In the search for and debate over objective indices of farm animal welfare, productivity is regarded by many animal scientists and others in the livestock industry as the most reliable measure of an animal's overall well-being and adaptability. On the surface, this would seem to be so, as productivity--in terms of growth rate, milk yield, feed-conversion and egg production--can be easily quantified. However, there are serious flaws in this assumption.
Fox, M.W. (1981). Productivity and farm animal welfare. International Journal for the Study of Animal Problems, 2(6), 283-284.