Drench resistant worms in sheep - breeding worm-resistant sheep

Sheep, farming, husbandry, disease, animal health, worms, internal parasites, breeding, breeding for resistance
By Dr Clive Dalton

Breeding - the only solution

Question: How come in the same flock, some sheep
are always clean and others always dirty?

Increasing numbers of farmers are starting to accept that breeding sheep, that are genetically resistant to internal parasites is the only way towards a sustainable future.

The precedent was set 30 years ago, when a pioneering group of breeders started to select sheep resistant to Facial Eczema (FE). It’s been enormously successful, and shows that if you have a clear aim, and you can keep the programme simple, it will bring results.

• Whatever are the rights or wrongs of past drenching practices, we have got to make an aggressive start to breed sheep resistant to worms. For some, this is now an urgent priority.
• It’s sobering to think that over the last 100 years in New Zealand when trouble arose, there was always a quick escape to grab what was needed through outcrossing to a new breed.
• For example we used Finnsheep to fix low fertility, different breeds provided scope to change wool types, and different meat breeds were used to meet market changes. New Zealand’s sheep history right up to the present shows how sheep importations and outcrosses were used as quick-fixes to change genetics.
• But when drench resistance really takes hold, where do we go? There is no readily-available outside genepool that is going to work this time. Countries like Australia, United Kingdom and South Africa have serious drench resistance too so it’s no good rushing there for solutions. They could be heading here for help.
  

Commercial sheep farmers have two clear options.

1. They can either buy in the genetics they need from the current industry’s stud breeders who have been selecting for host resistance, or
2. If these sheep are not available, they will have to breed their own. Those farmers with severe FE were forced to breed their own.
   

There’s no time to wait
Farmers in tough times cannot wait around for slow changes to take place in the stud industry. Predictions are that some farmers may only have 2-5 years left before most drenches will fail on their properties.

These implications are massive for the stud industry, and could cause as big a revolution in sheep breeding as Group Breeding Schemes did in the 1960s and 1970s. We could be heading for interesting times again with the ghosts of many old scientists rising from their graves to join in!

Genetic improvement works

• Enough work has been done already by AgResearch and a few top breeders to confirm that breeding host-resistant sheep works, while recognising that it takes a long time to get there - around 20 years to reduce FEC by 50%.
• Hopefully, genetic technology will speed this up in future (depending on cost which is never mentioned).
• If you delay making a decision and do nothing, expect problems, as remember resilience is weakly inherited (5-15%), whereas drench resistance in the worms has reasonably high heritability (23-25%).
• So if you do nothing the parasites will become drench resistant faster than you can improve resilience. But much more work needs to be done on the genetics of these traits as heritability estimates can jump around a lot and are not set in concrete.
• Clearly the aim is to improve host resistance to worms and not just resilience, but initially both of these will be happening at the same time and you won’t know how much of each.
• Don’t worry, as FEC/FCS will help identify animals that have developed host resistance in the long term.
  

Challenge to the stud industry

Stud breeders annual sale - how many can
offer you guaranteed dag-free and worm resistant genes?

• It would be nice if commercial farmers could simply ring up a SIL breeder and order all the rams they need to fix any approaching drench resistance crisis. (SIL stands for Sheep Improvement Ltd).
• Well they can’t, because at present not enough SIL breeders have rams available that have been tested. You can’t blame the SIL breeders as they have not been asked for them and demand creates supply.
• A big change is needed and fast but don’t hold your breath.
• So commercial farmers in trouble need to give serious consideration to starting a simple flock improvement programme including keeping their own rams.
  

A programme to think about

• I’ve set out below a breeding plan. You’ll have to decide if it’s an option for your farm and if you are prepared to accept the challenge!
• Don’t go into it half-heartedly as you’ll end up with a shambles and there will be plenty to say “I told you so”. It needs your full commitment.
• Key point: If you are a commercial farmer with a decent-sized flock and cannot buy the rams you want that are dag free and with proven host resistance to worms, then you cannot afford to wait around for stud breeders to respond to your needs. Get started!
  

