On Wed, 19 Feb 2014 21:50:45 +0000, JD wrote:

I have a lawn which was planted on very poor gritty, clayey soil. I'm
wondering if I can just spread a load of this stuff over the lawn,
thinly enough not to smother the grass, and mother nature will take it
down beneath the surface and enrich the soil a bit.... or will I get a
load of fresh weeds coming up from seeds contained in the 'manure'?

If I were you I'd hot-compost it before use. Try to get a trailer-load in
one go - add nothing else except water occasionally if it is getting too
dry. Straw tends to reduce the nitrogen content but this should be more
than made up for by the horse-urine, which will also act as an
accelerator. I may be wrong but I wouldn't rely on the horses' digestive
systems to destroy all the seeds in their diet. A good, hot compost heap
should and composted manure will be less likely to smother as it will be
broken down and friable.