Kinetics of solutes and particles of different size in the digestive tract of cattle of 0.5–10 years of age, and relationships with methane production

Summary The digestive physiology of cattle is characterised by comparatively long digesta mean retention times ( MRT s), a particle sorting mechanism (difference in MRT s of large vs. small particles) and a distinct digesta washing (difference in MRT s between particles and fluids) in the reticulorumen ( RR ). How these processes mature during ontogeny, and how they link to other digestion characteristics and methane production, is largely unknown. We used a set of passage markers (Co‐ EDTA for fluids and hay particles of 2, 5 and 8 mm length mordanted with Cr, La and Ce, respectively) to measure MRT s in 12 heifers (0.5–2.1 years; hay only) and two groups of 15 lactating cows (2.4–10.0 years; forage‐only vs. forage‐concentrate diet). The MRT s differed between markers (Co < Cr < La < Ce) and were longer in heifers than cows, consistent with the lower feed intake in heifers. MRT s were mostly similar between cow groups and increased with age. Digesta washing was not affected by group, age, feed intake and number of chews per unit of feed. The degree of digesta washing was not related to CH 4 measures. Particle sorting was more prominent in cows than heifers but did not differ between cow groups or change with age in cows. This could be the consequence of the abrupt increase in intake from heifers to cows at a time when gut capacity is not yet fully developed; particle sorting might then clear smaller particles from the RR sooner allowing a higher intake. Surprisingly, CH 4 yield per ingested feed did not correlate with MRT s, and CH 4 yield per unit of digested fibre decreased with increasing MRT s and with increasing fibre digestibility. As this pattern occurred in heifers and both cow groups, it appeared independent of age, indicating a mechanism that has not been described in the literature so far and requires further investigation.