Unfortunately, luvs2ride's comment that the freeze kills most of the parasite larva and eggs is incorrect. There is no such thing as a killing frost when it comes to ascarid and strongyle eggs/larva.
Strongyle eggs will not grow into infective larva when temperatures are below 45 degrees, but they do remain viable on the pastures and will turn into infective larva when temperatures get above 45 degrees. Strongyle larva that are already in the infective 3rd stage will remain in that stage while the weather is cold and can infect your horses. So if you are spreading manure on pastures in the winter months, you are only going to increase the risk of infections come spring when horses begin grazing again and temperatures are conducive to the strongyle larva maturing into the infective 3rd stage.
Ascarid eggs are not affected by cold or heat and can remain viable on pastures for up to 10 years or more.
By spreading manure on pastures, you confound the horse's natural instinct to not eat near feces. This avoidance behavior helps keep horses from picking parasites from feces....but when those feces and eggs/larva are spread all over the horse can't avoid them. Spreading manure should only be done at temps below 85 degrees when horses are not going to be on the pasture for 6 months or more. Spreading manure when the temps are above 85 degrees can be done and horses removed from pastures for 3-4 weeks because strongyle larva survive for a much shorter time when temps are that warm.
In pastures where foals are kept, spreading manure simply shoudn't be done because of the extremely long survival period of ascarid eggs.
With the quickly growing parasite resistance problem, we need to focus on preventing parasite infections by pasture management rather than by "just deworming". Ascarids are showing resistance to ivermectin and pyrantel. Strongyles are showing widespread resistance to fenbendazole, moderate resistance to pyrantel and instances of resistance to ivermectin. With the similarities between ivermectin and moxidectin, it's only a matter of time before resistance to moxidectin begins occuring. And there are no new deworming drugs coming on the market, so we already have limited choices as to what drugs may be used in particular situations and the choice is only going to become more limited. If we continue to "just deworm" and not consider appropriate pasture management options then it won't be long before we don't have any effective deworming drugs.
Licensed Veterinary Technician
Last edited by Ryle; 01-10-2010 at 10:05 PM.