Hi andy1, yes I've heard of copper being used as preventative, but as you say it can be dangerous. I have supplemented with it before, but not for years, precisely because it's too variable. I didn't have any losses or obvious problems because of it, just grew too chary of the possibility, lol. I usually prefer to err on the side of caution and didn't have a method at the time to reliably give each animal a standard amount.
Sulfur is another one that can be good to supplement with or use as a preventative but also potentially dangerous in overdose. I want to grow and test natural herbs for anti-parasite usage in their pens, feed etc in the near future.
You can get worthless specimens of herbs that look, smell and taste identical to their medicinally valuable contemporaries, just because one is a commercial cultivar and the other is not. Better to test them than just assume the species will perform as it used to before we began breeding for commercial, economically driven traits.
One place you can get bulk herbs of high quality is from herbalists or supplementary shops, online or 'brick and mortar', which specifically supply horses. Generally they'll also mail it out.
My experience with the horse industry is that they do not generally fool around with low quality, unlike so many other animal industries, where just about any shoddy dealer can open shop and get away with selling sub par crap for years before the backlash hits or they become bankrupt through customer drought.
Those shops selling bulk herbs for horses have a fair bit more riding on their quality than most other shops selling herbs for whatever other species. Just my experience, I'm sure it's not true across the board, generalizations never are.
In the near future I'm intending to patronize a shop known as Country Park Herbs, myself. You get a 5 kg bucket of dried, granulated garlic for under $40, works out at less than $8 per kg, and being dried it's concentrated and won't take as much weight wise as raw garlic to supplement the same sulfur levels, and since it's for my rather small current flock it'll last for ages. I'll still use fresh raw garlic for certain issues of course. I also intend to buy some bulk herbs from them for my other livestock. I used to use garlic from China until my chooks decided never again. I didn't tell them about the Fukushima incident, don't know who did LOL! But, it was soon after that incident that they permanently went on strike about Chinese garlic, which was sometimes around $2 a kg. So I'm looking for alternatives, and Country Park will do until I am settled in a location where I can grow my own garlic etc.
Best wishes.