The impact of rumen function on neutral detergent fibre degradation across the diurnal period in grazing dairy cows in a pasture based system

The objective of this research was to investigate the effect of diurnal variation in rumen environment on fibre disappearance in lactating, grazing dairy cows in a typical South Island dairy production system. This was achieved in three experimental activities of in vivo, in sacco and in vitro methodologies. For the in vivo study the characterisation of rumen environment by two hourly sampling and rumen evacuations with and without fasting periods between rumen emptyings was carried out. Lactating, ruminally fistulated dairy cows resident in a typical, commercial, South Island herd grazing 2.5-3.0% of their LWT daily of high quality ryegrass via a single 1700h allocation were used. A clear pattern was observed in all seasons where low rumen pH and high concentration of rumen metabolites were detected approximately 8h after the initiation of new grazing each day. Furthermore, a clear difference in rumen environment (pH, VFA mmol/L, and NH3-N mg/L) was found between fasted and grazing cows in the period between two successive rumen evacuations. Generally, rumen fill (fresh weight) at 0100h (120±15kg) was significantly higher than in the morning (0900h; 84±6.7kg) and afternoon (1700h; 89±7.2kg) in each season. However, rumen NDF pool was only significantly different between any times in spring (P < 0.05) and autumn (P < 0.001). The in vivo fibre degradation was estimated by two successive rumen evacuations (0100 – 0900h) in grazing cows, as in this period intake was limited, rumen pH was low, and concentration of rumen fermentation end products and other metabolites high. In autumn, apparent NDF and pdNDF disappearances were higher than in summer. But no differences in these were found between spring and autumn despite the lower rumen pH found in spring (5.5 vs. 5.9) and the differences in NDF content of pasture. Additionally, NDF fractional disapperance rate between 0100 and 0900h was highest in autumn (11.5%/h), followed by spring (5.3%/h) and then summer (2.7%/h). Using the in sacco technique the effect of diurnal rumen variation on fibre disappearance was investigated by incubation of high quality ryegrass (cultivar Bealey) used as frozen minced grass (MG) and fresh chopped grass (FG), with low quality ryegrass hay (H) as a control. Nylon bags incubations were undertaken in two periods of 6h each (from 2000h to 0200h (PM) and from 0800h to 1400h (AM)) where the rumen environment had previously been identified to present a diurnal variation. Frozen MG and H dry matter disappearance in the AM period was higher than in the PM period (P = 0.05 and P < 0.001, respectively). Additionally, MG and H NDF disappearance in the AM period were also higher than in the PM period (P < 0.05 and P < 0.001, respectively). In contrast, DM disappearance of FG cut to a different mass levels (residuals of 1800kgDM/ha for FGpm and residuals of 1450kgDM/ha for FGam) was higher in the PM period (45 vs. 36%). However, no effect of incubation period was found in NDF disappearance after 6h incubation. Further, the impact of K concentration with high and low rumen pH on in vitro NDF degradation was measured to test if high K concentration (via artificial saliva buffer infusion) could reduce the effect of low rumen pH on fibre degradation. High K concentration did not increase either DM or NDF in vitro disappearance 6h post feeding. In contrast, DM and NDF concentration at high rumen pH were 0.6 and 6.7% lower at that time than at low rumen pH (P < 0.05 and P < 0.01, respectively). This research has shown that in vivo rumen pool fibre disappearance of ryegrass in typical grazing cows was not significantly affected by the diurnal variation of rumen environment, although a trend toward reduced DM and NDF degradation was observed. Fibre disappearance was significantly increased when rumen pH was higher and concentration of rumen metabolites lower, when three different feeds were incubated in sacco in two different rumen environments within the diurnal cycle. In in vitro conditions high K concentration did not reduce the effect of low rumen pH on fibre disappearance. Therefore, the typical rumen environment at different diurnal periods in the grazing system of the South Island does appear to impact fibre degradation rates. However, in this type of system, the diminutive overall impact on fibre degradation, confirmed by the consistently high DMI achieved and consequent milk yields suggests there is an effective, dynamic adaptation to the diurnal variation of rumen environment that minimises the overall impact in functional biological terms