I am sure there is lots of research done on this topic, and I know there has been quite a few studies done in numerous research facilities over the years. My graduate thesis in college was " The Economic Importance of Beef Cow Size in Relation to Reproductive Performance". While this was not entirely regarding milk production, it certainly was a part of the many research papers I had to review to complete this paper. I did this back in the dark ages, before many on SP were even born, so I am sure there is much more that has been done since then.
My definition of what a great milking cow is, has changed over the years, and a few dry years have assisted me in coming up with thoughts on this. In my world , a great milking cow, is not the cow that produces the most milk. I am a very strong believer that milk like all other traits,( and virtually everything else in our lives,) is best in optimum amounts. Milk EPDs, vary in most breeds, and I see some breeds that put way too much emphasis on high milk than I think is desirable. I have friends who raise Angus cows that have piled so many high milk genetics on top of each other than they have developed thin skinned, hard doing cows that milk great, raise a big calf, come in from pasture and require additional supplement to get them back in shape before they calve again. This makes little sense to me. I see some breeders weaning their calves earlier every year so the cows can gain some flesh before winter sets in. I guess this makes more sense than having to sort the thin cows off and feed them during the winter, but it still does not seem to be the way things are supposed to work. The cows should work for us, rather than us just work for the cows.
I think the Shorthorn breed has had way too many cows of this type, and I do think there is a trend to move away from this... to which I say, it is about time! I honestly believe the number of cows like this that come in open each year, is higher than from cows that milk less and still raise a strong calf. Of course, environment conditions will dictate how much milk you can tolerate in your cows. My views differ from many on here in regards to a cows frame size and her ability to stay in condition, milk well, and calve every year. I have seen as many small framed cows that were frail, bone racks , that had no ability to stay in condition as I have seen large framed cows that do the same. There are exceptions to every rule, and for that reason, I do not agree that we can put all females of certain frame in the same basket, when we are talking about efficiency, and ability to flesh, and produce a valuable calf each year. It almost always comes down to Dollars produced compared to dollars invested.
In the Shorthorn breed, negative milk EPDs do not scare me in using a herd bull. I would probably be careful not to stack negative milk genetics, but an occasional lower milk EPD usually works well in our conditions. Getting back to your original question.... I think a strong argument could be made for either milk being a maternal growth or a straight growth trait, again partially dependent on what conditions your cattle work in. Where I am, I think I would consider milk as being a maternal growth trait rather than just a growth trait. But then, we may all have slightly different definitions to what these terms mean.
In the same vein, I have often thought that the femininity is the most misused and misunderstood word in the beef business. Many breeders completely confuse femininity with fraility. What is a good definition of a feminine cow? Is it a smoooth fronted, angular shaped female, that is goose necked and with little to no brisket? Or is a feminine cow , simply a cow that consistently brings in a calf that weighs 60% of her body weight, while she stays in good condition and regularly calves within a 12 month window for several years without missing? Does femininity have more to do with a females production than it does her phenotype? Again, the definition may differ slightly as to what conditions your cows have to perform in.... but I think it is good food for thought.
