I'm really hoping to hear opinions together with pros and cons of using shavings/sawdust in trailer. I do use a little now and thinking if I can stop (as it makes lots of mess in trailer). My concern is if the mats become too slippery whey they pee or poop (although pee happened just once so far in 2 years).
I wouldn't use any if it's an open style trailer or if you have the windows open while traveling. The wind can pick up the dust and the horse can breathe it in. In an enclosed trailer, you could use some but I don't think you need to. We have an open, slant load 2 horse, looks like a stock trailer, and we don't use any. We've never had any problem with the rubber mats getting slippery from pee or poop.
Bedding in a trailer is generally not a good idea. Staw or shavings make the floor much more slippery than just a good rubber mat. If you don't believe me ride in the back of your trailer while it's being pulled down the road.
I would put bedding in, it can get very slippery if your horse pees. I know of a horse who fell while backing out of his trailer and broke his leg and was put down. It is way too much of a risk in my opinion especially since it's so easy to just put shavings in the trailer, I don't see any benefit to not putting them in except that it might save you some money.
The only time I ever put bedding in a trailer is for very long hauls, or if the horse is going to be standing in the trailer for long periods of time. I want the shavings to absorb the pee so it doesn't get down to the floorboards of the trailer. Most horses won't pee in the trailer unless they really, really, have to, so most of the time, bedding isn't necessary. When I do use bedding, I only put it in the back half of the area.
We always use bedding. I think it gives them extra traction. If I was pulling a stock trailer with tread plate floors and open sides I may think differently. If you have rubber mats, they can get really slick with a little horsey pee pee on them.
I only spread it aft of the front feet putting it heavier in the rear.