If you're confused about the most effective time to deworm cows, perhaps you'll find some solace in the fact that even the worms don't know.

Fact is, Ostertagia ostertagi (brown stomach worm), which researchers say is responsible for a majority of damage caused by internal parasites, has shown remarkable adaptation to new environments. It even transforms its life cycle to match a new and opposite environment when transported inside of cattle.

“In general, deworming mature beef cows will benefit in enough extra milk production and enough extra condition to re-breed that it's an economically sound decision most of the time,” says Tom Craig, professor of pathobiology at Texas A&M University (TAMU) College of Veterinary Medicine.

For perspective, Ostertagia imbed themselves in the abomasa of cattle, irritating cells in the gastric glands, and inducing extra production of a hormone that suppresses appetite. The parasite also interferes with protein metabolism so that less protein is available for production and maintenance. So, cattle infected with Ostertagia consume less, and are less able to utilize the nutrients they do consume. Plus, their immune systems are impaired.

Steve Wikse, DVM, associate professor of large animal medicine at the TAMU College of Veterinary Medicine, says parasitologists believe 75% of the losses stemming from Ostertagia infection come in the form of reduced weight gain and reproductive performance. Keep in mind that researchers estimate 70% of the reduced gain is due to appetite suppression.

So, plenty of folks agree it pays to control Ostertagia in mature cows. Less obvious is exactly the best time to deworm cows in order to achieve the best economic benefits.

Nail Them Inside

For one thing, climate and geography means the optimum time to worm cows in one part of the world is exactly the wrong time in another.

The lifecycle and transmission path of Ostertagia goes something like this: Cattle infected with adult Ostertagia shed eggs in their feces. With warmth and moisture (as opposed to extreme heat or cold), the eggs develop into infective larvae, moving from fecal pats onto growing grass. Cattle ingest the larvae while grazing, and those larvae develop into mature egg-laying adults, and repeat the process.

During periods of extreme heat or extreme cold, ingested larvae are arrested in growth and delay development until favorable environmental conditions return. As a result, cattle are exposed to the greatest assault when arrested larvae emerge to resume development and have the opportunity to shed the eggs that grow into larvae on pasture. Those larvae can be ingested by cattle before extreme weather sets in again. At the same time, renewed moisture can bring out other larvae that have survived in fecal pats.

With that in mind, Wikse points out that generally this period of heaviest exposure occurs in spring and early summer (May-June) in the southern U.S., and in early fall (September and October) in the North.

Wikse's broad recommendation is: “Treat cattle when the greatest proportion of the total parasite population is in the cattle and not on the ground.”

In the South, Texas specifically, Craig says, “For the worms primarily in the stomach, I think late spring or early summer is the best time. Ostertagia can't stand the heat and are active in pastures here between September and early May.” Treat in May or early June, and Craig says producers can kill the arrested larvae before they can emerge and begin laying eggs.

Generally speaking, many researchers would say just the opposite about the North. By treating cows in October or November, producers have the chance to rid cows of the larvae they would otherwise carry through the winter and infect spring pastures.

For instance, according to University of Minnesota Extension, “If the decision is to deworm the cow herd one time, it should be done in the fall as cows come off grass. Treating at this time will kill any parasites picked up during the grazing season, help the cows get through winter without the physical stress of worms, and prevent egg shedding for the first part of the next grazing season.”

The “In-Between” Areas

In parts of the world that are transition areas, choosing the most effective time for a single annual treatment can seem like changing a fuel-pump blindfolded and wearing mittens.

“Essentially, what we see in the central U.S. is that the worm populations can't make up their mind, either,” Craig says. “So, in the central U.S., it may really make the most economic sense to deworm cows twice each year.”

Besides the lines of climate that can drift from season to season, Craig mentions a study conducted by Louisiana State University in which cows in Louisiana were moved to Ohio and vice versa. Although the cows carried Ostertagia with them — they were infected in the state of origin — the parasites' subsequent lifecycle mimicked that of the new environment.

“It's a continuum, and it can get confusing,” agrees Mike Hildreth, DVM at South Dakota State University (SDSU). A professor in both SDSU's Biology and Microbiology Department and in the Department of Veterinary Science, Hildreth explains this is even true in parts of the world where the north-south rule of thumb would seem to make timing obvious.

For example, in South Dakota, Hildreth says the idea of spring-time deworming wasn't even a consideration until recent years. Traditionally, he explains, producers would deworm in the fall for the reasons mentioned earlier.

However, as producers have seen the economic benefit of treating cows in the spring — the reduced parasite infection in calves — Hildreth says more are opting for spring over fall treatment, or adding spring to their annual control program.

“For years, the assumption up here was that spring deworming wasn't worth the effort,” he says. “But we've shown you can easily recover the cost of treatment in the weight of the calves.”

Hildreth says stocker trials show calves are losing 10-15 lbs./100 days of grazing.

“Since calves are even more susceptible to parasites, my guess is we're losing at least that if not more in the calves during their first grazing season.” He adds, “If you can keep calves from picking up worms, you should be able to recover most of that loss. And the way to keep calves from getting infected is by treating the cows.”

When producers planning to deworm cows only once ask Hildreth for his recommendation, he tells them: “Spring… After weaning, cows will come in and won't have access to grass or worms over the winter. So, if you're talking about eliminating worms in the adult cow, it may not be as effective in the fall.” Hildreth points out he lives in a part of the world where the challenge of internal parasites is less severe than in the southern parts of the nation.

Deworming Nursing Calves

Incidentally, when it comes to deworming for calf performance, Wikse says there's growing evidence that deworming calves while they're still nursing can offer added net return. In Texas field trials, he says controlling parasites in nursing calves along with their dams is yielding additional average daily gain in the calves of 0.1-0.2 lbs./day. Across trials, the average increase in weaning weight is about 25 lbs. against an average cost of $3.50 for deworming each cow-calf pair.

No matter exactly when producers choose to deworm mature cows, besides agreeing that the practice typically pays, everyone mentioned in this article emphasize the need for using products with long-lasting residual activity.

“With persistent-killing products, you have more leeway,” Wikse says. “It's not as critical that you treat cattle on a specific day.”

In fact, he points out, with some of today's long-lasting products, cows treated 30 days ago can be host to fewer parasites than those treated a day earlier with a short-duration wormer. In other words, if the treatment doesn't last long enough to kill parasites still to be ingested, little is being done to decontaminate the pasture itself.

For the same reason, spot-worming offers little return, says Pat Holland, TAMU veterinary research associate. While doing so offers initial relief to treated animals, untreated herd mates will continue shedding eggs and re-infecting those that were treated.

Likewise, those cited here point out the need for producers to define the goals of their parasite control programs.

For instance, when Hildreth suggests spring as the best time to deworm once during the year, he often hears, “Yeah, but I need lice control in the fall.”

Hildreth explains you've got to separate which parasites you're trying to control and when you need to control them to be effective.

Similarly in the South where flukes are prevalent, Craig says fall is the most effective time to treat them. Holland adds that different kinds of flukes require different treatment, demonstrating once again that there are no universal approaches.

Finally, as basic as it sounds, appropriate product selection and timing can be overwhelmed by shoddy application.

“Whatever product or timing is chosen, you have to make sure it actually gets on, or in, the animal,” Holland says, “and at the right dosage.”