Dust, Noise, Machinery, Steam, will tend to be the biggies, as well as the usual suspects e.g. fire, workplace transport. Hope that gives you a starting point, can't give you much more help without knowledge of what type of feed mill it is.

Dust includes explosion risk as well as respiratory problems. Also add in work at heights to your list - elevator motors are usually at the top of them, way up in the air. You might also like to enquire about medicated feeds and how they get compounded. As noted in post 2 above, there are a lot of variations in size and process across this sector so it's hard to be more precise.