Q: Is there any OBJECTIVE data proving that smaller cows eat that much less than larger cows? i.e. is this whole "efficiency" argument based in fact?

A: In fact smaller (lighter weight) cows do need less for maintenance than do larger cows (heavier weight) if other factors, such as genetic potential for milk production, are equal. So, if weight is the only difference between them (i.e. all other contributing factors are equal) they will eat less. There are several published examples (work by Jenkins and Ferrell from MARC for example among others) that illustrate this. The often "hidden" factor of energy requirements of females is milk production since more is obviously needed during lactation but also when she is dry due to larger visceral organ size.

In terms of efficiency, input is only part of the equation. Levels of output must also be considered in order to capture true differences in efficiency between animals.

Source: Dr. Matthew Spangler, Assistant Professor of Animal Science, Animal Science, University of Nebraska - Lincoln