Endogenous fraction and urinary recovery of purine derivatives in Nellore and Holstein heifers with abomasal purine infusion

The excretion of purine derivatives (PD) as an index of microbial biomass production in the rumen can be biased by the endogenous fraction and the incomplete urinary recovery of absorbed purines. Moreover, several authors have suggested differences for endogenous fraction of purine derivatives between genetic groups. The objective of this study was to estimate the endogenous fraction of PD and the urinary recovery of purines in Nellore and Holstein heifers fed diets containing 60% corn silage and 40% concentrate daily (13g dry matter/kg body weight). Daily creatinine excretion, intake and digestibility of dry matter and nutrients were also evaluated. Four Nellore heifers and four Holstein heifers fistulated at the rumen and abomasum, body weight of 270±7.76 and 225±7.16kg, respectively, were allocated in two 4×4 Latin squares. The experimental treatments consisted of four doses of RNA infusion (Torula Yeast): 0, 33, 66, or 100mmol/day. The endogenous losses of PD and the recovery of purine bases were estimated using a regression of the daily excretion of PD (Ŷ) and the abomasal flow of purine bases (X). There was no difference (P>0.05) between the regression equations for PD excretion and daily abomasal flow of purines for each genetic group, resulting in the following regression: Ŷ=0.405+0.923X, where 0.405mmol/kg⁰.⁷⁵ and 0.923 represent the endogenous fraction and the recovery of purine bases in the abomasum, respectively, for both genetic groups. The infusion and genetic groups did not affect (P>0.05) the daily creatinine excretion, which averaged 27.23mg/kg BW. We concluded that there are no differences between Nellore and Holstein heifers in the endogenous fraction of PD, the urinary recovery of purines, and creatinine excretion, with average values of 0.405mmol/kg⁰.⁷⁵, 0.92, and 27.23mg/kg BW, respectively.