Several issues back, Acres USA had an issue about Vechur cattle (
https://en.wikipedia.org/wiki/Vechur_Cattle), which is an Indian breed which has been brought back from the brink of extinction. They average ~280 lbs. for adults, produce around 3 liters of milk daily. Of special interest is the claim that "there has been some research claiming that the milk from such cattle helps lower the risk of many chronic conditions such as diabetes, asthma, autism, allergies, schizophrenia, SIDS, and cardiac disease... because the Vechur breed carries the A2 beta casein gene variant which has been linked to a lower incidence of the conditions mentioned above."
sources:
http://www.realmilk.com/how-to/meet-the-tiniest-dairy-cow-in-the-world-vechur-cattle/
http://www.ncbi.nlm.nih.gov/pubmed/16403684
https://en.wikipedia.org/wiki/Vechur_Cattle