Sheep worms – breeding worm resistant sheep

Page last updated: Friday, 30 June 2017 - 11:41am

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How to select worm resistant sheep

The overall worm resistance of a sheep flock can be increased in two ways:

  • measuring worm resistance in your own flock and selecting those animals with greater worm resistance to include in breeding programs
  • sourcing stock (usually rams) from breeders who have already been selecting for worm resistance. Sheep Genetics provide breeding values for a wide range of production traits on animals including WEC. This makes it possible to identify suitable rams from different breeders who measure WEC. Selecting for worm resistance can be incorporated into normal sheep breeding practices by comparing WEC of individual sheep. The number of worm eggs in the sheep’s faeces is a guide to the number of adult worms that have developed inside the sheep.

It will generally be most cost effective to concentrate on the young rams, with faecal sample collection at hogget age/weaning time. Combining information from measurements taken at weaning and hogget age will increase accuracy, but in most cases, one measurement will be most cost effective if pedigrees are available.

The choice between weaner and hogget samplings depends on the level of the local worm challenge. For example, in low worm challenge environments the natural immunity of the hoggets combined with the lower challenge can often result in inadequate faecal WEC levels to allow good differentiation between animals. In many situations, faecal WEC testing at hogget age could reduce costs because a proportion of the flock is likely to have already been culled based on other production traits. However, as worm resistance is more important in young lambs, it will be more beneficial over the long term to invest in measuring WEC at weaning. This information can be submitted to Sheep Genetics who will provide a breeding value for worm resistance at an early age to assist in making sound selection decisions.

Testing procedure

Checking the average WEC of the mob before testing

Sheep need to be exposed to worms in order to show their level of resistance. The best time to do this is during the green feed period in the winter rainfall regions of Australia. Worm egg counts of the test mob should be monitored as a group before commencing testing of all the individuals for selection.

Monitoring can be done simply by collecting three to five fresh dung pellets from individual piles deposited by 12-15 sheep in the paddock.

Procedure in lambs

In lambs the ingested infective (L3) worm larvae start to establish themselves in the intestines when the lambs become functional ruminants at about five to seven weeks of age in a normal season. It then takes about four weeks for these larvae to mature into egg producing adults. Therefore the lambs need to be at least 10 weeks old to have the WECs reflecting their worm burden. Faecal sampling should preferably be carried out on the day of weaning, or at least two weeks after weaning. Lambs should only be sampled if the average WEC of the group is at least 300 epg faeces. Thus, start monitoring faecal WECs of the lambs from 10 weeks of age, then re-check every week until the faecal WEC has reached an acceptable level. Don’t sample lambs during the first two weeks after weaning as the stress associated with weaning will temporarily suppress the immune response thus removing genetic differences between resistant and susceptible individuals. This practice was followed in the Rylington Merino flock that has been selected for low WEC at Mount Barker Research Station.

In flocks where mating is longer than five weeks, age differences between lambs can reduce the accuracy of WECs. This can be reduced by giving the lambs a short acting ‘even-up’ drench when the youngest lambs are at least 10 weeks of age. The lambs should then be exposed to a contaminated pasture for a couple of weeks to allow them to pick up some worm larvae. The worm larvae will mature to become adult worms in four weeks and lay eggs. The lambs can then be sampled five to six weeks post-drenching. In a late lambing scenario with an early pasture senescence, this may result in insufficient larvae pickup after drenching. However, if WEC is high enough for faecal sampling then this practice will also require an extra drench.

Procedure in hoggets

In hogget age sheep, start monitoring faecal WECs four weeks after the beginning of the winter rainfall season, then re-check every two to three weeks until the faecal WEC have reached an acceptable level as indicated below, before all the sheep in the flock are sampled.

The two main purposes of monitoring are:

  • to ensure an adequate worm challenge is present that will allow identification of resistant and susceptible individuals. For the non-barber’s pole worm areas, this is achieved when the average faecal WEC of the mob reaches about 300 epg for weaners, and 500 epg in the case of hoggets, and where only a few animals will have a zero count and only a few will have a WEC of over 1000 epg
  • to ensure that the worm challenge does not increase to levels where the production and welfare of the animals are compromised.

Sampling individual sheep

When the target faecal WEC level has been reached, individual samples from each of the candidate animals are collected, over as short a period as possible. Contact the laboratory a few weeks before collection is planned, to notify them when the samples can be expected. That will allow the laboratory to allocate enough time to process the samples as soon as possible after arrival. Samples must also be kept cool (refrigerated but not frozen) and should be sent to the laboratory within 48 hours of collection. Usually a drench will be needed on the day of sampling, as the counts required are at a level where some worm effects will occur in the lambs.

Co-selection against scouring and dagginess

Most cases of winter scours (dags) in grazing sheep are due to the effects of either immature and/or adult worms in the sheep. When scouring is due to immature worm larvae, this will not be expressed as an increased faecal WEC. Also, there is a tendency for some highly worm resistant sheep to be more predisposed to the hypersensitivity (low WEC) scour. Therefore, it is advisable to treat scouring as a separate trait.

Scouring is a heritable trait, which implies that it can be selected against to assist in reducing dagginess. Some breeders have expressed interest in selecting sheep for reduced scouring (as well as worm resistance) on the basis of visual recording of individual dag scores or faecal consistency scores in their sheep using a five point scoring scale. This has been standardised in a Visual Sheep Score classification available on the Australian Wool Innovation (AWI) and Meat and Livestock Australia (MLA) websites.

Using the information

To maximise the gain from selecting for worm resistance whilst maintaining other breeding objectives on the farm (such as lower micron, increased fleece weight), a geneticist should analyse the faecal WEC results. There are a number of service providers in the industry who can do this. Contact the authors (details below) for assistance. A geneticist will convert the raw faecal WEC data into a faecal WEC estimated breeding value (EBV) or Australian Sheep Breeding Values (ASBV) published by Sheep Genetics. The breeding values for each animal are calculated from its own measurements, incorporating information from an animal’s relatives, and correcting for other non-genetic factors such as the age of the ewe and whether the individual was a twin or single lamb. This process increases the accuracy of each individual’s estimate and will therefore improve the rate of genetic gain. The individual ASBV of fleece weight, body weight, fibre diameter, WEC, etc. can be incorporated into an overall selection index. This will allow maximum overall genetic gain in all of the economically important traits.

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