From The Ethics of Eating Animals, p. 326-7
To give up eating animals is to give up on these places as human habitat, unless of course we are willing to make complete our dependence on a highly industrialized national food chain. That food chain would be in turn even more dependent than it already is on fossil fuels and chemical fertilizer, since food would need to travel even farther and fertility–in the form of manures–would be in short supply. Indeed, it is doubtful you can build a genuinely sustainable agriculture without animals to cycle nutrients and support local food production. If our concern is for the health of nature–rather than, say, the internal consistency of our moral code or the condition of our souls–then eating animals may sometimes be the most ethical thing to do.
Since the utilitarian is concerned exclusively with the sum of happiness and suffering, and the slaughter of an animal with no comprehension of death need not entail suffering, the Good Farm adds to the total of animal happiness, provided you replace the slaughtered animal with a new one. However, this line of thinking does not obviate the wrongness of killing an animal that “has a sense of its own existence over time, and can have preferences about its own future.” In other words, it might be okay to eat the chicken or the cow, but perhaps not the (more intelligent) pig.