Ingestive processes and digestion of highly digestible pastures by strip-grazing dairy cows

© 2003 Dr. Yvette Joy WilliamsThree grazing experiments and one stall feeding experiment were conducted to gain an understanding of the relationships between grazing behaviour and rumen fermentation pattern in lactating dairy cows strip-grazing highly digestible pastures. An understanding of these relationships is fundamental to elucidating the mechanisms underlying the interaction between grazing cows and supplements which impacts upon the profitability of supplement feeding in the wide range of feeding systems in Northern Victoria. Grazing experiments examined the effects of pasture type, pasture allowance and cereal grain supplements on daily intake, intake rate, rumen fermentation patterns, pasture degradation rates and rumen digesta pools. The stall feeding experiment attempted to determine the effect of ruminal fluid pH on neutral detergent fibre (NDF) degradation in highly digestible pasture. The type of pasture grazed was found to have important influences on the fermentation patterns which suggests that different supplementary feeding and grazing management strategies may be required for annual clover-based pastures compared to perennial ryegrass-based pastures. The pasture allowance offered to the cows also had important influences on the diurnal grazing and rumen fermentation patterns. Work in this thesis hilighted the need for methods that provide accurate estimation of intake as interactions between pasture allowance and intake and supplementary feeding and intake are used in decision support models to predict production responses to and profitability of different feeding decisions. An important finding was that ruminal fluid pH's were below 6.0 for a large part of the day in all grazing experiments however there were no conclusive signs of clinical or subclinical acidosis. Important also was that ruminal conditions (concentrations of volatile fatty acids and ammonia, digesta pools) showed large diurnal fluctuations related to grazing patterns in a strip-grazing management system. The effects of these low ruminal fluid pH's and large fluctuations in rumen conditions on the degradation rate of NDF in highly digestible pasture are still unclear. However, it was clear that cereal grain supplements reduced the degradation rate of NDF. While research in this thesis has provided new knowledge on the effects of pasture type and supplementary feeding on the fermentation in the rumen or grazing dairy cows, it has also emphasized the complexity of the interctions between the sward, supplementary feeding and the cow. A more complete understanding of the processes involved would be required to better predict cow performance across the range of management systems that exist. For example the relationship between plant cell wall chemistry and the rumen microbe is complex and not completely understood under rumen conditions when cows are grazing highly digestible pastures. Characterisation of the populations of bacteria at the low ruminal fluid pH observed when cows graze highly digestible pastures is also required