Getting started

• It might be a good idea to take a few deep breaths before reading on –as there’s a fair chance you may get lost on the way through.
• Go through it slowly with your own flock in mind, and remember what the aims are in each part. Maybe don’t read all the parts in one go?
• Remember the overall aim – to breed sheep with a high natural (genetic) immunity to internal parasites. Keep remembering this if you get lost in the detail and lose track of what’s going on.
  

Two clear challenges

• The following breeding programme will bring results in commercial flocks of around 1000 – 1500 ewes.
• It probably would not be worth starting with fewer than 500 ewes because of the intense selection needed.
• But you may like to try it with a small flock and see how you get on. The challenge is twofold:

1. To keep the job simple and above all avoid getting bogged down in paper records.
2. To keep costs down to a bare minimum.
   

Genetic theory
The genetic theory behind the breeding plan is notnew. It was used by Thomas Bakewell in the 1700s to improve English Longhorn cattle and Leicester sheep, so it has been tested!
As students we were sick of hearing about him.

His painting showed a little fat man wearing a black hat, with a pigtail and fancy jacket sitting on a little fat pony.

I bet he never dreamed we’d still be using his simple principle of ‘breeding the best to the best’. The job is to sort out what is the best. So let’s start.

Four pathways
There are four ‘genetic pathways’ to flock improvement. Below is not the way these paths are usually listed, but this order is for a sheep flock starting at scanning in mid pregnancy. Here they are:

• PATHWAY 1: Selecting females to breed females
• PATHWAY 2: Selecting females to breed males
• PATHWAY 3: Selecting males to breed females
• PATHWAY 4: Selecting males to breed males
  

So start in mid pregnancy, but if this is not convenient, don’t worry – just dive in where you can. The key point is – make a start somewhere!

PATHWAY 1: Selecting females to breed females

Ewes in mid pregnancy

• Start by scanning the flock to identify (ID) the twin-bearers. Run them separately and feed them well.
• Do NOT drench any ewes in the flock from now on. Avoid any long-acting drenches like the plague!
• Lamb these twin-scanners separately and give them any extra care you can afford at lambing.
• Cull any that ‘pack up’ between scanning and lambing, that grow dags, or have any lambing or health problems. Be ruthless and if in doubt – throw it out!
• If you are ‘easy-care’ lambing – then don’t go near them.
• This is the start of the journey, as out of these twin-scanned ewes you’ll be continually selecting the very best for commercial traits that keep you in business.
  

At docking

• Do NOT drench any lambs on the farm at docking – and this means in the entire flock unless you have an animal welfare crisis.
• Do NOT drench any ewes on the farm at docking either.
• The twin-scanned ewes. Dock their lambs separately and mark all the ewe lambs (Use ear-markers to cut a bit out of the ear or punch a hole).
• Mark any twin-scanners for culling that are wet/dry or have any other problems that you don’t like. Be ruthless!
• If the twin-scanners go back with the main flock (or in case of escapes), identify them with a temporary mark that will last till weaning, or better still, give them a permanent ID with a small cheap plastic tag.
• This cost of this small tag (and not a big one) is justified at this stage and then your past work is protected.
  

At weaning

• Plan to keep many more ewe lamb replacements than normal, as you are going to put intense ‘selection pressure’ on them (i.e. cull heavily).
• As a target, plan to cull around 30% of these lambs – but this will depend on the final numbers you’ll need.
• Do not drench any ewe lambs wanted for replacements.
• The ewe lambs from the twin-scanners will have been earmarked at docking, and will be the core of your future replacements.
• Hopefully you’ll get all the replacements needed from the twin-scanners and will not need to go too deeply into the non-twin-scanners to make up numbers. But this will depend on twinning rate and the season.
• Any lambs for sale from the remainder of the flock can be drenched with a conventional combination drench if they look wormy, and if you think it will improve their sale prospects. If the bidding stalls at the sale tell the auctioneer that they’re ‘newly-drenched and they’ll shift well’!
• Before sale and after conventional drenching, run these sale lambs separately from the replacement ewe lambs. You don’t want any cross-contamination between worms.
• If you see tapeworms in any lambs, ignore them and do not drench unless you are sure they are doing harm. Use a drench specific to tapes.
  
