Is a "free martin" only a female twin to a bull calf or are there other "free martins"? If so, how do you identify these visually or can you even identify these visually?
A freemartin is a bull and heifer combination of calves carried at the same time. The male hormones in the uterus of the pregnant cow prevents the heifer's reproductive tract from forming. Your veterinarian can determine at 3 to 6 months of age if the tract is normal by examining or passing a test tube in the tract. Normal vaginal lengths are 14 cm (5.5 inches) or greater. Typical freemartins will have vaginal lengths of 7 to 11 cm (2.8 to 4.3 inches.
Mike Hutjens, Extension Dairy Specialist, U of IL - Illini DairyNET
Hugh Chester-Jones, Uof MN dairy and beef specialist