Wormwise's Mary Bowron outlines the long-term risks and short-term rewards associated with pre-lamb drenching and ways to reduce pre-lamb worm pressure without resorting to the drench gun.
Pre-lamb drenching remains a common practice on many sheep and beef farms. The goal? Less dags and better ewe performance, plus less worm eggs shed by the ewes over lambing (the periparturient rise or PPR). Less eggs from the ewes, results in less infective L3 larvae for new spring lambs to ingest. That’s the theory!
With long-acting capsules and injections available for pre-lamb drenching, it’s easy to see the appeal of giving ewes extra protection during a high-demand period. The question is not whether these products can work — they can — but what they may cost you in terms of future drench effectiveness, and whether you can get the same benefits another way.
Research by BSI (AgResearch) and others have shown that long-acting drench treatments can be beneficial on some farms some of the time. Most farms saw ewes heavier at weaning by 1-3 kg, and 50% of farms had heavier lambs at weaning. However, lamb survival was variable (and sometimes lower) and total weight of lamb weaned/ha didn’t necessarily increase, where long-acting treatments had been used.
But what can increase is the speed at which drench resistance develops, especially if drench is used as the primary tool to manage parasites.
So why can long-acting drenches speed up the development of drench resistance?
Resistance to drenches speeds up when there is no or minimal refugia in your system.
Refugia is having a pool of worms that have not been exposed to drench – worms in untreated animals and some of the worm larvae on pasture.
When refugia is low – because most or all animals are being treated – resistant worms that have survived drenching make up a much larger share of the potential breeding pool over time.
Long-acting products reduce refugia for an extended period after treatment, far longer than short-acting drenches.
If that long-acting treatment is only partially effective, the surviving worms in ewes can continue to contribute disproportionately to the next generation of worm larvae on pasture. These are part of the infection picked up and amplified by the new season's lambs.
How long does a long-acting treatment work and how do we know it's working?
The answer – it depends! A long-acting injection isn't killing every worm species for the same length of time. Different worm species can be affected for different lengths of time, which means some parasites can reappear much sooner than others.
For long-acting moxidectin products, initially there is a peak of chemical which tapers off over time. For Teladorsagia the length of protection can be up to 112 days, up to 91 days for Haemonchus but only around 42 days for Trichostrongylus colubriformis. This means T. colubriformis can build up in some situations and this may limit subsequent lamb performance if lambs are weaned back onto areas where treated ewes have been.
Capsules currently on the market have been revised from 100 days of action to a range of 85 to 115 days, if working effectively.
If some of the worms on your farm have become highly resistant to the drug/s in the long-acting product you are using, you may be facing a situation where, for the entire payout period, the product is letting that type of worm survive and reproduce. You won't necessarily see this – the ongoing control of other worm species may be enough to keep dags away, for instance, but the build-up of resistant larvae on pasture has the potential to cause problems later.
Check the long-acting product is effective while it’s within its payout period. A drench check should include a faecal egg count and larval culture or worm species PCR. For moxidectin injections do this within 4–5 weeks of giving the product, for capsules about 60 days after dosing.
Can we reduce the pre-lamb worm pressure without drench?
The worm pressure around lambing and the effects of the PPR are going to be a lot less when:
- Ewes are in BCS 3 or more.
- Feed covers around lambing don't drop below 1200kg DM/ha, (there will be high performing commercial flocks in some environments running higher covers than this).
- Lambing is at a time of year when pasture growth matches demand.
Ewes from genetic lines that have been selected for lower worm egg counts have been shown to have a lower PPR. But genetics are not an out-clause for good feeding!
For more details about PPR and how to manage it check out this article: Pre-lamb worm pressure – where it comes from and what to do about it?
Building confidence to use less drench
- Prioritise having ewes at BCS 3 all year round, starting at weaning with monitoring of body condition and drafting off light ewes to preferentially feed them. Cull repeat offenders, BCS is heritable.
- Protect spring lambing areas from worm challenge carrying through from the previous autumn:
- Graze cattle on lambing blocks in the autumn
- Sell trade lambs earlier – they can be worm factories!
- Prioritise growth of replacement stock. Faster they grow, quicker their immunity develops and less contribution to the worm challenge on pasture.
- Use of worm resistant genetics in your ewe breeding flock. They will cope better with worms and add less to pasture worm challenge.
When capsules weren’t available in 2022, initial panic was followed by quiet optimism. Many farmers took the opportunity to reassess their parasite management and made changes resulting in less reliance on drench.
Where to start?
- Avoid long-acting products where possible.
- Reduce reliance on long-acting products gradually as improvements in the farm system take effect. Abruptly stopping without having other measures in place may not end well.
- Where you need to use them, avoid blanket treatment of mobs. Target lighter ewes carrying multiple lambs. Two-tooths may need treatment but if well-conditioned and good feed on offer, the benefits of the drench maybe minimal.
- Mark ewes you’ve left undrenched so you can track their progress.
- If feed levels at set stocking aren’t where you want them to be, and ewes are lighter in body condition, then strategic use of long-acting treatments maybe warranted.
- Reset at weaning time to be in a better position next year. This includes an assessment of stocking rate, lambing date, feed management policies and lamb sale dates to prioritise ewe condition and feeding over late pregnancy and lambing.
The key is having a well-conditioned ewe flock – many farmers, backed up by research, have found that fat ewes are a great tool in the battle against worms. This maybe the year to have the confidence to give it a go.