[WormMail 20081210]
Stephen Love
Veterinarian/State Coordinator -Internal Parasites
NSW DPI, Armidale
Below are two perspectives on summer drenching: an article from Bruce Watt (District Veterinarian, Bathurst), representing a south eastern Australian view, and the other from Dr RB Besier, Principal Veterinary Parasitologist, Albany WA, sharing the latest on summer drenching in Western Australia
‘Summer drenching’ refers to one or two strategic drenches (in summer), based on work by CSIRO scientists such as Norm Anderson and others in non-seasonal and winter rainfall sheep raising areas of Australia. The idea is to ‘kick ‘em when they are down’: that is, drench the sheep when worms on pasture are in rapid decline due to hostile conditions (hot, dry summer weather).
The upside is good worm control, and this is achieved with just one or two summer drenches, as opposed to the several drenches that may be required if worm control was based on a tactical rather than a strategic approach once the rains began in autumn.
The downside, with the benefit of hindsight, is that summer drenching in these climactic zones may increase selection for drench resistance, particularly in Mediterranean climates and when sheep are drenched and moved onto cereal stubbles. This is because there are relatively few worms ‘in refugium’ (mostly those on pasture and thus escaping exposure to the drench) in the hot, dry summers of south eastern and western Australia.
One of the first widely adopted strategic programs employing summer drenching was the DrenchPlan program for central and southern NSW, launched in 1985; one year after WormKill in northern of NSW. These ground breaking strategic programs were a cooperative effort involving CSIRO, Pastures (now Rural Lands) Protection Boards, NSW Agriculture (now DPI), as well as the private sector (still the private sector).
Initially there were two summer drenches in DrenchPlan, one when pasture was haying off around November, and the second in February. In the version for lower rainfall areas (<350 mm p.a.), such as the Riverina (Deniliquin and other districts), the second summer drench was deleted, except in unseasonably wet summers.
As drench resistance escalated, DrenchPlan morphed into a modified strategic program around 1997, with the second summer drench being made conditional on the results of worm egg count monitoring.
About the same time in WA, there were moves afoot to modify the strategic program there as well, with a view to finding a better balance between worm control and selection for drench resistance.
Ivermectin resistance of Ostertagia (Teladorsagia), small brown stomach worm) developed very quickly, despite the drench being used only once a year. In some cases, resistance was detected after just 4 years.
Ways of increasing the number of worms ‘in refugium’ were explored, including leaving a small percentage of adult sheep un-drenched at the summer drench. More recently attention has focussed on not drenching adults at the beginning of summer, and drenching them instead in April. This approach has been tested by Dr Rob Woodgate and others in WA as part of the AWI-supported ‘IPM-S project’.
And now for the articles by Bruce Watt and Brown Besier. Keep in mind, as discussed by Love (2007), that while the principles may apply widely, the practicalities may not apply to all of the climatic types within non-seasonal and winter rainfall areas of Australia.
Summer drenching: south-eastern Australia
Bruce Watt, District Veterinarian,
Central Tablelands RLPB, Bathurst, NSW
[This article was a newspaper article, published recently throughout the Central West of NSW, and is republished with permission]
With pastures haying off especially in the more western and southern areas of the tablelands now is the time to plan summer drenching. Summer drenching has been the mainstay of sheep worm control in southern Australia for nearly thirty years. It has been a popular program as it has been highly effective and is easy to understand and to recommend to farmers. However, it has been criticised as contributing to worms developing drench resistance.
Parasite control in sheep is a compromise between effective control to prevent disease and production loss in the short term and management to prevent resistance developing in the long term. As each producer has a different set of priorities as well as a different farm and environment, blanket recommendations are a bit tricky. I caught up with some veterinary colleagues from a diversity of higher rainfall environments to get their opinions.
John Evers, District Veterinarian at Young, and parasitologists Stephen Love with NSW DPI at Armidale and John Larsen from the McKinnon Project at the University of Melbourne may not be keen on admitting it but between them, I estimate they have a century of experience in parasite control in higher rainfall areas of south-eastern Australia.
All agree that in the temperate parts of SE Australia a first summer drench for all sheep is close to mandatory. Give this drench after pastures have hayed off. For ewes with lambs at foot, this is most practical at weaning.
An alternate approach for producers with good management skills who are more concerned about the development of resistance is to sample some mobs of ewes before the first summer drench. Mobs with an egg count below 100 epg could be omitted from the drenching program. Producers taking this option need to monitor their sheep especially if it rains over the summer.
