Effect of nitrogen fertiliser supply and wintercutting on morphological composition andherbage digestibility of a Dactylis glomerata Lsward in springMichel Duru,* Vincent Delprat, Catherine Fabre and Estelle FeuilleracINRA Station d’Agronomie, BP 27, F-31326 Castanet Cedex, France

Abstract: The aim of this study was to improve the prediction of sward digestibility in spring as affected

by nitrogen supply and winter cutting, taking into account their effects on the leaf:stem ratio and the

digestibility of the plant parts. Two nitrogen treatments (120kghaÿ1 and nil) and two cutting regimes

were studied over three growing seasons. Herbage digestibility measurements were made about

2weeks before and after the heading stage. Nitrogen de®ciency always decreased the proportion of stem

in the herbage yield signi®cantly, mainly because it reduced or delayed the number of tillers which

reach the double-ridge stage. The stem digestibility was higher when no nitrogen was supplied. We

show that it decreased with increasing stem length, whatever the sampling date and the growing

season. Morever, for a given stem length we noticed a positive effect of herbage nitrogen status,

probably because the stem weight per unit length was lower when N was supplied. Green lamina

digestibility decreased more slowly than that of stem, but at a different rate according to the growing

season. At the whole herbage level, nitrogen supply always decreased the herbage digestibility, and

sometimes cutting in winter increased it. These trends resulted mainly from the effect of nitrogen

supply on the proportion of stem in the herbage and its digestibility.

# 2000 Society of Chemical Industry

Keywords: cocksfoot; digestibility; nitrogen; cut; stem

INTRODUCTIONThe digestibility pattern of herbage with high N input

is rather well known. However, little is known about

the effects of sward management, particularly those of

winter defoliation and nitrogen supply, while these two

practices might be expected to respectively increase

and decrease in Europe in relation to agricultural

policy.1

For forage crops it is well established that at least in

spring the decrease in herbage digestibility is largely a

consequence of a decrease in the leaf:stem ratio,2

because leaf digestibility decreases more slowly than

that of stems. However, there is sometimes a large

intraspeci®c variability in the leaf:stem ratio due to

growing conditions.3 For more accurate prediction of

the herbage digestibility it is necessary to analyse the

process which governs the proportion of the plant

components in the herbage. For a lucerne stand it was

shown that the digestibility increased with water

stress.4 Lemaire et al5 demonstrated that this increase

mainly resulted in a decrease in the weight of stem,

which is less digestible. This effect of growth condi-

tions on the morphological composition of the herbage

has more general applicability than for lucerne. For

Dactylis glomerata we have observed great differences

in reproductive tiller proportions depending on the

available nutrients.6 In pastures, for dicotyledons such

as Geranium sylvaticum, Fleury7 has shown that there is

a positive effect of nutrient availability on the propor-

tion of stem. On the other hand, other studies show

that the nitrogen effect is small8±10 and that it depends

on the duration of the growth period.11 Spring grazing

usually tends to increase herbage digestibility, because

stems are shorter.12 Winter grazing has a similar

effect.13 These examples show that some species

exhibit great plasticity in their morphological compo-

sition according to growing conditions, which could

thus in¯uence the digestibility of the whole plant.

To clarify the in¯uence of nitrogen and defoliation

and to explain some of their contradictory effects, two

aspects should be taken into account. First, nitrogen

and defoliation can have an effect on the number or

proportion of shooting tillers. Second, these factors

could have an effect on plant part digestibility,

particularly that of stems.

