Beef Herd Health

By W. Mark Hilton, DVM

 I received a few emails about the article that discussed internal parasite control and I wanted to follow-up on those questions. The most important statement I can make is to NOT change your parasite control program totally based on this article. The goal of this article is to have you think about your current parasite control program AND to talk to your herd health veterinarian about what is best for your specific farm.

In my many years of being a beef cattle veterinarian the ruminant parasitologists have given us numerous protocols for internal parasite control. Many of which we look back on now and shake our heads. The mantra of “Worms are bad. Kill them all” is outdated and wrong. If a cow has a very low number of worms and they are not causing any economic damage, do we really need to kill them? No.

Parasites have been on earth long before humans and they have evolved to survive. Our deworming products are rarely 100 percent effective, so some parasites generally survive. If a product is 99 percent effective at killing a certain worm, that sounds quite good. If a cow has 100 parasites before treatment and the product is 99 percent effective, it kills all but one. But what if the cow has 10,000 parasites? After treatment, 100 have survived and most if not all of these are resistant to the dewormer. This is like the problem of weeds that are resistant to herbicides, and having worms resistant to our anthelmintics is not good news.

The current recommendation from veterinary parasitologists is to leave some animals untreated. These untreated animals become the refugia for the herd, meaning the parasites these animals are harboring are not going to be exposed to the dewormer and should not possess resistance genes.

How do you choose which animals are the refugia?

If you have a winter/spring calving season, do not deworm calves until just before or at weaning. They are not picking up many parasites until late in the grazing season.

Cows develop some resistance to worms as they mature, and most recommendations are to leave cows over 4-5 years of age that are in Body Condition Score (BCS) 4-5 or better (out of 9) as refugia. If I have a 6-year-old cow that calves early in the calving season and is in BCS 5.5, even if she has some worms, I don’t care because they are not causing her any problem. I want these lowly pathogenic, “weakling” parasites reproducing and ending up on my pasture instead of the ones that are resistant to my dewormer. In my own herd I have cows I have not dewormed for five years.

Who should get dewormed?

Any cow that is less than 4-5 years of age and any cow that is thin (BCS under 4-5). Interestingly bulls are more susceptible to worms than cows, so in this case they are the “weaker sex.” My recommendation is to deworm your bulls every year.

Winter/spring born calves are likely among the most highly parasitized animals on the farm as they come off grass and should be dewormed in the fall soon before or at weaning. If they go to a dry lot for the entire fall/winter, this one treatment should suffice. Worms are picked up on pasture. The cow defecates and the parasite eggs mature in the fecal pat. After some time, they hatch into a larva, crawl a few inches away from the fecal pat and crawl up a blade of grass hoping to be eaten.

If weaned calves are placed back on pasture after weaning, a second treatment should be administered as they come off grass.

When is the best time to deworm the cows and bulls?

The answer is “it depends.” For winter/spring calving herds, the majority of parasitologists would advise treating the animals about six weeks after they have been turned onto pasture. This allows the animals to pick up some parasites that have emerged from the fecal pats and kill them. Others might suggest in the fall when cows come off pasture.

What product should I use?

This is where advice from your herd health veterinarian is vital. There are too many products on the market to simply guess. We have recently learned that the absorption of the pour-on dewormers is mostly from the cows licking themselves and others, not from absorption through the skin. Some parasitologists are worried that this becomes a dosing issue because a 1,400-pound cow might not actually get a dose that is sufficient to be effective. I love the convenience of the pour-on dewormers, but the dosing issue has me recommending them less and less. Both the oral and injectable products are more accurate in dosing.

One thing I see all too often is using a pour-on dewormer many times during the summer just to kill flies. The generic ivermectin products are very cheap and sometimes lower in cost compared to fly control products like permectrin/permethrin. If you are pouring cattle for flies, do not use a product that also kills internal parasites (worms). This will lead to parasite resistance building up quickly in your herd. We have not had a new class of dewormers in the U.S. for 40 years and we need to preserve the effectiveness of the ones we have.

Fall calving herds

The same recommendations for deworming only young and/or thin cows remain the same. The timing is more controversial with deworming after a hard freeze (under 28°F) favored by most. Do not deworm the calves until weaning.

I appreciate the emails about this subject. If you have questions or ideas for future articles, contact me at wmarkhilton@gmail.com.