Cows will eat approximately 2% of their body weight in grasses and/or hay each day. This should be a good quality grass or timothy hay that is free of mold and excessive dust. You can replace 20% of this with alfalfa hay for its added protein and minerals, or you can leave a protein / mineral lick out for them to self regulate those needs.
Cows will still require supplemental minerals and you can get these as tubs, loose minerals or blocks.
Cows can eat a variety of fresh vegetables and fruits from your garden and they have a serious sweet tooth, but I don't recommend going overboard on these options as they can upset your cows digestive system.
Our cows have access to fresh pasture grasses about 6 months out of the year. We augment that seasonally with quality second cut timothy hay and 10-20% of this is a cut alfalfa. Our older cows consume approx 16-20lbs of hay/alfalfa each per day. We will often times double this amount during the peak of winter when temps drop below zero.
Once a day, I make a mix of oats & cracked corn sweetened with molasses, organic kelp, organic flax meal, and salt with garlic. They each get approximately two cups of this mix. I also have various selenium and plain white salt blocks placed throughout the pastures.