To Kill A Worm, Go Where He Lives

Dewormers are still the No. 1 weapon for parasite control. But without good pasture management, you may not get the optimum return from that investment.

"I'm very much aware of the buildup of resistance in nematode parasites," said cattle producer and retired animal scientist John Stuedemann, of Comer, Ga. "Our underlying philosophy is to minimize the use of chemical dewormers. We try not to deworm more than twice a year. We also think of the total management system and try to minimize the handling of animals."

Tom Yazwinski, University of Arkansas animal scientist, said to control worms you've got to understand them. He explained worms only migrate 4 to 6 inches up on grass. This means the better quality the pasture and the taller the forage, the less likely cows are eating worms.

Stuedemann factors the life cycle of the brown stomach worm (Ostertagia) into his management plans. "It is one of the most pathogenic nematodes we have," he said of Ostertagia. "The worms lay eggs in the cow, and those are excreted in her manure. Inside the egg, a first-stage larvae develops within 24 hours. A week to three weeks later, they are in the infective stage."

It is during that infective stage that water (rain or dew) can help the worms move from cow patty to grass, where they make their way up stems. Once eaten by a grazing animal, the cycle begins again, spreading the worms via their eggs, to new pastures.


If there's any good news here, Stuedemann said it's the fact mature cows are largely resistant to the parasites. They can eat the larvae and, almost like a natural vacuum cleaner, help clear up a pasture. The program starts in August, when cows come off fescue pastures and go onto bermudagrass. This is where cows will calve in September and October. Weather helps determine if it's time to deworm.

"Down here, Ostertagia is a winter parasite. The sun is their enemy. In a hot and dry summer, they'll desiccate," Stuedemann explains. So, unless the Georgia summer has been unusually wet, he'll skip deworming at this point.

Cows and their new calves stay on bermudagrass supplemented, as needed, with small amounts of whole cottonseed and hay until Stuedemann completes his artificial insemination (AI) breeding program, normally mid-December. At that point, two herds go to stockpiled fescue, while two herds go to fresh pastures of bermudagrass overseeded with ryegrass.


Stuedemann said stockpiled fescue is key in his battle against internal parasites. Typically, fall is dry in his area. No cattle have been on the stockpiled pastures for four months. This gives any larvae that were present time to die. In addition, stockpiled forage is carefully managed by rotating cattle to new paddocks when they start to eat it down. That gives the forage time to regrow above worm height before rotating.

In January or February, calves get their blackleg vaccinations and a first round of respiratory vaccines. Stuedemann deworms both cows and calves. "Even though the infection rate is not that great in the cows, we don't want them to contribute to the parasite load on the pastures," he said.

He is adamant about deworming first-calf heifers during this time frame. "Parasites are opportunistic. The stress of calving and nursing make her more susceptible than the mature cow."

While Stuedemann's management program works well on his operation, he stressed there is no one best approach to the problem of parasites. "When we're dealing with internal parasites in cattle, there isn't one recipe." He said it's important to ask, "What species of forage do I have? What are the weather conditions in my area?"

Yazwinski agreed each operation is different. "Every farm is its own little research lab. What might work on one farm might not work on the farm a mile down the road. Take a look at the fecal egg counts before and 14 days after treatment. Egg counts on Day 14 after treatment should be at least 90% lower than on the day of treatment. "If they aren't, then you probably killed fewer worms than you left in your cattle."


At the USDA Agricultural Research Service experiment station near Watkinsville, Ga., retired animal scientist John Stuedemann said researchers kept worm counts near zero for 12 years.

In 1993, they took fields that had been in row crops for more than 25 years, which meant they had virtually no parasites, and converted them into bermudagrass pastures. Each year, they dewormed a fresh set of stocker calves 72 hours before turning them out on the pastures. That interval allowed time for the calves to shed the worm eggs before they hit the grass.

"We never dewormed the calves while they were on the pasture," Stuedemann stressed.

While holding calves off a fresh pasture for 72 hours after deworming worked in the '90s, University of Arkansas animal scientist Tom Yazwinski said it's a risky move now since some parasites are becoming resistant to some dewormers. "You're only going to contaminate the new land with resistant worms."

He said your best tool is common sense. "Lower your stocking rate, fertilize and get rid of weeds. Do what you can do to make more forage and better-quality forage available."

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