By Julia Wyatt, Technical Ag Specialist, Otago/Southland
Our youngstock need a good, robust drench programme as they are the most vulnerable and at higher risk of infestation. Farmers should be doing faecal egg counts every time they drench to ensure they are only drenching when numbers are climbing. Also ensuring that the product they are using is still working!
If a product is less than 95% effective, then it is classed as a failure as this means 5% of the worms are resistant to that active ingredient. Youngstock run off blocks with only one class of stock have a much larger risk as there is often no undrenched stock to dilute survivors. However, including older stock on a rotation with younger stock will help reduce the number of larvae, as adult cows have more immunity to worms than calves. Older stock also requires less drenching so can act as a source of susceptible larvae (refugia).
Despite increasing levels of resistance, traditionally we would often use a dry off drench across the whole herd - but is this required? And if not, when should you be drenching and for what stock?
Withholding periods
Most drenches do have withholds, including orals, injections and pour ons. Some drench products have a milk and meat withholding period following application.
All farmers should check what drench they have on farm.
Pour on drenches containing abamectin have had a label change. There is now a 35-day milk withhold on these products for milk, so they cannot be applied to milking stock as residues may be detected in the milk, resulting in strict financial penalties.
Being selective at dry off
Younger or lighter cows in the herd are more susceptible to parasite infections resulting in greater sub-clinical losses.
Parasite infections have been shown to negatively impact reproduction in heifers and decrease milk production in older cows. When larval challenge is high (or feed is short, so exposure to larvae will be higher) a persistently acting product can be very beneficial in these light or younger cows.
An abamectin-based pour-on drench can be used once the cows have been dried off. It kills both internal and external parasites (lice) but remember the milk and meat withholding periods are 35 days, so it can't be used too close to calving.
Whole herd drenching?
If you feel that the whole herd needs to be drenched, focus on the why.
If condition is the concern, then look to drench 6 weeks before dry off with a persistent product that will kill the incoming larvae. These products will also treat the lice, so you get a monetary return via milk or weight gain before heading into the calving season.
Wintering and drenching
If cows are being wintered on crop, they will not require persistent activity for worm control as larval challenge will be minimal. Youngstock being wintered on grass may still require a drench.
A 'mectin' drench will help control Ostertagia, which can emerge en masse from the gut wall, causing severe damage to it, in the spring.
If cows are in good condition at dry off with no signs of lice and are going onto a winter crop, then there may be no need to drench them at all. However, it may be useful to target the lighter 10% of the herd after calving with a persistent acting product with a nil milk, nil meat, and nil bobby calf withhold period.
Quarantine drenches
All stock, and especially young stock, should have a quarantine drench to ensure that any resistant larvae do not get 'sprinkled' over the farm. A triple combination oral drench would be my choice or discuss with your vet.
Overall, when it comes to drenching, we need to look at how we use these tools and ensure to keep effective active ingredients available for the most vulnerable: our youngstock and our future.
For more information on drenching, chat to your Farm Source team who can ensure you get the right drench for your needs on farm.