Hi! We worm all our stock 2 times a year, spring and fall, using Dectomax which is almost identical to Ivomec and costs lot less--it is an injectible. Injectibles are longer lasting since they are systemic vs. topical. We have also used a product called "Safeguard" which is a pelleted monthly de-wormer mixed in with a feed for bovines only. Still feel the injectible type is more effective. Another note: If calves are kept in a pen or stall area, be sure to regularly clean out the area and haul out the manure (just like you would do with horses). Cleanliness goes a long way to animal health. Additionally, we use a molasses/mineral block with Rabon insecticide in it which helps to keep flies killed in manure piles for bovines only.