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This thesis examined the productivity and pasture herbage quality of cocksfoot (Dactylis glomerata L.) cv. 'Kara' and prairie grass (Bromus willdenowii Kunth.) cv. 'Matua', under leaf stage based defoliation management. Leaf stage (number of live leaves per tiller) is a generic measure of plant development and was used to investigate the physiology underlying the regrowth response of these species following defoliation, and to determine optimal defoliation management of cocksfoot and prairie grass plants. A survey was undertaken of Tasmanian dairy farmers, to establish the current and potential use of perennial ryegrass (Lolium perenne L.), cocksfoot and prairie grass, and to evaluate quantitatively farmer perception of the feed quality, palatability and dry matter (DM) production of these species under dryland conditions. The survey responses confirmed that perennial ryegrass is currently the dominant perennial grass species utilised in Tasmanian dairy pastures and that the herbage quality, palatability and annual DM production of perennial ryegrass under dryland conditions is generally considered to be substantially higher than for cocksfoot and prairie grass. An initial field study investigated the effect of defoliation management based on leaf regrowth stage on the productivity and herbage quality of perennial ryegrass, prairie grass and cocksfoot under dryland conditions. The DM yield and quality of cocksfoot and prairie grass were found to be comparable with perennial ryegrass under appropriate defoliation management. While the 2-leaf to 3-leaf regrowth stage was previously well-recognised as the most favourable defoliation interval for perennial ryegrass pastures, this study showed that a defoliation interval coinciding with regrowth of four leaves provided an optimal balance between pasture productivity and herbage quality for cocksfoot and prairie grass. The physiology underlying the regrowth response of cocksfoot and prairie grass following defoliation was further investigated in a series of four glasshouse studies. Changes in the physiology and herbage quality of prairie grass during regrowth were examined, and results provided further evidence that the optimal defoliation interval for prairie grass is the 4-leaf stage of regrowth, as this is when increased watersoluble carbohydrate (WSC) levels in the stubble coincide with a resumption of tillering and root growth, but is prior to the reduction of herbage quality due to increased senescent and stem material. The role of nitrogenous (N) and WSC reserves during regrowth of these species was further investigated, concluding that as with perennial ryegrass, the stubble (tiller base below 50 mm height) is the primary storage site for energy reserves and the priority sequence for allocation of WSC reserves follows the expected order of leaf growth, root growth and tillering for both species. Nitrogenous energy reserves were found to play a minor role in the regrowth of cocksfoot plants following defoliation. For prairie grass, although WSC reserves were identified as the primary contributor to plant regrowth following defoliation, there was a strong relationship between stubble N concentration and regrowth parameters. The distribution of WSC reserves within the stubble of cocksfoot and prairie grass was also determined. The pattern of WSC accumulation in the stubble of these species suggests that the previously adopted defoliation stubble height of 45-50 mm (optimal management for perennial ryegrass) is also suitable for the persistence of cocksfoot and prairie grass. However, while decreasing defoliation height to 30 mm may be acceptable for cocksfoot, prairie grass is more sensitive to defoliation severity, with defoliation below 45 mm not recommended. Differences between four cocksfoot and four brome cultivars (including Kara and Matua) under leaf stage based defoliation management in the glasshouse were xamined. Variation between cultivars indicated that there are some improvements resulting from selection and breeding within the cocksfoot and brome genera since the commercial release of Kara and Matua. However, whether the overall value of the newer cultivars to dairy pasture systems exceeds the value of the original cultivars is yet to be determined. The remaining potential limitations to the use of Matua and Kara in the dairy industry, as highlighted in this thesis, include slow establishment of Kara in the field, high rates of seeding for Matua, and relatively high fibre levels for both cultivars compared with perennial ryegrass. However, the overall results show that under defoliation management based on leaf stage, an optimal balance between pasture yield, persistence and herbage quality can be achieved, supporting the future use of cocksfoot and prairie grass in dryland dairy pastures of southern Australia. |
