May 12, 2009

Facial Eczema (FE).Farmer Information. Part 20. (Sheep). Breeding. Breeding resistant rams.

Agriculture, farming, animal husbandry, animal health, Facial Eczema, sheep, breeding, genetics, breeding resistant rams, GGT test.
By Dr Clive Dalton
Original 1991 information written by Dr Barry Smith and Dr Neale Towers, Ruakura Agricultural Research Station, Hamilton, New Zealand.

20. Facial Eczema: (Sheep). Breeding. Breeding resistant rams.
  • Start breeding FE resistance into your flock. 
  •  Why? Because it is a permanent gain,  adding value to your flock and making the other annual precautions easier and less critical.
  • Tolerance to FE is strongly inherited. About 40 percent of the differences in resistance to FE seen between individual animals is due to their sire. Therefore progress in selecting and breeding more resistant sheep can be rapid, and significant gains can be made in only a few years of concentrated selection. 
  • But remember always that breeding for resistance is a long term commitment - it will not provide an overnight solution to your FE problems. Once you start buying rams bred for FE resistance buy only from flocks using tested rams.
  •   Work in the Ruakura Resistant flock shows that the proportion of animals able to withstand a standard toxin dose can be increased by at least 2.5 to 3.5 percent each year.
  • A FE resistance testing service is operated by the Ministry of Agriculture under the name RAMGUARD.
  • Breeders who have used the service regularly have generally made good progress towards breeding more resistant sheep. Those breeders who have used the service for 5 or 6 years have doubled the resistance of their flocks to the toxin causing facial eczema.
  • Although this is rapid progress and breeders who have selected for facial eczema for several years will now be benefiting from their investment, it will still be necessary to continue selection for several generations yet to produce sheep resistant to the most toxic pastures. 
  • It is quite unrealistic to expect any real benefits if testing is carried on for only 2, 3 or even 5 years.
What does a Performance Test do?
  • A performance test identifies the most resistant of any group of rams tested. The test ranks rams on their ability to withstand a challenge with sporidesmin, the fungal toxin causing facial eczema. 
  •  Rams which are themselves resistant to the toxin have been shown to sire progeny with an increased resistance to the disease.
  • It is a relative - not absolute - measure of resistance. The rams identified as the most resistant in your flock may be much more or much less resistant than the most resistant in another flock.
What is Involved in a Performance Test?
  • Potential flock sires are dosed with small amounts of toxin and the susceptible animals identified by measuring the elevations in blood gamma-glutamyltransferase  (GGT) concentrations three weeks later. 
  •  GGT is an enzyme (protein) normally concentrated in the liver and present in only small amounts in the blood. In animals with facial eczema i.e. animals with livers damaged by the toxin, this enzyme leaks from the damaged tissues into the bloodstream.
  • The GGT test is particularly sensitive and can detect liver damage in animals showing no outward sign of the disease, this allows the use of very low doses of toxin in the test so that very few tested rams shown any sign of facial eczema.
  • In most performance tests a first round of dosing with a very low dose of toxin identifies the most susceptible rams.
  • The rams resistant to this first dose of sporidesmin are then re-dosed with a larger amount of toxin.
  • Rams with only a moderate resistance to the toxin react to this second sporidesmin challenge while the most resistant animals are unaffected. 
  • The complete performance test is spread over about two months.
  • The rams are weighed before the start of the test to determine dose rates, bled before dosing and are bled and weighed again 3 weeks after dosing.
  • If a second dosing of sporidesmin is required this is given after a 2 week interval and further blood samples collected 10 days and 3 weeks later.
What does RAMGUARD do?
  • RAMGUARD calculates the dose rates needed for your flock, provides the sporidesmin, analyses the blood samples and interprets the results. 
  •  RAMGUARD reports the results after each round of dosing, ranking the animals for FE resistance and recommending whether further testing is required.
  • Your veterinarian will normally collect the blood samples and dose the rams with sporidesmin.

What Does it Cost?
Ramguard charges from from 1991-92 (not include GST)
  • In addition there will be veterinary charges for the blood collection and sporidesmin dosing - discuss these with your veterinarian.
What to do now?
  • If you intend performance testing for facial eczema resistance discuss this with your veterinarian and ask him to contact RAMGUARD to make the arrangements for testing.
  • If you need help in structuring your FE resistance selection programme and incorporating it into existing selection objectives we suggest that you discuss the programme with your consultant.
Points to consider:
  • FE resistance is not linked to other production traits; therefore selection for FE resistance is done independently of selection for production traits.
  •  Select rams for FE testing first on the basis of their high ranking on selection index lists such as Animalplan etc.
  •  If progress in FE resistance is the first priority consider testing ram lambs as soon as the hogget selection index list becomes available. This will allow you to test more rams for the same total cost and speed progress.
  • If production traits are to have equal or greater importance than FE resistance,  arrange to test rams after the two tooth selection index is out.
  • Test only rams you are prepared to use as sires in your flock.
  • Test at least 5 times as many rams as needed for your ram team, more if you have FE as your most important breeding objective.
  • Maximise progress by using tested rams for only one year - the sale of elite tested rams after use in your flock will help meet testing costs.
What do I do with My Present Ram Team?
  • Breeders in FE prone areas can screen their existing ram team by having their vet collect blood samples after the FE season (or at any time that clinicals are seen) and getting a GGT test done.
  • Rams with the highest GGT values are the most susceptible rams and should be culled.
  •  Remember that fastest progress is made by using rams for only one season before replacing with the next crop of tested rams.

2009: For further information on performance testing contact:
Kees Wesselink
Facial Eczema Tolerance Testing Service
T +64 7 838 5259
M +64 29 838 5259

AgResearch Limited
Ruakura Research Centre
East Street, Private Bag 3123, Hamilton, New Zealand
T  +64 7 856 2836   F  +64 7 838 5012

This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

No comments:

Post a Comment