What forage will make a calf sick?

None. If there was a certain forage that will make a calf sick, it wouldn't be called forage, but rather a weed or simply a plant. Forage, by definition, is herbage that is grown and harvested for livestock to eat.

However, it is the anti-quality factors that are present in some forages that can make a calf sick. Alfalfa and clover, for instance, contain high protein and low fibre contents that can make a calf bloat. Tall fescue contains endophyte fungus that can cause Fescue Toxicity. Forages that are subject to sudden freezing or heat can develop nitrates causing Nitrate Toxicity. But the most common anti-quality factor to watch out for in calves is bloat.