The proportion of shooting tillers depends on the

number of tillers which were subjected to a ¯oral-

inductive period and to tiller birth and death. If

nitrogen and defoliation treatments were initiated

before or during the ¯oral-inductive period, they could

Journal of the Science of Food and Agriculture J Sci Food Agric 80:33±42 (2000)

* Correspondence to: Michel Duru, INRA Station d’Agronomie, BP 27, F-31326 Castanet Cedex, France(Received 9 March 1998; revised version received 14 June 1999; accepted 15 July 1999)

# 2000 Society of Chemical Industry. J Sci Food Agric 0022±5142/2000/$17.50 33

affect the number of tillers liable to ¯ower. Indeed,

¯oral induction depends on temperature and day

length, but also on the light, which could vary as a

result of the cutting regime.14 Secondly, the nitrogen

availability could change the apex development as

observed for wheat.15 In fact, nitrogen and defoliation,

besides their effect on the tiller status, could change

the dynamics of each of the tiller types. Tillering stops

when competition for light increases,16 and is low

when there is nitrogen de®ciency.17,18 Furthermore,

when tiller death is frequent, it usually affects the

smallest tillers, which are often the youngest.19

MATERIALS AND METHODSExperimental designThe study was conducted on a clay loam soil near

Toulouse (SW France). A sward of cocksfoot (Dactylisglomerata L, cv Lude) was sown in the autumn of

1994. There were four treatments: two levels of

nitrogen (120kghaÿ1 and nil) were applied at the

beginning of February, factorially combined with two

cutting regimesÐno cutting from autumn (NC) and

two or three cuts during the winter (CC) (Table 1).

Cuts in winter were done so that the canopy did not

intercept more than 70% of the incident radiation; this

was measured with a `Ceptometer' sensor of 80cm

length (Sun¯eck, Decagon, USA).

The four treatments were arranged in a randomised

block design with four replications. Measurements

were made in three consecutive years at different

places in the sward. Plot size was 4m�5m. Cuts

during winter on the CC treatments were done 5cm

above ground level with a mower. In spring, plots were

irrigated if necessary to prevent any water stress.

MeasurementsThe biomass of aerial parts was measured from

samples cut 1cm above ground level with a small

clipping machine two or three times from 2weeks

before to 2weeks after the heading stage. At each date,

subplots of 0.25m2 were randomly chosen. From

herbage collected on each subplot, we selected at

random 100 tillers and sorted them by eye into vegeta-

tive and reproductive (shooting or heading stages:

Rsh). For reproductive tillers the sheath length was

measured from the bottom of the stem up to the ligule

of the youngest leaf. Green and senescent laminae

were separated from the sheaths, stem and in¯ores-

cence. The proportion of stem was de®ned as the ratio

of the weight of stem� in¯orescence to that of the

above-ground herbage. Plant components were dried

at 80°C for 48h, then milled through a 0.8mm screen.

Total nitrogen concentration was measured at each

date for each plot and for the whole sample by a

Kjeldahl procedure. Whole green herbage, green leaf

and stem digestibility were analysed using an enzy-

matic method.20

Tiller density was estimated four or ®ve times during

the experimental period (Table 1) by weighing 50

tillers on a subplot of 0.25m2.

Two (1996) or three (1997) times we collected

scores (50cm�50cm) of soil and vegetation in order

to assess the tiller apex development. On 50 tillers

selected at random, we dissected out the sheath to note

the apex stageÐvegetative (V), `white streak' (R1)

when the apex gets longer,12 double ridges (R2),

appearance of the ®rst stem of the glumes (R3) and

stamens (R4)Ðand to measure the stem length. The

double-ridge stage is usually considered as the ®rst

unambiguous sign of in¯orescence initiation.21 For the

comparison of treatments we have considered the R1

and R2 stages and the stage `stem 10cm long' (R10).

Apex stages were noted as the ratio of the number of

tillers reaching a given stage to the number of observed

tillers. Knowing the tiller density, we calculated the

number of tillers at a given stage per square metre. If

the measurement of apex development did not

coincide with a measurement of tiller density, a ®gure

was found by interpolation between tiller densities.

Global radiation and air temperature were measured

daily and plotted for 10-day periods (Fig 1).

