PREDICTION OF MILK PROTEIN-CONCENTRATION FROM ELEMENTS OF THE METABOLIZABLE PROTEIN SYSTEM

Elements of the metabolizable protein system in the United Kingdom wer e examined for their suitability as potential predictors of milk prote in concentration. Models were based on data from 163 cows offered five forage mixtures for ad libitum intake plus concentrates at 3, 6, or 9 kg/d of dry matter. The models were then tested on a separate data se t of 100 cows offered seven forage mixtures for ad libitum intake plus concentrates at 6 kg/d of dry matter. To minimize problems with colli nearity, variables were arranged hierarchically; successive elements w ere components of variables at higher element levels. Variables from d ifferent element levels were not used in the same models. Models were constructed using ridge regression to minimize problems with collinear ity. The fit and precision of prediction were generally poor because t hese models did not take into account animal variables. Models using u ndegradable dietary protein performed slightly better than did those u sing digestible undegraded protein. The use of slowly degradable prote in and quickly degradable protein rather than rumen-degradable protein generally resulted in improvements in prediction. Models using neutra l detergent fiber and quickly fermented carbohydrate were better than those using total carbohydrate. We concluded that there was little to be gained from using the elements of the metabolizable protein system considered here for the prediction of milk protein concentration.