Argument number one is that growing animals costs us more plants than growing plants for ourselves. Meaning that even if we wanted to assume that plants have feelings and can feel pain, the best way to make sure the least amount of plants and animals feel pain is to eat plants ourselves.
if we believe plants suffer, then how can we quantify their suffering against another things suffering? and should we? it seems, if we could establish that plants do suffer, then we must resign ourselves to the fact that some suffering is necessary to eat, and there is no reason, in my mind, to make a million stalks of wheat suffer, but not make a cow suffer for food.