dun is right; Yes they can. It is hormonal, BUT, nature does something that allows them to not let their milk down; has to do with the oxytocin in their system. One of the vets could do a much better job of explaining the "how tos" of it. But I have found it with the "multi-purpose" nurse cows. If I have a nurse cow that the calf runs with all the time, and I bring her in to milk, she often will not let her milk down, wanting to "save it" for her calf. So what I do is to leave the calf with the cow 12 hours, take it away and milk 12 hours later and then put the calf back in with the cow. Usually they will try to hold it for a day or two, but will learn the system and be fine to milk. Alot of dairy cows will hold their milk for a few days after they calve if the calf is within "earshot" and she can hear it hollering and she is talking back to it. And some don't have an ounce of "mothering" and will let their milk down as soon as they walk in the parlour.