I have noticed that sometimes horses prefer one bucket over another... dont know if one plastic smells more than another, or just doesnt seem right so they pass it by for another?? Not sure there, but have had a couple of tubs that nobody seemed to want to drink from, so got rid of them as water buckets.
I would imagine if the horse weighs at least the same as a human, a whole yogurt would not hurt?
I use the paste probios when travelling- it is quick and easy! That was something new we tried this year- not that we had problems in the past, but when they are travelling for two long days, I figured it couldnt hurt.