The so-called P2 fat thickness measurement, made in the middle of the back, is the basis for payment to Britain's pig producers. Higher payments for thinner fat led to boars replacing castrates and have encouraged genetics companies to produce leaner and leaner breeding stock. Some of these new breeds, particularly those based on the Belgian Pietrain, have 'blockier conformation' than the original standard Large White, which means shorter carcases, thicker muscles and, potentially, different relationships between fat thickness and percentage lean in the carcase.
Various sophisticated measuring instruments have been developed for measuring fat thickness at different points on the carcase and combining the information, using software to provide the best overall prediction of percentage lean for the multibreed pig population we now have. Automatic grading probes measure fat thickness and muscle thickness by detecting differences in colour as the probe is inserted into the carcase.
The Danes have developed the Carcase Classification Centre, which robotically inserts several of these probes into different parts of the carcase; and the Autofom, which records fat thickness using ultrasound sensors. This new technology does increase the accuracy of predicting percentage lean over use of the simple hand-held optical probe, but the difference is very small. This probably explains why the optical probe, which records fat thickness at the P2 position, is still the instrument of choice in most British pig abattoirs.