Sources of energy for the lactating dairy cow

The overall efficiency of energy conversion by the lactating cow is briefly discussed, together with some effects of variation in the amount of concentrates fed due to variation in productivity. The diurnal changes in the concentration and ratios of rumen volatile fatty acids, associated with a particular feeding routine, were shown to have considerable effects upon the relative caloric contributions of the different volatile fatty acids. Changes in management may therefore be important, and the difficulties of measuring these are discussed. Additions of orthophosphoric acid to diets containing urea depressed the digestibility of dietary organic matter and crude fibre, but had little effect upon rumen pH. Subsequent rumen-ammonia levels, reflecting rate of release from these diets, were directly related to rumen pH at the time of feeding.