Why you should be using WormFEC Gold Genetics

A collection of powerful insights from farmers,
breeders, veterinarians, and scientists.

A collection of powerful insights from farmers, breeders, veterinarians, and scientists.

Breeding for host resistance to internal parasites in sheep has been shown from more than 40 years of research in both Australia and New Zealand to be a low-cost sustainable approach to reduce the effects of parasitism.

John McEwan

AgResearch Scientist

Parasitism: a perspective from a retired veterinarian

Parasitism has consistently been ranked the No. 1 Animal health problem by sheep farmers in NZ

In 1962 the first truly broad spectrum anthelmintic was released onto the market. The Benzimidazoles (Thibenzole, Telmin, Panacur etc.) or white drenches were followed by the clear drenches, Levamisole (Nilverm) and the avermectins (Ivermectin , Abamectin) and Milbemycin (Moxidectin or Cydectin). Finally, and relatively recently Monepantel (Zolvix) and Derquantel.

Over the last 60 years worms have developed significant resistance particularly to the white, clear and mectin drenches and now even to Zolvix. Triple drench resistance is not uncommon.

What is now clear is that the use of drugs alone will not solve the parasitism problem in NZ pasture-based farming systems. Even if a new class of drugs is discovered then if history is anything to go by, overuse will result in rapid development of resistance by worms.

Climate change is likely in some cases to enhance larval survival and development increasing the challenge to sheep.

To maintain our current farming system there is no silver bullet, and it is obvious that the problem must be attacked from many directions, i.e., an integrated approach.

Breeding sheep that are resistant to internal parasites is an essential tool in this battle. It has positive benefits to many of the recommended procedures advocated in Wormwise.

For instance, should the worm challenge be such that a drench is warranted, the interval between drenches would be longer in resistant sheep and the proportion of the flock requiring a drench is smaller. These practices reduce the development of drench resistance in worms.

Farmers are often very reluctant to leave sheep un-drenched in a mob but having worm resistant sheep can give them confidence.

Ewe drenching is not a recommended practice if you want to avoid drench resistance but personal knowledge of two flocks in this area with FEC’s of 2500 show that parasitism is still a major problem. One of these flocks had been drenched a month before the samples were taken indicating possible worm resistance to that drench. Both flocks were in very poor condition with abysmal scanning results. Resistant ewes are far less likely to develop such a high worm burden.

Safe pasture in practice is a relative term and resistant sheep are able to graze areas that are more contaminated with infective larvae than susceptible sheep without production loss.

Finally breeding sheep that are resistant to worms is a sustainable practice looking to the future in these uncertain times.

Danny Hajdu

Retired Veterinarian

If breeders don’t objectively select for resistance and dag score they are passing on the costs of drenching and dagging, directly to their clients.

Allan Richardson

Avalon Genetics

Running our own worm fecal egg counts using the FECPAC kit from Techion Group, we are able to ascertain the worm burden in an individual animal and/or a whole mob within a short space of time.  This has proved to us the vast differences in individual animals that can occur across a mob.  With continued selection for high worm resistance genes, we have been able to dramatically reduce the frequency of drenching.  This has been a time and cost saving and it has also ensured that we don’t have any drench resistance on our farm.

Adrian and Jenny Savannah

Glenbrook Romneys

Currently, control of parasites in sheep relies heavily on the use of anthelmintic treatment. The increasing prevalence of drench resistance throughout the world suggests that this strategy is unsustainable. Sustainable, long-term management of parasites requires an integrated control program, and breeding sheep with an increased ability to resist infection is an important part of this strategy.

Kathryn McRae


Drench resistance is the number one animal health issue affecting sheep performance across New Zealand. Introducing worm resistant genetics into the maternal ewe flock will be an important tool to help manage the issue. It will take time to build this trait into the flock and see the flow on effects, so start now.

Dave Warburton

Vet Services, Hawkes Bay

Gut parasitism is a major problem for sheep farming.  It has been estimated that upto 30% of modern sheep production is reliant on effective parasite control.  The shift from wool production to lamb production, and increasing intensification, are all increasing the pressure of worms on the farming system. 

Unfortunately chemical control of worms (drenching) is deteriorating rapidly with the spread of multiple drench resistant worms through sheep flocks.  Triple drench resistance in worms is now not uncommon and although “new actives” are available they are much more expensive and will only have a limited life before resistance get to them as well.  The only sustainable solutions are through management, and worm resistant sheep is a major tool for this.

We know that there is a genetic component to resistance, so it can be selected for.  As with all genetic traits the rate of progress depends on a number of factors including, how long you have been selecting for this trait and how serious you are about it (how much emphasis you put on this trait as opposed at all the other traits that you are selecting for). 

When selecting a breeder you need to ask the hard questions about when they started selecting, how they are selecting and what future plans do they have.  A group of ram breeders who are serious about selecting for parasite resistance have formed WormFEC Gold. WormFEC gold members have a track record and a passion and the knowledge to produce rams with a genuine genetic advantage.  Remember however that using one “resistant” ram will not suddenly mean that you do not have to drench the lambs.  This is a long project with incremental gains but one that could ultimately keep you able to farm sheep profitably.

Mark Anderson MVSc