What weather conditions heat does your horse have need of to wear a coat /rug and what season do u metamorphose them?
My honest answer is that most horses do not need a winter blanket,whether that's what you're asking. Horses are built for surviving cold weather,and if you've ever seen a horse's winter coat,you'd be positive.:) If it's going to be below -10 then I'd definately start thinking around it...Horse blankets do cut down on hay intake too. The hay gives the horses the energy to stay warm,but whether they're not using as much energy,they'll eat less. That's another thought. After around January/February I'd bring the blanket off. To add onto that. Older horses and younger horses would be more likely to obligation a blanket than a middle aged horse in perfect condition.
As far as rain, I don't blanket or put a sheet on them. I always have a run within shelter for my horses even when its not raining. But actually my horses are always outside standing within the rain...Even in a down pour of rain. They would fairly be outside grazing than worry about getting out of the rainfall. Most horses I know are the same way. Rain doesn't bother them much.
For blankets I go by the simple rule for ever 5 degree Celsius the warmth drops, add 100grams of insulation to the blanket, this is any time of year.
above 15 C (if windy or raining) a simple rain cover, no nourishing
15-10 C(59-50 F) 100gram
10-5 C(50-41 F) 200gram
5-0 C (41-32 F) 300gram
bellow freezing 400gram
You don't have to buy one of each gram of insulation, a horse can be turned out with 2 blanket, one lower than the other, to achieve the desired insulation. If you don't work your horse as much and want a thicker coat, simply drop the list 5-10 degrees so you start at 10-5 C near 100grams :) The blanket should be removed daily and the horse brushed to remove dirt or the blanket might rub. I always stick with marine proof blankets as I'm not always home to get the horses contained by if it rains.
My horses never wear them. they are out with a run surrounded by shelter all year round, in all weather which get down to 30 degrees below 0 F, and still choose be be outside of the shed most of the time.
Mine also only have a run in. Mine are surrounded by WY & they don't get blankets there, 151 acres.