The reason the barns are only accepting 2 of the 3 ingredients of food is because you fed enough of the first 2 to fill the total capacity to 100%. Now there's no more room for any amount of any of the ingredients until some is consumed. Also, the production of milk is going to vary because typically only the Brown Swiss and Holstein are used as dairy cows. Limousin and Angus are beef cows that are generally raised for meat or sale.
I grew up on a beef farm, Staw is bedding, they like to lay on it, if they don't have bedding they won't lay down because it'll just be concrete and manure on the ground, we also used to use shredded news paper as bedding when we would run out of or run low on straw
Love your beginners guide. And you need straw in the barns because they sleep on them so they don't sleep on concrete. And you don't need straw in the open because there's no concrete. That's why it's at conditions not food. 😁