Influence of sward condition on leaf tissue turnover
in tall fescue and tall wheatgrass swards under
continuous grazing
M. G. Agnusdei*, S. G. Assuero†, R. C. Fernández Grecco*, J. J. Cordero†, and V. H. Burghi†‡
*Instituto Nacional de Tecnologı́a Agropecuaria (INTA), Balcarce, Argentina, †Facultad de Ciencias Agrarias,
Universidad Nacional de Mar del Plata, Argentina, and ‡Instituto Nacional de Tecnologı́a Agropecuaria, Manfredi,
Argentina
Summary
Introduction
Two experiments were carried out on a tall fescue
sward in two periods of spring 1994 and on a tall
wheatgrass sward in autumn 2001 and spring 2003 to
analyse the effect of sward surface height on herbage
mass, leaf area index and leaf tissue flows under
continuous grazing. The experiment on tall fescue was
conducted without the application of fertilizer and the
experiment with tall wheatgrass received 20 kg P ha)1
and a total of 100 kg N ha)1 in two equal dressings
applied in March (autumn) and end of July (midwinter).
Growth and senescence rates per unit area increased
with increasing sward surface height of swards of both
species. Maximum estimated lamina growth rates were
28 and 23 kg DM ha)1 d)1 for the tall fescue in early and
late spring, respectively, and 25 and 36 kg DM ha)1 d)1
for tall wheatgrass in autumn and spring respectively. In
the tall fescue sward, predicted average proportions of
the current growth that were lost to senescence in early
and late spring were around 0Æ40 for the sward surface
heights of 30–80 mm, and increased to around 0Æ60 for
sward surface heights over 130 mm. In the tall
wheatgrass sward the corresponding values during
spring increased from around 0Æ40 to 0Æ70 for sward
surface heights between 80 and 130 mm. During
autumn, senescence losses exceeded growth at sward
surface heights above 90 mm. These results show the low
efficiency of extensively managed grazing systems when
compared with the high-input systems based on
perennial ryegrass.
The model first proposed by Bircham and Hodgson
(1983) offers a conceptual framework to analyse the
effect of management on both components of net
production, i.e. gross growth and senescence. It was
shown that net production of perennial ryegrass
(Lolium perenne L.)-dominated swards grazed by sheep
had a marked degree of insensitivity over a wide
range of sward state conditions (Bircham and Hodgson, 1983; Grant et al., 1983; Binnie and Chestnutt,
1994). This insensitivity was mainly explained by
compensating changes in tiller population and production per tiller as well as by the existence of a close
link between growth and senescence rates. In most
temperate swards some degree of adaptation to
management is expected when sward state conditions
are maintained during a certain period of time. The
degree to which such responses could be extended to
other forage species will depend on their ability to
regulate size and density as well as on the stability
of tissue turnover.
A study designed with the purpose of extending
and complementing the studies of Bircham and
Hodgson (1983) and Grant et al. (1983) to different
edaphic and climatic conditions was carried out by
Binnie and Chestnutt (1994) on perennial ryegrass
swards. When swards were vegetative, the authors
also observed a relatively stable net growth rate over
a wide range of sward surface heights. However,
before swards developed a proportion of stem, they
found a sustained increase in net growth rate over
the whole range of sward surface heights maintained.
In the last decade efforts have been made to investigate the variation that exists in temperate grass
species with respect to the effects of sward conditions
on the rates of tissue turnover and on their plasticity
to grazing management. Results obtained by Mazzanti
et al. (1994) on vegetative tall fescue (Festuca arundinacea Schreb.) swards continuously grazed by sheep
under different rates of application of N fertilizer
Keywords: Festuca arundinacea, Thinopyrum ponticum,
leaf tissue flows, tiller population, sward state
Correspondence to: M. G. Agnusdei, EEA INTA Balcarce, C.C.
276, 7620 Balcarce, Argentina
E-mail: [email protected]
Received 5 September 2006; revised 17 October 2006
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
55
56
M. G. Agnusdei et al.
confirmed the previous results on perennial ryegrass
swards on the dynamics of leaf tissue flow. Xia et al.
(1994), working with prairie grass (Bromus willdenowii
Kunth cv. Grassland Matua), an erect species with
year-long production of seed heads that forms low
tiller density swards, observed a limited tiller size and
density compensation in response to increased severity of grazing.
