We heard years ago about resistance with fly tags. I believe the first tags were called Bovaid and with no other tags on the market, researchers noticed resistance developing after a few years.
Soon other companies were making tags with a different family of chemicals so producers could rotate them and ward off resistance.
Horn flies continually feed, so if tags or other fly control methods become ineffective, flies become immediately visible on the animals’ backs.
This is especially true on bulls because they attract more flies and hundreds or thousands will be visible if control is inadequate.
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In the last several years a product called Cylence was developed as a pour-on for flies. Its effectiveness lasts about two-thirds of the time for which the fly tags are good. It is commonly used because it is easy to apply and cows did not have to be caught.
We are now starting to see resistance to the Cylence. For producers who have used it several years in a row, flies can be seen on the backs of cattle for short of the 60 day effectiveness window.
We are not getting the bang for the buck, so to speak.
If producers see flies sooner than expected, it is time to change products. The best remedy is to keep cycling through the different fly tags and Cylence.
This prevents resistance from developing and weight gains are better on the cattle, which is what producers strive for.
At this time of year, producers should use binoculars and check for flies on cattle in pastures, or have a closer look at animals during processing. At the minimum, producers should treat bulls being turned out for breeding.
With internal parasites (worms), resistance to endectocides has been shown in the southern United States, where internal parasites are treated several times a year. Canada doesn’t have nearly the same worm problem so cattle are usually treated once to twice yearly.
The main culprit in Western Canada is lice. To my knowledge, research was done in this field a few years ago and no resistance was found.
TREATMENT TIPS
To avoid flies and parasites gaining resistance to key control treatments, producers can take several steps:
• Don’t over treat. If cattle need it, treat them but not every time they go through the chute.
• Do not under dose because that can lead to resistance. This may have occurred when the endectocides were pricey.
• Producers apply control according to weighto accurate scales are needed or someone who is able to estimate the weight well.
• If unsure whether treatment is necessary, consult a veterinarian. If worms are the problem, the vet may change to an older generation dewormer, such as Safeguard (fenbendazole) or an even older product Tramisol (levamisole) if still available. In the past, there was some resistance developing to these, but because they have been used little for the last several years they may be valuable.
In many cases, fecal counts to test for worm loads may be negative or low enough whereby deworming may not be necessary. As with most parasitic conditions, a low level of infection elicits some natural resistance in the animal.
I have seen this with tapeworms. Young bison may have a heavy load and yet the adult bison are clean and this was without deworming them for the tapeworms. Fortunately in most of Canada, winter puts parasite transmission on hold, making control easier. If producers use diligence and don’t overuse good products, we hopefully can avoid resistance.
Occasionally checking fecal counts on the least thriving animals will identify whether internal parasites are a problem.
