A freemartin is a genetically female calf that is born with genitalia resembling those of a male. The freemartin is, naturally, sterile, and by extension she will not lactate.

This birth defect occurs only in the case of male/female fraternal twins. In a small percent of such pregnancies, the blood vessels of the two embryos will overlap, so that androgens (male sex hormones) produced by the male calf to trigger the development of his reproductive tract, diffuse into the blood supply of his sister, and influence the development of hers. Because the male reproductive organs differentiate before the female reproductive organs do, his androgens get to her before her estrogens might get to him.

...More than you thought you wanted to know from your friendly neighborhood Animal Science major.