How To Get Started Breeding Parasite Resistant Sheep

If you are interested in reading other parts of this series:

Barber Pole Worm Series Part 1-The #1 Killer Of Small Ruminants

Barber Pole Worm Series Part 2-Controlling Worms Without Dewormer

Barber Pole Worm Series Part 3-How To Properly Deworm

Barber Pole Worm Series Part 5-How To Do a Fecal Egg Count

Looking to breed a parasite resistant sheep?

You probably know just how important it is that we breed out our reliance on chemical dewormers. Thankfully, breeding for parasite resistance is something that is well within the abilities of every small ruminant owner. That is if (and only if) you are willing to keep good records and cull animals that should not be bred.

Record keeping and culling is a must, even if you originally purchased parasite resistant stock. Parasite resistance is something that is relatively easy to select for. Conversely, parasite resistance is something that can easily be bred out if you are not actively selecting for it. After all a parasite resistant sheep and a susceptible sheep might look and perform identically if we deworm all our animals frequently

Where do we start?

First and foremost, buy the best ram you can afford. It is much cheaper in the long run to buy a good quality ram to improve your flock than to work your way up from the bottom. At this point in time, there are many breeders out there who are seriously selecting their flocks for parasite resistance.

Before you purchase the ram ask them for their records and proof of their parasite resistance. If the farmer is actually actively selecting for parasite resistance, then they will have data to prove it. Otherwise they might be breeding ‘parasite resistant’ animals simply because they own animals of a breed (such as the Katahdin) that has a reputation for parasite resistance. Not every flock of a ‘parasite resistant breed’ is actually parasite resistant. Thus it is important to verify that the breeder is actually making tangible efforts to select for this trait.

If you have a breed of sheep that submits data to the National Sheep Improvement Plan (NSIP), look for a ram from a breeder that has NSIP data on their sheep. This will provide you valuable parasite resistance data, and also other useful production data on the sheep. This is one of the best ways to make large improvements on your flock quickly.

‘Buy local’ applies to sheep as well as produce

There is one more thing to consider when buying in sheep. It is best to select sheep from a breeder that is local or semi-local, if at all possible. They have already been selecting animals that perform well in your local environment.

Why is buying local sheep so important? Because parasite resistance is relative. For example. A parasite resistant sheep kept in a feedlot in Montana is likely NOT parasite resistant on a pasture in Florida. Without intervention, that ‘parasite resistant’ sheep might be dead of parasites within a month of moving to that Florida pasture. Why? Because the parasite pressure in Florida is exponentially higher than in Montana.

There are a few other things we need to keep in mind when selecting for parasite resistance.

When we are evaluating animals for parasite resistance, there are two kinds of immunity we need to consider. Innate immunity is the immunity the lambs are born with. They are just plain less hospitable environments for parasites from the day they are born. The second type of immunity is acquired immunity. This is the immune mediated response similar to how you get sick from a cold virus only once. This is a learned immunity to parasites.

For most parasite resistant breeds acquired immunity starts to kick in at 3-4 months of age. For non-parasite resistant breeds it is closer to 6-8 months of age. This is useful information to use both as you are selecting replacement ewes, but also as you are making management decisions.

We also need to keep in mind is that sheep can be both resilient and/or resistant to parasites. Resilience and resistance are not the same thing. A resilient sheep is one that is better than average at withstanding a high parasite load. They are better at making more blood faster. A resistant sheep has a low parasite load. Their immune system is really good at getting rid of barber pole worms.

Both of these animals will look identical out in your pasture. They will have identical FAMACHA scores when you check them for anemia. However, we would prefer to select for resistance because resistant animals are benefiting the rest of your flock. Resistant sheep are not shedding tons of live parasite eggs all over your pastures but resilient sheep still are.

Breeding for parasite resistance is done through a lot of culling

The bare minimum approach to selecting for parasite resistance includes culling poor performing animals. Cull ewes and lambs that require more deworming than the rest of your flock. Also cull any animals that seem to grow slower, or just do poorly compared to the flock average. Getting rid of the worst performing animals will slowly improve your flock.

To make progress faster. Do fecal egg counts (FEC) on your animals.

You can hasten the process to increased parasite resistance in your flock by regularly doing fecal egg counts (FEC) on your animals.

Cull animals that shed a lot of eggs around lambing, and keep those that don’t. We now know that periparturient rise is closely correlated with the overall parasite resistance of the ewes. This can also help you significantly reduce your farm’s overall parasite load.

Cull animals that consistently shed a high number of eggs, keep those that don’t. Keep in mind that animals in different life stages (eg. a lactating ewe), and high performing animals might have higher egg counts. It is important to track animal performance and FEC long term (over months or years) rather than using just one snapshot in time.

The thin ewe with a higher FEC who is feeding quads, might be just as resistant as the lower FEC chubby ewe feeding a single lamb. Her body is just much more stressed, and she is working much harder for you. If you checked again when both ewes are dry and in good body condition, you might find that your harder working ewe has a lower FEC than the other ewe. This is why we said ‘consistently shed a high number of eggs’.

Keep replacements from those that have less than average parasite egg shedding and who require less deworming than the average. Keep in mind the 80/20 rule. 20% of your animals are shedding 80% of the parasite eggs on your pasture. Getting rid of that 20% (or at least not keeping offspring from them) will reduce the parasite load on your pasture by 80%. Cull, cull cull, and before you know it you will have a more resistant flock!

One last thing to keep in mind

If we are going to be selecting our most parasite resistant animals to keep, we need to make sure that we are making fair comparisons. We know that bigger, larger sheep are better at withstanding parasites than smaller animals. A large single lamb will have less problems with parasites than a group of quads all other things being equal. Not because the single lamb is actually more resistant, but simply because it is just much bigger.

Another example of an unfair comparison would be comparing a January born lamb to an April born lamb. Don’t do it. Why? The January born lamb was born when there weren’t many parasites around. Your April born lamb will have been exposed to many more parasites. So for them to look like they ‘performed equally’ the April born lamb would actually need to have performed much, much better.

We need accurate data for it to be useful

So to get useful, accurate comparison data to select for parasite resistance, it is important to look at a contemporary group of lambs. That is a group of lambs that were born about the same time, from the same size litters, and who are fed and housed together in the same environment.

Take FEC from your lambs several times over the course of the summer, and use those numbers to select lambs to keep. Choose replacement lambs who have both high growth performance and consistently low egg counts.

Don’t base your choice on just 1 FEC

Look at how the lamb compares to the group average, and also how it compares to itself over time. Did your youngest/smallest lamb have the highest FEC in June at weaning, but then the lowest one a couple months later when the worm burden should have been greater? If so, you might have seen their acquired immunity kick in. Did you have a lamb who’s FEC stayed the same or went down when everyone else’s went up? This lamb probably has excellent parasite resistance. Ideally you want to select lambs that have been consistently out performing others all season.

The one drawback to using FEC data is that it requires either more time or expense than just culling based on FAMACHA scores. Nonetheless, it is worth the investment. It will get you to your goal faster.

We would also like to note, that if you wish to use FEC to select within your flock, but feel that you cannot afford to pay a vet or lab to perform that many FEC for you, this tool is not out of your reach. With a basic microscope and some practice, it is actually quite easy to perform FEC for yourself at home. That is going to be the topic for our last article in our barber pole worm series.

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