Data analysisAs cutting regimes can induce different levels of

nitrogen availability, it is necessary to assess the

herbage nitrogen status to compare accurately the

different treatments. In order to do this, we could not

simply use the herbage nitrogen content, as it

decreases linearly as the plants grow. Lemaire et al5

showed that for grasses growing with non-limiting N

supply, the N content of a sward (N%) could be

Table 1. Treatments, characteristics and dates of measurements

DM in February

Herbage nitrogen status Sampling dates

Growing season Treatment Precuts in winter (gmÿ2) N1 N0 Digestibility Apex development

1995 NC No 240 73 46 3/5; 17/5; 31/5 No

CC 19/10; 23/11; 23/12 56 67 31 3/5; 17/5; 31/5

1996 NC No 190 79 43 6/5; 22/5 23/4; 16/5

CC 18/10; 19/12 35 81 36 6/5; 22/5 23/4; 16/5

1997 NC No 167 92 37 6/5; 20/5 15/4; 29/4; 13/5

CC 8/10; 4/11; 16/12 55 95 32 6/5; 20/5 15/4; 29/4; 13/5

No, no cut or no observations.

34 J Sci Food Agric 80:33±42 (2000)

M Duru et al

related to the above-ground dry matter herbage yield

(DM): N%=a DMÿb. Numerous works22 have shown

that for optimum nitrogen nutrition the a and bparameters are the same regardless of year or species.

We used the parameters of this control curve to

calculate an index of sward nitrogen status (Ni), taken

as the ratio of the measured N concentration (Nm) of

the above-ground dry matter (DMm) to the optimum

N content.23 The optimum nitrogen concentration in

the herbage for C3 species is N=4.8DMÿ0.32, with

DM and N being the above-ground biomass (thaÿ1)

and the nitrogen concentration in the herbage (% dry

matter) respectively.24

In order to assess the effect of N supply, cutting

regimes and their possible interaction on herbage

characteristics (yield of herbage mass, tiller density), a

two-way analysis of variance was performed. When

characteristics were expressed as a ratio (stem propor-

tion, tiller proportion at a given stage), we used a log±

linear model.

Treatment effectsThe effect of nitrogen supply on the herbage nutrient

status was always marked and was greater for the

treatments which were subjected to cutting in winter

(Table 1). The effects of N varied throughout the

growing season. For the treatments with N supply the

herbage nitrogen status was the highest in 1997. This

is why, once statistical analysis had been done to

compare the effect of treatments, sward characteristics

were expressed in relation to the herbage nitrogen

index. Just before N fertiliser was supplied, stand

herbage mass was signi®cantly higher for treatments

which were not subjected to cutting over the winter

period (P<0.001). The heading stage was reached on

5 May, 6 May and 4 May in 1995, 1996 and 1997

respectively.

RESULTSHerbage yield and proportion of stemThere was a signi®cant effect of nitrogen supply on the

herbage yield (P<0.001). The effect of nitrogen

supply was greatest for the CC treatments, as it was

for the herbage nitrogen status. The effect of cutting in

winter was signi®cant, but less so than that of nitrogen

(Table 2).

Nitrogen supply increased the stem proportion

(P<0.001) signi®cantly (Table 2). For a given

nitrogen supply there was not always a signi®cant

effect of the cutting regime. As for herbage yield, the

proportion of stem on the NC0 treatment was greater

than on the CC0 treatment. This stem proportion

increased with time, but at a given date there were

Figure 1. Average 10-day (Oct1,1–9 October; Oct2, 10–19 October;etc) temperature (°C) and radiation(MJ mÿ2) over the three growingseasons: * 1995, ~ 1996, & 1997.

J Sci Food Agric 80:33±42 (2000) 35

Nitrogen and cutting effects on stem proportion and digestibility

large differences in stem proportions according to

treatments and growing seasons (Fig 2).

Herbage yield signi®cantly increased with time

(P<0.001), except in 1995 (X axis on Fig 3).

