Well I can give you several that I use but be warned, once they know the signs of a treat coming they can be somewhat dangerous trying to get to said treat!
1. Once I pick my sweetcorn, I will cut the stalks down with a machete and feed the stalks to the cattle slowly. You do not want to feed them large amounts of it at one time.
2. Cattle also love to eat the leftovers from my thinning process of spring turnips, radishes, etc. If I don't
compost all of them I will feed them to the cattle.
3. As has been said before cattle will love carrots and feeding them whole large turnips is quite a treat as well. Make sure to cut large turnips in several smaller pieces so the animal won't choke.
4. I will feed some of the bolted lettuce in my garden to the cattle as well.
There are a lot of things deemed undesirable in the garden that cattle will make great use of. So if you don't find yourself composting things feed them to the cattle. A word of caution though make sure you look to see if you can feed the leaves of some plants to them. One in particular I know can't be feed to cattle or livestock in general is Rhubarb leaves; it is toxic to the animals.
Once again, please exercise caution when feeding cattle "treats". Once my cattle know they have corn stalks coming them will literally run after me and pull them from the vehicle. So please be careful as to not let yourself get run over by the animals. Otherwise enjoy it, cattle are quite entertaining!