A noted horseman recommended Roundup in pastures as a way of reducing grass/weed growth around the base of fenceposts. Is Roundup toxic to a horse that may graze around the fenceposts? Is it safe to allow horses back into pastures that have had the ground around fenceposts treated with Roundup?

You could use 2,4 D instead (Crossbow). This will work on broadleafs but not grass. Some horse owners have reported that horses that lick Roundup can get colic. For 2,4 D, there are no restrictions for horses on pasture, but you should spray when plants are actively growing (not during hot drought), using 4 quarts per acre broadcast sprayer or 2 ounces per 1 gallon spot sprayer.
Roundup can be used as it has a relatively low toxicity as long it is applied according to directions. It should not be allowed to puddle, just like any other herbicide.