However, even in the latter case there was an increase

in stem weight (Y axis on Fig 3). This trend means that

the increase in stem proportion resulted only from a

greater stem weight. In the other cases the stem

proportions increased whereas the total herbage yield

also increased. A power model (S=aWb) was estab-

lished between stem (S) and total herbage (W) yields

for the set of data where there was a signi®cant increase

in herbage yield between two sampling dates (all the

data for 1996 and 1997, but only the data from the ®rst

sampling date in 1995). The ®t was more signi®cant if

each of the growing seasons was considered separately.

Table 2. Herbage yield, stem proportion and plant component digestibility according to growing seasons, treatments andsampling dates

Treatments Signi®cance of effects

Growing season Sampling date CC1 CC0 NC1 NC0 Nitrogen Cut Nitrogen�cut

Herbage mass (g mÿ2)

1995 1 607 115 968 596 *** ***2 807 211 965 613 *** *** **3 857 252 961 599 *** **

1996 1 627 115 722 237 *** *2 765 194 895 298 *** **

1997 1 467 131 579 236 *** **2 775 164 966 351 *** **

Stem proportion (g per 100g)

1995 1 29 0 26 17 *** ** ***2 48 21 45 28 ***3 51 29 57 39 ***

1996 1 50 30 53 29 ** ***2 72 43 72 46 ***

1997 1 40 9 41 3 ***2 43 22 53 35 *** *

Stem length (cm)

1995 2 41 9 56 28 *** *** *3 64 28 71 54 *** *** *

1996 1 30 9 38 14 *** *** *2 51 24 62 30 *** *** *

1997 1 42 13 47 16 *** ***2 64 25 72 33 *** *** *

Stem digestibility (gkgÿ1)

1995 2 531 654 510 613 ***3 421 520 421 525 ***

1996 1 664 771 636 774 ***2 458 584 476 510 ***

1997 1 619 748 584 714 *** **2 458 567 457 524 **

Green lamina digestibility (gkgÿ1)

1995 2 650 700 576 631 *** ***3 572 671 542 579 *** *** ***

1996 1 760 765 738 780 ** *2 734 792 726 769 ***

1997 1 683 718 676 675 *** *2 594 634 618 630

Whole green herbage digestibility (gkgÿ1)

1995 2 577 670 572 620 *** * *3 482 601 461 538 *** ** *

1996 1 741 768 699 779 **2 516 596 515 551 ***

1997 1 657 721 638 677 ** *2 535 619 535 592 ***

* P<0.05, ** P<0.001, *** P<0.0001.

36 J Sci Food Agric 80:33±42 (2000)

M Duru et al

However, for a given herbage yield there were large

differences in stem weight throughout the growing

season (Fig 3). For example, when the herbage yield

reached 800g mÿ2, the stem weight varied from 200 to

550gmÿ2.

The sheath length of reproductive tillers increased

signi®cantly with nitrogen supply and when there was

no cut in winter (Table 2). This variable was positively

correlated to the stem proportion in the aerial biomass

(P<0.05). The sheath mass per unit length decreased

signi®cantly following N supply (P<0.01) and when

the sward was cut in winter (P<0.05). On average it

was 1.62, 1.51, 1.43 and 1.31g mÿ1 for treatments

NC0, CC0, NC1 and CC1 respectively.

Stem and green lamina digestibilityStem digestibility was always signi®cantly reduced

following N fertiliser supply (Table 2). Winter cutting

had a signi®cant effect only at one sampling date. Stem

digestibility decreased throughout the study period. It

was negatively related to stem length, whatever the

sampling date (Fig 4). However, for a given length the

stem digestibility (Ds) for the treatment with N supply

was higher than those without fertiliser N. Considering

herbage nitrogen status (Ni) and the sheath length of

the shooting or heading tiller (Lsh), we have

Ds � 784ÿ 6:67Lsh� 1:6Lsh�Ni;

r2 � 0:78 �P < 0:001�If N1 and N0 treatments, which have contrasting

herbage nitrogen status, were considered separately,

the same variables were selected using a stepwise

regression (P<0.001 and 0.01 respectively for N1 and

N0 treatments). This means that slight variation of

herbage nitrogen status (Table 1) could induce

differences in stem digestibility for stems of the same

length.

