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-fermentable energy is limiting) in the diet and three levels of difference between rumen-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 conducted using Latin-square designs and all cows were fed a total mixed ration. Dry matter intake, net energy intake, and milk yield were significantly 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 were 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 the 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 decreased as the rumen-degradable PDIN deficit increased (P < 0.01). The milk protein N yield/N intake ratio increased with the increase in rumen-degradable PDIN deficit (P < 0.01). (C) 2000 Elsevier Science B.V. All rights reserved.