Parasitism is a major cause of production loss in livestock. Based on current information, many New Zealand farmers are using anthelmintics in a manner which will result in drench-resistant worms and drench failure.
Once present on a farm, worm resistance to anthelmintics is permanent. Therefore, selective drench use and herd management is key to delaying the presence of drench resistant parasites.
Every farm is unique, so effective worm management may differ for each farm. Complete eradication is neither an appropriate goal nor an achievable one. Most of the year there are more worms in the pasture than inside the animals in various life stages and healthy animals harbour worms (and always will). However, cattle and sheep may be affected differently by a parasite burden.
Well-fed animals are less affected by worms than those under nutritional stress and some animals have a genetic variability to being less- susceptible to parasites (and can therefore be selectively bred for resistance to roundworms).
Older animals also build more of an immunity than younger ones, which is why the most vulnerable cattle (and sheep) are youngstock when first introduced to worm burden on pasture. This also means older animals can be used to reduce the number of infective larvae on pasture with effective grazing management.
Anthelmintic drenches are a way of managing worm burdens in cattle and sheep. However, each anthelmintic is a finite resource and should be used to achieve the greatest sustainable benefit for each farm. Use of long-acting drenches may hasten the development of drench resistance.
Routes of administration also vary in the effectiveness of each drench. Pour-on drenches have a higher difficulty to enter the animal’s system, due to the protective nature of skin, compared to oral or injectable products. However, due to the size of the animals and numbers being drenched, pour-on's are a greater convenience product because of their ease of application.
For a drench to be effective, it must be administered correctly. Key points to remember include:
Under-dosing will result in an increase in drench resistance. Over-dosing puts the animals at risk of drench toxicity. Weigh representative stock to ensure the correct dose is given.
Select a dose volume based on the bodyweight of the largest animals in the group.
Read drench labels carefully (active ingredients, dose rate, withholding periods, expiry date and storage conditions).
Set and check the reliability of the drench gun by squirting several doses into an accurate measuring vessel.
Re-check the dose delivery each day the gun is used, or after every 200 doses, whichever comes first.
Ensure your drench gun receives regular maintenance.
Wormwise factors to consider when drenching cattle
Adult stock should not require routine drenching. Adult cattle develop an immunity, making them less susceptible to parasitism. Only those suspected of ill-thrift should be considered for worm drenching.
Worms are only one reason stock may be thin or scouring (other reasons could be Johnes, lameness, bacterial infection, viral diseases etc) - make sure you know what you are treating.
Drenching intervals should seldom be less than 28 days.
Aim to keep drenching to a minimum.
Consider stock age/class, condition, feeding levels, stress.
Drenching needs to be combined with appropriate grazing management.
Drenching plays a valuable role in animal health. Its use should be part of a worm management strategy, with advice from your animal health adviser.
Source: Wormwise; Beef & Lamb NZ.
