“A heavily stocked pasture can have more internal parasite problems,” Dr. Robert Wells, livestock consultant with the Samuel R. Noble Foundation in Ardmore, Okla., told Ozarks Farm & Neighbor.
“A heavily stocked pasture is going to be grazed shorter, which allows those internal parasites to more easily complete their life cycles. They have to have short grass for the larva to hatch out of the ground, migrate up the stem of the plant and to the tip of the leaf of the plant while it’s got water on it, so either on a heavy dew or after a rainfall. Then, an animal has to come by and graze on that plant, and ingest that larva. If we’ve got grass that’s very tall, it’s difficult for that larva to make it to the top of that grass before it dries off.”
There is no one-size-fits-all formula for ideal stocking rates, which can depend on the type of forage and the growth stage of the animals.
“Cool season-type grasses that are going to be grazed during their growing time frame typically can be stocked a little bit heavier than warm season grasses,” said Wells. “Also, keep in mind if we’re grazing a cool season grass when it’s actively growing, the likelihood of internal parasite infestation is going to be lower because of the fact that the internal parasites have to have temperatures above 70 degrees for them to hatch out of the soil and out of the ground.”
Parasite loads are less of an issue on winter annual pastures, particularly if they have been frequently or recently tilled, which helps disrupt the parasite’s lifecycle.
The most common species of internal parasites that cause problems are the brown stomach worm (Ostertagia ostertagi), small intestinal roundworms and the barber pole worm. If deworming, Wells recommended rotating chemistry classes to prevent resistance; if using an anthelmintic in the avermectin family like Cydectin, Ivermectin and Dectomax, switch to an orally administered product the next time.
Rotation can also reduce parasite loads by letting pastures rest grass to grow taller. Eldon Cole, University of Missouri Extension regional livestock specialist at Mt. Vernon, Mo., told OFN, “Internal parasites probably do build up and tend to be more of a problem in those pastures that are being overly grubbed into the ground; it gives cattle more of a chance to pick up parasite eggs. That is the reason that we like to see rotation of pastures.”
He said while older cattle seem to be able to adapt to parasites even in a densely stocked pasture, problems will rise to the forefront in the younger animals.
External parasites, like hornflies and face flies should also be taken into consideration.
Cole passed along an interesting theory he’s heard to the effect that if cattle stocking populations are particularly dense, it will reduce fly problems; the constant trampling will disturb manure piles, leaving them less productive for flies depositing eggs. But he added, “I’m not sure that I buy into that, necessarily.”
Excessive stocking rates may not be as much of an issue as it has been in the past; Wells said most ranches he’s visiting are understocked.
“Because of the high prices, most people in the stocker business can’t really afford to buy all the cattle they need to, and on a cow-calf operation most of the folks that I’ve been talking with are still recovering from drought situations,” he said. “It’s very difficult for them to buy all the cattle back that they need to, or would like to, to be able to capitalize on these high prices.”


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