Drench resistant worms in sheep – getting the best from a Worm-Resistant ram

Sheep, farming, husbandry, disease, animal health, worms, internal parasites, rams, exploiting genetic gain

By Dr Clive Dalton

'The Colonel' - a ram who carries dag-free and worm-free genes

Wasting a ram’s potential

• If you buy genuine WR rams (judged by zero or very low FEC) and just turn them loose in the flock expecting to see the dags disappear and the drench bill drop – you are dreaming. Be realistic.
• Joining a WR ram with a random group of ewes will not get you far, as his genetic influence will be diluted out of sight, and you and your offspring won’t live long enough to see big changes in the frequency of worm resistant genes in the flock.
• The ram can only contribute half his genes to his offspring – the other half comes from the ewes he is mated to.
• But an individual ram makes a much greater genetic contribution to the flock than any single female – which is surely obvious.
• And you won’t get far if you only buy one WR ram from a breeder and expect it to do wonders. It will provide skite value for a short time if you paid enough for it, but that’s about all. You need to buy a team of WR rams.
• So you’ll have to get serious about a genetic improvement programme for your whole flock recognising the contribution the male and female side make to overall genetic gain.
  

Remember the 3 parts to genetic gain
They go together like this:

Genetic gain = (heritability x selection differential) / generation interval

• You can’t change Heritability or the strength of inheritance of the trait (which is generally low for worm resilience/resistance traits).
• You can’t shorten the Generation Interval much beyond using ram hoggets over ewe hoggets.
• But you can put maximum pressure on the Selection Differential (SD), which is how much the selected parents are above the flock average.
• You add the male SD to the female SD and divide by two to get the average. If the female SD is zero – you can see how you slow up genetic gain.
  

Mating options for a Worm Resistant ram
Here are some options from lowest priority to best – written with a scientist’s hat on! So you’ll have to work out what’ s the best practical option in your flock.

Before you start doing anything - stop drenching all females in the flock for at least two months and preferably longer, unless there is an accurately diagnosed animal health or welfare problem.

Stopping drenching will remove any complications caused by anthelmintics till you get things sorted out.

Mixed age ewes:

• Sort out a mating group from MA ewes that are structurally sound and have totally clean backsides.
• Insert your finger in each sheep's rectum to see what FCS you can feel.
  
• Using the faecal probe, sort out a mating group with a FCS of 1 (marbles) or 2 (hand grenades).
• FCS 1 is the top priority if you can get the numbers.
  

Two tooth ewes:

• Repeat the above actions.

Cast-for-age ewes:

• These are the best genetically proven sheep in the flock.
• Repeat the above actions.
  

Twin scanners:

• These sheep will be of all ages. Twin-scanned two tooths would be an excellent genetic resource to start with.
• Regular twin scanners over a number of years would be even better.
• Repeat the above actions.
  

Ewes that have reared a lamb to weaning:

• You could only judge this by udder development at weaning. Not the best of options – but it’s better than doing nothing!

FEC/FCS on potential mates in the above classes of sheep:

• This would involve the cost of FECs.
• Only keep sheep with FEC under 500 epg and FCS of 1 (marbles) or 2 (hand grenades). Make zero FEC and marbles the ultimate aim.
  

Join each WR ram to as many ewes as you dare!

• Why not offer him 400 for one cycle?
  
• Farmers exploiting high-fertility rams did it 20 years ago.
• Or offer him 200 for two cycles. Don’t put a harness on the WR ram as it may restrict his activities, but harness the ram that follows or use a colour-marking terminal sire.
  
• Then you can ID the WR-mated ewes.
  

Form a nucleus flock

• The progeny from these WR ram and ewe matings could form a nucleus to increase the gene frequency for worm resistance and dag-free in the flock.
  
• Make them feel special by calling them “elite”!

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.