A properly applied clay poultice will have the most long lasting effect. My personal favorite now, after trying it, is the Sore No More Poultice.
Standing you horse in ice water or applying ice boots is a very short term method, Gets the legs very cold very quickly but they will warm right back up and fill with fluid if the work was really hard. I'm assuming when you say cooling your horse's legs you mean to use "cold" to prevent or reduce inflammation and stocking up. so what you really want is cooling that lasts around 12 hours and support which also helps prevent the stocking up.
A lot of people will do a "quick" poultice where they smear a clay or gel poultice onto the hair of the leg and walk away. This method appeals to people as it is simple, quick and easy. It also doesn't really do much at all other than make the person feel like they've done something for their horse.
A clay poultice woks by evaporation. The clay needs to be in contact with the skin of the leg, not just the hair. the hair isn't hot, it isn't filled with fluid, it's just hair. Apply moist slppy clay to the leg and work it down to the skin so the SKIN is covered with clay. Do this for the entire cannon and ankle area. Add more clay evenly and smooth you want at least a 1/4"-3/8" on clay in a constant layer over the entire lower leg. As the water evaporates SLLLLOOOOWWWLY out of the clay the clay and the leg that it is contacting will cool down. that's the cool thing about evaporation and was the principle behind old swamp coolers, early AC. You want the leg to be cooled for hours so you slow the evaporation by covering the clay with a paper sheet that has been soaked in water. A paper grocery bag or an inner layer of paper from a feed bag works well. wrap the wet paper over the clay then wrap a firm quilt, snugly over the paper and top this off with a decent and real leg bandage, not a polo wrap, not vet wrap, not a narrow short knit tail bandage. You are trying to create something like a flexible and supportive cast. It should be firm. Don't use those squishy pillow wraps. You need a firm solid bandage to prevent the filling.
I recently had a horse hurt a tendon and I resorted to clay poulting as it was the only practical way to cool the leg over a long period of time as I work and the ability to ice or cold hose my horse's leg is limited. I had a free tub of the Sore no More Poultice and decided to use it. I was very impressed with how cool and tight the leg got and very quickly. I use their liniment all the time and sometimes have used it under a poultice. This stuff is pretty handy. Washing a poultice off after it dries can be time comsuming but this stuff came off pretty easily.
My guy is on 24x7 turnout so wrapping is usually not an option. I have a set of leg soakers that he "chills" in for 20-30 minutes. I do not ride all that hard/long but we are both not young so I try to keep him comfy and happy.
Thanks for the responses. I am looking for something to use on site at a ride, so hosing is not an option. I see no one is voting for any of the cooling gels on the market. Clay seems to be the product of choice?
12 - 24 hours - since when it's dry, it's done doing it's thing. I myself like to splash a teeny bit of DMSO on before I apply poltuce, since it keeps it wet for longer, but be careful doing that, since it can cause irritation.
As for brands, I like making my own, using Bowie Clay for the base, of course.
I always pass on "wraps" or "quilts" that have foam inside them, typically called No Bows. I don't buy saddle pads with foam inside, other than thick removeable foam inserts. Foam doesn't hold up well to repeated washing and is too soft for leg wraps. The old style traditional cotton sheets were outstanding for standing bandages but only lasted for a few uses. So I look for quilted leg wraps, with a poly interlining, cotton fabic outside or drilex. there are really thin cheap ones which can be doubled up but Toklat and Roma make some decent quilted warps. You don't want big fat wraps that compress and squish as you wrap around them. This isn't supportive. OK maybe as replacements for shipping boots but really, when doing a wrap over a poultice or liniment for a horse who's worked really hard, who's legs have taken a pounding, you want compression, firmness.
I only leave a wrapped poultice on for 12 hours. As said, once it's dried out it serves no further purpose. If I put the poultice and wrap on at the end of the day, I'll remove the wraps and papaer in the morning. If I have to go to work (I leave really earlie!) I'll just leave the dried clay on the leg and remove it when I get home. The SNM poutice actually flakes off pretty well then I cold hose and rub the rest out of the hair.
I am amazed by the Sore No More liniment. On Sat I went to a CTR and had to pound electric fence posts into hard rocky ground with a hammer. I missed with one swing and smashed the back of one hand! It swelled and filled with a lot of blood which spread out across the back of my hand. I just ignored it until Sunday evening when I got home. It occurred to me to use some SNM liniment on it. I wanted to keep a lot of the liniment on it over night so I soaked a guaze square with the liniment and held it in place with a huge bandaid and tape. It covered most of the blood filled area. This morning I took the bandage off and laughed. I had a perfect square of normal colored skin, all the blood and bruise was gone. But all the area around the bandage that hadn't been covered with the liniment soaked gauze is still purple red with blood. Really amazingly clean line between the two areas. buy this stuff, two bottles!
If at home, hosing each leg for a few minutes is my preferred approach. For travel (and for longer lasting effect at home), I use Green Jelly- now marked by Farnam I think, but they say they are using the original formula as made by a small outfit in NJ when I first started using it, back in the 1970s. Easy to apply, for more serious issues one can bandage it- but as an example, returning from several hours of foxhunting in CO one day, my horse had a bit of swelling in a hind ankle- I generously applied some green jelly, when to enjoy the hunt breakfast, swelling gone when I returned to the trailer.
I'm not convinced that the poultices work. I like my ice horse boots. Like, not love, but they work. I keep extra packets, becuase I find that one set of ice packs won't stay cold on the legs for the 20 minutes. moldable ice packets you use in lunch boxes work.
20 min on/20 minutes off is what's usually reccomended. I suually do ~2 cycles, but it depends on the ride difficulty etc.
I'm thinking about experiementing with a technique I read in a tom ivers book that they use on the track - polos with ice water/rubbing alcohol, resoaking the polos as they warm up. This seems like it would be pretty effective.
After a vigorous lesson or show day, we brush clean, poultice and wrap our horses' legs. Sheets of Spitfire's Poultice Paper soak in water while we poultice. Using wet paper over the poultice keeps the poultice working extra long. We also prefer a nice firm pillow wrap covered with a stable bandage for good even pressure on the wrapped legs. When the legs are unwrapped in the morning the paper crumbles away easily and the poultice can be brushed or hosed right off. Poulticing has been the method we've used to help ensure sound legs for our horses. A roll of Spitfire's Poultice Paper is great to have in the grooming box. No need to hunt for feed bags!