Effects of sampling time on internal markers concentration and fecal recovery in dairy cows

Abstract

We aimed to evaluate the effects of sampling time points on fecal contents of indigestible DM, NDF, and ADF, fecal recovery, and apparent digestibility in cows fed different levels of rumen degradable protein (RDP) without or with supplementation of calcium salts of fatty acids (CSFA). Eight multiparous Holstein cows (207 ± 23.6 days in milk, 618 ± 39.0 kg body weight; mean ± SD) were randomly allocated into a 4 × 4 Latin square design with 21 days of experimental period. A 2 × 2 factorial treatment arrangement was used in this study to evaluate the effects of calcium salts of fatty acids [CSFA: -CS, basal diet without fat supplementation; and +CS, with 33.2 g CSFA/kg DM] and rumen degradable protein (RDP) levels [LP: low RDP, 98.0 g/kg DM; and HP: high RDP, 110 g/kg DM]. Cows were fed twice daily, and total collection of feces was carried out for 72 h (from D15 8:00 to D18 8:00 h). In addition, spot samples were collected every nine hours, from D15 09:00 until D18 00:00 h. Samples of each sampling time were analyzed for iDM, iNDF, and iADF content. Internal markers showed a partial fecal recovery and, consequently, overestimated fecal excretion, and underestimated total-tract apparent digestibility. Indigestible NDF showed lower fecal recovery than iADF (855 vs. 935 g/kg). An interaction effect between marker and CSFA was observed for marker recovery where fat supplementation had greater influence on iADF recovery compared with iDM recovery. In addition, the fecal concentration of internal markers was affected by sampling time where the lowest concentrations of iDM were observed at 9:00 and the highest concentrations were detected between 15:00 and 00:00. Thus, experimental procedures need to consider diurnal variation of internal markers excretion. Internal markers, especially iDM, show partial fecal recovery.