Worm resilience is the ability of the sheep to grow in spite of worm burden. Worm resistance is the sheep using natural antibodies to fight worms.
Positives for resilience, the sheep are not using any energy to produce antibodies putting all energy into growth. Negatives are larger worm residuals on the pasture.
Positives for resistance are less worm contamination on pasture. Negatives are sheep transferring energy from growth to producing antibodies.
Our aim is to have a ewe that never needs drenching and their lambs minimal drenching with weight gains that are industry acceptable in conjunction with other physical traits.
It is widely now understood that there is a strong genetic trait for worm resilience, the following excerpt is from Beef and Lamb NZ. There are differences between individual animals, and between breeds, in their susceptibility to or tolerance of worms. For example, Merinos are generally more susceptible to worms than the down land Romney or Composite breeds of sheep. These differences are inherited and can be altered through genetic selection. Research on breeding and selection for resistant or tolerant animals has identified sires that produce progeny more resistant or tolerant to worms. Worm Wise resource book Beef and Lamb NZ
In the year 2000 the ARDG and WRIG with advice from AgResearch scientists set out to find a protocol for breeding worm resilient sheep. After six years with funding from MAF the protocol was set.
Ram and or ewe lambs are weaned, weighed and drenched, depending on FEC counts somewhere about 6-8 weeks the lambs are weighed again and a faecal egg sample is taken. Any lamb that hasn't put on the same weight gain as the control group (which is drenched every four weeks) is drenched. This procedure is repeated every 10-14 days until the mob is down to about 10% of the original. At this point all lambs are drenched; the last 10% sometimes go up to 16 weeks without a drench, whilst achieving substantial weight gains.
DNA breeding values are now available from Zoetis on FEC.
Vet Mark Anderson from Vets North Helensville provided the following quote in October 2021.
Breeding for Parasite Resistance
Gut parasitism is a major problem for sheep farming. It has been estimated that up to 30% of modern sheep production is reliant on effective parasite control. The shift from wool production to lamb production, and increasing intensification, are all increasing the pressure of worms on the farming system.
Unfortunately chemical control of worms (drenching) is deteriorating rapidly with the spread of multiple drench resistant worms through sheep flocks. Triple drench resistance in worms is now not uncommon and although “new actives” are available they are much more expensive and will only have a limited life before resistance get to them as well. The only sustainable solutions are through management, and worm resistant sheep is a major tool for this.
We know that there is a genetic component to resistance, so it can be selected for. As with all genetic traits the rate of progress depends on a number of factors including, how long you have been selecting for this trait and how serious you are about it (how much emphasis you put on this trait as opposed at all the other traits that you are selecting for).
When selecting a breeder you need to ask the hard questions about when they started selecting, how they are selecting and what future plans do they have. A group of ram breeders who are serious about selecting for parasite resistance have formed WormFEC Gold.
WormFEC gold members have a track record and a passion and the knowledge to produce rams with a genuine genetic advantage. Remember however that using one “resistant” ram will not suddenly mean that you do not have to drench the lambs. This is a long project with incremental gains but one that could ultimately keep you able to farm sheep profitably.