As pointed out by Hodgson (1985), there is a need for
comparable information for morphogenetically differing species and for environmental conditions where
growth and senescence processes are not highly coupled. This is particularly evident for the case of regions,
like the Humid Pampas of Argentina, where pastures
are frequently grown in marginal soils for cropping and
where perennial ryegrass is not well adapted. Only
some cultivated species grow and persist under such
restrictive ecological conditions that include alkaline
clay soils, cool season flooding, summer droughts and
the minimal use of fertilizers. Tall wheatgrass (Thinopyrum ponticum (Podp.) Barkw. & Dewey) and tall fescue
are the most commonly-used species, occupying around
0Æ50 of the area sown with perennial pastures (Molina,
1988; INDEC, 1993). Both species have low rates of leaf
turnover, with leaf lifespans of around 540–600 GDD
(growing degree days) for tall fescue (Lemaire, 1985;
Labreveux, 1998) and 800 GDD for tall wheatgrass
(Borrajo, 1998) when compared with 300 GDD for
perennial ryegrass (Davies and Thomas, 1983). Similar
to what occurs in Australia with tall wheatgrass (Smith,
1996), in Argentina the lack of management strategies
for this species, as well as for tall fescue, generally
results in laxly-grazed pastures, with development of
large tussocks that are not grazed by livestock.
While some information is available concerning leaf
tissue flows and sward structure of tall fescue under
continuous grazing (Mazzanti et al., 1994; Assuero,
1998), at present this information is lacking for tall
wheatgrass. Information on the latter species is scarce
despite it being the dominant component of pastures
sown into alkaline and/or saline soils or low-rainfall
environments and its high potential in soil conservation, as it has been pointed out by Smith (1996). The
available information is limited to a study on leaf and
tiller demography under grazing in a temperate environment of Argentina (Bertı́n et al., 1987) and to
another one on herbage production and survival in
environments with prolonged summer droughts in
the USA (Malinowski et al., 2003).
The aim of this research was to provide information
necessary to develop appropriate management strategies for tall wheatgrass and tall fescue. Both grass
species are also interesting because their morphology
and growth characteristics contrast with those of
perennial ryegrass. In these contexts, two experiments
were carried out on tall fescue and tall wheatgrass
swards continuously grazed by sheep or cattle with
the objective of analysing the effect of sward state on
the dynamics of leaf tissue flows under restrictive
edaphic conditions and fluctuating climatic environment of the Humid Pampas of Argentina.
Materials and methods
Experimental procedures
Two experiments were carried out in Balcarce
(3745¢S, 5818¢W, 130 m above sea level). The first
one was set up on a sward of tall fescue (Festuca
arundinacea Schreb.) cv. Maris Kasba established in
autumn 1992 on a typic Argialboll (Soil Survey Staff,
1999) soil and fertilized at sowing with triple
superphosphate (0-46-0) at a rate of 100 kg ha)1.
In two grazing periods, early spring (20 September to
6 October 1994) and late spring (6 December to
26 December 1994), three sward state treatments
(low ¼ L, intermediate ¼ M and high ¼ H) with two
replicates were randomly allocated to six paddocks of
variable size (0Æ75–1Æ25 ha). Treatments were
established in each plot in order to generate the
maximum possible contrast between extremes in
terms of sward surface height and its correlated
variables, herbage mass and leaf area index (LAI).
Taking into account that this species tends to form
tussocks when leniently defoliated, the highest
sward surface heights were constrained to avoid the
loss of a sward type structure. Swards were continuously grazed by castrated male lambs with an initial
live weight (LW) of 30–35 kg by ‘put and take’
grazing.
The second experiment was carried out on a tall
wheatgrass (Thinopyrum ponticum (Podp.) Barkw. &
Dewey) sward established in 1985 on a Typic Natraquoll
(Soil Survey Staff, 1999) soil fertilized every year with
20 kg P ha)1 and a total of 100 kg N ha)1 in two equal
dressings applied in March (autumn) and end of July
(mid-winter). In two grazing periods, autumn (15 May
to 28 June 2001) and spring (15 October to 7 November
2003), three (L, M and H) and two (L and H) sward
state treatments with two replicates were randomly
allocated to six and four 1Æ5 ha plots respectively. To
define the range of sward state treatments, similar
considerations to those stated for the tall fescue experiments were taken into account. Swards were continuously grazed by heifers with an initial LW (s.e.m.) in the
autumn of 314Æ1 (5Æ0) kg and 238Æ8 (4Æ1 kg) in the
spring by ‘put and take’ grazing.
Meteorological conditions for the experimental
periods, as well as long-term means for these periods
are presented in Table 1.
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Leaf tissue turnover in tall fescue and tall wheatgrass swards
57
Table 1 Mean air temperature, rainfall and global radiation registered at the INTA Balcarce Experimental Station during the
experimental periods. Historical mean air temperature and rainfall correspond to the periods between 1984–93 and 1990–99 for tall
fescue and tall wheatgrass experiments, respectively (s.d. of mean in parentheses).
