For many producers, deworming has become a routine part of herd management. Cattle are processed, products are administered and the expectation is that parasite control is handled for another season.
But across the industry, cattle continue to underperform despite regular treatment. In many cases, the issue is not a single product failure, but a combination of resistance pressure, hidden production losses and management habits that gradually reduce the effectiveness of parasite control programs over time.
On the most recent episode of “The Bovine Vet Podcast,” Megan Bollin, a technical services veterinarian with Norbrook, and Nancy Jackson, a field veterinarian for the Mississippi Board of Animal Health, outlined several reasons why deworming programs may not be delivering the results producers expect.
1. Subclinical Parasites May Be Hurting Performance Before You Notice
Parasites do not need to cause obvious disease to affect productivity. In many cases, the biggest losses are occurring quietly through reduced digestion, feed efficiency and weight gain.
“Those parasites are going in and doing damage to the lining of the abomasum, and so what normally should be a lower pH is actually becoming more neutral. That impacts protein digestion, nutrient absorption and even appetite. It reduces voluntary feed intake, and then that cascades into average daily gain, feed efficiency, milk production and reproductive performance,” Bollin explains.
Because those effects develop gradually, they are often difficult to recognize without measurement.
“They’re those silent robbers that are there. We can’t really see them, and that’s why it’s called a subclinical impact, but they’re doing major damage,” Bollin says.
Jackson notes some calves may visibly underperform, but many losses remain subtle enough that producers underestimate the impact.
“You can see it in some cases, calves just standing there, not grazing, not performing, but a lot of times producers don’t realize what they’ve lost because they’re not measuring it,” Jackson says.
2. Resistance Is Already Present on Many Operations
Reduced dewormer efficacy is no longer considered a future concern. Parasite susceptibility can now vary significantly between farms, even within the same geographic region.
“Even from one side of the county to the other, recommendations might be very different depending on pasture type, parasite exposure and treatment history,” Bollin says.
That variability makes it increasingly difficult to assume a protocol that works well on one operation will perform the same way elsewhere.
At the same time, few replacement products are expected in the near future.
“We’ve routinely given the same things over and over, and we don’t have any new molecules on the horizon,” Bollin explains.
As resistance pressure increases, reduced efficacy in existing products can have growing consequences for cattle performance and long-term parasite control.
3. You May Be Underdosing More Often Than You Think
One of the most common management issues contributing to reduced efficacy is underdosing. As cattle size has increased over time, dose estimates have not always kept pace.
“Our producers still think they have a 1,000-lb. cow, but cows have been getting bigger for years. So, we’ve probably been underdosing cattle, especially those larger animals and bulls.” Jackson warns.
Underdosing exposes parasites to a drug without fully eliminating them, increasing the likelihood that surviving worms contributes to future resistance problems.
4. Some Dewormers Are Being Used Like Fly Control Products
Convenience can also create problems when products are used outside their intended purpose.
According to Jackson, some producers are administering pour-on dewormers at partial doses primarily for fly control rather than at labeled doses intended to control internal parasites. Repeated exposure to subtherapeutic drug levels creates ideal conditions for resistant parasites to survive and spread.
5. Poor Record-Keeping Makes Resistance Harder to Detect
Inconsistent product tracking can make parasite control decisions much more difficult over time. Without knowing which active ingredients or drug classes have been used previously, producers may unknowingly rely on the same class repeatedly or struggle to evaluate whether a protocol is still effective.
“I’ll ask what they used, and they’ll say, ‘It was the blue one’ or ‘I got it off the shelf at the co-op.’ But we need to know the active ingredient to make good decisions,” Jackson says.
That lack of detail can make it harder to identify emerging resistance patterns before they become more significant problems.
6. Application Problems Can Look Like Resistance
Not every apparent treatment failure is true resistance. In some cases, the problem lies in how the product was administered.
“There are a lot of things that have to go right with a pour-on for it to work. If the animal is dirty, that product isn’t going to get absorbed. If it rains, it can dilute it. Oral products can be spit out. There are a lot of factors that can look like resistance but aren’t,” Bollin explains.
Without recognizing those factors, producers may incorrectly conclude that resistance is solely to blame.
7. Parasite Problems Don’t Stay on One Farm
The effects of ineffective parasite control can extend well beyond a single operation. As calves move through the production chain, resistant parasite populations can move with them, affecting downstream performance and management decisions.
“When those calves leave your place, you’re passing that parasite load on to someone else. If it’s resistant, it affects the feedlot and performance down the line,” Jackson warns.
That interconnectedness means small failures repeated across multiple operations can gradually reshape parasite pressure across the industry.
Why Parasite Problems Keep Building
Many deworming programs do not fail because of one dramatic mistake. Instead, they lose effectiveness gradually through repeated small issues: underdosing, inconsistent application, misuse of products and resistance pressure that goes unnoticed until performance has already been affected.
Routine treatment schedules alone are no longer guaranteeing consistent outcomes, particularly when the surrounding management practices remain inconsistent. This means parasite control is becoming less about whether cattle are treated and more about how those treatments are being used and how the results are being monitored over time.
To hear more from Bollin and Jackson on how deworming strategies may be falling short, and how strategies are evolving, listen to the full conversation on the latest episode of “The Bovine Vet Podcast.”