Mark Anderson MVSc
The following is further information written by Mark Young in 2008
The reference to 'Dag scoring' is a visual score given to the animals prior to crutching based on the absence or amount of dags present. This is another trait our animals are ranked on
1 of 2 Resilience to internal parasites Part 1. Rationale SIL Technical Note Relates to: Selection to reduce the impact of internal parasites on sheep performance Written by: Mark Young Date: 27 January 2008 Summary •
Resilient sheep may carry worms in their gut, but their performance is not noticeably affected. These worms are a source of contamination for other sheep who may suffer reduced performance because they are not resilient. • Resilience to worms is less heritable (c.15%) than resistance to worms (c.30%). • Genetic assessment of resilience requires a specific protocol to be followed for worm challenge and data collection. • Genetic improvement through selection offers one of the best long-term solutions to the increasing problem of drench resistance by worms. Background Control measures for internal parasites, or worms, have a very significant effect on farm profit and on farm management. Susceptibility to worms leads to loss of production and to the maintenance of a large population of worm larvae on pasture that can reinfect stock later. Increasing incidence of worm resistance to drench is looming as a critical limiting factor to productivity on many farms. One strategy to reduce the impact of worms on animal performance is to select sheep that “tolerate” an infection with no noticeable drop in performance due to susceptibility or to mounting a resistance response, to the worms. Such sheep are “resilient”. The main criterion defining this is productive performance in the face of a challenge. Some of these sheep may carry large worm burdens and be a significant source of infection for other sheep. An alternate strategy is to select for “resistance” whereby the host, the sheep, “fights” the worm infection. This has been shown to incur a cost to the sheep and can lead to a reduction in performance as the sheep diverts nutrients (energy & protein) to this task. There is some controversy with regard to selecting for “resistance” or for “resilience”. However the two strategies share some common features. Those wanting to consider and compare the two strategies are directed to the documents under “Related information” below. Definition of resilience In practice, resilience in the face of a challenge from worms is measured as no noticeable drop in production. Parasite loads are not relevant, so some resilient animals may carry large worm burdens. The important thing is that they are performing as well as animals without parasites. Responses to challenge can be dependent on the level of challenge. Animals that appear to be resilient at low levels of challenge may be susceptible or exhibit resistance at higher levels of challenge. We do not know enough about the ways animals cope with or fight worms. However, we can use existing knowledge to enhance genetic merit for resilience through selection. Some of the genes selected for may be involved in both resistance and resilience. 27-jan-2008 Resilience Background001A06.doc mjy - sil 2 Biology and genetics of resilience As animals grow from birth to adulthood their ability to cope with disease typically increases as the immune system matures and is challenged. Since farmers market many animals before maturity, the impact of parasites on performance during early life is critical. Prior to weaning, the impact of parasites on young lambs is reduced due to passive immunity the ewe passes through her milk and by the dominance of milk in the diet reducing the rate of infection from parasite larvae on pasture. Later, the lamb is challenged more and must rely on its own physiological processes to reduce the impact of worm infections. Assessment of resilience should take place at times similar to those when worms impact on commercial animal production. Resilience is heritable (c.15%) but at a lower level than resistance (c.30%). Higher figures can be obtained when consistent challenges are obtained from year to year. Since resilience is measured in terms of productivity, it is positively related to other important traits under selection. It has a weak association with dag score (more resilient = tend to have fewer dags). Measuring resilience in practice It is more complicated to assess resilience than resistance. Breeders should be aware of this when contemplating selection to increase resilience. Like selection for resistance, breeders wishing to select for resilience must be registered before genetic evaluations can be carried out by SIL. This is to ensure data collected meet minimum standards. SIL uses a protocol developed by AgResearch. Measuring performance under challenge is the key to selection for resilience. The best time to do this is after weaning and the most practical criterion is liveweight gain. Resilient animals continue to grow well when other sheep show reductions in growth rate due to worms. Animals are assessed for two critical measures. Firstly their gain from the beginning of the challenge period until the first sub-group require drenching because their growth rate has dropped below that of unaffected animals. Secondly, for the time after the start of the challenge before they need to be drenched. This time is longer for those animals that show signs of a drop in performance later. A separate SIL document describes in detail the protocol for assessing the resilience of sheep. It is titled “Resilience to internal parasites – Part 2. Protocol for measurement”. Related information “Internal parasites – Selection to reduce drench requirements of sheep” considers the different ways to reduce the impact of worms on sheep performance. “Selection to increase resistance of sheep to internal parasites” and “Internal parasites – WormFEC protocol” consider selection for resistance to worms. “Dag Score” describes how to reduce dags through selection. All documents mentioned are on the SIL website (www.sil.co.nz) under Technical Information. Need more information? • Contact your SIL bureau, local SIL adviser or call 0800-745-435 (0800-SIL-HELP) or email to firstname.lastname@example.org • Queries related to collecting data, the genetic evaluation of resilience and registration for this service can be obtained from Neville Amyes at AgResearch (07-838-5421) or by email to email@example.com 17-May-06 Resilience Background001A06.doc mjy - sil
Breeding for Parasite Resistance