Mean air
Historical mean
Mean global
temperature air temperature Rainfall Historical rainfall radiation
(C)
(C)
(mm)
(mm)
(MJ m)2 d)1)
Tall fescue
Early spring 1994 (20 September–13 October)
Late spring 1994 (5 December)
Tall wheatgrass
Autumn 2001 (15 May–28 June)
Spring 2003 (15 October–7 November)
12Æ7 (2Æ7)
21Æ6 (3Æ1)
11Æ7 (0Æ8)
18Æ4 (1Æ9)
35Æ0
143Æ3
44Æ5 (29Æ3)
65Æ5 (50Æ8)
12Æ4 (5Æ7)
20Æ3 (5Æ8)
10Æ1 (3Æ4)
15Æ3 (3Æ3)
9Æ3 (0Æ7)
14Æ5 (0Æ8)
103Æ2
69Æ2
86Æ4 (72Æ8)
84Æ5 (51Æ4)
5Æ6 (2Æ1)
17Æ9 (8Æ4)
Sward measurements
Herbage mass was measured three times during each
experimental period (beginning, mid and end) by
cutting to ground level with battery-powered clippers
ten quadrats (200 mm · 500 mm) per paddock. Before
cutting, three sward surface height measurements were
performed on each quadrat with a Hill Farming
Research Organization sward stick (Bircham, 1981;
Barthram, 1986). Herbage samples were processed in
the laboratory for the following botanical composition
determinations: number of grass tillers and contribution
of green leaf lamina, sheaths, dead material and weeds.
Fractions were oven-dried and the proportion of each
component calculated. The green surface area of leaf
laminae was determined using a leaf area meter (Model
LI-3100; LI-COR, Lincoln, NE, USA) and the specific
leaf area was calculated on a DM basis (m2 g)1). LAI
values were estimated as the product of herbage mass
per unit area (g DM m)2), the proportion of leaf
laminae and the specific leaf area. Linear regressions
between the average sward surface height of each
quadrat and LAI were fitted for each sward surface
height treatment. Additionally, the average sward
surface height per paddock was estimated twice weekly
by performing twenty-five random sward surface
height measurements. These data were applied to the
linear regressions mentioned above in order to estimate
the evolution of LAI during the experimental periods.
in the tall fescue experiment, and in series of seven,
equally spaced along a 1Æ4 m transect in the tall
wheatgrass experiment. Ten transects per paddock in
each period were randomly allocated in both experiments. Twice weekly the green lamina length of
individual leaves was recorded on each tiller. From
these measurements it was possible to estimate the leaf
elongation rate and leaf senescence rate as described
previously by Mazzanti and Lemaire (1994). Leaf
elongation rate and leaf senescence rate, expressed in
mm tiller)1 d)1, were converted to gross growth rate)
and senescence rate per unit area (kg DM ha)1 d)1)
using the average dry weight per unit of leaf length and
tiller density.
Statistical analyses
As sward state treatments were not equivalent between
experiments or between periods within experiments, all
four experimental periods were statistically analysed as
independent data sets. A completely randomized design
with a single factor (sward surface height) with two
replicates was used. Analyses of variance (A N O V A ) were
performed using the GLM procedure from SAS (1990).
Means were compared using least significant differences
(LSMEANS statement) at the P < 0Æ05 significance
level. Linear regressions were fitted using the SAS
GLM procedure and slopes were compared using
orthogonal contrasts. Nonlinear regressions were fitted
using SigmaPlot (SPSS Inc. Chicago, IL, USA).
Leaf tissue fluxes
Measurements of leaf tissue turnover were made on
tagged tillers (Davies, 1993; Mazzanti et al., 1994).
Tillers were identified by a loop of coloured plasticcoated telephone wire. To keep the wire loop in
position, one end of the wire was rolled onto an
80-mm length nail buried in the soil at approximately
50 mm from the tiller. Labelled tillers were grouped in
series of ten, equally spaced along a 2Æ0 m transect
Results and discussion
Sward state conditions
In both experiments a gradient of sward states was
achieved in all the experimental periods. (Table 2). Tall
wheatgrass swards had lower sward surface heights in
autumn than in spring. However, mean herbage mass
values were markedly higher in the former season.
2007 The Authors
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58
M. G. Agnusdei et al.
Treatment
Tall fescue
Early spring 1994
Late spring 1994
Tall wheatgrass
Autumn 2001
Spring 2003
SSH
(mm)
Herbage mass
(kg DM ha)1)
LAI
L
I
H
L
I
H
61Æ0
83Æ5
131Æ2
34Æ1
65Æ0
191Æ0
(2Æ0)
(13Æ6)
(11Æ7)
(2Æ4)
(13Æ1)
(5Æ6)
1038
1185
1636
720
1226
1997
(36)
(117)
(127)
(55)
(308)
(205)
1Æ54
1Æ61
2Æ52
0Æ68
1Æ23
2Æ71
L
I
H
L
H
61Æ0
77Æ1
105Æ7
93Æ5
124Æ0
(1Æ6)
(1Æ6)
(7Æ2)
(2Æ0)
(20Æ0)
1647
2328
2561
1105
1498
(21)
(184)
(487)
(40)
(217)
1Æ42 (0Æ07)
1Æ96 (0Æ13)
2Æ60 (0Æ46)
n.a.
n.a.
