Feed intake and urinary excretion of nitrogen and purine derivatives in pregnant suckler cows fed alternative roughage-based diets

This study compared intake of alternative roughage-based diets and of common late-cut grass silage and related intake to urinary nitrogen (N), urea-N and purine derivative (PD) excretion, where PD is an indicator of rumen microbial crude protein (MCP) synthesis. Total urine was collected from 36 Hereford cows, blocked into three groups based on expected calving date. Cows within calving groups were randomly assigned to one of four roughage diets: common mixed grass silage (MGS), festulolium silage plus urea (FLS), reed canarygrass silage (RCS) and barley straw plus urea and rapeseed meal (BRM). Diet crude protein (CP) content was classified into five fractions (A, B1, B2, B3 and C), based on degradability characteristics. Feed intake and urinary excretion data were analysed by ANOVA in a randomised block design. To further explain the ANOVA results, multiple regression analyses were conducted to study relationships between intakes of total N (g/d); sum of the CP fractions A, B1 and B2 (AB1B2; g/d), most of which is considered rumen-degradable; digestible organic matter (DOMI; kg/d); protein balance in the rumen (g/kg dry matter); and urinary excretion of N, urea-N (g/d) and PD (mmol/d). Urinary N and urea-N excretion was positively related to N intake and was better explained by N intake than intake of AB1B2. Feeding BRM resulted in the lowest N intake and urinary N output (P < 0.001). Cows fed MGS, FLS and RCS had similar N intake, but urinary N and urea-N excretion was significantly higher (P < 0.001) in cows fed RCS, which probably was attributable to the significantly lower DOMI of this diet (P < 0.001). Furthermore, addition of DOMI to N intake in the multiple regression analysis increased the proportion of explained variation in urinary N and urea-N excretion. The MGS and FLS diets stimulated rumen MCP production to a greater extent than the BRM diet, as indicated by the higher urinary output of PD in cows fed the grass silage-based diets (P < 0.001). Diet had no significant effect on urinary PD excretion when expressed per kg DOMI. Overall mean urinary creatinine excretion was 0.197 ± 0.047mmol/kg body weight, with no significant effect of diet. This study showed that intake of both N and DOM need to be assessed when choosing a suitable alternative roughage diet for suckler cows, in order to prevent undesirable losses of urinary N.