You should link the product you are using - Nutrena has several lines and several senior products. Read the feeding instructions. Are you feeding within the recommended range? Read the ingredients list - it probably contains rice bran, alfalfa and beet pulp (most senior feeds have those ingredients). So you are taking a balanced product and adding more stuff to it that are already in the feed, and possibly affects the balance. Why not just feed more of the senior feed?
The diarrhea is probably due to the hay. Some horses react to a change in diet even if the products are good quality. Iād add probios as soon as the horse starts getting hay, and add the hay to the diet in small amounts, increasing over time.