Table 2 Mean sward surface height
(SSH), herbage mass and leaf area index
(LAI) for tall fescue and tall wheatgrass
experiments (s.e.m. in parentheses).
(0Æ06)
(0Æ01)
(0Æ16)
(0Æ01)
(0Æ01)
(0Æ27)
L, I and H denote low, intermediate and high sward surface height treatments
respectively. n.a., data not available.
Figure 1 shows the relationship of sward surface height
with herbage mass and LAI for both species in each of
the experimental periods. In autumn, slopes for tall
wheatgrass were approximately twice those observed
for both species in spring. In practical terms, these
results indicate that, in order to properly manage this
species through control of sward surface height, relationships between this variable and herbage mass or
LAI for each season would be required.
While the highest sward surface heights in the two
experiments (Table 2) largely exceeded the 80–100 mm
reported for perennial ryegrass swards under approximately non-limiting growing conditions in the UK
(Bircham and Hodgson, 1983; Grant et al., 1983; Binnie
and Chestnutt, 1994), the corresponding LAI values
(Table 2) were markedly lower (4–5 in the UK studies).
These results reveal a comparatively more open and less
leafy structure for the tall fescue and tall wheatgrass
swards. It is worth noting that most of the average LAI
values in these experiments were below 2, which may
reflect the less favourable agronomic and edaphic
conditions relative to those in the UK experiments.
Tiller density
A general feature that emerges from the data is an
overall lower density of tillers than those normally
observed for perennial ryegrass swards in the UK (in the
order of 10 000–50 000 tillers m)2 in pure grass
swards) and even when compared with the values
estimated for perennial ryegrass swards under New
Zealand conditions (4000–11 000 tillers m)2) by Xia
et al. (1990). In part, the markedly lower tiller populations presented here may reflect limitations in the
availability of mineral nutrients. Laidlaw and Steen
(1989) observed significantly lower numbers of tillers
(s.e.m.) of perennial ryegrass growing in mixed swards
with white clover in low-nitrogen swards with average
values of 9600 (1126) tillers m)2 for an application rate
of 60 kg N ha)1 and 14 150 (883) tillers m)2 for an
application rate of 360 kg N ha)1. Despite this probable
main effect of mineral availability, Mazzanti et al.
(1994) found tiller populations of around 6000 tillers ha)1 for tall fescue swards fertilized with
360 kg N ha)1 under continuous grazing. The lower
tiller density of tall fescue relative to perennial ryegrass
swards could be due to their lower site-filling capacity
(Kemp et al., 2001) and their longer phyllochron
(Lemaire and Chapman, 1996). Long phyllochrons
have also been reported for tall wheatgrass (Borrajo,
1998).
In tall fescue swards the number of tillers tended to
be higher on the L and I treatments relative to the H
treatment, though no decline with increasing sward
surface height was evident in the case of the tall
wheatgrass sward (Table 3). While the results on the
tall fescue sward agree with those reported for perennial ryegrass swards under continuous grazing (e.g.
Bircham and Hodgson, 1983; Grant et al., 1983; Binnie
and Chestnutt, 1994), where a negative relationship
between increasing LAI, herbage mass and sward
surface height, and tiller density is observed, those for
the tall wheatgrass sward would appear to be contradictory. However, Matthew et al. (2000), analysing data
on tall fescue swards obtained by Onillion et al. (1995),
showed that size-density compensation would only
occur beyond a critical LAI that depends on species
morphology and environment. Moreover, up to that
critical point, empirical data show a consistent increase
in tiller density and size. This is in agreement with the
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
2700
2400
LAI
2100
1800
2
1500
1200
1
900
0
600
20
40
60
3000
4
2700
2400
3
2100
1800
2
1500
1200
1
900
0
80 100 120 140 160 180 200
600
20
40
(c) 4
3000
2700
2400
3
LAI
2100
1800
2
1500
1200
1
900
0
600
20
40
60
80 100 120 140 160 180 200
Sward surface height (mm)
Herbage mass (kg DM ha–1)
Sward surface height (mm)
60
80 100 120 140 160 180 200
Sward surface height (mm)
(d)
3000
2700
2400
2100
1800
1500
1200
900
600
20
40
60
Herbage mass (kg DM ha–1)
3
(b)
LAI
3000
4
Herbage mass (kg DM ha–1)
(a)
59
Herbage mass (kg DM ha–1)
Leaf tissue turnover in tall fescue and tall wheatgrass swards
80 100 120 140 160 180 200
Sward surface height (mm)
Figure 1 Leaf area index (LAI, –d–) and herbage mass (HM, - -s- -) of tall fescue in (a) early and (b) late spring 1994 and
of tall wheatgrass in (c) autumn 2001 and (d) spring 2003 under continuous grazing at different sward surface heights. The
equations for LAI and HM are the following: (a) LAI ¼ 0Æ5788 + 0Æ0143x R2 ¼ 0Æ91; HM ¼ 484Æ5865 + 8Æ7241x R2 ¼ 0Æ99;
(b) LAI ¼ 0Æ3255 + 0Æ0126x R2 ¼ 0Æ98; HM ¼ 570Æ7825 + 7Æ6893x R2 ¼ 0Æ92; (c) LAI ¼ )0Æ1971 + 0Æ0270x R2 ¼ 0Æ94;
HM ¼ 527Æ0741 + 20Æ3231x R2 ¼ 0Æ74; (d) HM ¼ 314Æ4649 + 9Æ0762x R2 ¼ 0Æ70. LAI values for tall wheatgrass in spring
2003 not available.
