It has been suggested (Kyriazakis and Emmans, 1992a, b) that the material efficiency with which the pig can retain protein (ep), is directly proportional to the energy:protein ratio of the diet (R), so that ep = k.R. This hypothesis accounts for the finding that the rate of protein deposition can sometimes be increased by increasing the energy supply at a constant rate of protein supply.

It is also possible that the constant of proportionality, k, is a genetic variable, although it was proposed, partly for the sake of simplicity, that there would be no genetic effects on the value of k. To test the propositions: (i) that ep is proportional to R, and (ii) that k does not differ between genotypes, an experiment was designed and carried out on two pig breeds which were expected to be very different from each other.