Why would a cow give milk to her first two calf's and not the third calf at time of birthing?

Answer 1

There is some confusion here. Are we talking about triplets that this cow of yours has given birth to, or two consecutive calves from two consecutive births over a two- (now three) year-period? Whatever the answer is, I'll cover both.

As far as triplets are concerned, a cow has a sixth sense of telling that one of her calves, regardless if it's a single calf born, a calf from a twin birthing or triplets, may be sick or unwell that it will not survive, and that it may be not worth having that calf around because of its health status. There is no explanation for this, other than sometimes cows just do that because they have some sort of intuition that we humans cannot understand nor know about. So when that happens, that calf that gets rejected by the cow will be reliant on the humans to care for it and to work hard to ensure its survival. That is what you will have to do if the case is a cow caring for triplets, including regular bottle-feedings.

If, however, this is a case of a cow allowing two consecutive calves from past births to continue to suckle on her, you really need to analyze your management criteria to the care of this cow, because that would be exactly the reason why this third calf wouldn't get milk from its dam. These "first two calves" should have been weaned a long time ago, especially her first calf. By now her first calf would be at least 24 months of age, and this is far too old to have it continue to suckle on its momma. This animal needs to be weaned as soon as possible (ASAP). The second calf no doubt would be around 10 months old, also overdue for weaning. You should have weaned this second calf two months ago before the third came along. Now this cow does not have enough nor adequate colostrum to give to this new and third calf of hers, and because you have failed in proper weaning management of her calves, you may have compromised the health and welfare of this third calf. If you want to fix this, wean those two older animals (they are no longer "calves" at 24 and 10 months of age, respectively) immediately and get some colostrum in that calf IF and ONLY IF that Number Three calf is less than 12 to 24 hours old. I say "if" because beyond that, colostrum is no longer effective to the calf as a form of passive immunity.

Hopefully, however, that the situation you are in is the former (triplets) and not the latter (older "calves" still on her).