The main item on the worm control menu at present is summer drenching: what sheep, when, and what with.
What sheep: In most sheep areas of WA, weaners and hoggets (red and yellow tag) should receive a summer drench. Lambs are still developing their worm immunity and in most parts of WA they usually have significant worm burdens by early summer. Some sheep from the previous year’s drop will also not have developed a complete immunity.
In low-rainfall cropping zones, there may be no need for summer drenches if lambs have had a drench at weaning and went onto relatively low-worm pastures. Hoggets may also have low worm counts in areas of lower worm intake. A worm egg count will quickly show the need for treatment at this time.
Adult sheep: in all regions in WA adult sheep should be drenched in autumn, not in summer. Treatment is rarely justified in summer on worm burden grounds, and it markedly increases the level of drench resistance. If it is considered essential to give summer drenches to these sheep, leave the fattest 15–20% (or more) undrenched. This ensures some non-resistant worms are carried through summer and can dilute out resistant ones—as with autumn drenching. (See the DAFWA webnote under the section ‘Livestock Parasites’ or WA worm control programs at www.wormboss.com.au.)
When: Summer drenches should be given as soon as sheep are running on a low-worm pasture, either a crop stubble after harvest, or when the pasture has dried off. Beware of delaying these drenches unless you know the sheep have low worm burdens, as we often see severe worm problems in lambs that last had a drench 2 months or more ago.
What drench: it is essential that summer drenches are as close to 100% effective as possible. Recent drench resistance tests indicate that the macrocyclic lactone class (abamectin or moxidectin) are no longer always fully-effective.
The main options still likely to be effective in WA are:
In all cases, follow-up worm egg counts are advised on at least one mob, to ensure the summer drenches worked and as a guide to the future use of the particular drench.
Drench resistance testing: it’s not too late to test lambs, if they haven’t had a drench for at least 8 weeks, and haven’t been put onto a known worm-free paddock after their last treatment. A small group of lambs can be left for this purpose if summer drenches are given before the 8 week post-drench period. A drench test kit is available from DAFWA offices, taking the pain out of organising the drenches and equipment.
With a wide range of effective chemical groups, there’s no reason not to achieve lice eradication. How long the flock stays lice-free depends on some factors you have less control over (mostly stray sheep getting through fences), but many producers treat only when pre-shearing checks show lice are present. In most cases, break-downs occur and treatment is needed every few years, but as no lice control chemicals have a prolonged protective period there is little ‘insurance’ to be gained from routine annual treatment.
Beware of the older chemical groups: the synthetic pyrethroids (SPs) and insect growth regulators (IGRs). Resistance by lice is very common to these, but there are several other effective chemical groups. Effective treatment then depends on getting a clean muster, and ensuring correct application (with dipping, it is essential that sheep are thoroughly wetted).
After that, lice freedom is based on keeping stray sheep out and avoiding introductions of sheep of unknown lice status: biosecurity applies to lice as much as to other potential problems.
Significance of worms in late spring to early summer
This year, ewes after weaning have often returned to paddocks heavily contaminated with winter worm larvae. The resilience of adult ewes to re-infestation has been tested on two properties weaning lambs in mid spring with high rates of larval availability. It has been very pleasing that 6 and 8 weeks after ewes were drenched and returned to contaminated paddocks, their faecal egg counts remain low. However, we will continue to monitor these mobs into next autumn.
Lambs have little resiliance or resistance and on a number of properties have been found with very high counts 5–6 weeks after being effectively treated at weaning. Some lambs weaned onto spelled paddocks, have subsequently developed high egg counts from larvae surviving over a considerable period since the areas were last stocked more than 2 months before- demonstrating the importance of properly prepared weaning paddocks.
However, if weaners are showing early evidence of illthrift at this time of year, it is important not to assume that worms are the sole cause, but to ensure that other likely causes or factors are investigated including selenium, vitamin B12 (cobalt) or vitamin E deficiencies. Illthrift is often complex, involving more than one contributing cause. From experiences in similar years, it is also worth considering that lambs chasing even a small amount of residual green pick may have increased exposure to Nematodirus spp (thin-necked intestinal worm) whose larvae have the potential for prolonged survival. An accurate diagnosis is important. These conditions will respond well to appropriate targeted treatment, which is preferable to relying on unnecessary broad spectrum responses.
In the last month most farmers have been busy with harvest. In the mobs that we have tested, counts have been low.
A rain event last week saw most properties receive between 20 and 60 mm of rain. This has been great for perennial pastures. Farmers will need to watch for Barber’s pole in the next few weeks.
As stubbles become available, mobs should be monitored to determine if a strategic effective drench is required. Weaners and hoggets will require a summer drench. Older sheep should be drenched if they have counts of 200 epg or more. Any mobs that are not performing as expected should also be monitored.