• Be especially vigilant to mark for culling any ewes and lambs anywhere in the flock that have any physical defects.
• At this stage the lambs are starting to develop their natural immunity so there will be great variation in the number of daggy backsides you’ll see. Some may go through a daggy stage and then permanently dry up.
• So make your final decision in the autumn when they are hoggets (see below) and they have had more time to show early immune development.
• Pay special attention to the twin-scanned ewes at weaning and their replacement ewe lambs. Cull any that fail to meet your standards. Be ruthless, especially on dirty backsides.
• f you didn’t tag the twin-scanned ewes at docking, then make sure you give them a good permanent ID now. You cannot afford to lose them now as they are going to be the core of an ‘elite’ flock (see Pathway 2).
  

Ewe Hoggets

• When these weaned ewe lambs are hoggets, go through them every 2-3 weeks (a month is too long) and draft off the ‘tail-enders’ that have not measured up for culling.
• Pay special attention to those that have been frequently daggy and get rid of them.
• If you think they need it, give the tail-enders a conventional combination drench and finish them for sale. Again tell the auctioneer the good news about them having just been drenched!
• Before sale and after drenching, don’t mix them with the replacement ewe hoggets. The drench status will have to go on the ASD form if they are within the withholding period for the product.
  

Take a moment
Now take a moment and evaluate how far you have come from weaning, and guard against the urge to conventionally drench the hoggets. What the hoggets look like will be a early sign of any progress made towards resilience and/or host resistance.

• Satisfy your curiosity and boost your faith in the exercise so far, by doing a FEC from each of 10 different (fresh) heaps of dropping in the paddock. Remember to correct these for FCS.
• Discuss the results with your veterinarian provided that they are up with the play on recent developments, and that drench sales are not the main priority.
• Use a trigger level of 500 epg to see how things are going, and conclude that fewer than 500 is resistant and above 500 could be anything, resistant or resilient or a bit of both. You don’t want them.
• But remember to look at the animals in light of these trigger levels.
• Be strong and guard against the urge to drench these hoggets by the calendar or because the dogs need a run!
• Mid winter (May-June) is an ideal time to start putting even more selection pressure on these replacement hoggets. Cull heavily any that fail to meet your standards – and especially any with dirty backsides.
• Buy, borrow or share some scales and weigh the hoggets to decide a cut-off (cull) weight in the light of how many you need to go into the flock. Find the range by weighing a few of the smallest and a few of the biggest, and then decide a cut-off ‘target’ weight.
• Keep on being ruthless and cull any hoggets that are even slightly daggy, lame, or have any other physical defects you don’t want in the flock. Be ruthless.
• It would be a great idea to go through the hoggets with the faecal probe and identify those with FCS of 1 (marbles) and 2 (hand grenades) – or mark for culling any with clearly soft faeces (3,4 and 5) to get rid of any with genes for dags.
• Keep the drench gun locked away unless there is a worm blow-up (e.g. Barber’s pole) and animal welfare is compromised.
• After all the work you’ve put in so far, it’s time to put a reliable permanent tag in them showing individual number and year born. They are now your most valuable animals – and you had better believe it!
  

Hogget shearing

• In the past it would have been worthwhile weighing the wool and keeping those ewe hoggets with high fleece weights – but don’t bother unless you can do it easily and you think it will give an objective measure to reduce numbers further.
• Hoggets with high liveweight will also have the genes for high fleece weight (the genetic and phenotypic correlations between the two traits are highly positive).
• If wool was worth more in overall returns, then things may be different. However, if you breed fine wool then it would certainly be worth culling on hogget fleece weight.

Two-tooth ewes

• These ewe hoggets have now become two-tooths and should look a picture because remember their background and the intense selection pressure you have put on them so far.
• Also remember that so far, their individual FEC is not known. Apart from the odd sample to monitor progress, it’s not worth doing any more FEC. It doesn’t matter at this stage whether they are resilient or resistant or a bit of both.
• Keep on being ruthless and culling any that develop problems like footrot.
• If you want a confidence booster, do another FEC on 10 of them, just to see how their host resistance is building up.
• The next job is to plan their mating, so you’ll have to decide what kind of rams to buy and where are you going to get them from?
• These top two-tooths certainly deserve something special that can improve their genetics for host resistance to worms, and not just to get them pregnant. There are two things you can do:
  

Option 1.
You may already have a ram team on the farm purchased from SIL or other stud breeders in the past. Check their FEC/FCS making sure they have been drench-free for at least 2 months (preferably 3).