findings on the tall wheatgrass sward, as well as with
those reported by Mazzanti et al. (1994) in tall fescue
swards maintained within a range of LAIs from 1Æ6 to
2Æ7. In this context, the decline observed in tall fescue
swards suggests that under restrictive environmental
conditions, size-density compensation could also occur
at LAIs lower than the critical level, e.g. under limiting
nutrient availability. Results reported by Bertı́n et al.
(1987) in a study carried out on tall wheatgrass swards
under natural field conditions and with a comparable
experimental layout to the present study, where significant differences in tiller (1703 and 1035 tillers m)2 for
herbage masses of 1500 and 3500 kg DM ha)1 respectively) in early autumn (end of March) were observed,
support this. Nevertheless, a deeper insight would be
needed on tillering plasticity of tall fescue and tall
wheatgrass swards, particularly under the limiting
environmental conditions in which the species are
usually cultivated.
Tiller morphology
Total green lamina length per tiller showed a consistent
increase with sward surface height (Figure 2). While in
spring the increase in total leaf lamina length per unit
sward surface height was similar between tall fescue
and tall wheatgrass swards, in the tall wheatgrass sward
such an increase tended to be lower in spring than in
autumn (P ¼ 0Æ08). This seasonal difference could have
been determined by a greater aerial biomass partitioning to sheath and stem fractions at the expense of
lamina growth as a consequence of the reproductive
behaviour of the species.
Leaf tissue flows per tiller
Lamina elongation or growth rates per tiller are expected to increase with sward surface height. Examples
have been reported for perennial ryegrass (Bircham and
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
M. G. Agnusdei et al.
Tall fescue
Early spring 1994
Late spring 1994
Tall wheatgrass
Autumn 2001
Spring 2003
Treatment
Tiller density
(tillers m)2)
L
I
H
s.e.m.
L
I
H
s.e.m.
3571
3858
2718
890
1791
1858
1099
401
L
I
H
s.e.m.
L
H
s.e.m.
2939
3671
3343
282
3408
3410
265
(a)
450
Total green leaf length
per tiller (mm tiller –1)
Table 3 Mean tiller density for tall fescue and tall wheatgrass
experiments.
400
350
300
250
200
150
100
0
L, I and H denote low, intermediate and high sward surface
height treatments respectively. No significant differences
(P < 0Æ05) were found between treatments in any period.
Hodgson, 1983; Grant et al., 1983; Laidlaw and Steen,
1989; Binnie and Chestnutt, 1994), tall fescue (Mazzanti et al., 1994) and prairie grass (Xia et al., 1994).
This response reflects the positive association that
generally exists between canopy height and tiller leaf
area and, in turn, between the latter and lamina
elongation rate. The behaviour observed for tall fescue
and tall wheatgrass in the spring and autumn experiments, respectively, was in agreement with the response mentioned above, although nonsignificant
differences were found (Table 4). In the case of tall
wheatgrass in the spring experiment, lamina elongation
rate was insensitive to variation in sward state. This
should not necessarily be interpreted as a lack of
response in terms of total growth per tiller. A proportionally higher contribution of assimilates to the sheath
fraction with increasing sward surface height could
have occurred in the tall wheatgrass spring pasture
when compared with the autumn one, as well as to the
tall fescue ones in both spring periods.
