There's an awful lot of loving attention to detail in the descriptions of the cows at Talbothays Dairy, don't you think? Not only are individual cows frequently referred to by name, we get these vivid descriptions of the shape, size, and color of their udders. Just look at this example: "The red and white herd nearest at hand […] now trooped towards the steading in the background, their great bags of milk swinging under them as they walked" (16.25).
OK, the udders are "great bags of milk" that "swing" heavily. Fine. If it were just this one description, we'd try to ignore it. But here's another, from the very next paragraph: "their large-veined udders hung ponderous as sandbags, the teats sticking out like the legs of a gipsy's crock; and as each animal lingered for her turn to arrive the milk oozed forth and fell in drops to the ground" (16.26).
Udders and breasts usually represent fullness of life and fertility – just look at images of earth goddesses from almost any culture. (Here's an image of the ancient Greek goddess Gaia, the goddess of the earth. Notice how prominent her breasts are?)
So the graphic descriptions of the cow udders in Tess probably have something to do with Hardy's interest in nature, or maybe even in the ancient worship of fertility goddesses that he references elsewhere (see "Club-Walking" below).