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How To Rid Sheep Worms
Without Drug Dependence

By John Kirchhoff

Kirchhoff Katahdins

A parasite is something that lives off of something else without making any useful return. That word describes many organisms; ticks, fleas, tapeworms, roundworms, brother-in-laws and so on.

Parasites cost the sheep producer money in two ways:

  • Parasites not only depress gains, but
  • Each pound of gain requires more feed and forage to produce than it should…A hidden loss so to speak. The average sheep producer in the Midwest or Southeast knows full well worms can turn a hidden loss into a dead loss when lambs become so anemic they die. Conversely, parasite control costs the producer money in wormers and time; parasite control that doesn’t work doubly costs the producer money.

However, I’m not suggesting you rush out and buy some high-dollar wormer and pump everybody full of it. Wormers work, at least initially, are expensive and time-consuming to use and then to top it all off, not all wormers are created equal.

In my opinion, that expensive bottle of wormer should be at the end of the control spectrum and not the beginning. Pasture management needs to be there.

Managing Your Pastures for Parasite Control

Common Sheep Stomach Worms, or 'Barber Pole Worms' (Haemonchus Contortus) Actual length of female, about 3/4-inch.
Common Sheep Stomach Worms, or ‘Barber Pole Worms’ (Haemonchus Contortus) Actual length of female, about 3/4-inch.

So, let’s start at the beginning. Your animals don’t just “get” worms. Eggs are shed from the host animal onto your pasture, they hatch into larvae, crawl up a leaf blade where they are swallowed by the grazing animal and start the cycle all over again.

Temperature, humidity, sunshine and grass height affect how easily and how quickly your animals become infected.

Fact: During hot, dry weather, eggs go dormant, thus reducing the infestation potential.

Often times, animals “get wormy” in hot dry weather, usually July and August. What actually happens is host animals shed eggs earlier in the spring and the warm moist conditions at that time allowed the eggs to hatch and infest any animals grazing that area, including the original hosts. By June, the worms have produced several generations of offspring, the animals are heavily parasitized and by July the physical signs are showing. Poor quality forage during that time of year only compounds the problem. Remember that summertime parasite control starts in spring.

Fact: In most cases, larvae will climb the leaf blade no higher than approximately two inches.

When working with my grazing system clients, I recommend leaving four inches of stubble when moving animals to another paddock. Four inches is kind of a magic number for several reasons. That height leaves enough leaf area to keep the photosynthesis process going, helps hold soil moisture and most parasite larvae will be below that height, thus reducing infestation opportunities. Sheep are better able to graze at shorter heights, which greatly increases the opportunity for larvae ingestion.

Fact: Most intestinal parasite’s life cycles extend no longer than 30-32 days.

This means if you graze a week on and three weeks off, there’s a very good possibility the larvae will be waiting for your animals when you move them in. Rest periods longer than a month allow many of the larvae to perish before host animals return.

With a rotational grazing system, a month is much too long between grazing periods in spring. The best course is to move animals through the system quickly, just taking the tops off. This strategy helps prevent grass from becoming overly mature and very few larvae are going to be that high on the plant.

Bear in mind that in times of drought, the eggs can lie dormant in the pasture for extended periods of time.

Managing Your Animals for Parasite Control

Fact: In most flocks, it’s common for 20% of the animals to shed 80% of the eggs.

ust as some breeds are more parasite tolerant than others, so are individuals within that breed. Parasite tolerance is a heritable trait and something to consider when selecting animals to cull or use as replacements. Good records on individual animals as to body score, hair or wool coat, eyelid color and other parasite symptoms make the selection process much easier.

Larval stage of Barber Pole Worm (Haemonchus Contortus) at tip of grass blade, waiting to be eaten by sheep.
Larval stage of Barber Pole Worm (Haemonchus Contortus) at tip of grass blade, waiting to be eaten by sheep.

Because weather has much to do with the severity of parasite pressure, look for animals that are better or worse than average. Some years only the most susceptible animals will have any problems while other times all will, with the most susceptible individuals having very serious problems.

Using the FAMACHA system to assess anemia will level the playing field between animals regardless of environmental conditions. It’s an excellent way to compare one individual to another.

Keeping replacements from animals more highly affected is a practice that perpetuates the profit-killing trait of parasite susceptibility. This is particularly important with ram selection. Remember the sire makes up half of your entire lamb crop while the dam is half of her offspring only.

When evaluating the sire, take into consideration the environment he’s been exposed to. An animal in a very small grass pasture all summer long is more likely to ingest larvae than a dry lot animal eating hay out of a bunk all summer. In addition, animals browsing on tall weeds and brush are much less likely to ingest larvae than are animals grazing pasture that looks like a golf course.

Fact: There is a pre-partum rise in worm numbers several weeks before giving birth.

