Development of a pasture model for grazing studies

The object of this thesis was to study those aspects, mainly of pasture origin, which affect the herbage intake of the grazing ruminant.Literature was reviewed on the growth of grass, its nutritive value, the manner in which pasture is grazed, and its regrowth after defoliation. The nutritive value of grass was examined largely in terms of its digestibility.Two experiments were carried out to obtain quantitative informationon sward structure and the digestibility of the components. The first experiment used S24 perennial ryegrass growing in field conditions.The weights, heights, digestibilities and ages of parts of the tiller were monitored throughout the year and under different levels of nitrogen fertilizer. The organic matter digestibility of the two youngest leaves (89% 1st leaf, 86% 2nd blade) varied to only a small extent with season. Major changes in leaf digestibility (from about 82% to about 65 or 70%) were associated with a change in colour from green to brown. Nitrogen fertilizer had a small temporary effect in increasing leaf appearance rate, reducing leaf lifespan, and reducing leaf digestibility. It had a substantial effect on the vertical distribution of dry matter in the sward.The second experiment examined leaf growth and digestibility following the removal of a particular leaf or leaves from tillers of S24 ryegrass. The investigation took place in controlled environment rooms under three temperature regimes (6°C/3°C; 10°C/4°C; 19°C/4°C)_and low light intensities (40 - 90 Wm-2) . There was a reduction in the weight of new leaves only when defoliation was severe and included the removal of the two youngest leaves. Digestibility was unaffected.The discussion develops a conceptual framework for a grazing model. Pasture is represented in terms of the spatial distribution, weight and digestibility of its component parts. Some of the quantitative relationships are provided by the information obtained from the two experiments reported. The model provides a possible means of testing alternative hypotheses as to how the ruminant selects its diet