Metabolizable protein supply (PDIE) and restricted level of ruminally degradable nitrogen (PDIN) in total mixed rations: effect on milk production and composition and on nitrogen utilization by dairy cows

Three levels of metabolizable protein supply (108, 98-95 and 85 g PDIE kg(- 1) DM) (PDIE: protein digested in the small intestine when rumen-fermentabl e energy is limiting) in the diet and three levels of difference between ru men-degradable energy and N (PDIE - PDIN = 0, 10 and 20 g kg(-1) DM) (PDIN: protein digested in the small intestine when rumen-fermentable nitrogen is limiting) were used in two trials with 24 and 32 dairy cows. Trials were c onducted using Latin-square designs and all cows were fed a total mixed rat ion. Dry matter intake, net energy intake, and milk yield were significantl y higher (P < 0.05) with increasing PDIE level. The PDIE level of the diet did nor, affect milk true protein content but the results for fat content w ere different for the two levels of PDIE increase: fat content decreased (P < 0.05) between the low and medium PDIE levels and did not vary between th e medium and high PDIE level. Milk NPN and urea contents increased with the increased level of PDIE. The milk protein N yield/N intake ratio decreased (P < 0.10) as the level of PDIE in the diet increased. A difference of 10 g PDIN kg(-1) DM between rumen-degradable N and energy affected neither dry matter, energy or PDIE intake, nor milk yield or composition. However, dry matter, energy and PDIE intake decreased (P < 0.10) for a rumen-degradable PDIN deficit of 20 g/kg DM. In this case, milk yield was not significantly affected bur milk fat content increased significantly (P < 0.10) and milk true protein content decreased (P < 0.01). Milk NPN and urea contents decre ased as the rumen-degradable PDIN deficit increased (P < 0.01). The milk pr otein N yield/N intake ratio increased with the increase in rumen-degradabl e PDIN deficit (P < 0.01). (C) 2000 Elsevier Science B.V. All rights reserv ed.