Many people report it as being more digestible for them - same with ewes' milk.
Nothing definite but this may be an issue with the proteins. Some, but not all, breeds of cows yield milk with a form of beta-casein that has what seems to be a rather unfortunate mutation:
http://fourfoldhealing.com/2009/03/1...09-newsletter/Some five thousand years ago, a mutation occurred in this proline amino acid, converting it to histidine. Cows that have this mutated beta casein are called A1 cows, and include breeds like Holstein.
The side chain that comes off this amino acid is called BCM 7. BCM 7 is a small protein (called a peptide) that is a very powerful opiate and has some undesirable effects on animals and humans.
Not all cows' milk has this problem - Jersey milk doesn't, for example. No goats' milk has the problem.