Rate of passage (PROTIEN)

The extent to which a protein is broken down in the rumen depends not only on its innate degradability but also upon the length of time for which it is exposed to breakdown and therefore upon its rate of passage through the rumen. The rate of passage of food from the rumen is affected by a complex of food and animal factors. Passage is faster for:
• smaller particles,
• particles of higher density,
• more highly hydrated particles,
• more highly digested particles.
It will thus increase as digestion and rumination proceeds. Rate of passage increases with increased dry matter intake and is by affected by a number of animal and environmental factors:
• advancing pregnancy limits rumen fill and increases rate of passage,
• lactation increases intake and rate of passage,
• excessive body condition can reduce intake and rate of passage,
• high environmental temperatures will reduce intake and throughput.
The rate of passage may be determined by treatment of the protein with dichromate. The treatment renders the protein completely indigestible, there is no loss of chromium from the protein subsequent to treatment and particle distribution is not affected. The rate of dilution of chromium in samples of rumen contents taken over a period of time can therefore vide an estimate of the rate of passage of the protein from the rumen. The rate of passage of forages may be estimated from the dry matter intake (DMI) of the animal and the proportion of this provided by forage.