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of A. latifolia plants being grazed more than twice in the dry season, (compared to 25% in the wet season), and 56% of P. decompositum plants in the wet season (compared to 29% in the dry season). In the dry season 21% utilisation was achieved during day five when 48% of C.
fallax, 24% of forbs, 12% of D. fecundum and A. latifolia and 5% of P. decompositum volume had been removed (Fig. 3a). In the wet season 21% utilisation occurred at day six when 30% of P. decompositum, 25% of A. latifolia, 21% of forbs, 18% of C. fallax and 12%
of A. latifolia volume had been removed (Fig. 3b).
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These findings are important in understanding: the response of the pastures to varying seasonal conditions; the changes in grazing that occur with season, pasture composition and degree of defoliation; and in managing plant/animal interactions. For example, if a rotational grazing system was used, in the wet season voluntary intake and nutrition levels (and presumably liveweight gain) would be better maintained if cattle were moved to the next pasture after three days. In the dry season rotational grazing would seem to confer little benefit. (Extrapolating trial results to commercial situations should be done with caution as the trial was undertaken in small paddocks over short periods with high stocking rates and utilisation levels).
Measurements of pasture height and basal area revealed distinct defoliation patterns in each trial. Species selection was apparent, with C. fallax and forbs preferred initially in the dry season, and P. decompositum in the wet season (Fig. 3c & d). Consistent in both seasons was leaf height removal at the first defoliation. For C. fallax and A. latifolia plants, 60 to 70%
was removed, greater than for P. decompositum or D. fecundum plants where 45 to 53% was removed. Field observation of the grazing of C. fallax and A. latifolia suggested this was due to use of the tongue to grip the longer lamina (Gordon & Benvenutti, 2006), not evident with the other species.
Dry season pasture removal of 48% reduced perennial grass basal area and pasture biomass in the following wet season under this grazing system (White 2012). This indicates that pasture utilisation exceeding the recommended rate of 21% could prove to be unsustainable. At 21% defoliation, large variability in grazing was evident, with plants of the same species ranging from being not grazed, to some 100% defoliated, with no consistency between species. Similarly, large variability was evident in the number of plants grazed either once or multiple times, and for plant removal levels and species preference. This variability probably contributes to the sustainability of the grazing strategy without over-grazing.
The ability to restrict the frequency of plant defoliation is an important management factor in reducing the stress on individual plants and contributing to sustainable grazing (Gammon & Roberts, 1978; Heitschmidt & Walker, 1996). The data showed that, at 60%
pasture removal in the wet season, most stress was placed on P. decompositum, 56% of plants being grazed more than twice, the most being six times. This suggests that P. decompositum is most at risk of over-grazing and multiple defoliations should be restricted to ensure its retention in the pasture.
The different seasonal intake and defoliation patterns highlighted in this study suggest that sustainability could be enhanced by adopting more flexible grazing management with a seasonal basis that responds to the variable seasonal conditions and is sympathetic to key pasture species responses. Being the first study of its type in the Australian tropical savannas, it provided a greatly enhanced level of understanding of the dynamics of the system while also highlighting where management would benefit from additional knowledge.
Acknowledgements
Thanks to P Isherwood (University of Queensland) for the alkane analyses and his field assistance with M Poucin and F Debouche, and NT Department of Resources for access to the Victoria River Research Station. This study was part of the PhD of I. A. White, funded by Meat and Livestock Australia and the AW Howard Memorial Trust.
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