How a New Zealand sheep stud added 720g to carcass weight

Carcass yield has been increased on a New Zealand sheep stud by using myostatin in a balanced breeding programme that has also lifted eating quality and worm resistance.

Nithdale Station belongs to Andrew and Heather Tripp, who introduced Texel genetics in 2005 as an outcross to improve lean meat yield on their South Island farm.

The Tripps must have functional and easy-care sheep that lamb outside unassisted, so going gung-ho at carcass yield could have resulted in big losses and difficult lambings.

It has taken 17 years to infuse myostatin into the flock and 30 years to see big gains in parasite resistance.

See also: 13 tips for lambing outdoors

Now that the ewes are 15-20% Texel, carcass weights are up to 18.5kg deadweight. This is a lift of 4% (720g).

Farm facts: Nithdale

  • 5,600 ewes
  • 1,900 hoggets and 750 ram hoggets
  • 850 dairy cows take up 275ha (680 acres)
  • 150 beef cows and up to 300 finishing cattle
  • Baleage and kale or swedes for wintering
  • Sells Sufftex and Romney rams off farm
  • 1,409ha (effective) (3,481 acres)

“This doesn’t sound much, but across 5,000 lambs this is an extra 3.6t of meat a year, so it does add up,” explains Mr Tripp. At a UK deadweight price of £6/kg, this increase is worth £21,600.

The Tripps stress that a balanced breeding programme must:

  • Find ewes that rear twins unassisted to improve mother ability, ease of lambing and lamb survival
  • Be based on structurally sound sheep
  • Make progress in other key areas such as growth rates and worm resistance.

This approach to breeding has increased profit for the Tripps’ customers, with an £8 a ewe benefit and 200% faster genetic gain on the maternal index, worm resistance, meat and dag score in June 2021.

Andrew Tripp with dog in field

Andrew Tripp of Nithdale Genetics © Nithdale Station

Where is Nithdale?

  • Nithdale Station is in the far south of New Zealand, near Gore and about 90 miles south-west of Dunedin
  • It lies on the eastern edge of Southland
  • Rainfall averages 1,050mm/year, typically distributed across the year
  • The farm covers a range of north- and south-facing hills and flat and rolling paddocks

Adding myostatin

By the early 2000s, carcass weights seemed to be plateauing at 17.5-18kg. So, in 2005, two Texel rams carrying the GDF8 gene were introduced into the Romney flock.

Both Texels were “double MyoMAX” (had two muscling genes) and were carefully selected on the highest meat index possible, with good growth and fertility scores.

One Texel was from the award-winning Waikite Texels on the North Island, which is run by the state government-owned Lancrop Farming.

The other was from the Blackdale stud, Riverton, which scores tups on lameness and feet and is breeding the GDF9 fertility gene into its tups.

“There were faults that came through with the Texels,” admits Mr Tripp. “We had lambing difficulties, crooked feet, lower fertility and black spots. There was also less wool.

“However, now we are back to about 80-85% Romney in the mix, those faults have mostly gone.”

Romney ewes

© Nithdale Station

The resulting progeny is an open-faced Romney which has a little more meat.

Each cross to a Texel has been followed with a cross back to a Romney. The aim was to breed a sheep that is 80-95% Romney with two copies of the myomax gene.

About 20% now have double copies and 40% have a single gene.  

Focus on breeding for myostatin has eased a little because of its effect on lowering intramuscular fat (IMF), explains Mr Tripp.

This would have started to affect eating quality, had the issue not been identified in the farm’s Sufftex flock and the South Island Genomic Calibration flock at the research farm of AgResearch at Duncraigen.

The farm is not paid for meat quality on its processor grid, but looks at IMF as a way of safeguarding the future by breeding lambs that “eat well”.

 “We have lines coming through that are good for both meat yield and meat quality, so now that we can select for both, we can make progress on both,” he says.

Worm resistance

Drench resistance is as big an issue in New Zealand as it is in the UK, says Mr Tripp. “Many vets are seeing triple drench resistance and some farms even have resistance to orange and purple wormers.”

To breed resistant ewes, the farm only drenches ewe lambs that need a dose, based on faecal egg counts (FECs), and ram lambs are left undrenched until FECs are done at five months old.

Improved daily liveweight gain also means lambs are on the farm for a shorter time and not drenched as much.

A big factor that has helped reduce worming requirements has been selecting sheep that still perform when carrying internal parasites (assessed by FEC).

This has taken 30 years, but it is starting to show in reduced drenching.

“We have found an average mob FEC of about 500 is the trigger point for dosing – any higher and it starts to affect the animals that aren’t as worm resistant,” says Mr Tripp.

Drenching has been at such a low level that a 98% reduction has been seen in white, yellow and clear wormers.

This has resulted in adult FECs nearly halving in number. Overall, the industry has not changed a great deal, but Nithdale sheep produce 40% fewer eggs than 30 years ago.

This means buyers are now coming to Nithdale because of parasite resistance in the ewes.

Rundown of Nithdale farming system

Aerial view of Nithdale Station

© Nithdale Station


All ewes lamb outdoors, and most of them on a hill. Shepherds drive round to check every other day for any issues, but the flock gets on and lambs itself.

Sufftex ewes are checked daily and are tagged at birth. Beef cows are spread out to calve among the sheep.

About 1,000 commercial ewes lamb to terminal sires in paddocks that are checked daily.

Having DNA parentage of the flock since 2012 means the sheep can be left alone to lamb (like a commercial farm), but parentage is known to an accuracy of almost 100%.

Flock genomics has been ongoing since 2017, meaning accuracy and genetic gain is.

Summer grazing 

Stud ewes are tissue-tagged for genotyping at the end of lambing. Cattle are mobbed up to mate in December and graze blocks over the summer for pasture control on the hill.

Dairy cows are on their own specific platform.


Lambs are weaned in December at 30kg at 10-12 weeks and sold in March to average an 18.5kg carcass.


Tups are out for 45-50 days. Terminal sires and Sufftexes start tupping on 8 April. The rest of the ewes are tupped on 26 April and hoggs are mated  from 15 May.


Winter is budgeted at 100 days – from 1 June to 10 September. Hoggs are wintered on grass and ewes are either on swedes or kale, plus baled hay.


About £235,000 is tied up in two tractors and cultivation equipment. The business employs a tractor driver and is more capital intensive than others which would use more contractors.