As the stem mass per unit length depended on both

herbage nitrogen status and cutting regime, we

investigated to what extent we could substitute the

Ni in the previous relationship by the stem mass per

unit length (sml) considering the two cutting regimes

separately. We obtained

Ds � ÿ�Lsh� sml

rÿ2 being 0.76 and 0.72 for NC and CC treatments

respectively and a being a constant >0. This means

Figure 2. Stem proportions (g per 100g) according to Julian days: * NC1,~ CC1, & NC, ! CC0; 1995 (open symbols), 1996 (full symbols), 1997(shaded symbols).

Figure 3. Stem yield according to herbage yield (g mÿ2): * NC1, ~ CC1,& NC, ! CC0; 1995 (open symbols), 1996 (full symbols), 1997 (shadedsymbols).

Figure 4. Stem digestibility (gkgÿ1) through stem length (cm): * NC1,~ CC1, & NC, ! CC0; 1995 (open symbols), 1996 (full symbols),1997 (shaded symbols).

J Sci Food Agric 80:33±42 (2000) 37

Nitrogen and cutting effects on stem proportion and digestibility

that nitrogen may act on stem digestibility through a

decrease in the sheath mass per linear metre.

At the sampling dates when comparisons were

made, the digestibility of green lamina was always

reduced following nitrogen fertiliser supply and

usually increased in the case of winter cutting (Table

2). The most signi®cant fact was that at the ®rst

sampling date, differences in digestibility between

stem and green lamina were very low, at least for the

N0 treatments (Fig 5). Between the two sampling

dates the lamina digestibility decreased more slowly

than that of the stems, particularly in 1996. The

digestibility of the whole herbage depended on both

the stem proportion and the stem digestibility, these

two sward characteristics being related to the herbage

nutrient status.

Tiller density and tiller typeAt the beginning of the experiment in autumn, tiller

densities were not signi®cantly different between

growing seasons (1730 tillers per square metre on

average, P>0.05). At the beginning of April there was

a signi®cant effect of nitrogen supply (P>0.05),

except on CC1 in 1995 (Fig 6(a)). From this time

there was a general trend towards a decrease in tiller

density (Fig 6(a)). At the end of the experiment there

were no signi®cant differences between the treat-

ments, except in 1995 for N1.

There was a signi®cant effect of nitrogen supply on

Figure 5. Stem digestibility (gkgÿ1) versus green leaf blade digestibility(gkgÿ1): * NC1, ~ CC1, & NC, ! CC0; 1995 (open symbols), 1996 (fullsymbols), 1997 (shaded symbols).

Figure 6. Number of tillers per square metre—(a) total and (b) those reaching the R1 stage: * NC1, ~ CC1, & NC, ! CC0; 1995 (open symbols), 1996 (fullsymbols), 1997 (shaded symbols).

38 J Sci Food Agric 80:33±42 (2000)

M Duru et al

the proportion of shooting or heading tillers (Table 3),

and the number of tillers per square metre reaching

this stage varied greatly according to the nitrogen

treatment. The proportion of shooting or heading

tillers did not vary very much between the two

sampling dates, except on the N0 treatments in

1997, where it increased. In 1996 and 1997 when

sampling dates were similar, we observed a greater

proportion as well as a greater number of shooting or

heading tillers in 1996. Data collected in 1997 were

similar to those obtained in 1995.

The effect of treatments on the tiller percentages

reaching at least the R1 stage was signi®cant in 1997

but not in 1996, except for nitrogen on the second

sampling date (Table 3). On the other hand, the

percentage of tillers passing the R2 stage differed

signi®cantly between treatments for the two growing

seasons. The same was true for the percentage or

number of tillers where the stem length was greater

than 10cm. Finally, the differences between treat-

ments, mainly nitrogen, became greater as the apex

stages or the sampling dates advanced.

