lambs each to be measured for meat and wool.

Predicting a sheep's performance

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In more recent years high prices for sheep meat have seen the merino industry move towards breeding a dual-purpose animal — adding meat traits such as growth rate and intramuscular fat to their wool growing capacity.

Genomics is now enabling producers to use individual DNA profiling of animals to gain more improvements to sheep breeding values, according to the Cooperataive Research Centre (CRC) for Sheep.

CRC director professor Julius van der Werf said researchers could now predict the later performance of sheep from an early age, even just after birth.

It means producers can improve their flocks with better selection of traits — such as reproductive rate, lifetime wool performance, and disease resistance — but the biggest change was in meat traits.

"You cannot slaughter an animal and then breed from it," Professor van der Werf said.

"But you can slaughter some animals and, for other animals, you can predict its breeding value from that animal. So you can now select for meat quality and eating quality like intramuscular fat [and] the quality of the meat.

"[Basically] you can select for those traits much better than you could do before."

Watch Landline's story on merino sheep breeding on ABC TV at midday on Sunday.