The validity of the critical size theory for particles leaving the rumen

TLDR

The study measured particle size and flow resistance in three fistulated sheep fed three grasses and two legumes at two maturity stages, using wet sieving of feed, faeces, reticulo‑rumen, abomasum, and ileum samples and calculating modulus of fineness for particles >0.15 mm. Results showed that the modulus of fineness was higher in feed (5.72) than in reticulo‑rumen (2.56) and downstream compartments (~1.6–1.8), that particles >1.18 mm passed the reticulo‑rumen with only 1–3 % retained, and that flow resistance correlated with size but not with forage type or maturity, leading to the recommendation of a non‑compartmentalized model or a two‑compartment model using a critical sieve size of ~1.18 mm.

Abstract

Summary Three grasses and two legumes, each at two stages of maturity, were fed to three fistulated sheep and samples of the feed, and faeces and contents of the reticulo-rumen, abomasum and ileum were collected for the determination of particle size by wet sieving. Modulus of fineness (MF) was calculated for all particulate matter that failed to pass a 0·15 mm screen. Resistance to flow from the rumen of feed particles of different sizes was also calculated. The mean MF of the reticulo-rumen contents was 2·56 compared with 5·72 for the feed. Material leaving the rumen and found in the abomasum had a MF of 1·67 and subsequent changes were small and not significant; ileum 1·63, faeces 1·80. Particles greater than 1·18 mm passed out of the reticulo-rumen although the quantity was small (1–3%). The resistance to flow of particles of different sizes from the reticulo-rumen was closely related to particle size with no difference between grasses and legumes or between young and mature forages. It was concluded that in modelling the flow of particles from the reticulo-rumen a non-compartmentalized approach should be adopted, but if a simple two-compartment model is required then a critical sieve size of about 1·18 mm may be useful, since less than 5% of the particulate material is retained on this sieve size.