I’m not saying there is no difference - they are different species so we wouldn’t expect them to have an identical experience of pain and deprivation - but rather that the difference is not easily quantifiable. We are not able to access the interior lives of these animals. And in practice, I suspect many people would be as reluctant to rip the wing off a live chicken as the ear from a cow, while showing few misgivings about squashing an ant. That is to say, in practice most people are likely to behave as if the moral stances toward cows and chickens are roughly equivalent.
In terms of being forced into decisions to value one human life over another, that doesn’t change the fact that morally and ethically we value human lives equally. We don’t extrapolate from emergencies to posit that one mom = three children. In any case, what bearing does this have on the problem at hand? If one cow’s life is morally equivalent to a non-singular number of chickens, how many chickens are we choosing, and why?