Abstract
Permanent pasture within and adjacent to a mature, widely spaced stand of Populus serotina was continuously grazed to a range of sward surface heights for periods during 1990 and 1991. The effect of sward surface height on the herbage biomass, herbage biomass density, tiller population density, botanical composition and net herbage accumulation in swards maintained at 3,5,7 and 9cm within the tree stand (SWT) , and 5 and 9cm in the adjacent open pasture (SWo) was investigated. Within each sward, herbage biomass varied proportionately with sward surface height, as the herbage biomass density was found to be similar at each height. However, the herbage biomass density was found to be lower in SWT than SWO when the trees were in leaf and hence comparisons between swards of equal surface height in the two areas represented differing herbage biomass. Sward surface height had no effect on the tiller population density (Td) within each area, although closely controlled continuous grazing management during the two seasons' caused the Td of each sward area to become similar. During this period, although sward surface height had no effect on the botanical composition of the swards, there was a proliferation of the higher yielding grasses in the sward (eg. Lolium perenne and Ho1cus lanatus) in line with that found by Td assessment.Net herbage accumulation was unaffected by sward surface height in either sward area, although it tended to be greater in swards maintained at 7cm in SWT and 5cm in SWO and was consistently lower in SWT compared to SWO. Only minor differences were found between rates of tissue turnover (growth, senescence and net production) of L. perenne and H. lanatus when compared at a similar sward surface height (5cm) in SWT and SWO. The grazing exclusion technique and tissue turnover measurement were found to give a similar estimation of the pattern of sward net production, but differed in absolute values at each sward surface height, largely due to the variation in Td.
It was shown that the microclimate in SWT during spring where soil temperatures were lower than SWO limited the growth rate of each species. The leaf senescence rate in SWT was low in autumn so that tiller net production was not reduced to the same extent as in SWO.
The leaf extension rate of H. lanatus was found to be more limited by continuous PAR reduction than L. perenne. However the leaf extension rate of H. lanatus responded more positively to the fluctuating light climate within the tree stand compared to L. perenne. Both L. perenne and H. lanatus became morphologically adapted to the presence of shading during regrowth from tillers which had been previously shaded for a period of 4 weeks. This included a fall in leaf biomass and LAI in the lower strata of the sward canopy and an increase of each in the upper regions of the canopy under severe shading. Stem extension increased, specific leaf area increased, root: shoot ratio fell and final biomass was reduced with increased continuous shading. Although each of these morphological adaptations were also noted in tillers growing within the tree stand, the final biomass was unaffected. It was concluded that these adaptations, together with the more efficient utilization of intermittent PAR within the tree stand, led to a maintenance of final herbage yield.
The main limitation to herbage biomass production in this silvopastoral system was concluded to be sward tiller population density and botanical composition, which were found to be strongly influenced by grazing management.
| Date of Award | Jul 1993 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Department of Agriculture, Environment and Rural Affairs |
| Supervisor | James McAdam (Supervisor) |
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