RUMINAL PASSAGE RATE AS AFFECTED BY CRNDF PARTICLE-SIZE

Fractional rumen passage rates of three particle sizes of CrNDF (less than 0.3, 0.6-1.0 and 15-25 mm) and of CoEDTA were estimated in an experiment with three lactating and three non-lactating rumen-fistulated cows, fed grass silage ad libitum and 7 kg and 1 kg of concentrates, respectively. Total feed intake was higher for the lactating cows, but silage intake did not differ significantly. Fractional passage rates of CrNDF and CoEDTA were derived from the descending parts of the faecal excretion curves and from CoEDTA measured directly in the rumen fluid, following a pulse dose into the rumen. The faecal excretion curves of the 15-25 mm CrNDF particles showed no decline until the last sampling time at 86 h after dosage, so fractional passage rates of these particles could not be determined. Fractional passage rates did not differ significantly between lactating and non-lactating cows, and were on average 2.0% h-1, 4.1% h-1, 8.6% h-1 and 14.9% h-1 for CrNDF (0.6-1.0 mm), CrNDF (less than 0.3 mm), CoEDTA-faeces and CoEDTA-rumen, respectively. The results of this experiment indicate that the particle size of the CrNDF marker has a great influence on the determined passage rates.