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Practical Advice For Growing a Parasite Resistant Herd

Some time back, I wrote about grazing management to reduce parasites, and in particular liver flukes. These modest management changes appears to be a simple way to reduce parasite populations and help us when parasites have developed resistance to the wormers we relied on in the past. To refresh your memory, here are the practices we can use to reduce worms:

Tall may not be as tall as you imagine. This graph shows the distribution of larvae on grasses. At 15 centimeters, or about 6 inches, the number of larvae drops to almost 0. Below 5 cm or about 2 inches, their numbers increase dramatically.

Leave taller residual.
The taller the leaf, the harder it is for the worm larvae to reach the top and the harder it will be for the larvae to end up in the animal.

Provide longer rest periods.
Worm larvae have a limited life span. It only makes sense that a longer rotation will result in fewer live larvae to infect your stock when they come back to that pasture.

Graze for shorter periods in each paddock.
Moving your stock out before they begin to re-graze plants keeps their mouths from moving closer to the ground where the larvae populations are highest.

And, if you’re dealing with liver flukes, keep your animals out of the creek or pond.
This keeps them from encountering the snails that carry the immature liver flukes. You can read all about that life cycle in my previous article.

We can also manage our herd’s genetics.

Beyond simply managing our grass, we can also make changes to the way we manage our animals. So let’s take a look at that.

There appears to be increasing evidence that genetic factors in individual cattle allow some cattle to thrive, even in worm-infected landscapes. Some individual cattle are more prone to have heavy internal worm populations and some prone to have less. That means, selecting against problem genetics is a fairly simple partial solution to the presence of worms in our pastures.

The 80/20 Rule of Culling

Yearling steer with worms from “Tapeworm and roundworm parasites of cattle” by D. Porter, 1942

I recently read an article that called this the 80/20 rule: 80% of your worms will inhabit only 20% of your herd. The strategy they suggest, then, is to identify the 20% of your cattle that have low resistance to worms (or that suffer the worst effects of worms, perhaps) and aggressively treat those individuals. So, look your cow herd over and sort off the skinny, poor-doing individuals and give them a dose of wormer.

For the life of me, I can’t help but wonder what might happen if we replace the word “treat” with the word “cull.” In other words, if we buy into the idea that there is a genetic force that allows some cattle to function just fine while other individuals struggle (under precisely the same conditions), why don’t we simply select against the weak genetics by sending the weaklings to the butcher pen?

I know, I know. Selling off 20% of your cows each year sounds pretty draconian. But consider this: the actual cull rates on most ranches hovers around 15%, especially if we count dead cows. By aggressively culling any poor-doing, poor producing cow as a function of genetically selecting against worm problems, we will probably be picking out quite a few cows that would be culled for some other production or fertility reason.

Besides, if there truly is a genetic factor that affects resistance to worms, the necessary culling rate should fall in a very few years. At that point, you have a healthy herd and you might, like Greg Judy, be able to market your animals to others for their superior genetics.

Happy Grazing!

Want more?

Chip Hines, also wrote on this topic here:

Genetic Parasite Control in Cattle

Note from Kathy: Sadly, Chip passed away on October 12, 2020 after a battle with Parkinson’s.

I met Chip at a workshop about ten years ago where I was presenting about teaching cows to eat weeds. We hit it off right away and he took me under his wing, introducing me to folks and encouraging them to try the training technique. When I started On Pasture, he immediately volunteered to write and provided lots of great articles, ideas, and support. I am grateful for everything he did for me, for On Pasture, and for all the readers who learned from him.

Chip was passionate proponent of sustainable, profitable ranching and he share his “outside-the-box” ideas in four books. Check them out. You’ll be glad you did! And, if you’re a subscriber to Kindle unlimited on Amazon, you can read these books for free.

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John Marble
John Marble
John Marble grew up on a terribly conventional ranch with a large family where each kid had their own tractor. Surviving that, he now owns a small grazing and marketing operation that focuses on producing value through managed grazing. He oversees a diverse ranching operation, renting and owning cattle and grasslands while managing timber, wildlife habitat and human relationships. His multi-species approach includes meat goats, pointing dogs and barn cats. He has a life-long interest in ecology, trying to understand how plants, animals, soils and humans fit together. John spends his late-night hours working on fiction, writing about worlds much less strange than this one.

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