Through clever use of chains and genetic culling, only the largest, healthiest, whitest-skinned will be allowed to breed.
By which I mean cows, of course. I've begun a selective breeding program to get cream or white-colored cattle with gigantic musculature and heavy fat, since these traits seem best for the freezing biome and all of its snowy landscapes. I've taken my cream-colored bull and my white-colored gigantic cow and chained them a tile apart, then slaughtered all the russet or weak males and am keeping the females seperate from breeding partners to harvest the milk until my new breed becomes a large enough herd to support the fort's needs, at which point we'll be eating hearty, my lads!
Interestingly, this is the first feature of Dwarf Fortress my mother took an interest in. When she dropped by to pick up her grandbaby I mentioned I was trying to get an all-white herd bred from a mix of russets. Immediately she starts getting that 'you should be focusing on more important things' look, only to my surprise SHE REMINDS ME THAT MUSCULATURE IS OF UTMOST IMPORTANCE. To which I assured her I had gigantic cattle in the breeding pens and butchered a white bull calf that was described as weak. She then commended my willingness to butcher immature animals that didn't fit the desired genotype; 'you get a better price for veal, after all'.