Option 2.
Check what rams are available for the current season from breeders using SIL.

1. Using your current ram team

• Your current ram team bought from SIL or other stud breeders in previous seasons will have been a considerable investment, and it would be well worthwhile to see what their worm resistance status is by doing a FEC on all of them. Make sure they have not been drenched for at least 2 months (preferably 3) before the FEC.
• Rank them on FEC/FCS and decide what trigger levels you can afford to accept. Whatever the FEC, don’t use any ram that has ever had a dirty backside or not thrived over winter.
• Aim for nil FEC or certainly not higher than 500 epg and with FCS of marbles or hand grenades.
• If you have high-Index rams that you paid a lot of money for, and they have higher-than-acceptable FEC, test them again before considering their future as culls.
• This exercise will be a very good investment as you could be either shocked to see what possible genetic worm resistance you had bought in, or you could be pleasantly surprised.
• It’s going to be vital information for when you talk to your ram breeder for next year, either to order more or cancel the lot! At least you have valuable factual information for your decision which may bring about some industry change. Pigs may fly before this!
• If you want to buy more rams from the stud breeder next season, then demand to see FEC/FCS from the rams on offer – making sure they were not drenched for at least 2 months before the test or never drenched at all. The response could be interesting!
  

2. Buying in rams from SIL breeders

• Contact your nearest consultant from Sheep Improvement Ltd (SIL) by phoning 0800-745-435, or Email .
• Find out which breeders are formally selecting for host resistance to worms and incorporating it with other productive traits into an overall Index.
• Make sure the breeders are doing this formally though SIL which will show in an official logo (WormFEC™) on their Ram Selection List.
• But be prepared for snags. Despite the SIL WormFEC protocol in the Breeders’ Manual (1994), some breeders have developed a few variations of their own, so for example, check how many of their total rams on the selection list are included in WormFEC data to go into their DPO index. Others may be using the SIL dag score too to add into the overall DPO index.
• The FCS is not accepted by SIL information. So for your own protection, it would be a good idea when you have your potential purchases sorted out to check them with the faecal probe for marbles or hand grenades, and avoid like the plague any with soft faeces (FCS or 3, 4 or 5).
• Note they should not have been drenched for at least two months before sampling – which may be an unrealistic request from a client.
• Because of the low numbers of ram suppliers doing this at present, expect to pay a premium for their rams.
• Make sure you use these rams to their maximum and certainly much more than accepted mating ratios.
  
• See blog on how to get the most from a Worm Resistant (WR) ram.
  

PATHWAY 2: Selecting females to breed males

• Your main task now is to search for dams of future rams (ram mothers), as these are going to be critical. They need a special status so call them your ‘elite’ ewes. They will be thrilled about this!
• You won’t have to look far to find them, as the ewes that started off as twin-scanners and have been ruthlessly culled right through to weaning their lambs, have got to be the core of the elite flock.
• Your ram supplier or agent won’t like this idea, especially when you say you have closed your flock and want to severely reduce or cancel your ram order for next season.
• You had better make sure you have some sound genetic and economic reasons to support your actions. Tell them what you are doing and especially why, and invite them to come and see your elite ewes. Use their reaction –positive or negative as motivation.
  

