There’s a connection between parasites and pasture management

Sheep and goats raised in a pasture-based system present some additional pasture management challenges to the grazier.

Specifically, the grazier needs to be aware of the parasite status of the herd or flock and how grazing management and pasture rotation are connected to parasite management.

The parasite of primary concern is Haemonchus contortus, commonly known as the barber pole worm. This is a blood-sucking internal parasite that does its damage in the abomasum (the true stomach) of the sheep or goat.

Growth stages

In order to understand how pasture management decisions can affect the parasite status of the herd or flock and vice versa, it is necessary to have some understanding of the parasite biology and life cycle.

From spring through fall the adult Haemonchus contortus worm is feeding on blood and laying eggs that pass through the animal’s feces and are deposited on the pasture.

Under most of our pasture growing conditions, those eggs soon hatch and the resulting larvae pass through two molts or instar stages termed L1 and L2.

The nasty stage

The L3 larvae are the infective stage. L3 larvae move up and down grass stems and blades and are ingested by grazing animals.

Generally, the migration up and down the grass blade is confined to the lower 3 to 4 inches of the plant. Once inside the animal, these larvae molt into an L4 stage and begin to consume blood by slashing openings in the abomasum wall. The L4 molt into adult worms, mate, and begin to lay eggs and the entire life cycle starts over.

Need to know

Some key questions we must answer within the context of this life cycle include: How many days are required to go from deposited egg to the L3 stage? How many days are required to complete the entire life cycle from egg to egg?

How many eggs can the adult Haemonchus contortus produce each day? What is the infection level of my pasture paddock? How long can that L3 larva survive on the pasture if not ingested?

Think fast

Under ideal conditions of moisture and temperature (think late spring/early summer) the L3 stage can develop from an egg in about 3.5 to 4 days. Under those same optimal conditions, the entire life cycle from egg to egg can be completed in about 23 days.

The challenge

Now here is the big challenge to parasite control in terms of pasture management: The reproductive capacity of the adult Haemonchus contortus worm is tremendous; each worm can produce 5,000 or more eggs per day.

The infection level of a paddock depends upon the amount of adult worms in the animals, the number of animals on a pasture paddock and the time spent on each paddock.

If you play around with some numbers, it doesn’t take long to see that literally millions of eggs per day can be deposited on pastures. Thus we come up with the term of a “hot” pasture when talking about the parasite load and potential to infect an animal with a large dose of parasites.

The Haemonchus contortus parasite does not multiply within the animal. The severity of parasite infection is dependent upon the quantity of eggs produced and deposited on the pasture.

In our parasite control workshops, we teach that 95% of the parasite load is out on the pasture and only 5% is found inside the flock or herd.

Added challenge

On top of this, there is the added challenge of the fact that the L3 larva can survive at least 60 days and possibly as long as 90 days on pasture under our typical climatic conditions here in Ohio.

What can you do

What does all this mean in terms of pasture management for sheep and goats? Looking at the life cycle, it says that if we could rotate to a fresh pasture every 3 to 4 days, potentially our sheep and goats would not be consuming any infective L3 larvae and we could keep them parasite free.

This, of course, assumes that those fresh pastures do not have infective L3 larvae waiting to be ingested. On most, if not all farms, this would mean rotations of 60 to 90 days, which is not practical or economically feasible.

The point here is that the grazier can’t avoid parasite infection by practicing good rotational grazing management that produces high-quality forage. In practical terms, the grazier can start out with a 3-4 day rotation to outrun the parasites, but when that second grazing pass begins, the grazier has to be thinking about the potential infective nature of those pastures. How “hot” is that pasture paddock?

Answer

The answer goes back to the number of worms, the parasite load inside the animals, and the stocking density that was used on that paddock previously.

One tool that can be used to get some indication is a fecal egg count. Fecal egg counts provide the grazier with the number of eggs/gram that is being shed at the time of the fecal collection.

It is a snapshot in time and should be done at regular intervals. Although I have not seen any hard, fast rules, I use 250 or less eggs/gram to indicate low infection levels, 250 -750 eggs/gram as moderate infection levels and 1000 eggs/gram and higher as high infection levels.

Immune response

Young lambs and kids do not have any immune response or protection and are a parasite replication machine. Putting them on a “hot” pasture is a formula for disaster, and possibly a death sentence if they are not monitored closely and if an effective chemical de-wormer is not available for a rescue treatment. Ewes and does under lactation have a suppressed immune response to parasites and can suffer from parasitism.

When lambs and kids are weaned, the immune system of healthy ewes and does kicks back in and they exhibit resistance to parasite infections, evidenced by fecal egg counts dropping to very low levels.

From a pasture management perspective, dry ewes and does can best utilize previously grazed pastures where eggs have been shed.

Pasture is key

For sheep and goat owners, pasture management is intertwined and connected with parasite management. It is a complicated topic with no single correct answer or strategy.

More help and information is available at: http://go.osu.edu/parasitecontroltools.

Another good publication covering this topic in greater detail is: Internal Parasite Control in Grazing Ruminants by Joseph Tritschler of Virginia State University, available online as a pdf publication.

What’s a ‘safe’ pasture?

Young, weaned lambs and kids should be moved to safe pastures.

A safe pasture, in terms of parasite management, can be defined as a pasture where the number of L3 larvae are likely to be very low or non-existent. It is either a pasture where very few eggs were deposited in previous grazing passes or it is a pasture paddock where a long enough period of time has passed that most or all of the L3 larvae have died.

If a pasture paddock was grazed the previous fall, for example in early October, it will probably be the end of June before most or all of the L3 resulting from eggs deposited at that time have died.

A safe pasture can be one where L3 larvae have been killed or removed. For example, if a previously grazed area is tilled up and planted into an annual forage crop, that will be a safe pasture. Tilling the soil will destroy the L3 larvae.

A no-till planting is more risky. I have seen Haemonchus contortus survive in those situations.

Haymaking is a strategy that can remove the L3 stage larvae but probably can’t be counted on to produce a 100% reduction.

For goats, which are top-down grazers, a safe pasture contains lots of brushy plants or taller forage where goats will graze with their heads up and above a 6 to 8 inch height.

This does not work with sheep, which will put their head down even in taller forage.

— Rory Lewandowski