Whenever I ask someone why they buy a particular product, the answers are invariably the same: because I've always used that dose; or because the sales rep told me it was a great dose; or the more honest answer, it's cheap.
This was of no real consequence 30 years ago, when stocking densities were very low, intensive farming hadn't even been heard of yet and worm dosing only happened if a problem was identified.
As we all know, in 2016, it is quite the opposite. Stocking densities, especially on dairy farms, are very high. Paddocks are being grazed much tighter and worm dosing is part of our routine herd health plans. In this age of intensive farming, resistance to certain dosing products has developed.
Increasingly, we are seeing cases where cattle have been dosed and the farmer contacts us to say that the dose "hasn't worked". The animals that were coughing or scoury pre-dose have remained the same or gotten worse. This is a classic case of resistance.
Why is resistance becoming an issue?
Despite the myriad of products on the market, there are only a few basic families of worm doses. This means that, although one product may have a completely different trade name to another, it may have the same active ingredient.
Not only that, but an injectable product may have the same ingredient as a pour-on product. So, even if you change from your regular dosing product to a new one for a variation, you may still be using the same class of wormer.
The parasites on the pasture are being exposed to the same wormer on a regular basis and this allows them to develop and adapt to protect themselves against it.
Another issue is under dosing. It is vital to accurately estimate the weight of the animal that is getting dosed. If too little of the product is used, all the parasites may not be killed.
Those that aren't killed will adapt and learn how to survive when the next dose is given.
Incorrect use of product is also a problem. This is especially the case when it comes to pour-ons. Cattle should be dry before application and remain dry for at least six hours afterwards. If some of the product is washed off, the parasites are under dosed and can develop resistance.
You may argue that this doesn't apply to you because you dose every month. However, dosing too regularly can also be a problem. The animals own immune system can be affected by repeated dosing.
Cattle need a certain level of exposure to worms to develop an immune response to them. If cattle are repeatedly and regularly dosed, especially in their first grazing season, they do not develop any immunity to worms. This can lead to clinical problems in their second grazing season.
Routine faecal sampling should be the cornerstone of any worm dosing plan. Take samples from each different group of cattle on the farm. Results can tell us what parasites are present and allow us to an appropriate dosing strategy.
If you suspect that resistance may be an issue on your farm, talk to your vet. Samples can be taken from individual animals before and after dosing to determine if the worms are resistant to a particular family of wormers.
This applies equally to liver fluke where cases of resistance have been reported in recent times.
"The easiest" or "the cheapest" doesn't make sense any more when it comes to dosing.
This attitude could already be developing resistance on your farm. If you are buying a dosing product, ask how it compares with the ingredient of the last dose you used.
If in any doubt, a quick call to your vet will clear up any confusion. Devising a dosing strategy for all cattle on your farm with your vet will eliminate any confusion and reduce the chance of resistance developing.
Eamon O'Connell is a vet at the Summerhill Vet Clinic, Nenagh, Co Tipperary; email: firstname.lastname@example.org