Physiological and morphological responses of tall fescue (Festuca arundinacea Schreb.) and perennial ryegrass (Lolium perenne L.) to defoliation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph. D.) in Plant Ecology and Physiology, Department of Plant Science, Massey University
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Date
1993
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Massey University
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Abstract
Tall fescue (Festuca arundinacea Schreb.) has been suggested as an
alternative to perennial ryegrass, particularly in conditions of moisture limitation,
but there is little comparative information on the plant characteristics influencing
regrowth in the two species, particularly under continuous stocking. The objectives
of this study were to :i) examine the response of tall fescue to continuous stocking
in terms of tillering activity, leaf growth and competition, and ii) determine which
physiological or morphological factors are important in influencing regrowth after
defoliation, using perennial ryegrass as a reference standard. Experiments were
undertaken in field, glasshouse and controlled environment conditions.
In the field sown swards of tall fescue (Festuca arundinacea Schreb. cv.
'Grassland Roa') and white clover (Trifolium repens L. cv. 'Grassland Tahora'),
with volunteer grasses mainly consisting of perennial ryegrass (Lolium perenne L.
cv. 'Grassland Nui') were continuously stocked with varying numbers of sheep to
maintain sward surface heights 90-100 mm (Lax, L), 50-60 mm (Medium, M) and
30-4 0 mm (Hard, H). Measurements were made on areas with and without clover
by removal of white clover with clopyralid. Tiller population density, tiller weight,
and leaf growth and productivity of tall fescue were all reduced under hard grazing.
Tall fescue was susceptible to competition from companion species, particularly
perennial ryegrass, and tended to be replaced by other species under hard
grazing.
Under glasshouse conditions individual plants of tall fescue and perennial
ryegrass were defoliated to stubble heights of 100 mm (Lax, L), 60 mm (Medium,
M), 30 mm (Hard1, H1) and 30 mm (Hard2, H2) 9 times with 5 day intervals over
a period of 4 5 days. Treatments L, M, and H1 were initiated at an average of 1 1
tillers per plant for each species; treatment H2 commenced at 6 tillers per plant.
Both species showed sensitivity to severe cutting treatments by reduction in tiller
number, tiller weight, leaf growth and less shoot and root growth. Tall fescue
showed lower leaf growth, tillering activity and herbage harvested per plant than
perennial regress, but it produced larger tillers.
The comparative response of tall fescue and perennial ryegrass to leaf
defoliation was studied under controlled environment conditions at both the
vegetative and reproductive stages of growth. The oldest leaf lamina was
defoliated regularly to maintain four, three, two or only one live leaf per tiller for six
or seven leaf appearance intervals. In both species repeated removal of older
leaves had little effect on tiller production, tiller weight, leaf growth rate and
consequently total accumulated shoot and root weight, and mean shoot and root
relative growth rates. Removal of all fully expanded leaves resulted in significant
reduction in the above components, though leaf elongation rate was little affected.
Leaves were shorter, narrower and lighter under hard defoliation, but leaf
appearance rate was not affected. Hard defoliation affected tall fescue tiller weight
more than perennial ryegrass. Water soluble carbohydrate concentrations in stem
bases of plants decreased with increasing severity of defoliation especially for tall
fescue. Leaf photosynthetic capacity per unit area was not influenced by
defoliation intensity, but photosynthetic capacity per unit leaf weight increased
under hard defoliation. Hard defoliation decreased the proportion of root mass to
shoot mass, and increased the proportion of leaf mass to shoot mass. Plants
showed relatively similar sensitivity to defoliation at vegetative and reproductive
phases of growth. At both phases tall fescue again produced larger tillers with
longer leaves and had a longer leaf life-span than perennial ryegrass, but it had
lower leaf growth and appearance, and produced fewer tillers per plant.
Photosynthetic activity per unit leaf area was similar for the two species, but tall
fescue often had lower photosynthetic rate per unit leaf weight than perennial
ryegrass.
In conclusion, hard defoliation intensity reduced both tiller population
density and tiller weight and consequently decreased pasture regrowth through a
reduction in LAI, life-span of leaf area, photosynthetic efficiency, and shortage of
carbohydrate reserves. The factors that resulted in the regrowth of tall fescue
being less responsive to hard defoliation than perennial ryegrass were slower leaf
turnover, slower leaf appearance rate, lower tillering capacity and longer leaf lifespan.
These resulted in lack of plasticity in tiller population density of tall fescue
in response to hard defoliation.
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Keywords
Pastures, Defoliation, Fescue arundinacea, Ryegrasses, Grasses, Growth