this will be one half head cannon and one half speculation on the cattle of stardew valley, i hope you enjoy and let me know what you think
first off, lets cover what we know. for instance the cows come in 2 main colour's white and brown, but are treated as the same entity as well as at times, cows will birth calves of the opposite colour. we also know they produce 1 jug or regular or large milk a day depending on happiness
generally we can see a few things right away, the two colours belong to the same breed or grouping, and they have a milk production affected by care and age. all of that gives us a good starting point
my general headcannon is that stardew valley cattle belong to a breed of cattle with highly varying colours, something like shorthorns. or in place of being a proper breed they are a local landrace, which fits well with the games themes of a village stuck in the past. either way making the assumption the cows origins are quite local which could explain their milk production, based on size and dimension of the milk jug sprite/item in game, the best estimatr of milk output i can come up with (and verified by a dairy farmer and 2 farm workers) is about 9-14 litres or 2-3.5 gallons of milk a day, an output thats more inline with typical heritage/dual purpose breeds than commercial dairy animals
on top of that, making some estimated guesses based on the ingame sprites, these cows have short leg's with short broad square bodies, somewhat like jersey cattle, yet with more outward meat like a hereford cow, this body shape is logical, as their bodyplan is quite alike to cow breeds that tend to both graze pasture and forage brush
putting all this together, stardew valley cattle are seemingly a local landrace whos colour ranges from white to brown, that is well adapted to the regions forested hills and meadows while supplying an ample supply of meat and milk to local families, making these often overlooked animals all the more special to the area