I think I answered most of your questions on the phone a while ago, but I think there are some salient points that can be made.
Most of the research around the world, UK, NZ & AU find 20-25% heritability of FEC at 20 weeks of age.
Dr Charles Parker has advocated for years that hair sheep with West African ancestry (e.g. Katahdin, Barbados & St Croix) and Gulf Coast & Florida Natives are different. They can express detectable resistance at 8 weeks that is heritable. Recent research on Katahdins does hold that up and that heritability is 40-50%.
Is the heritability in Katahdins higher than other breeds due to it being a wool hair composite with some sheep very susceptible (Suffolk & Hamp resistant genes) and some sheep very resistant (Virgin Island hair sheep resistance genes)? That could make the heritability appear high. We don't know. Probably not worth the effort, but a study of the heritability of St Croix or Florida Natives would tell us a lot. Dr Jim Miller of LSU found 20-25% in Gulf Coast Native.
This is worth bringing up because it is a great example of BLUP.
The initial Katahdin research by Dr Notter found this. It also shows the importance of Kathy Soder's example of 120 day wt in the middle of winter.
The following two columns are a) average FEC of lamb contemporary group & b) heritability of FEC
< 250 epg 8%
250-500 epg 20%
In other words, if exposure is low (e.g. < 250 epg contemporary group), FEC is not very heritable. Many of the lambs are not being exposed and have low FECs due to lack of exposure. If exposure is high (FEC > 500 epg for contemporary group), basically no lambs are escaping ingestion of worm larvae and it is quite heritable.
So, if most of your lambs have very low FECs, we can't tell much. LambPlan uses a filter of 500 epg for the FEC contemporary group. LambPlan is set to not results of FECs if the contemporary group has a FEC average of less than 300.
There have been several research projects with Katahdins, NE SARE grant in Maine to Drs Tom Settlemire & Dick Brzozowski, three Farmer/Rancher SARE grants in Ohio to Kathy Bielek, a On-Farm Research Grant in the SE USA & an Organic Systems grant both to Dr Joan Burke and with Drs Dave Notter & Charles Parker collaborating on many of these.
There are several take home messages from the research.
First, selection for parasite resistance using FECs can occur within flock
Second, rate of selection for parasite resistance should occur faster than selection for any other trait that I know of (but one needs a challenge).
No other production trait that I know of can have 3 orders of magnitude difference with in the contemporary group.
60 day wt - 20-30 lbs top to bottom in most cases (factor of 1/2 & 10% heritable)
number born - 1 to 4 or 5 lambs/litter (factor of 5 and 10% heritable)
FEC (range may be 0-2000 or maybe even 15,000 & 20-40% heritable).
Therefore if exposure of the contemporary group is high and there is a range from top to bottom, selection on FEC can have rapid results.
Enough for now. If anyone wants a protocol for selection for parasite resistance, we can do another topic for doing it with EBVs or without. The big advantages of EBVs is that with across flock, we can compare FECs between flocks and thus can related a FEC of 500 in one flock to 1000 in another. Also, increased accuracy.