The elite flock

• Aim to end up with from 7-10% of your total ewe flock in an elite group. Be flexible though.
• Keep going back into these elite ewes and (depending on numbers needed) keep culling them hard for any physical defects and health problems (especially any that are daggy, had flystrike, are lame or have simply packed up). Keep searching for structurally sound robust ewes.
• Continue to be ruthless, and if you have any doubts about a sheep, cull it.
• YOU need to have great confidence in these sheep, as they’ll be great motivators for you and your staff as the programme proceeds.
• Expect to get cold feet from time to time over what you are doing – and you’ll doubtless have criticism from some quarters. Use any criticism as motivation to succeed.
• In your doubting moments, just go and walk through the elite ewes and see if you can fault them or if you have seen any better ones on the way home from town! This will restore your faith in the exercise.
• If you know of any skeptics, invite them out to the back of the farm to see the elite ewes as that’s where they’ll be. They won’t be grazing the hay paddocks by the road!
• If you need more ewes to boost elite flock numbers, select them from two-tooths that scanned twins, i.e. if you marked them and kept them separate. If not remember to do it next time.
• These elite twin-scanned two-tooths need to be screened for FEC. It will be worth spending money on taking individual samples and keeping the best sheep with low FEC corrected for FCS (below 500 epg).
• This would also ID any that had very high FEC for future culling.
• If you can’t afford the FECs, then at least use the faecal probe to cull any that don’t have a score of 1 (marbles) or 2 (hand grenades). If you have the numbers, only go for those with clearly-defined marbles.
  

Maintaining the elite flock
This is a vitally important part of the exercise. To maintain the elite flock, use the same simple rules that you used at the start of the programme.

To gain entry into the elite flock in the first place, or to regain entry at any subsequent lambing, then a sheep must have completed all of the following:

• Scanned twins.
• Not been wet/dry at docking.
• Survived all the culling on constitution, health and structural soundness – and not caused any extra work or cost you money!
• Have shown some clear evidence of being host-resistant to worms through good performance and low FEC.
• Never shown any sign of a dirty rear end and have consistently produced marbles or hand grenades.
  

Docking ram lambs from elite ewes

• Keep all the ram lambs entire from these elite ewes.
• Do NOT drench any of them.
• Permanently ID the ram lambs at docking by taking a bit out of the ear or punching a hole.
• Or put a small plastic tag in them if you have enough helpers.
  

Weaning ram lambs from elite ewes

• Do NOT drench any of these ram lambs at weaning.
• Treat them as for the ewe lambs (above).
  

Ram hoggets

• When these ram lambs are hoggets, manage them the same as for the ewe hoggets, except put a lot more selection pressure on them. A good starting target is to cull 80% and end up with 20% for the final selection.
• Every 2-3 weeks (a month is too long) after weaning, draft off any ‘tail-enders’ and if you think they need it, give them (the culls) a conventional combination drench and mark them for sale.
• They’ll make good money during the winter and remember the auctioneer will proudly tell buyers they’ve just been drenched and will shift well!
• OR before drenching - advertise them as ‘chemical-free’ lambs or see if your meat company is interested in them at a premium.
• Don’t mix the drenched culls with those kept as replacements.
• Be very strict on what you keep, and get rid of any that don’t meet your standards.
• In mid winter, do the same as for the ewe hoggets, weigh them and cull the tail-end on a target weight.
• Now give them a permanent ID (number and year born) with a reliable plastic tag if you didn’t do it at docking. They are very valuable stock.
• Keep culling any animals that fade and get daggy. Look for those animals consistently passing marbles and hand grenades. Be ruthless!
• Record their live weight twice. First in May (winter) and then in August (spring).
• As there will not be large numbers at this stage, it would be worthwhile recording fleece weight at hogget shearing (usually in October). Cull those with the lowest fleece weight and off-type wool.
• When they are weighed in May, do a FEC and correct it for FCS. Then repeat this at the August weighing.
• Use this information to start sorting out a top team. Make up a simple index by adding the two live weights to the fleece weight and put them in order from highest to lowest. Put the FEC corrected for FCS alongside each animal.
• Come down the list looking for individuals with the highest combined weights and Nil FEC. If you can’t achieve nil FEC then go for the next lowest figures.
• See what the numbers look like to meet your needs, aiming to end up with around 20% of the original group to proceed to the two-tooth stage.
  

Two-tooth rams

• When the ram hoggets are two-tooths, things are starting to get exciting and any doubts about the wisdom of starting this exercise should have long gone. They should look a picture!
  
