March 5, 2010

Breeding sheep to eliminate dags and worms. 1.

By Dr Clive Dalton


  • Farmers used genetics to improve fertility in their commercial sheep flocks in the 1970s - 1980s, and this greatly improved income.
  • But with recent inflation and currency fluctuations, they realise that more lambs are not all profit.
  • Rising costs, especially for animal health are now a major issue, but veterinary science has not come up with ways to reduce these; so-called progress seems to be to use more!

Dagging and drenching

  • These are the two main killers in terms of cost and physical effort, as sheep farmers’ average age is now creeping up towards 60, with fewer young people entering the industry, or who want to spend their time wrestling with today’s 80kg sheep.
  • Many sheep farmers have back, hip and knee problems from hauling sheep around, and the veterinary profession are only offering more drug and chemical therapy as future solutions, which will not reduce costs.
  • This approach is not sustainable – neither economically nor for the environment.
  • So the obvious conclusion is that we have to stop, (or at least greatly reduce), dagging and drenching sheep.
  • If you don’t agree, and are happy to get the handpiece out to clean up sheep like the one below, then don’t read further.
What a prospect to have to clean up this sheep's rear end!

Major mind change needed

Sheep are never keen to cooperate
  • Dagging (to prevent blowfly) and drenching (to kill worms) have always been accepted as part of sheep husbandry.
  • So has crutching, most of which is done to prevent dags forming.
  • You never see these chores featured in a shepherd’s job description: they are taken for granted.
  • If the boss had no jobs planned, then he/she could always find some dagging or drenching to do to keep staff active and give the dogs a run!

This advert in a 1970 New Zealand Farmer was certainly convincing.
Back then, nobody thought about long-term implications of the
new wonder chemicals!

Breeding is the solution

  • Breeding is the way to get rid of the endless chore of dagging and drenching, which inevitably is followed by more dagging and drenching.
  • The chemicals used in drenches cannot be sustainable, and consumers will start to demand ‘drug-free’ products for their health and welfare.
  • Genetic gains are cost effective and they are not lost.
  • But breeding as a solution has been hard to get through to farmers, as since the modern anthelmintics came on the market in the 1960s, farmers have been bombarded by myths and untruths.

Myths & untruths

A typical range of anthelmintic chemotherapy products on the market.
Sold in bright coloured packs with regular promotional specials.

  • Breeding is too slow. Breeding for Facial Eczema (FE) resistance for example took nearly 30 years. True but there are reasons – see below.
  • Drench is faster, and with the new products on the market that kill everything, the problem of drench resistance has been solved. Not true.
  • Vets who sell drench hardly ever mention genetics to their clients. If they do, it’s always as a last resort. Drench sales are a significant part of vets' income and the more they sell the bigger markup they can negotiate from the manufacturer supplying them.
  • Genetics doesn’t seem to figure strongly in their university training, and in any case they don’t sell rams from their clinics!
  • If you start a breeding programme, you’ll be buried in paper and will need a computer. Not true.

Breeding for FE resistance – why it took so long?

Hogget with FE which is clearly suffering pain and distress.
It also cannot see as its eye lids are swollen up.

There were plenty of reasons:

  • The sporidesmin toxin harvested from the fungus was so horrendously expensive that stud breeders could only afford to dose a few of their rams.
  • It’s a very dangerous poison and has to be handled carefully by veterinarians, and if not done correctly valuable animals can be killed.
  • Farmers couldn’t afford to drench any females, so halving the overall ‘Selection Differential’ (selection pressure) and slowing up genetic gain, despite the trait being highly heritable (about 25% or the same as fleece weight).
  • Selection on the ewe side had to rely on ‘natural challenge’ to the toxin, which varied greatly between seasons.
  • Commercial sheep farmers only bought FE tested rams after a bad season, and expected instant solutions the next season. They didn’t have a policy of buying resistant rams every year.
  • Farmers turned their purchased FE resistant rams out with their commercial ewes and expected to see rapid gains. This random mating approach could never bring rapid improvement.
  • Because tested rams were expensive, farmers only bought one or two, when they should have bought a team of rams to bring about positive change.
  • So, many stud breeders gave it away, as there was no profit in the business. Only the dedicated kept going to benefit their own flocks, which they certainly did. Now in bad years they hardly ever see a clinical case of FE.
  • With today’s knowledge and technology, success could be achieved in a quarter of the time.

How did FE resistance work?
  • Scientists worked hard to find out how resistant sheep could break down the toxin’s complex chemical structure in the liver, and get rid of it. It was a fascinating bit of detective work at Ruakura, which I don’t think was fully completed.
  • In simple terms, the sheep’s immune system did the work, so breeders were actually selecting for sheep with better genetic immune systems.
  • This is very interesting, as the pioneer breeders today (along with their ram buying clients), are finding their sheep don’t get grass staggers (another fungal toxin), and have fewer internal parasites as judged by less drench needed.
  • So to select against dags and worms, we need to identify sheep with a high natural (genetic) immune system, and manage them to allow it to be fully expressed.
  • It’s a simple objective and is really a repeat of breeding for FE tolerance, but in a very much shorter time.

Dags and worms – why fix them in this order?
  • Because this is the order in which they cause pain to the farmer.
  • With dagging you have to wrestle with sheep before staying bent over, to remove a worthless product. And then being in constant contact with faeces is also a health hazard.
  • Even if you have modern sheep handling equipment, physical effort is still needed and costs didn’t disappear.
  • Drenching is not so bad, as you stand upright but with no automatic drenchers invented yet, stretching and being bumped by sheep, and perhaps even being bitten are still hazards!

Are these traits inherited?
  • Yes they are.
  • The heritability of dags is around 25% and resistance to worms (also called host resistance) is around 23-25 %.
  • You will see ‘resilience’ used which is where a sheep can live with worms and still keep performing.
  • If this is a separate trait, (and scientists state that it is), we don’t want it as these sheep produce dags.
  • We only want sheep that are Worm-Free and Dag-Free.

Are the traits linked?
  • Technically, dags and worm resistance are separate traits so are not linked through common genes.
  • But there are farmers who have cleaned up the dags in their flocks (measured in barrow loads), and are finding that worms, (measured by the number of drenches given) are very apparent.
  • This may be entirely environmental and not genetic, but at this stage why worry – it’s saving work and money. So keep an open mind at this stage.

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