Buying Parasite Resistance: considerations for greater success

  • Post author:
  • Reading time:27 mins read

Often producers interested in adding genetics for parasite resistance to their flock are faced with deciphering the fecal egg count (FEC) EBVs listed on a stall card or catalog at an auction (online or in person). Is that enough information to identify a parasite resistant animal? In my experience, no.

Estimated Breeding Values (EBVs) provide a wonderful tool to predict animal performance, but when it comes to parasite resistance, the stated EBV often doesn’t provide the whole answer. More than other traits, FEC EBVs can be subject to some pretty big changes over time. Partly that’s because there is such a wide range in values, from -100 to +400 and more. Parasite resistance may be the hardest trait to measure because it involves a highly individualized immune response and because there can be tremendous variations in the FEC used to measure the trait. Like all EBVs, they become more accurate as more data (in this case FEC) is added to the system. For a buyer, it can be pretty disappointing to purchase a high FEC EBV ram only to have it tank upon collecting data on his offspring. Understanding the nuances of the EBV and accuracies will help avoid this.

Range of EBVs: FEC EBVs estimate the percentage of change from the breed average in the FEC of the offspring. The change can be either above (+) or below (-) the breed average. The lower (or more negative) the EBV the better if you’re looking for parasite resistance (-100% is as good as it gets). It means the animal you’re looking at has the ability to decrease the FECs of their offspring. For example, a ram with a -90 PFEC EBV is predicted to reduce the post weaning FEC of his lambs by 45%, since a lamb gets half of its genetics from its sire. The EBV doesn’t correlate to an exact level of parasite resistance or a specific FEC, but instead is used to identify relative differences between animals.

Immune Response: When first exposed to parasite larvae, the body must first recognize an invader and then mount an immune response. This immune response has to occur in order to identify and select for parasite resistance. While each animal is different, the FEC of each lamb often “spikes” or increases as their immune response is triggered, and will usually return to a lower level in time. When the spike occurs (age and/or time of year), how high the spike is and how fast they return to normal varies by lamb and is affected by genetics, nutrition and other environmental factors. When performing FECs, all the fecal samples on a group of lambs are collected on the same day, yet each lamb could be in a different stage of developing and expressing their immune response, which affects their FEC. For that reason, two FECs, at least a month apart, are much more predictive of the true level of parasite resistance. Even if only one FEC fits the submission criteria for NSIP, the raw data is still useful. The real test, though, is comparing the average FEC of each ram’s offspring.

Variations in FEC: FEC values can range from 0 epg (which simply means “no eggs detected”) to over 20,000 epg. Management and environment can affect the FEC. For instance, lambs that are older and heavier when first exposed to parasites, or when a FEC is collected, will typically have a more mature immune system resulting in lower FECs. This can make them appear more resistant than they actually may be. Lambs with better nutrition (i.e., those given supplemental feed, or singles vs triplets) are generally better able to withstand a parasite challenge. They will often have a lower FEC while being supplemented, which again can mask an animal who is less resistant. FEC EBVs are based on comparisons between an animal’s individual FEC and the average for the group. For example, a lamb with a FEC of 1000 epg may be considered high if the group average is only 500 epg, but the same FEC would be considered low if the group average is 5000 epg. In order to correctly identify differences between animals, a challenge, or group average of at least 500 epg (more is better) with a range of at least 1000 epg between animals is essential. If the group average is less than 500 epg, it will still be used by NSIP to calculate the EBV, but that EBV will be much less reliable. The number of animals in the group matters, too. It’s much easier and more accurate to identify differences in a group of 50 or 100 lambs managed the same than in a group of six.

Pedigree: Parasite resistance is a complex trait associated with multiple genes. The more generations of negative FEC EBVs, the more likely that the genetics for the trait have been passed down to the next generation, and the more reliable and predictable the EBVs become. Ideally, there will be multiple generations of ancestors with negative FEC EBVs. Reliability and predictability are also increased when there is less variation in the EBV values between individual ancestors and between FEC submissions. Note though, that an animal who was only recently enrolled in NSIP or who hasn’t had FECs submitted in the past may have less predictability but may still be resistant, so don’t rely entirely on the pedigree. The NSIP searchable database is a valuable tool when considering pedigree EBVs.

Accuracy: NSIP provides an accuracy value for every EBV (it’s the number in grey below the EBV value on the Individual Listings report and in the searchable database). The accuracy is the correlation between the estimated breeding value and the true breeding value. Accuracy is improved every time more data (FEC) is added on an animal or it’s close relatives. It surprises some people to learn that an animal can have FEC EBVs without ever having any FECs submitted themselves. NSIP calculates a “pedigree” EBV if parents or close relatives have had FECs submitted, although the accuracy of the pedigree EBV will be lower. A “pedigree” EBV will
change (up or down) over time as data is submitted on the animal and close relatives. As a rough estimate, an accuracy value for FEC EBVs less than about 55 often indicates a pedigree EBV. That accuracy increases to around 60 as actual FECs are submitted as lambs, and to over 70 as FECs are submitted on offspring. Don’t confuse accuracy with parasite resistance. A higher accuracy is good, but doesn’t correlate to the degree of parasite resistance; it simply means that the FEC EBV value (whether high or low) is predicted to be more accurate. For instance, a ram whose offspring have had FECs submitted will always have higher accuracy than a ram lamb, but the ram lamb may be more (or less) resistant.