In 1996 the number of tillers passing the R1 stage

increased between the two sampling dates (Fig 6(b)),

and at the same time the tiller density decreased,

mainly for treatments receiving N (Fig 6(a)). For the

four treatments in 1997 we ®rst observed an increase

in the number of tillers passing the R1 stage, then a

decrease in a second phase. The ®rst phase coincided

with different trends in tiller density resulting from the

treatment. However, during the second phase the tiller

density always decreased. The decrease in tiller density

was greater than the decrease in the number of tillers

passing the R1 stage. This means that, in contrast to

1996, tiller death in 1997 applied mainly to tillers that

had already passed the R1 stage.

If we compared in pairs the number of tillers passing

over one of the four apex or stem stages of develop-

ment (R1�R2, R2�R10, R10�shooting,

shooting�R2), we observed that in each case the

overall data (treatment�growing season) could be

®tted to a line (Fig 7). This was true for the ®rst

comparison, although the R2 stage is the ®rst one

which could be shown unambiguously to be repro-

ductive. Moreover, even if the number of tillers

passing R10 was lower than the number of shooting

or heading tillers, there was a rather good relationship

between these measurements collected independently

(slope=1.08, intercept=1.58, r =0.945, n =10). In

other words, there was not too much error in the

sampling procedures, even though shooting tillers

were assessed by eye. This is why we could use data

Table 3. Tiller and apex characteristics according to growing seasons and treatments

Shooting or heading

Stage of apex or in¯orescence

Growing season (% or (number)) R1 R2 R1 R2 R1 R2 R10a

1995 17/5 31/5

Treatments CC1 33 (730) 27 (535)

CC0 9 (162) 9 (170)

NC1 32 (523) 41 (562)

NC0 19 (378) 17 (268)

Signi®cance of effects Nitrogen *** ***Cut **Nitrogen�cut

1996 6/5 22/5 23/4 16/5 16/5

Treatments CC1 60 (1075) 59 (715) 26 (444) 19 64 (961) 48 40 (601)

CC0 22 (326) 17 (171) 27 (358) 4 51 (566) 24 10 (111)

NC1 77 (1275) 61 (629) 27 (465) 19 62 (830) 59 53 (710)

NC0 34 (629) 25 (239) 33 (407) 23 58 (812) 35 14 (196)

Signi®cance of effects Nitrogen *** *** ** ** ***Cut *Nitrogen�cut *

1997 6/5 20/5 15/4 29/4 13/5 13/5

Treatments CC1 27 (356) 28 (423) 20 (498) 10 26 (635) 22 26 (367) 14 11 (155)

CC0 4 (59) 10 (100) 11 (203) 4 18 (423) 8 12 (148) 8 2 (27)

NC1 27 (341) 37 (425) 28 (896) 18 32 (1024) 22 47 (568) 34 27 (326)

NC0 10 (12) 22 (284) 22 (406) 16 26 (455) 18 20 (244) 11 3 (37)

Signi®cance of effects Nitrogen *** *** ** ** ** ** *** ** ***Cut * ** *** ** ** *Nitrogen�Cut

* P<0.05, ** P<0.001, *** P<0.0001.a Stem >10cm.

J Sci Food Agric 80:33±42 (2000) 39

Nitrogen and cutting effects on stem proportion and digestibility

from dissected tillers to explain the effect of treatments

on the proportion of stem in the sward. When the same

graphical representation was made for the tiller

percentage instead of the tiller number, similar trends

were observed, because the tiller density varied less

than the tiller stages within treatments and growing

seasons.

If the same comparison was made for R1�R2 stages

at the end of April (not shown), the overall data could

be ®tted to a line as before, but for a given treatment

the number of tillers was always greater in 1997

compared with the following sampling date.

