How do I select my sheep/goats to be more resistant to internal parasites (worms)?


According to the 70:30 rule, 30 percent of the flock/herd is responsible for shedding 70 percent of the worm eggs onto pasture. If you can eliminate these 30 percent (or at least not save replacements from them), you will move towards a more resistant flock/herd. Selecting the most resistant animals (the ones with the lowest fecal egg counts) will result in even more rapid progress.

Where internal parasites are concerned, there are two traits of importance: resistance and resilience. Resistance is when an animal prevents parasites from being established, inhibits their egg laying ability, and expels worms from its gut. The immune response varies by species and breed (type). Resistance is estimated with fecal egg counts: number of worm eggs (strongyle-type) per gram of manure. It is assumed that more worms lay more eggs and vice versa. You get a number like 1000 eggs per gram (EPG). What is high depends on the parasite species, individual animal, and parasite challenge. Fecal egg count is a moderably heritable trait. Heritability is usually higher in more resistant breeds and lower in goats.

Resilience is when an animal is able to produce and stay healthy despite having a worm burden. Resilient animals do not require deworming. They are not anemic. They maintain body condition. They scour less and have cleaner backsides. A resilient animal may or may not be resistant. While it may not require deworming, it may still be shedding a lot of eggs onto the pasture. At the same time, there is a correlation between resistance and resilience. Resilient animals usually do not have as high fecal egg counts as those that require deworming. Resilience is less heritable than resistance. It is harder to quantify and has less variability, making it harder to select for.

There are three primary ways for sheep/goat producers to select for parasite resistance in their flocks/herds: on-farm selection, central performance testing, and estimated breeding values.

On-farm selection
On-farm selection is the most common method of selection. The goal of on-farm selection is to select the best animals for breeding and to cull the inferior ones: breed the best to the best. It works the same for all traits. Genetic progress (improvement) depends upon the percent of variability that is due to genetics (heritability), the reach (the difference between the selected animals and the rest of the flock/herd), and the generation interval (how long you keep animals in the flock/herd).

Many sheep/goat producers select more resilient animals by eliminating the ones that require more frequent deworming. Selecting animals with better FAMACHA© scores, body condition scores, or rate-of-gain is selecting for resilience. This will make some progress towards establishing a more resistant flock/herd, but selecting on the basis of fecal egg counts is a more accurate method. Fecal egg counting is costlier and takes more work, but it will result in a more resistant flock/herd faster. It should also add value to seedstock.

In order to accurately select for parasite resistance (fecal egg counts), several things must happen. Obviously, animals need to be individually identified so that samples can be matched to individual animals. It is important to compare animals in the same contemporary group. A contemporary group is a group of animals of similar age that have been managed the same. You cannot compare lambs/kids born in January and creep-feed with those born in the spring and put out to pasture with their dams. You cannot compare lambs/kids from dams that have been separated for preferential feeding with those that haven't. Artificially reared lambs/kids don't fit into contemporary groups.

You must have enough animals to compare. Twenty or more is ideal. You must have a sufficient challenge. It is recommended that the average fecal egg count of the group of animals being compared be at least 500 epg. Higher is better. You must have a sufficient spread in the fecal egg counts; for example, 0 to 4000 or 250 to 6000 EPG. Selecting for parasite resistance isn't for the faint-hearted, as some animals will become clinicallyl parasitized during the selection process.

The age at which to select for parasite resistance varies. Obviously, it is affected by the age at which lambs/kids first become exposed to parasites (begin grazing). You can select for resistance at younger ages in more resistant breeds (as early as 40 days), but for most breeds, selection decisions should be made post-weaning when lambs/kids are 90 days of age or older. In fact, goats develop immunity to parasites later than sheep and you may not be able to make accurate selection decisions until they are six months of age or older. Using fecal egg count data from periparturient females has been proposed as an alternative to evaluating offspring, as they are less susceptible to the negative effects of parasitism.

When selecting for parasite resistance, it's important not to select against reproduction. Ewes/does raising triplets usually have higher fecal egg counts than those rearing twins and especially singles. High producing dairy does are more likely to need deworming. Triplet born lambs/kids usually have higher fecal egg counts than twins and twins usually have higher egg counts than single births.

Central performance test: "ram/buck tests"
A central performance test is when rams/bucks are brought to a central location for performance recording. Because the rams/bucks are managed the same at the test station, they are considered to be in the same contemporary group. Thus, differences in performance are attributed to genetics.

There are several ram/buck tests in the US. Several evaluate the rams/bucks for parasite resistance (fecal egg count). These include Southwest Virginia AREC (rams), West Virginia University (bucks), University of Florida (ram), and Oklahoma Buck Test. If you don't participate in a ram/buck test, you can still purchase males with performance test data or purchase from flocks/herds that participate in central performance testing.

Estimated Breeding Values
Estimated breeding values or EBVs estimate the genetic value of an animal. They are the most accurate method of selection because they include records from other animals (relatives) and are better able to eliminate environmental influences. For sheep/goats in the US, estimated breeding values (EBVs) are provided by the National Sheep Improvement Program (NSIP). They are calculated by Sheep Genetics Australia. NSIP is mostly for purebred flocks/herds, though animals do not need to be registered. Crossbreds cannot be compared to purebreds. Different sheep breeds cannot be compared.

In order to get fecal egg count EBVs calculated for your sheep/goats you need to submit fecal egg count data for lambs/kids at weaning and post-weaning. Fecal egg count EBVs are reported as percentages. Unlike most traits, a negative number is more desirable. A fecal egg count EBV of -50 percent means the animal's fecal egg count is 50 percent less than the breed average. Each EBV will have an accuracy value. A higher value means there is more confidence in the EBV. If you don't participate in NSIP, you can still purchase rams with favorable EBVs for parasite resistance.

In order to compare the parasite resistance of animals in different flocks/herds, producers must share genetics with one another; for example, use the same ram/buck (or son) for breeding. Without genetic connections between flocks/herds, EBVs are limited to within-flock/herd comparisons, which still have value.

When selecting for parasite resistance, the most important thing is to breed with rams/bucks that have been proven to be more resistant. A ram/buck will influence the genetics of far more offspring than a ewe/doe. The females that are the most susceptible to parasites should be considered for culling. Select the best males; cull the worse females. It makes little sense to select resistant females only to breed them to a susceptible male or one with unknown genetics.

You can learn to do your own fecal egg counts, but it's probably better to have them done at a lab, especially if you are submitting data to NSIP. Three land grant universities offer low cost fecal egg counting (only $5/sample) for genetic selection and fecal egg count reduction testing. Sometimes, there are grant projects that can provide free fecal egg counting to participating producers.


Additional resources

Genetic selection: using crossbreeding and estimated breeding values
On-farm selection for resistance to parasites
Blueprint for selecting parasite resistant sheep: a shepherd's perspective