How Much Parasite Resistance: Consider how much parasite resistance you actually need, given your management and environment. If you live in Vermont or Minnesota, or if you have excellent forage, or 20 ewes on 60 acres (low stocking density), or if you lamb in late fall or winter so your lambs are 4-6 months old before they’re exposed to parasites or you raise your lambs in the barn and/or feed a lot of supplements, parasite resistance may not be as critical. It’s possible that you could make do with a less negative EBV or with less accuracy in this trait. However, if you live in a hot, humid part of the country (the south for sure, but also other parts of the country), or if you wean lambs in the peak worm season, are strictly grass based, have a high stocking density, and/or poor-quality forage, parasite resistance may be very important to prevent excessive pasture contamination and lamb losses. In that case, a ram with a lower FEC EBV and more accuracy for that trait would be a better choice. Consider also how fast you want to make improvement relative to your other breeding goals. You can generally make faster progress with more extreme EBVs (top 10% of the breed), but beware of single trait selection. If it becomes a life-or-death situation on your farm, consider selecting a proven ram with excellent (very low) FEC EBVs and high accuracies for those traits.

Example: Here’s an example of how the FEC EBV and accuracies can change over time as more data is submitted. The chart below shows two yearling rams I’ve used, as well as their parents and grandparents. PFEC EBV values are given for each ram at three points:1) pedigree EBV as a lamb before any FECs submitted; 2) as a lamb after two FEC submissions; and 3) as a sire after two FECs were submitted on their offspring. In addition, each ram’s sire, dam and grandparents are included.

Ram A has multiple generations of strongly negative FEC EBVs on both sides of his pedigree which makes his personal performance much more predictable. His EBVs haven’t changed a lot from his pedigree EBV as a lamb, through submission of his own FECs, to what he’s passing on to his lambs.

Ram B on the other hand, has much more variation in his pedigree and in his own EBVs. His pedigree shows much lower levels of parasite resistance (the EBVs are less negative) and the very high accuracy suggests that the lower parasite resistance is real. It’s possible that the higher estimation of resistance as a lamb based on FECs was due to better nutrition, timing of the immune response or other environmental factors. Once FECs were submitted on his offspring, his true genetic potential became more evident.


Ram A

Ram B








As lamb – pedigree







As lamb – after 2 FEC







As Sire – after 2 offspring FEC





















Sire’s Sire







Sire’s Dam














Dam’s Sire







Dam’s Dam







As you can see, there can be quite a bit of change over time. The more data behind the EBV and the better quality the data is (i.e., sufficient challenge), the more reliable the EBV will be and the less likely to change over time. As more offspring data is added in subsequent years and accuracy improves, Ram A’s FEC EBVs will probably remain consistently low. It’s hard to predict if Ram B’s EBVs will continue to go up (become more positive) or go down, but it’s very unlikely that he will ever be as strongly parasite resistant as he appeared as a lamb.

Now that you’ve identified a parasite resistant ram, can you expect him to solve all your parasite problems? Unfortunately, no. Remember that 70-75% of what you see on your farm and in your lambs is due to management and environment. And the ram is only half the genetics of each lamb after all. Buying a single ram, even one who is strongly parasite resistant, won’t solve all your problems in one generation, but it’s a good start and it will help. The more often you stack those genetics over multiple generations using different parasite resistant rams, the more reliable the resistance becomes. In order to really know if it’s working, you need to be able to evaluate your lambs. Keep records of FAMACHA scores, deworming history, body condition scores and/or FECs. Using more than one ram in separate breeding groups is the best way to observe sire differences. Only compare lambs that are around the same age and managed together in the same environment. Comparing lambs born this year in May to those born last year in March won’t tell you anything. There are too many differences in age, weather, nutrition and more to see differences in genetics.

What about the ram himself? If you’re buying a ram, especially a lamb, from an environment different from your farm you may need to make some allowances while he adjusts to his new environment, just like you would any new addition. The stress of moving, especially if combined with an auction or sale, new and unfamiliar feed and forage, and potentially a different subgroup of parasites can take a toll. Remember that “parasite resistant” doesn’t mean “parasite proof”. You can overwhelm even a strongly resistant animal under the right circumstances. His genetics won’t be affected though, and his lambs, born on your farm under your management should reflect the traits you’re seeking.

To summarize, if you’re looking to improve parasite resistance in your flock, start with the FEC EBV, but don’t stop there. Check the NSIP accuracy values. Look at the FEC EBVs of the sire and dam, and even the grandparents. Consider how much parasite resistance you need or want. Then identify available rams with those traits. There are many auctions, both online and in person, as well as on-farm sales. The NSIP searchable database can help. Once you’ve narrowed down your search, contact the producer. If you’re really serious, you’ll want to find out if FECs were submitted for the ram in question as a lamb. How many FEC collections were submitted? What was his FEC each time? What was the group average FEC, and what was the range? How many lambs were in the group (a larger number improves accuracy)? If it’s a mature ram, ask the same questions about his offspring. Did his EBV change after submitting data on his offspring? Don’t be afraid to ask the breeder for this information. If a producer has been collecting FECs, they will likely be happy to share this data with you. Collecting and submitting FECs is a lot of work and expense; they’ve gone to a lot of effort. An educated buyer who appreciates the data makes it worthwhile.

By: Kathy Bielek, EAPK Communications Committee

Additional Resources:


Print Friendly, PDF & Email