Whatever the cause-be it hormone changes, increased nutritional need or a weakened immune system-animals begin shedding increased numbers of eggs at this time. This sets the stage for a parasite “explosion” months later after they have gone through several reproduction cycles. It’s a good idea to worm ewes several weeks before giving birth. Talk to your vet about which wormer to use since some products kill adult worms only while others kill dormant eggs also.

Fact: As animals mature, their natural immunity to parasites increases.

This is particularly true of lambs. Even breeds with high parasite-tolerance can have problems when less than a year of age.

This lends additional credence to the “leader-follower” method of grazing where young animals are run through a paddock first followed by old animals afterwards. Not only do young animals with higher nutritional needs get better forage, they are less likely to ingest larvae.

Managing Your Worming Program

When do you worm animals? If you wait until your animals show physical signs of parasites, you’re already way too late. It’s something akin to closing the barn door after the horse gets loose. Physical signs can include rough coat, diarrhea, going off feed, weight loss, staggering when walking, bottle neck and pale or white gums and eyelids, a sign of anemia. Waiting for those signs to show before treatment is like waiting for the ceiling to fall in as a sign it’s time to fix a leaking roof.

Many producers have found that worming every animal early and often seems to get the problem under control. However, this is merely a temporary cure. No wormer on the market will kill 100% of the parasites in an animal. When they worm “early and often,” all they are doing is selecting for resistance. The worms that survive the treatment impart resistance to their offspring and eventually the wormer that worked so well a couple of years ago doesn’t.

So, they switch to a product with a different mode of action that works at first but soon fizzles out also. Ever hear of “survival of the fittest?” That’s what happens when you use the same family of wormers repeatedly over a period of time.

Common Sheep Tapeworm (Moniezia Expansa) About one-half inch wide; up to about 20 feet long. Egg-filled segments break apart and can be seen in sheep droppings.
Common Sheep Tapeworm (Moniezia Expansa) About one-half inch wide; up to about 20 feet long. Egg-filled segments break apart and can be seen in sheep droppings.

There are many different brands of wormers, but all fall into three “families” depending upon their chemical make up. It’s just a matter of time before all current products are ineffective as parasites build resistance to them. The more often you use a product, the sooner resistance will develop. As you can see, depending upon chemical products for parasite control is a temporary measure at best.

There’s the dilemma; if you don’t worm you have problems and if you do worm repeatedly, it quits working and you still have problems.

I raise sheep and my mother raises goats, so I’m experienced with all of what I’ve written. With my operation, parasite control is very manageable. Yes, parasites are a problem, but actually rather minor.

In my pastures, I have a large number of paddocks; move the sheep promptly with fairly long rest periods and try to leave an average stubble height of four inches.

I have hair sheep, a breed with good tolerance to parasites due to their ancestry in humid, tropical regions favorable to parasites.

I usually worm all lambs two to four times a summer starting in May before symptoms are noticeable. Last year was a dry year and I wormed early born lambs once and summer lambs three times. Adult animals are not wormed and in the past, any animal that needed worming was culled from the flock.

This type of management does two things:

  • It selects for individuals tolerant to parasites, and
  • While they do harbor some worms, these worms make up a reproductive pool with genes sensitive to current wormers. This dilutes the frequency of resistant genes being present in succeeding generations of worms.

If I worm the ewes, I then select for resistant worms that produce resistant offspring.

I rotate anthelmintic families yearly to reduce the possibility of building a resistant population because rotating drugs with each treatment speeds the development of resistance. Use all three families consecutively and you’ve selected for resistance to all three in short order. Keep it up and eventually you’ll have worms resistant to all products available.

My mother hasn’t selected for parasite tolerant breeding stock, doesn’t rotate pastures, must worm adults and kids and now must use the most potent product of the Ivermectin family available. When it fails, I don’t know what she’s going to do. On the other hand, I still get good results with Levamisole, one of the oldest and possibly least effective products around.

Be wary when you purchase animals, especially those from farms or sellers you are unfamiliar with.

If you’re not careful, the purchase price could also include a belly full of resistant worms at no charge. There are methods of “cleaning” an animal out, but you should talk to your vet concerning this matter.


So there you have it. Don’t worm your animals just because a magazine ad or I said to. Talk to your vet, weigh the advantages and disadvantages and know what end result you want.

Pharmaceutical agents are a good source of information about their particular product, but keep in mind they sell drugs for a living. Talk to your vet for a more realistic, down-to-earth view before choosing a particular product. If you think you have resistant worms, a fecal egg reduction count or larval development assay should be considered. The latter will cost around $150, and while pricey, a jug of non-effective wormer or dead animal can easily exceed that amount.

I know people who quit raising sheep altogether because of animal losses and the time and expense involved in parasite control. It doesn’t have to be that way. I see parasite problems as the result of a series of management decisions. Change the way you look at parasite control and I bet the outcome will be much easier to live with.

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