Investigation of different levels of RDP in the rations of lactating cows and their effects on MUN, BUN and urinary N excretion

Twenty-one multiparous Holstein cows in the late stages of their lactation period were used in complete randomized design to investigate the effect of rumen degradable protein on milk urea nitrogen (MUN) and some blood metabolites. Experimental periods were 6 weeks in length, with days 1 to 14 used for adjustment and weeks 3 to week 6 used for sampling (urine, blood, and milk). Three concentrations of a rumen-degradable protein (RDP) supplement according to National Research Council recommendations (9.3, 11.4, and 14% of dry matter intake) were treatments. Dietary RDP content altered both total urinary N and urinary N concentration, leading to increased urinary output. Estimations for microbial protein yield were compared with the measured excretion of purine derivative as yeast RNA equivalent, in urine. No significant effect of concentration of RDP supplement was detected on microbial N production. Plasma cholesterol concentration decreased linearity by increasing RDP concentration in diets (P0.05). In this regard, milk urea nitrogen, as well as triglyceride concentration in plasma, was not associated with dietary RDP concentration. To ensure a correct balance between energy and protein available in the rumen and consequently higher N efficiency for late lactation cows, a MUN content of 15.1mg/dl milk is the upper margin. Milk urea N is a simple and noninvasive measurement that can be used to monitor N efficiency in dairy cows.