Passage kinetics of concentrates in dairy cows measured with carbon stable isotopes

Fractional passage rates form a fundamental element within modern feed evaluation systems for ruminants, but knowledge on feed-specific fractional passage is largely lacking. Commonly applied tracer techniques based on externally applied markers, such as chromium-mordanted neutral detergent fibre (Cr-NDF), have been criticised for behaving differently to feed particles. This study describes the use of the carbon stable isotope ratio (¹³C : ¹²C) as an internal digesta marker to quantify the fractional passage rate of concentrates through the digestive tract of dairy cows. In a crossover study, five dairy cows were fed low (24.6%) and high (52.6%) levels of concentrates (dry matter (DM) basis) and received a pulse-dosed Cr-NDF and ¹³C isotopes. The latter was administered orally by exchanging part of the dietary concentrates of low ¹³C natural abundance with a pulse dose of maize bran-based concentrates of high ¹³C natural abundance. Fractional passage rates from the rumen (K₁) and from the large intestine (K₂) were determined from faecal marker concentrations of Cr-NDF and of ¹³C in the DM (¹³C-DM), NDF (¹³C-NDF) and neutral detergent soluble (¹³C-NDS). No differences in K₁ estimates were found for the two concentrate levels fed but significant differences between markers (P<0.001) were observed. Faecal Cr-NDF excretions gave lower K₁ estimates (0.037–0.039/h) than ¹³C-DM (0.054–0.056/h) and ¹³C-NDF (0.061–0.063/h). The ¹³C-NDS was calculated by the difference of ¹³C in the DM and NDF, and K₁ values (0.039–0.043/h) were comparable to Cr-NDF. Total mean retention time was considerably higher for Cr-NDF (40.9–42.0 h) as compared to ¹³C-DM and ¹³C-NDF (32.0–33.5 h; P<0.001). The accuracy of the curve fits for Cr-NDF and ¹³C-DM and ¹³C-NDF was overall good (mean prediction error of 9.9–13.9%). Fractional passage rate of Cr-NDF was comparable to studies where this marker was assumed to represent the fractional passage of roughages. However, K₁ estimates based on the ¹³C : ¹²C ratio varied considerably from studies based on external markers. Our results suggest that the use of ¹³C isotopes as digesta passage markers can provide feed component-specific K₁ estimates for concentrates and provides new insight into passage kinetics of NDF from technologically treated compound feed.