Passage kinetics of 13C-labeled corn silage components through the gastrointestinal tract of dairy cows.

Fractional passage rates form a fundamental element within modern feed evaluation systems for ruminants but knowledge on feed type and feed component specific passage rates are largely lacking. This study describes the use of carbon stable isotopes ((13)C) to assess component-specific passage kinetics of 6 intrinsically (13)C-labeled corn silages varying in quality (2 cultivars×3 maturity stages) in a 6×6 Latin square design using 6 rumen-fistulated lactating dairy cows. An increase in maturity increased starch and decreased neutral detergent fiber and acid detergent fiber contents of corn silages. Passage kinetics were assessed for an external (chromium mordanted fiber; Cr-NDF) and an internal marker ((13)C isotopes) collected in feces and omasal digesta. The best fit was obtained with a deterministic multicompartmental model compared with stochastic Gn and GnG1 models with increasing order of age dependency (n=1 to 5) for both sampling sites. The Cr-NDF marker yielded higher rumen fractional passage rates (K1) than did (13)C in the dry matter ((13)CDM) in feces (0.042/h vs. 0.023/h). Omasal marker excretion patterns support the conclusions based on conventional fecal marker excretions. Component-specific passage was assessed for acid detergent fiber ((13)CADF) in feces and for starch ((13)CST) in omasal digesta. The fractional passage rate based on fecal (13)CDM and (13)CADF did not differ. Omasal (13)CST provided higher K1 values (0.042/h) than omasal (13)CDM (0.034/h) but lower values than omasal Cr-NDF (0.051/h). Fractional passage rates from the proximal colon-cecum (K2) based on fecal marker concentrations showed trends similar to K1, with Cr-NDF providing a value (0.425/h) more than twice as high as that of (13)CDM (0.179/h) and (13)CADF (0.128/h). Total mean retention time in the gastrointestinal tract was approximately double for (13)CDM (64.1h) and (13)CADF (77.6h) compared with Cr-NDF (36.4h). Corn silage quality did not affect any of the estimated passage kinetic parameters. In situ fractional degradation rates did not differ among corn silages, except for a decreased fractional degradation rate of starch with advancing maturity. Results indicate that isotope labeling allows assessment of component-specific passage kinetics of carbohydrate fractions in corn silage.