I used to do this on a regular basis 10 or so years ago as a better way of screening for FE resistance. I figured that proving a ram by dosing, then mating him to females of unknown resistance status didnt make as much sense as GGT screening both sexes, utilising the best and getting rid of the worst.
Its important to note that the GGT values shown in the table were obtained from no particular effort to induce or reflect a challenge. I'm often questioned on the validity of GGT screening like this, but believe me, variation is there, even if you dont see it visually, or register a challenge season. The response might vary from season to season, but the more important observation to my mind is that the "good" animals show no, or very little response by way of their GGT's elevating.
Recent and ongoing DNA research work by Phua and Dodds, AgResearch, Invermay, detailed in the 2011 AAABG Conference Proceedings, adds significantly to understanding the resistance problem, what to do about it in the breeding program, and is why I've resumed the GGT screening.
Sheep cope with sporidesmin in their system three ways:
1- non-absorption from the gut
2- de-toxification by the liver
3- or by the anti-oxidation process.
The researchers have so far found eight "chromosomalities" affecting these processes. They're gene groupings that dont overlap, and may be present, or not present, and in differing states of effectiveness, depending on mutation.
Which suggests the modes of resistance offered by respective flocks might differ.
There's also a suggestion some of them may be X-linked, ie. travel with the female X chromosome.
This latter point interested me, so I had a look at how SIL's FE index, DPX, related to ewe performance in the lambing paddock. In the Waione flock DPX is based not just on Ramguard dosing result, but moreso on the ongoing supply of annual GGT blood-test of all MA and incoming 2th sires, commercial included.
Lambing Group Mean Ewe DPX
Single, survived 10
Single, lost -27
Twin, both survived -6
Twin, lost one 6
Twin, lost both -31
Triplet, all survived 1
Triplet, lost one -51
lambed as a hgt 20
not lambing as hgt -6
From just one season's data there appears to be a fair relationship between foetal/lamb survival and FE resistance as indicated by DPX. I hope the relationship will repeat this coming lambing.
I suspect the healthier ewes may better safeguard their on-board litter through the last few weeks of gestation.
Phua's research shows an ongoing ability for resistant genes to evolve from continued use of tested sires over ewes of even unknown status, over the last 27 or so years a steady heritability of .38 has shown no sign of abating.
By including incoming 2th ewes GGT I hope to make the flock DPX even more robust.
I'll be able to select the very best DPX sires to use in the breeding program, and drop off the worst females.
Its more than likely I'll discontinue Ramguard dosing, the rams I'm using are invariably passing .6 dosing anyway.