All herbicides will list any grazing restrictions for livestock, including horses. Some postemergence broadleaf herbicides have a grazing restriction of seven or more days.
If there are no grazing restrictions listed for horses and the herbicide is applied according to directions, it should be safe for horses. You can check with the herbicide manufacturer (phone number and Web site are listed on the product label). To ease uncertainty in this situation, keep horses off the pasture until the herbicide has dried.
Also, some herbicides may increase the palatability of weeds and poisonous plants. Therefore, it is important to read the herbicide label and follow all grazing restrictions.
Bottom line: Always read the herbicide label to determine what the grazing restriction for the herbicide is.
