Refine Your Deworming Program

In his cow herd, Missouri rancher Mike John uses a long-acting injectable and pasture rotation to keep parasites under control. ( James Fashing, MFA, Inc. )

Advice to seek professional help might seem presumptuous, but for your parasite-control program, expert guidance could pay off with better health, weight gains and returns on your input investments. 

Like so many aspects of beef production, deworming cattle has become more complicated. There’s a growing awareness that generic, calendar-based recommendations do not necessarily provide the best control in every production environment. Also, cattle movements have facilitated worm movements, and your herd might now harbor completely different worm species than in the past. Finally, the risk of promoting drug resistance in worm populations adds a new wrinkle, as traditional deworming programs could become progressively less effective. 

Retired Texas A&M University veterinarian Tom Craig still puts in time at the veterinary diagnostics laboratory, focusing on parasite issues. He says regional and local environment, weather and biosecurity practices influence control priorities. Traditionally, Craig says, strategic deworming has meant treating cows and calves in the spring, before turnout, to prevent shedding of parasite eggs onto green pastures, then treating again in the fall to remove parasites they picked up during the grazing season. He adds though, that this approach does not fit all production environments, and no single treatment or control strategy will work for every operation.

Among all a cow-calf manager’s input investments, parasite control potentially offers some of the highest returns. 

In an often-cited economic analysis from 2006, Iowa State University economist John Lawrence and his team identified the numbers behind parasite control. The analysis, “Economic Analysis of Pharmaceutical Technologies in Modern Beef Production,” using 2005 cattle prices, estimated that without using dewormers, cow-calf producers would sacrifice more than $165 per head in lost returns. 

For the beef-production system overall, the impact of eliminating dewormers on break-even prices totaled $190 per head.
Mike John, a Missouri rancher and director of MFA Health Track, says the program verifies health and preconditioning practices to help ranchers market their cattle, including a 45-day on-ranch weaning period. The certification program includes parasite control, partially to help ensure buyers that the calves will perform well, and also for on-farm benefits. 
John stresses deworming calves at or before weaning helps protect their overall health, boosts their response to vaccines and maximizes daily gains over the 45-day weaning period, resulting in heavier sale weights and higher calf values. 

John says his parasite control program has evolved over time. He also sees increasing customization of programs among his clients across Missouri and into neighboring states. Across that geography, he says, production environments, management practices, forage types and parasites vary widely.


While some producers resist change and continue to deworm based on calendar dates, many have become more strategic and flexible, John says. They monitor the efficacy of products they use and in many cases consult with professionals in designing their overall herd-health programs, including parasite control. 

On his ranch, John primarily uses injectable dewormers, and sometimes an oral drench product, rather than pour-on products. Many ranchers in the MFA Health Track program do the same, he says, for more reliable dosage and efficacy. Ranchers on extensive operations have, in many cases, turned to a longer-acting injectable dewormer due to the difficulty of gathering cattle for treatment during the grazing season.

A high degree of deworming efficacy is critical. Overuse of a particular class of dewormer, especially if the dosage is ineffective, leads to drug resistance among parasite populations. Work with your veterinarian to select the right product applied at the correct dosage at the right time for your operation. 

University of Florida veterinarian Max Irsik says many producers routinely deworm all cows when they are gathered without much thought as to whether they need to be dewormed. “Today we need to really consider the appropriate timing, with regard to the life cycle or pasture buildup of infective larvae when we deworm cows, along with the age of the cows, the overall body condition of the herd and individuals within the herd,” he says.

Older cows in good body condition might be best left untreated, but if nutrition for the herd is adequate, thin cows or poorly conditioned cows become likely candidates for deworming. This, Irsik says, could save the producer significant pharmaceutical costs while helping minimize parasite resistance within the herd. Craig agrees, saying distribution of a parasite usually will not be consistent across the herd. “It’s the 20/80 thing,” he says. “Twenty percent of the herd will have 80% of the worms.”

Know the types of worms you are battling for best control. Craig says Ostertagia ostertagi, or the brown stomach worm, remains the primary nematode parasite causing production losses in cow-calf herds across the U.S. Within herds or local areas though, ranchers also could face other types of worms including Cooperia species, Haemonchus, Nematodirus, liver flukes and others. The ideal timing for deworming varies from one type of worm to another, as does the degree of drug resistance.

Ostertagia, Craig says, reproduce and spread primarily during the winter in the South and summer in the North. They become active when temperatures are above 50°F, and go dormant during the hottest, driest weather, only to emerge later.

Cattle ingest Ostertagia larvae while grazing, the eggs hatch and mature worms damage the stomach lining and impair digestive function. In Type II ostertagiosis, the worms encyst in the abomasum during cold or hot seasons and re-emerging later in tremendous numbers.

Most cows develop a tolerance to Ostertagia by around four to five years of age. Depending on their tolerance level though, cows infested with Ostertagia can experience weight loss, loss of feed efficiency and body condition and possibly reduced reproductive inefficiency.


Craig stresses that veterinarians and producers need to understand how their local climate and production system affects the Ostertagia life cycle, pattern of infection, and the duration of efficacy for the anthelmintics they use. 

In the southern Plains, for example, Ostertagia tends to enter its dormant stage in late spring or early summer as the weather turns hot and dry, with adults emerging and egg shedding occurring from the fall into early spring. In the North, the pattern is reversed, with cold weather causing the worms to enter their dormant state and type-1 infections taking hold in spring and summer. 

In the cowherd, Craig generally recommends deworming in the late spring to early summer with injectable avermectin products to control Ostertagia. For suckling or weaned calves, he turns to white drench dewormers to also control Cooperia and Haemonchus species. Cooperia, he notes, have become resistant to macrocyclic lactones, such as avermectin, as have Haemonchus populations in many areas.

On his fall-calving operation, John usually deworms cows and calves 15 to 20 days after initial turnout in the spring, just before rotating them to the next pasture. This allows time for the larval worms picked up on the first pasture to mature. The treatment kills them and thus prevents egg shedding on subsequent pastures throughout the season. With calves weaned in the spring, once he turns cows out, he won’t have them in a chute again until calving time, so he uses a long-acting injectable and pasture rotation to keep parasites under control through the grazing season.

Craig stresses producers should work with a veterinarian in devising and continuously monitoring an overall parasite-control strategy. If performance and reproductive efficiency decline in a herd, strategic changes in timing or product selection could make a difference. If a producer suspects resistance has developed among local worm populations, they can work with specialists to conduct fecal egg count reduction tests to evaluate efficacy of deworming products.  

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