A new system for the protein feeding of ruminants: The PDI system

Abstract To replace the digestible crude protein system (DCP) a new system is proposed which is named the PDI system (Proteines Digestibles dans l'Intestin grele). It estimates the quantity of amino nitrogen N × 6.25 absorbed in the small intestine from the dietary protein which has escaped fermentation in the rumen, and the microbial protein arising from that fermentation. The extent of N degradation in the rumen and the microbial protein yield (135 g/kg of digestible organic matter intake) were obtained using an equation describing the flow of non-ammonia N entering the duodenum. The true digestibility of microbial true protein (80% of microbial crude protein) was considered equal to 0.70 and that of dietary protein was calculated from nitrogen and organic matter digestibilities of each feed. Two PDI values are ascribed to each feed: one (PDIN) calculated from both its non-degradable N and degradable N contents and the other (PDIE) from both its non-degradable N contents and its content of rumen available energy (in fact its digestible organic matter content). When calculating the PDI value of a diet, the PDIN and PDIE values of the different ingredients are summed separately: the PDI value of the diet is the lower of the two values. The PDI requirements of animals have not been measured previously. They were calculated from nitrogen balance trials on non-producing animals and from feeding trials on producing animals. The mean efficiency of PDI utilization was found to be 0.67 for milk production in cows, 0.60 for body gain in cattle and 0.60 for pregnancy in sheep. These values have also been used with animals of other species. The advantages of the PDI system over the DCP system, as well as its shortcomings, are discussed. It is compared with alternative systems which have been proposed recently in other countries.