Short communication: Comparison of 3 solid digesta passage markers in dairy cows

This study investigated the usefulness of acid-detergent fiber-bound ¹⁵N [acid detergent insoluble (ADI)-¹⁵N] as a solid digesta passage marker in dairy cows compared with chromium (Cr) and ytterbium (Yb) (as labeled fiber or forage, respectively). Intrinsically (ADI-¹⁵N) or extrinsically (Cr, Yb) labeled alfalfa hay was pulse-dosed intraruminally to 7 lactating dairy cows. Following marker administration, spot fecal samples were collected for up to 72h for marker analyses. Urine and milk samples were also collected and analyzed for Yb and Cr. Fecal marker excretion data were processed using 2-compartment mathematical age-dependent/age-independent (G_n→G₁) models. The rate of passage of the marker in the first, age-dependent compartment tended to be slower for Yb compared with Cr and ADI-¹⁵N, which resulted in a trend for longer mean retention time (MRT) in this compartment when Yb was used as a marker (19.0h) compared with Cr and ADI-¹⁵N (14.5 and 13.9h, respectively). The rate constant of marker disappearance for the second or age-independent compartment tended to be greater for Yb compared with Cr and ADI-¹⁵N, which led to a shorter MRT of Yb in this compartment (15.6) versus ADI-¹⁵N (32.1) and Cr (24.8h). The cumulative MRT was greater for ADI-¹⁵N versus Cr and Yb (46.0, 39.3, and 34.4h, respectively). Total MRT of marker tended to be greater for ADI-¹⁵N than for Yb (46.6 vs. 36.6h, respectively). Urine and milk analyses data suggested no measurable losses of Yb along the digestive tract, but about 0.79% of Cr dosed intraruminally was secreted or excreted in milk and urine in the 48-h period following marker administration. Collectively, this study confirmed previous observations that ADI-¹⁵N can be used reliably as a solid digesta passage marker for ruminants, producing pre-duodenal and total-tract retention times similar to that of Cr-labeled fiber. Retention time in the age-independent compartment was underestimated when Yb was used as a marker, emphasizing the need to process forages to isolate fiber before labeling with Yb.