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To characterise diurnal patterns of rumen pH, fermentation end products and rumen fill in traditionally grazed South Island dairy cows, and the concomitant fibre disappearance rates, an intensive field experiment was carried out in the 2009 – 2010 lactation and in the spring of the 2010 -2011 lactation (Chapter 3). Rumen evacuations, 24h rumen sampling and a liquid phase marker were used to assess rumen function and diurnal fibre degradation in typical grazing conditions.

It was clear from the results of this study that intake patterns typical of South Island dairy systems were the driver for distinct sub-diurnal periods of rumen activity (Chapter 3). The principal parameters of rumen function – pH, VFA concentrations, NH3 concentration– showed

a decline (pH) or rise post prandially, to levels similar to or above that observed in previous pastures studies (Beever et al., 1985). This pattern was conserved between seasons, although there were baseline differences between seasons, further indicating stability of these patterns. The same drivers appeared to alter both faeces and urine measured parameters across the diurnal

145 cycle (Figure 3.9 and 3.10), which is further evidence of the magnitude of these rumen changes, and there was also evidence for sub-diurnal variation in rumen fluid fractional disappearance rate (Figure 3.6), which was seen most clearly in spring.

This work demonstrated clearly the existence of ‘active’ (low rumen pH and high concentration of rumen metabolites) and ‘less active’ (high rumen pH and low concentration of rumen metabolites) sub-diurnal periods in dairy cows grazed under this system. The available literature around fibre degradation would then suggest that these active periods are most likely to be associated with reduced fibre digestion rate (Bergen, 1972; de Veth & Kolver, 2001b). Given the clear differences between seasons (Figure 3.5), it would also be reasonable to expect changes in fibre degradation between these.

Rumen evacuations (in vivo technique) enabled fibre disappearance and clearance rate to be estimated from the entire rumen DM pool, on the basis of evidence for a markedly reduced intake at certain diurnal windows even in unrestricted grazing cows in this system. Traditionally, periods of fasting have been used between successive rumen emptyings reducing the need to use markers to identify either the initial rumen pool and/or any additional intake over the period when disappearance was calculated (Gregorini, Gunter, Beck, et al., 2008; Taweel, 2004; Taweel et al., 2005a). However, in Chapter 3, experiment two there was evidence that feed denial between rumen evacuations in diurnal windows where grazing was otherwise typical influenced rumen environment (Table 3.15 and Table 3.11) thereby potentially influencing fibre degradation.

A non-restricted grazing system between rumen evacuations was used to ensure that the rumen environmental conditions against which fibre degradability was assessed were similar to those of non fistulated grazing cows. Rumen evacuations were carried out every 16h to minimise any potential negative effects of short intervals between consecutive rumen evacuations on rumen microbes and normal rumen function. However, calculations of fibre degradation were undertaken every 8h (Section 3.2.5) by assuming pasture allocation and grazing conditions were similar, by strict management, throughout the experimental period. This procedure permitted, with definable limitations, some assessment of in vivo rumen environment on apparent fibre disappearance and clearance rate in periods within a day and among seasons with as little interference as possible with normal grazing behaviour. As a consequence the effects of the “real” rumen environment on fibre disappearance of lactating cows grazing temperate pasture were investigated in concert with rumen environment quantification across the diurnal cycle, and then between seasons.

Rumen evacuations in non-restricted grazing cows revealed a clear diurnal pattern of rumen fill among seasons (Table 3.2) with the highest rumen fill approximately 8h after the new daily pasture allocation. Similar diurnal patterns of rumen fill in cattle and sheep in grazing conditions have been described previously (Gregorini, Gunter, & Beck, 2008; Taweel et al., 2004; Thomson et al., 1985). Furthermore, mean rumen DM and NDF pool sizes (g/kg BW) reported were similar to those calculated from previous studies that used either grazing lactating cows with overnight starvation (Chilibroste et al., 1997), or cut and carry protocols with some concentrate supplementation (Taweel, 2004). This gave confidence in the approach taken in this study.

Apparent fibre disappearance between two successive rumen emptyings was estimated in the period (0100 to 0900h) when it could be reasonably assumed from the previous high rumen fill and existing grazing behaviour studies that intake was markedly reduced, rumen pH was low but increasing and concentration of metabolites high and declining (Table 3.13). This period was then considered to hold an ‘active’ rumen sub-diurnal period and, therefore, was available for use in comparing between seasons.

Unexpectedly, apparent NDF disappearance was similar in spring and autumn cows in spite of the differences in pasture composition (Table 3.1) and the lower rumen pH in spring cows (5.5 vs. 5.9, respectively). Apparent fibre disappearance in summer however was lower than in autumn, even though pasture composition was similar, especially NDF, in both seasons. It is possible that an increased number of cows, with the additional resources required for sample collection and analyses, would have increased the chance of identifying the impact of specific rumen fermentation end products on fibre degradation in experiment one where three cows, three seasons and one period were used (Figure 3.8). It is worth noting that the standard errors of those parameters recorded were low, therefore this is not automatically the case. However, in Chapter 4, the in sacco fibre disappearance method, with a greater specificity, revealed relationships with rumen pH, NH3-N and osmolarity (Table 4.10), despite the

proportion of the total variability explained being very low. This suggests that the in vivo relationship of rumen function and fibre degradation is clearly more complicated than simple assessment of individual parameters can elucidate.

High rumen fluid fractional disappearance rates were estimated in each season (Figure 3.6) and were higher than values reported by Berzaghi et al. (1996) on lactating grazing dairy cows in summer pastures or on fresh cut grass in summer and autumn (van Vuuren et al., 1992). This is the first such report of disappearance rates of this magnitude in grazing cows, and is an example

147 of the extreme rumen environment changes associated with high intakes of highly managed pastures (Table 3.1). Such high fluid passage rates may cause greater washout of small particles (both pdNDF and iNDF) from the rumen to the lower GI tract, which may contribute to important differences from other grazing dairy production systems reported on. This increased transport of potentially degradable material to the lower GI is also is responsible for the faecal parameter changes across the diurnal period (Figure 3.9).

Overall, estimations of fibre disappearance by this method (rumen evacuation technique with unrestricted grazing at windows of identified minimal grazing) suggested that fibre degradation was not strongly affected by the in vivo rumen environment, although there was a trend to that effect, and despite the obvious extremes of pH and fermentation products and metabolites across that diurnal period. This is an indication, perhaps, of the complexity and plasticity of rumen function in this environment. However, two periods with the greatest contrast in the rumen environment in this grazing system were identified and a more detailed study where replicate standard feed samples were incubated in these periods was undertaken Chapter 4.