• If they don’t then review what you have done and accept that the challenge may be bigger than you expected. Use this as motivation to do better next season.
• Keep studying the selection list developed above and if you still want to reduce numbers, you can always increase selection pressure by doing another FEC/FCS on the very top ones as a reassurance that they are host resistant.
• Or you could take another live weight.
• Keep culling on structural soundness as there is always a few last-minute cases of footrot or scald injuries. Or teeth going wonky!
• Select a first team to mate with your elite ewes to speed up generation turnover, and make sure they all had nil FEC and FCS of 1 (marbles).
• Keep a second team to mate to the flock ewes – again if possible with nil FEC corrected for FCS. Nil FEC or FCS of 1 has got to be the standard for all two-tooth rams eventually used.
• Graze them now and again in the hay paddock by the road so passers-by and your critics will see them! It will also help challenge the rams’ immunity and host resistance as these paddocks tend to be regularly overstocked and rich in larvae.
• Have an outrageous price ready for anyone brave enough to ask! This is no joke as what is the value of a ram that is very likely to be host resistant to internal parasites with a nil FEC will not grow dags and certainly has never had a drench?
• You may easily get some offers – perhaps from your critics or via your vet from clients who have stopped buying drench because they have ‘hit the wall’ over drench resistance.
  

PATHWAY 3: Selecting males to breed females

Using the two-tooth rams

• This genetic pathway is the ‘power stroke’ in flock improvement because a ram has greater genetic influence than any single female.
• These rams are worth big money and if in doubt, ask yourself where you could buy better ones with proven host resistance to worms and dag-free?
• They have been intensively selected on your farm, so there are no doubts about how their progeny will suit your farming system.
• Their mission is to sire as many future females as possible in the flock to spread their genetic potential for host resistance to worms and dags. They will be delighted to accept the challenge.
• If AI had been a commercial reality, then this would have been the way to use them. But a lot can be done using single-sire natural mating.
• Past experience has shown that in situations like this where genetic improvement is ‘urgent’, top rams can be joined for single–sire mating with 400 ewes for one cycle. You’ll be surprised how many lambs will result. But for more ‘normal’ use- join a ram with 100 ewes.
  

PATHWAY 4: Selecting males to breed males

• This is the final genetic pathway and comes as part of pathway 3.
• The male lambs produced by these top sires will go around the loop again described in Pathway 2.
• Inevitably the top sires of this generation will sire the top sires of the next generation, and breeders often get concerned about the build-up of inbreeding. With a large flock this is not a major problem in the early stages.
• When you become concerned, then one outcross to sheep from a breeder with similar objectives will restore enough genetic variation for you to continue making progress.
• Another method is to divide your main flock into small sub-flocks (on paper) and keep moving rams around these in a planned rotation each year.
  

Final points to ponder

• There is no need to panic! You cannot do any ‘genetic’ harm in your flock, because if you’ve been buying rams from progressive stud breeders in the past, you’ll have enough of their good genetics in your flock to stay where you are.
  
• You should in theory only be a couple of generations behind them.
• You certainly cannot go genetically backwards without some very serious negative selection which would be hard to do. Everything you have done so far is positive!
• With the basic selection programme as outlined, the chances are very high that you’ll go forward on to a new ‘genetic plateau’ – and keep going.
• You have used the four classical ‘genetic pathways’ for flock improvement – and you’ve shown that the dull theory (that has bored so many generations of students) does actually work in practice! Oh if they had only appreciated this when they slept through the lectures!
  

What to do if you hit a crisis?

• This could happen of course – an example would be an unexpected outbreak of Barber’s Pole (Haemonchus contortus) worms or some other species.
• Again don’t panic, as all you need do is use an appropriate conventional drench, and because your sheep have never been drenched and are either resilient and/or resistant, the drench will be highly effective.
• Such an outbreak should be a one-off event and will not compromise your breeding programme. But be vigilant as under the law you cannot compromise animal welfare.
  

Whew! Did you make it?

• Congratulations if you came through all that lot!
• You deserve a plasma-screened TV or an iPod!
• Remember what it was all about - to breed sheep with a high natural (genetic) immunity to internal parasites.
  

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.