By Dave Scott
NCAT Agriculture Specialist
Published June 2018
Genetic selection can limit the occurrence of bottle jaw in your flock. This is an advanced stage of Barber Pole Worm infection in which the ewe FAMACHA scored F5. Early spring periparturient rise and stress of lactation pulled this ewe down. Note, in contrast to her lamb, the droopy ears, swelling under the jaw, and thick upper eyelids—all signs of a sheep not feeling well. Photo: Dave Scott, NCAT
Genetic-selection tools can lift a seemingly grim parasite situation into a landscape that has hope. It will not happen in a year, but after several generations of careful breeding, you will begin to see some improvement in the resistance of your flock to the Barber Pole Worm, Haemonchus contortus. As in any other successful breeding program, focus on selecting for a balance of resistance and the other important economic traits, such as the number of lambs born, and maternal weaning weight. There are three genetic-selection tools we producers can use to lessen the impact of Haemonchus in our flocks: fecal egg counts, FAMACHA© scoring, and National Sheep Improvement Parasite Estimated Breeding Values. This tipsheet is intended to supplement the informative ATTRA publication, Tools for Managing Internal Parasites in Small Ruminants: Animal Selection.
When selecting individuals within flocks for potential replacements, there is one primary rule: your animals must be exposed to a parasite challenge. For example, if your average fecal egg count is not greater than 500 eggs per gram (epg), you do not have enough infection in the flock to genetically select for Haemonchus-resistant sheep. Similarly, if you are using FAMACHA scores to select for breeding replacements, at least 25% of your lamb population must score F3 to F5 (Morgan, 2018).
This rule also applies when purchasing breeding stock from other flocks. If your breeder is advertising sheep that are seldom dewormed while on irrigated pastures, you don’t know if they are parasite resistant or if they have never been challenged. For this reason, parasite challenge is a requirement of all NSIP-proven sires. Sometimes the most obvious is forgotten.
The sires we use provide one-half of the genetic makeup of our flocks. A small number of rams are thus largely responsible for the genetic resistance our flocks have to the Barber Pole Worm. In fact, in four generations, 90% of the genetics is determined by the sires that we use (NSIP, 2015). Although not as important from an overall perspective, the ewe also determines one-half of the genetics of her own offspring. Selection for parasite-resistant sheep is well worth the trouble of identifying superior individuals in our flocks. Here is how:
|Lamb ID||7/27 FEC||8/13 FEC||Change in FEC epg||Replacement KEEP/Cull||Reason|
|5987||1200||4520||3320||Cull||Change in FEC too high|
|5741||1200||1500||200||Keep||Low change in FEC|
|6545||500||1375||875||Keep||Low change in FEC|
|6782||2000||2200||200||?||Low change, but too high FEC?|
|6627||3700||3300||-400||Cull?||FEC too high|
|6710||280||490||210||Keep||< 500 epg|
FAMACHA is another technique that can be used to identify sheep that are less susceptible to the Barber Pole Worm. It actually measures the sum of resistance and resilience, but for large flocks, it is a practical alternative to egg counts. You can FAMACHA score 150 animals an hour, which is 25 times faster than performing fecal egg counts. Here is one procedure:
FAMACHA scoring is fast, easy, and definitive. It can be used as a practical alternative for selecting ewe and ram replacement off spring within a large flock. Photo: Rich Myers, NCAT
Dynamic biology is the driver of parasite infection. It is very hard to prescribe a certain set of rules, so you must monitor and adapt to what you see. In some years, on some farms, the F4 and F5 cases may continue for 60 days post-weaning. As an example, here is what we had at Montana Highland Lamb (total lambs = 258) in a particularly bad year:
|FAMACHA Score||7/23 Weaning||8/13||9/4 Off Pasture||9/4||9/11||9/26||10/15||Total|
|F4 and F5||0||15||29||2||4||0||50|
As previously mentioned, sires have an incredible amount of influence on flock genetics. Estimated Breeding Values developed by the National Sheep Improvement Program provide the fastest way to make tangible improvements in parasite resistance. Coupled with grazing practices to limit worm ingestion (as described in the tipsheet Grazing to Control Parasites), these two strategies represent a long-term solution to Barber Pole Worm control. Purchasing outside rams with elite Estimated Breeding Values in parasite resistance (PFEC) is not only easier than selecting with fecal egg counts and FAMACHA scoring, it is a much more powerful way to make genetic progress in this trait. Currently, Katahadin and Polypay breeds have estimated breeding values for parasite resistance. Hopefully, in the coming years, more breeds will follow.
To select sires for ewe replacements, determine your production goals for the primary traits you wish to improve. Since genetic progress will be slower by selecting for a large range of traits, try to focus on two or three of the traits in the flock with the highest priority. For example, a sheep producer primarily selling lambs on irrigated pasture needs a large number of lambs born, enough milk in the ewes to raise twins successfully, and parasite resistance in both the lambs and the ewes. If losses due to the Barber Pole Worm are significant, make that your number-one priority and choose a ram with a parasite PFEC EBV in the top 20% of the breed. The number of lambs born (NLB) and maternal weaning weight (MWWt) can be assigned secondary priorities, and rams with trait EBVs in the top 40% nationally should also be included in your index. These rams can be purchased through private treaty (respective breeders are listed in the NSIP database) or in national ram sales. The searchable NSIP database and a guideline on how to use it are available.
The Polypay and Katahdin breeds that currently have PFEC EBVs will admittedly not fit into everyone’s breeding program. The sheep industry needs to push for more breeds to develop PFEC-proven rams. Until then, breeding-flock selection using fecal egg counts and FAMACHA within flocks and in flocks with contemporaries (a minimum FEC mean of 500 epg and, ideally, similarly-sired offspring) are the tools that can be used. If not ideal, they are still a giant and pragmatic step forward.
Selecting for genetic resistance for parasites is one of the leading tools we can employ to combat the Barber Pole Worm. For flocks on irrigated pasture, future success may depend upon it.
Special thanks to Dr. James Morgan, Katahdin Breed Representative, National Sheep Improvement Program; and Dr. James Miller, Veterinary Pathologist, Louisiana State University, for their help with this tipsheet.
Gauly, M., and G. Erhardt. 2001. Genetic Resistance to Nematode Parasites in Rhön Sheep Following Natural Infection. Veterinary Parasitology. Vol. 102, Issue 3. p. 253-259.
Morgan, Jim, Katahdin Breed Representative, National Sheep Improvement Program. 2018. Personal communication.
National Sheep Improvement Program (NSIP). 2015. NSIP Ram Buying Guide.
Simple Genetic-Selection Strategies to Manage the Barber Pole Worm
By Dave Scott, NCAT Agriculture Specialist
Published June 2018
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