The effect of protein supplementation on nitrogen utilization in lactatingdairy cows fed grass silage diets

The study set out to examine the effects of supplementing grass silage with various levels of protein concentration and degradability on dietary nitro gen (N) excretion in lactating dairy cows consuming at least 60% forage. Si x Holstein/Friesian cows in early to midlactation were offered six diets co mprising two levels of crude protein (210 and 290 g/kg DM) and three levels of protein degradability in the concentrate achieved using different amoun ts of untreated or formaldehyde-treated soybean meal. Despite a difference of almost 100 g/d in N intake, apparent fecal and milk N outputs were not s ignificantly affected. Protein degradability also had no effect on N output s in feces and milk. However, there was a major effect of both level and de gradability of CP on urinary N output. Moreover, an interaction between lev el and degradability of CP was detected, such that the rate at which urinar y N increases with increasing CP degradability was higher on the high-CP th an on the low-CP diet. A low level of protein (150 g/kg DM in the diet) and medium to low rumen-degradable protein supplements provided a significant reduction in N excretion without compromising lactational performance (mean 24.8 kg/d), in terms of both milk yield and composition. This study also d emonstrated that a high efficiency of N utilization could be achieved on lo w-CP diets (supplying less than 400 g N/d), with feces being the main route of N excretion, whereas an exponential excretion of urinary N was observed as N intake exceeded 400 g N/d.