Is Neutral Detergent Fibre (NDF) degradation in grazing cattle affected by diurnal variations in the rumen?

Fibre represents a significant fraction of forage diets, and the extended time required to break down the fibre for passage out of the rumen can limit intake. Rumen pH and concentration of rumen metabolites have been described as factors that could interfere with the rate and extent of fibre degradation, but there is no work describing the effect of diurnal variation in rumen environments on fibre degradation. An in sacco trial was conducted at the Lincoln University Dairy Farm from 2010 to 2012 to measure the effect of the diurnal variation of rumen environment on the extent of fibre degradation. High quality perennial ryegrass was cut and frozen and later minced for fibre degradation assessment. The extent of fibre degradation was measured in two diurnal periods of 6 h each (from 2000h to 0200h (PM) and from 0800h to 1400h (AM)) where rumen environment had previously been identified to present a diurnal variation. The mean of the four runs showed that minced grass fibre degradation in the AM period was 22% higher than in the PM period. Rumen environments with a lower rumen pH and higher concentration of rumen metabolites reduced fibre degradation when compared with those with a higher pH and lower concentrations of rumen metabolites