Parasites pose a problem in pastures


Summer grazing management is generally about slowing paddock rotations and providing the grass plant with an adequate recovery period before another grazing pass is made.

As if that were not enough of a management challenge, the pasture-based sheep and goat producer faces another twist on top of that. They must also manage summer grazing schedules to avoid loading up their animals with debilitating and potentially fatal burdens of Haemonchus contortus, the barber pole worm.

In order to do this, the grazier must know which pasture paddocks are likely to be harboring high parasite numbers and manage pastures within an understanding of the parasite life-cycle.

Life cycle

Haemonchus contortus survive and begin their life cycle each year through larvae that overwinter on pasture or through larvae “hibernating” in a process called hypobiosis in the abomasum of the host animal.

Hypobiosis is actually an arrested development stage of the L4 larvae. In early spring, generally around lambing/kidding time, these L4 larvae resume the normal life cycle, become adults and start laying eggs that are passed in the feces of the animal.

So in the spring, adults and lambs can begin to accumulate Haemonchus contortus from consuming overwintered larvae, or from larvae emerging from newly deposited eggs.

Once an egg is deposited on pasture, if there is moisture and warmth, the egg hatches and a larva, termed an L1 larva, emerges. Larvae need to go through a couple of growth stages to reach an L3 larva, which is the infectious stage.

L3 larvae migrate up and down grass blades in films of moisture. For the most part, this migration pattern is confined to the lower 3-4 inches of the grass blade.

The time to go from egg to L3 infectious larvae can be as short as four to five days under ideal temperature and moisture conditions.

Once L3 larvae are ingested by grazing animals, they travel to the abomasum, or true stomach, of the host animal. Larvae attach to the abomasum wall and begin feed on blood. Within two or three days, L3 larvae develop into L4 larvae and then adults.

Once adulthood is reached, about 14 days are required before egg laying begins. The entire life cycle from egg to egg can be completed in as little as 21 days.

Once egg laying begins, the female Haemonchus contortus can lay up to 5,000 eggs per day. As life cycles are completed, pastures can become heavily contaminated.

It is estimated that as much as 95 percent of the total Haemonchus contortus farm burden is contained on the pastures. It is important to realize that the L3 infectious larva can survive on pasture for up to 90 days in the summer and up to 180 days when they develop in the fall.


This obviously presents some difficulties in a rotational grazing system. For example, let’s say a pasture was grazed by ewes or does in late October or early November. This means that eggs being deposited at that time could result in some overwintered larvae still being around in late April or early May.

If that pasture is grazed in late April, and particularly if it is grazed below a 3-4 inch height, then there is a good chance some overwintered parasitic larvae are being ingested. In addition, by April most ewes and does are depositing new parasite eggs on pastures.

By the time that paddock has been rotationally grazed a couple of times, watch out! The number of potential parasitic larvae that could be ingested is astronomical.

This is what would be termed a “hot” pasture. It might look like good green, succulent grass, high in quality, but it can be teeming with infectious L3 larvae. Even a pasture paddock that started out more than 180 days removed from a previous season grazing pass can become a hot paddock after a couple of grazing passes with ewes, does, lambs and kids shedding eggs.

How to manage

What are management options? They must revolve around trying to keep parasite levels low.

— Keep a pasture log or record of when paddocks were grazed and the length of time pasture paddocks were grazed.

— Start the grazing season on pastures that are likely to have very low levels of overwintered larvae.

— Shorten pasture rotations.

Rotate to a new paddock within a five-day period to ensure animals are not ingesting infectious L3 larvae. In practical terms, for most graziers, this will mean decreasing paddock size. Temporary electric netting is a valuable tool.

Graziers must become aware of where heavy parasite numbers are, where the “hot” spots are, and avoid letting animals graze in these locations.

For example, I have seen a number of graziers that work hard at managing their pasture rotations, but then bring their goats or sheep up close to a barn paddock every evening. If there is grass growing in that paddock, you can bet it is loaded with parasites.

All the good pasture management work being done in the daylight hours is for naught in this situation. If animals are going to be brought in for the night, make sure it is a dry lot, with no grass.

Save some

A critical piece is to have some safe pastures that have very low or no parasite larvae on them available to graze in July and August. This could be a pasture that has not been grazed since the previous fall, a warm-season perennial grass pasture, or a summer annual forage such as sudangrass or a sorghum-sudangrass hybrid.

Another option to reduce the parasite load on cool season grass pastures is to take an early grazing pass, then let the paddock re-grow and harvest the forage as a hay crop. This should remove a large number of larvae from the pasture.

The next re-growth after the hay crop can once again be grazed.

For graziers interested in cattle, interspecies grazing might become a pasture and parasite management strategy. Haemonchus contortus is not a serious parasite of cattle. If a number of cattle can be used to either graze before or after a sheep or goat grazing pass this could, in theory, reduce parasite numbers.

I say in theory because I doubt that just a couple of head of cattle either grazing with sheep and goats or used as a pre- or post-grazing pass will make a significant difference. There is probably some critical mass or ratio of cattle that must be maintained.

Weaning time can also play a role in how pastures are used. Once a lamb or kid has been weaned from the ewe or doe, and the stress of lactation removed, the ewe or doe can tolerate some parasite infection. The immune response provides some protection.

So keep some safe pasture for weaned lambs and kids and then the ewes and does can go back and graze some of those pasture paddocks that contain infectious L3 larvae.


Currently in Athens County there is a sheep producer experimenting with early weaning as a component of his pasture and parasite management. I’ll have more to say about the results of that study in a later column.

Pasture management is now the centerpiece of parasite management for sheep and goat producers because of the well-documented problems of resistance by Haemonchus contortus to all available classes of chemical de-wormers currently on the market.

It’s July. Do you know where your parasites are?

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