Fall deworming: the pour-on is not working as well as it used to, and here is how to find out

For thirty years fall deworming meant running the cows through and pouring a macrocyclic lactone down their backs. It was cheap, it was easy, and it worked. The first two are still true. The third is a question you need to answer on your own place, because on a lot of places the answer is now no.

What has changed

The pour-on avermectins, ivermectin and its relatives, have been used so heavily and so casually that the parasites they target have selected for resistance. Surveys of U.S. cattle operations have found Cooperia, the small intestinal worm that dominates in young cattle, resistant to macrocyclic lactones on a majority of farms tested, and reduced efficacy against Ostertagia, the brown stomach worm that is the real economic parasite in cows. Pour-ons are the worst offenders because the dose actually absorbed varies so much with hair, weather, and licking.

The white drenches, the benzimidazoles like fenbendazole and oxfendazole, and levamisole still work against most populations. They are more work to give. That is the trade.

Find out on your own cows

The test is a fecal egg count reduction test. Collect fresh manure from 15 to 20 animals in a group the day you treat, treat them, and collect from the same animals 14 days later. Your vet or a diagnostic lab counts eggs per gram before and after. A working dewormer drops the count by 95 percent or more. Under 90 percent, you have resistance to that product in that group. The lab work runs a few dollars a sample, and it is the only way to know. Treating with something that does not work is not free; it costs the product, the labor, and the weight the calves were supposed to gain.

Who to treat this fall

Weaned calves and yearlings carry the highest worm burdens and get the most from treatment. A calf that grazed all summer with its mother has been picking up larvae since May. Treat at weaning with a product you have tested, or with a combination.

Cows are the harder question. Adult cows develop immunity and carry lighter loads. The reason to treat cows in the fall in the northern half of the country is that Ostertagia larvae go dormant in the stomach wall over winter and emerge in a wave in late winter, when the cow is in late gestation and least able to afford it. A fall treatment after the first killing frost, with a product that reaches inhibited larvae, prevents that. In the South, the timing flips, and the treatment that matters is in spring.

Thin cows, first-calf heifers, and anything that scored a 4 at weaning are the cows to prioritize. A fat 8-year-old cow in good condition probably does not need it.

Combination treatment

The current recommendation from parasitologists is to give two products from different classes at the same time: a macrocyclic lactone injectable and a white drench, for example. Each class kills what the other misses, and the parasites that survive both are rare enough that resistance builds much more slowly. It costs more per head. It is still cheap next to the calves.

Refugia: leave some untreated

The counterintuitive part. If you treat every animal, the only worms that survive to breed are the resistant ones. If you leave the animals that need it least, the fattest 10 to 20 percent of the cows, untreated, they keep shedding susceptible worms onto the pasture, which dilutes the resistant population. This is standard practice in sheep and goats now, where resistance got bad first, and it is coming to cattle.

Dose by weight, not by eye

Under-dosing is the fastest way to build resistance. Weigh a few animals or use a tape. Set the gun for the heaviest animal in the group, not the average. And if you are using a pour-on, apply it on dry hide along the back, not in a lump on the shoulder.

The cost of getting it wrong

A weaned calf carrying an untreated worm burden gains 0.1 to 0.2 pounds a day less through the backgrounding period. Over 60 days, that is 6 to 12 pounds at $4 a pound. The fecal test is $50 for the group. It is not a close call.