The rate of lamina senescence per tiller in a grazed
pasture is a function of the probability for an individual
leaf to be defoliated before it senesces which, in turn, is
primarily determined by the stocking rate (Mazzanti
and Lemaire, 1994). Therefore, as pointed out by
Lemaire
and
Agnusdei
(2000),
the
typical
linear increase in senescence rate per tiller (or per unit
area) with sward height, normally observed under
40
80
120
160
200
Sward surface height (mm)
(b)
450
Total green leaf length
per tiller (mm tiller –1)
60
400
350
300
250
200
150
100
40
80
120
160
200
Sward surface height (mm)
Figure 2 Total green lamina length per tiller of (a) tall fescue
in early (–s–) and late spring (–d–) 1994 and of (b) tall
wheatgrass in autumn 2001 (- -h- -) and spring 2003 (–m–)
under continuous grazing at different sward surface heights.
The equations for total green lamina length per tiller (TGLL)
are the following: (a) TGLL ¼ 116Æ9836 + 1Æ7592x R2 ¼ 0Æ93;
(b) TGLLA ¼ )55Æ1561 + 3Æ2427x R2 ¼ 0Æ93;
TGLLS ¼ – 50Æ8978 + 2Æ2006x R2 ¼ 0Æ83.
continuous grazing, is a result of a direct effect of
stocking rate on the lamina size that remains undefoliated before senescing. Lamina senescence rate per
tiller of both species tended to increase with sward
surface height increments in all periods (Table 4),
although statistically significant differences between
treatments were only found in the tall fescue sward. In
the case of the tall wheatgrass sward the corresponding
probability values were 0Æ06 and 0Æ13 for autumn and
spring respectively.
The overall consequence of lamina elongation and
senescence rates per tiller on net lamina elongation
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
Leaf tissue turnover in tall fescue and tall wheatgrass swards
61
Table 4 Lamina elongation rate (LER), senescence rate (SR) and net elongation rate (NER) per tiller for tall fescue in early and
late spring 1994 and for tall wheatgrass in autumn 2001 and spring 2003 under continuous grazing at different sward surface heights.
Tall fescue
Early spring 1994
Late spring 1994
Tall wheatgrass
Autumn 2001
Spring 2003
Treatment
LER
(mm tiller)1 d)1)
SR
(mm tiller)1 d)1)
L
I
H
s.e.m.
L
I
H
s.e.m.
4Æ11
4Æ49
5Æ53
0Æ47
4Æ98
5Æ49
8Æ24
0Æ64
1Æ23
2Æ07
3Æ12
0Æ23
1Æ53
3Æ05
4Æ20
0Æ31
L
I
H
s.e.m.
L
H
s.e.m.
2Æ84
3Æ47
3Æ51
0Æ24
4Æ44
4Æ19
0Æ44
1Æ83
2Æ93
4Æ13
0Æ41
1Æ76
3Æ18
0Æ41
b
b
a
b
a
a
NER
(mm tiller)1 d)1)
2Æ88
2Æ42
2Æ41
0Æ26
3Æ45
2Æ44
4Æ04
0Æ34
1Æ01
0Æ54
)0Æ62
0Æ39
2Æ68 a
1Æ01 b
0Æ09
L, I and H denote low, intermediate and high sward surface height treatments respectively. Letters indicate significant differences
between treatments at P < 0Æ05.
rates differed between species (Table 4). While nonsignificant effects of treatments were found in all the
experimental periods in tall fescue, net lamina elongation rate per tiller of tall wheatgrass significantly
declined between the L and H treatments in spring,
showing a similar trend in autumn (P ¼ 0Æ11). Moreover, in the latter period the average net lamina
elongation rate per tiller for L, M and H treatments
represented 0Æ36, 0Æ16 and )0Æ18 of the leaf elongation
rate per tiller respectively. These low and even negative
ratios indicate that pastures were not in steady-state
conditions. In this sense, as pointed out by Parsons
(1988), because tissue does not die at the moment it is
produced, there is always a delay before any variation
in growth rates brings about a concomitant variation in
the rate of tissue death. Consequently, transitions
between seasons may involve uncoupling between
leaf elongation and senescence rates associated with
the differential sizes of growing and senescing leaves
(Robson et al., 1988; Lemaire and Chapman, 1996;
Lemaire and Agnusdei, 2000).
Leaf tissue flows per unit area
Bircham and Hodgson (1983) described the effect of
increasing sward surface height, herbage mass and
LAI on tissue flows for continuously grazed perennial ryegrass-dominated pastures by using different
mathematical functions: inverse quadratic for herbage
growth and linear for lamina senescence. In their case,
main decreases in growth rates increments were
estimated at LAI values of between 2 and 3, where
more than 0Æ80 of the incident solar radiation is
intercepted (Hodgson et al., 1981). Maximum predicted
growth rates of around 120 kg DM ha)1 broadly agree
with the observations obtained by different authors for
perennial ryegrass-dominated swards continuously
grazed by sheep (Grant et al., 1983) or cattle (Laidlow
and Steen, 1989; Binnie and Chestnutt, 1994).