Another strategy to delay resistance is to leave a small percentage of the mob un-drenched. To many this sounds crazy but the theory is that a small number of un-drenched ewes carry their unselected worms through into the autumn and winter so the subsequent worm population is composed mostly of non-resistant worms. (This is potentially not good in areas with more Haemonchus and you get a wet summer).
John, Stephen and John agreed that this idea is most appropriate for good managers and in areas with hot dry summers. Further east we know that some worms survive quite well on pastures over the summer anyway and leaving a portion of the mob un-drenched may increase the risk of worm build-up either in a wet summer or in the following autumn or winter.
Even though we sometimes find very low egg counts in lambs at weaning, they should always be drenched. This is because we can’t be sure that they are harbouring immature worms, because little weaners need all the help they can get to survive the summer and autumn and because we can’t risk contaminating the weaning paddock.
Consider an egg count from a mob just before and 10-14 days after you drench to check the effectiveness of your drench. It is a quick and easy way of checking to see if you have some resistance, as an effective drench should reduce the egg count by 95%. If you have relied on the ML group of drenches consider drenching with a mix including an organo-phosphate such as Rametin® or Colleague®. I will discuss the OP drenches in a later article.
New rules for summer drenching of adult sheep: simple and sustainable (Western Australia)
Brown Besier, Principal Veterinary Parasitologist
Dept. Ag WA, Albany WA
[This article was written for 'AgMemo' in WA, October 2008, and is republished with permission].
For some years we have known that while summer drenching gives very effective sheep worm control, it enables resistant worms to survive in preference to others. Over time, this can lead to an increase in the level of drench resistance across a farm.
Research shows that a small change to the time of drenching adult sheep will significantly reduce the risk of this disadvantage. By deliberately ensuring that some non-resistant worms survive on the farm, resistant ones will no longer be in the majority and the development of resistance will decrease.
Summer drenching recommendations
Weaners and hoggets: No change, give a drench to all sheep once the pasture becomes dry, or as they move onto a crop stubble. These sheep are still growing and no risk with worms should be taken.
Adult sheep (2 ½ years onward): do not drench in summer, but give a drench in early April. Mature animals in WA rarely have significant worm burdens in summer, but immature worms sometimes develop to the adult stage by autumn and often justify removing.
Ewes before lambing: no drench if lambing from April to June, but later lambing ewes may need a pre-lamb drench 2-3 weeks before lambs are due to drop.
All drenches should be given with a fully-effective drench, ideally after a test to confirm that it will work.
More efficient control
Worm egg counts show whether a worm burdens justifies drenching, and this enables farmers to fine-tune worm control programs. This is especially useful to check whether an autumn drench to adult sheep is needed, or whether ewes need a pre-lamb drench. Scouring or poor growth is common in winter as worm burdens increase, and the need for a drench is easily seen with a worm egg count.
Other non-chemical methods of worm control should also be used as a routine. Pasture movements planned so young sheep do not graze paddocks with high levels of worm larvae will reduce the need for drenching. Sheep in good nutritional condition are less susceptible to worms, and buying rams selected for a superior resistance to worms will lead to a long-term reduction in drenching. Consulting an animal health adviser or private veterinarian is recommended to develop the most efficient worm control plan for a particular property.
References and further reading
Love S (2005). DrenchPlan 2005. Primefact 14, 1st Edition, September 2005. Retrieved December 2008 from http://www.dpi.nsw.gov.au/agriculture/livestock/sheep/health/internal/drenchplan-2005
Love S and Evers J (2005). DrenchPlan: the basics. Primefact 54, September 2005. Retrieved December 2008 from http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0011/38828/drenchplan-the-basics.pdf
Love S (2007). Sheep worm control: summer versus winter drenching in southern NSW. Primefact 479, 1st Edition, June 2007. Retrieved December 2008 from http://www.dpi.nsw.gov.au/agriculture/livestock/sheep/health/internal/sheep-summer-winter-drenching
© State of New South Wales through NSW Department of Primary Industries 2008. You may copy, distribute and otherwise freely deal with this publication for any purpose, provided that you attribute NSW Department of Primary Industries as the owner.
Check for updates of Primefacts at: www.dpi.nsw.gov.au/primefacts
Disclaimer: The information contained in this publication is based on knowledge and understanding at the time of writing (December 2008). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of New South Wales Department of Primary Industries or the user’s independent adviser.
