Friday, April 12, 2024

Breeder seeks bell curve edge for resistance

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What worked for sheep could work for cattle – if we had the numbers to develop it, says Gordon Levet.
Reading Time: 3 minutes

Over the next two weeks, the Farmers Weekly team unpacks what the drench resistance threat means for the sector, from revisiting farm systems to heeding the sheep farmers whose experience could offer a solution.

Self-taught scientist and respected Romney breeder Gordon Levet says the cattle sector could have something to learn from his work in developing worm-resistant sheep. 

With drench companies signalling no likelihood of new actives on the horizon, he maintains it has never been more critical to look within animals’ genetics to respond to chemical resistance to worms.

Levet speaks with the experience in a farming career where decades were spent honing an almost instinctive scientific ability, breeding sheep capable of tolerating high worm loads. 

His work started from simple observation, noting how some lambs were significantly less affected by worm challenges than others.

Working with the encouragement of AgResearch’s Dr Tom Watson, Levet started a comprehensive sampling programme monitoring faecal egg counts (FEC) and individual lamb responses to drench doses. 

Over the years, 300-400 ram lambs were tested twice with 10-12 sires used, and assessment was also done on ewe lambs.

Noting the average FEC of each sire’s progeny, Levet was greatly encouraged by a five-fold genetic variation between the sons of the best and worst sires.

In the late 1980s his genetic programme for breeding sheep with a level of worm resistance began in earnest.

“In the first 10 years progress was painfully slow, as there were only susceptible sheep, with some slightly more resistant than others,” he said. 

His lucky break came in 2000 with a genetic freak in sire 765-98. The ram had 22 of his sons ranked from No1 to No 22, and none in the bottom 60% of poor performers. 

“That one sire with his wide use greatly accelerated progress.” 

Levet said breeding focuses on finding the outliers along the bell curve of animal traits, isolating those outliers, and breeding up from them.

Making the big call not to drench between sampling later that decade was also a game-changer. To Levet’s surprise FEC numbers did not explode, and in fact dropped by week 16 of no drenching. 

He opted not to drench lambs after birth, while continuing to drench the tailenders, about 10% of the flock. 

From then on, he never drenched the ram lambs, a risky move in an environment with the blood-sucking haemonchus worm present.

His other major breakthrough came not long before he retired. In 2020 he witnessed a 30-fold drop in FEC counts – something genetics alone could not account for. 

He concluded the absence of pneumonia risk that year ensured lambs’ immune systems were more capable of handling the worm challenge.

“It illustrated the immense power an enhanced immune system has to control all internal parasites.” 

He cites it as a personal high over 34 years of work in the field. 

Looking at the cattle sector’s issues with resistance, Levet said the ability of cattle to tolerate a level of worm loading will also prompt a healthy immune system response, as he witnessed in his sheep.

“But the difficulty for cattle is that you need the numbers for testing and identifying the outliers. You would have to get several farms working together. 

“I was testing the progeny of 800 rams and had a few lucky breaks. I was totally dedicated. There were a lot of breeders who started out doing this but found no money in it and pulled out.”

Meantime he, like many in the sector, laments the lack of funding into pastoral parasites. 

This includes the need to better understand and prove his theory that a level of worm loading in animals is vital to stimulate the immune system in a healthy way, and therefore the animal’s ability to live with them.

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