Mazzanti et al. (1994), working with two cultivars of
tall fescue continuously grazed by sheep under adequate conditions of mineral nutrition (360 kg N ha)1),
observed growth rates in the order of 100 kg DM ha)1
for a LAI range of 1Æ6–2Æ7.
The associations between herbage growth rate per
unit area and sward surface height observed for tall
wheatgrass and tall fescue swards in these experiments
were described using an inverse quadratic equation
(Figure 3). Maximum estimated growth rates for the
tall wheatgrass sward were 25 and 36 kg DM ha)1 d)1
for autumn and spring respectively. Corresponding
values for the tall fescue sward in early and late spring
were 28 and 23 kg DM ha)1 d)1 respectively. The
estimated growth rates of both species in these swards
were markedly lower than those reported in the UK for
intensively managed perennial ryegrass swards at
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
62
M. G. Agnusdei et al.
30
30
25
25
kg DM ha–1 d–1
(b) 35
kg DM ha–1 d–1
(a) 35
20
15
10
20
15
10
5
5
0
0
20 40
60
80 100 120 140 160 180 200 220
20 40
1·0
1·5
2·0
2·5
3·0
60
3·5
0·75
1·25
LAI
1·75
2·25
2·75
LAI
(d) 35
30
30
25
25
kg DM ha–1 d–1
(c) 35
kg DM ha–1 d–1
80 100 120 140 160 180 200 220
Sward surface height (mm)
Sward surface height (mm)
20
15
10
20
15
10
5
5
0
0
20 40
–5
60
80 100 120 140 160 180 200 220
Sward surface height (mm)
–10
20 40
60
80 100 120 140 160 180 200 220
Sward surface height (mm)
1·3
1·7
2·1
2·5
2·9
3·3
3·7
LAI
Figure 3 Lamina gross growth rate (GGR; –d–), senescence rate (SR; - -.- -) and net growth rate (NGR; j ) of tall
wheatgrass in (a) early and (b) late spring 1994 and tall wheatgrass in (c) autumn 2001 and (d) spring 2003 under continuous grazing
at different sward surface heights. The equations for GGR, SR and NGR are the following: (a) GGR ¼ 28Æ024 – 36150Æ762x)2
R2 ¼ 0Æ83; SR ¼ )0Æ043 + 0Æ114x R2 ¼ 0Æ81; NGR ¼ 14Æ310 – 0Æ025x R2 ¼ 0Æ27; (b) GGR ¼ 22Æ681 – 6276Æ828x)2 R2 ¼ 0Æ96;
SR ¼ 6Æ260 + 0Æ037x R2 ¼ 0Æ55; NGR ¼ 11Æ023 ) 0Æ009x R2 ¼ 0Æ29; (c) GGR ¼ 25Æ447 – 35905Æ632x)2 R2 ¼ 0Æ60;
SR ¼ )9Æ564 + 0Æ334x R2 ¼ 0Æ81; NGR ¼ 18Æ214 – 0Æ207x R2 ¼ 0Æ80; (d) GGR ¼ 35Æ638 – 85464Æ140x)2 R2 ¼ 0Æ21;
SR ¼ )25Æ386 + 0Æ393x R2 ¼ 0Æ99; NGR ¼ 36Æ946 – 0Æ242x R2 ¼ 0Æ50. LAI values for tall wheatgrass in spring 2003 not available.
comparable LAI levels. This may denote the wide
contrast in terms of nutrients, mainly N, and, possibly,
water availability that characterized the different
experiments.
Growth rates predicted from sward surface heights
corresponding to LAI values of around 2 were 0Æ2–0Æ4
times higher than for the lowest sward surface
heights. These increments were much lower than those
predicted for perennial ryegrass swards within the same
range of LAIs (about 3 times, with growth rates
increasing from around 30 to 80 kg DM ha)1 d)1). In
the case of the tall wheatgrass sward, this phenomenon
is a direct consequence of only slight variations in tiller
growth and density with sward state condition observed
in the present experiment (Tables 3 and 4). In the tall
fescue sward, conversely to what it has usually been
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
Leaf tissue turnover in tall fescue and tall wheatgrass swards
observed in perennial ryegrass (i.e. Bircham and Hodgson, 1983; Binnie and Chestnutt, 1994) and other tall
fescue swards (Mazzanti and Lemaire, 1994), the
increase in lamina elongation rate with sward surface
height (Table 4) was not enough to compensate for the
corresponding decline in tiller density (Table 3). The
restrictive conditions for plant growth that characterized this experiment when compared with those mentioned above, particularly in terms of mineral nutrition,
are assumed to be the main reason for the response
obtained.
