Parasite prevention vital as dewormer efficiency fades

In the past, most pour-on products were primarily targeted at lice but for about the past eight years, the efficacy of pour-on dewormers on internal parasites has diminished.

This means that after a fall treatment, the internal parasites emerge in spring, excreting lots of worm eggs. This then carries onto the pasture season and pastures become more contaminated.

In the past, pour-on endectocides were about 100 percent effective on internal parasites but recent evidence indicates this has fallen to about 50 percent.

This raises the question of how to reduce parasite levels and decrease pasture contamination.

The first strategy is to turn the cattle out to pasture in spring, preferably parasite-free, and then initiate a mid-summer deworming program to kill the worms and stop shedding of more eggs. If done at the correct times, the larvae have become adults but have not excreted many eggs yet. This breaks the cycle and greatly reduce pasture contamination.

Cattle with worms excrete many eggs in a day with some species producing several hundred eggs daily and others producing thousands of eggs daily.

Pasture contamination from 100 cows with a moderate worm burden could be as high as 15 million eggs a day. Cattle then must ingest the larvae to be infected.

This is where intensive rotational grazing can increase the potential exposure to parasites, although it is good for many other reasons, so keep doing it.

One way to reduce parasites would be to have an entire season of desiccation. In pastures where cattle don’t see the same ground for an entire year, parasites are desiccated. Running sheep with cattle is another strategy that reduces parasites.

These strategies can’t be carried out at most cattle operations and most producers will have to deworm cattle six or more weeks after they’ve been turned out into pasture.

There are many formulations of benzimidazoles, the most popular being Safe-Guard, a product that can be given mixed in feed or minerals under a veterinary prescription. This dewormer is close to 100 percent effective on the adult worms.

A low percentage of cattle are dewormed mid-summer and yet that is the best time to attack the parasites, before they can lower weight gains in cattle.

If deworming is carried out correctly, weight gains can be 20-30 pounds or more, depending on the parasite load, quality of grass and other factors.

Studies on calves that had been concurrently implanted shows there is an additive effect between the gains with deworming and the gains with the implants. This one-two punch results in huge economic benefits compared to the costs of the implants and dewormers.

In past studies of parasite levels on pastures, it has been found that about two-thirds or more of herds would benefit from a deworming program.

Larvae do overwinter in Canadian winters and more so in pastures with good snow cover. In recent work at the University of Calgary Veterinary Medicine, about 90 percent of larvae perish, but the survivors can infect the first cattle grazing in the spring.

Several years of pasture treatment can gradually decrease the pasture burden.

By strategic deworming, you not only rid the cattle but significantly decrease the pasture burden.

If you are not sure about the parasite load, a check of older calves or yearlings, especially later in summer, will determine whether parasites are a burden or not.

Many samples are necessary to get an idea of the parasite load because parasite egg shedding varies a lot between cattle.

Some clinics will do the test in-house, so ask your veterinarian.

Another strategy is to use the benzimidazoles (Safe-Guard family) products in late fall when applying a lice treatment. The lice treatment could be specifically for lice. If using a pour-on ivermectin product, do it later in the fall combined with these other dewormers to take the internal parasite count to near zero.

Then cattle are worm-free all winter and will gain weight better, have a better immune system and better feed conversion.

Consider deworming cattle in mid-summer to boost productivity and herd health.

Roy Lewis works as a veterinarian in Alberta.

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