As expected, senescence rates per unit area increased
linearly with sward surface height for both species in all
the experimental periods (Figure 3). The slopes observed for the tall wheatgrass sward in both seasons
were significantly higher than those of tall fescue in the
spring. While in the latter the increase in senescence
rate per tiller with sward surface height was partially
compensated by a concomitant decrease in tiller density
(Table 4), such a phenomenon did not occur in the case
of tall wheatgrass where tiller density was not affected
within the range of sward states evaluated.
In the case of the tall fescue sward, predicted average
proportions of the current growth that were lost to
senescence in early and late spring were around 0Æ40 for
the range of sward surface heights of 30–80 mm (L and
I treatments respectively), and increased to around 0Æ60
for sward surface heights over 130 mm (H treatment)
(Tables 2 and 5). Within the narrow range of sward
Table 5 Mean herbage growth rate: senescence rate ratios
(GR:SR) for tall fescue and tall wheatgrass experiments.
Treatment
Tall fescue
Early spring 1994
Late spring 1994
Tall wheatgrass
Autumn 2001
Spring 2003
SR:GR
L
I
H
s.e.m.
L
I
H
s.e.m.
0Æ31
0Æ46
0Æ58
0Æ02
0Æ31
0Æ54
0Æ57
0Æ03
L
I
H
s.e.m.
L
H
s.e.m.
0Æ65
0Æ85
1Æ17
0Æ10
0Æ39 b
0Æ76 a
0Æ04
L, I and H denote low, intermediate and high sward surface
height treatments respectively. Letters indicate significant
differences at P < 0Æ05.
b
a
a
b
a
a
63
surface heights maintained on tall wheatgrass sward in
spring (80–130 mm, Table 3), the average lamina tissue
lost to senescence increased from around 0Æ40 to 0Æ70 of
the current growth (Table 5). During autumn, this ratio
was much higher with senescence rates exceeding
growth rates above 90 mm (Figure 3). In general,
average proportions of the current growth lost to
senescence were high compared with values reported
by Bircham and Hodgson (1983) and Binnie and
Chestnutt (1994) for perennial ryegrass swards maintained at LAIs below 3 under continuous grazing
(senescence rate: growth rate, <0Æ30). This difference
illustrates that extensive grazing systems with swards of
low productivity are, additionally, systems with a low
efficiency in terms of potential herbage utilization. This
finding can be largely attributed to the fact that the
probability of a portion of leaf lamina being defoliated
before senescing decreases with a reduction in stocking
rate (Mazzanti and Lemaire, 1994; Lemaire and Agnusdei, 2000). Accordingly, the much higher herbage
growth rates of perennial ryegrass swards compared
with those for tall fescue and tall wheatgrass swards
were necessarily associated with concomitant differences in terms of the stocking rates required to maintain
equivalent sward states.
The marked degree of insensitivity in net rate of
herbage production over a wide range of sward state
conditions generally observed for perennial ryegrass
swards in the UK contrasts in several aspects with the
results obtained in these experiments. For example, in
the tall fescue sward net growth rates only varied by
0Æ10 for the range of sward surface heights from 60 to
100 mm in early spring and from 30 to 200 mm in late
spring. The stability in net growth rate resulted from
compensating changes in tiller population and production per tiller. Nevertheless, this condition occurred
at LAI values below 2–2Æ5 whereas, in the case of
perennial ryegrass swards, gross and net growth rates at
this LAI were rapidly increasing. On the other hand, the
steep decline in the net growth rates of the tall
wheatgrass sward meant that maximum values were
limited to a remarkably narrow range of sward surface
heights. For instance, moving from 60 to 70 mm in
autumn and from 90 to 100 mm in spring resulted in
net growth rates of 0Æ64 and 0Æ84 of the maximum
values estimated for each season. This high sensitivity of
the net growth rate to grazing management is equivalent to what was found in prairie grass by Xia et al.
(1994).
When changes in tiller population are limited, as
observed in this study, the manipulation of tiller
dynamics has been proposed as an important objective
of grazing management to promote pasture production
and long-term stability (Lemaire and Chapman, 1996).
In this sense, long-term grazing experiments as well as
2007 The Authors
Journal Compilation 2007 Blackwell Publishing Ltd. Grass and Forage Science, 62, 55–65
64
M. G. Agnusdei et al.
detailed studies of tillering plasticity are needed to
elucidate the effect of different grazing managements
on the persistence and stability of herbage production of
grass species that are adapted to grow under restrictive
environments like those typically used for livestock
production in the Humid Pampas of Argentina.
Acknowledgments
The authors thank John Hodgson for helpful discussion
and encouragement in the early stages of the preparation of the manuscript, Adriana Cano for her valuable
help in programming and computing assistance, Alejandra Marino for help in the measurements of leaf
tissue flows, and Oscar Erquiaga and José Mendez for
technical assistance.
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