Finally, the growing season effect was very clear, but

it depended on the date of comparison. In 1997 the

tillering slump was more severe than in 1996 and

probably affected mainly tillers that had passed the R1

or R2 stage.

Herbage digestibility and sward componentsThe tiller status signi®cantly affected the morphologi-

cal components of the sward. On the other hand, the

stem percentage in the herbage yield depended both

on the percentage of shooting tillers (Rsh) and on the

stem length (Lsh):

S/W � 16� 0:45Rsh� 0:33Lsh

�r2 � 0:73; se � 7:8�Moreover, as the digestibility of the stems depended

on their length and the herbage nitrogen status (Ni),

and the lamina digestibility was correlated with that of

stems (Fig 5), we could assess the digestibility of the

whole green herbage (Dw) with the same variables:

stem length, Rsh and Ni. Using a stepwise regression,

we obtained

Dw � ÿLsh�Ni �r2 � 0:87; se � 33�Rsh was not signi®cant because it was correlated with

Ni and stem length (P<0.05). In other words,

whatever the growing season and the sampling date,

the digestibility of the whole green herbage depended

on the same variables as the digestibility of stem.

Considering the N fertiliser treatments separately, or

the sampling dates, the same variables or their

interaction were signi®cant, r2 being always greater

than 0.78.

DISCUSSIONNitrogen supply and cutting regimes both affected the

herbage digestibility through the digestibility of the

stem and leaf and the proportion of stem. Further-

more, a growing season effect was observed as else-

where,25 although it was not expected.

Effect of sward management on plant componentdigestibilityNitrogen supply had an effect on stem digestibility as

well as on stem development. In fact, young stem is as

digestible (or more so) as leaf in grasses.26 For

cocksfoot, stem and leaf have similar digestibility until

the end of the shooting period.27 This means that the

decrease in herbage digestibility in spring was also due

to the leaf digestibility pattern. However, digestibility

of stems decreased more quickly as they aged, because

stems differ from leaf blades in that their tissue

characteristics change greatly with age.28 Leaf lamina

digestibility decreased because of an increase in the

leaf insertion level.29 However, the growing season

effect was dif®cult to explain. The leaf sheath

digestibility was generally close to that of stem.26,30

For a given herbage nitrogen status a single relation-

ship between the digestibility of stems and their length

was found, whatever the sampling date and the

growing season. This correlation between stem digest-

ibility and tiller length has been observed before.31

The ligni®ed ring is the major component providing

structural strength and the least digestible fraction.32

For alfalfa it has been shown that high plant density

often decreases stem size and diameter. As a result of

this, lignin content decreased with increasing tiller

density.33 Gastal et al34 also showed that for lucerne

there was a negative relationship between stem

digestibility and length. They suggested that there

was co-ordination between the meristematic activity,

which determines growth in length, and the cambial

activity, which determines the increase in stem

diameter and the ligni®cation of cell walls. If the same

were true for grasses, it could explain the good

correlation we found between stem length and

digestibility. The fact that stem digestibility was lower

for a given stem length as the herbage nitrogen status

decreased could result from a higher stem mass per

unit length. This point must be studied more precisely,

because in this experiment we measured the sheath

length of the shooting or heading tillers. Therefore this

measurement was greater than the real stem length at

the shooting stage and less at heading when the spike is

higher than the sheath.

Figure 7. Relationship between numbers of tillers (tillers per square metre)reaching R1, R2, R10 and shooting stages: * NC1, ~ CC1, & NC,! CC0; 1996 (full symbols), 1997 (shaded symbols).

40 J Sci Food Agric 80:33±42 (2000)

M Duru et al

Effect of sward management on stem proportionAs suggested by Buxton and Marten,35 the proportion

of reproductive and vegetative tillers in swards, as well

as their stage of development, needs to be known to

more accurately predict forage quality. The passage of

the apex from the vegetative to the ®rst and second

reproductive stages (R1 and R2) takes place progres-

sively, as we show for 1996 and between the ®rst two

observations in 1997. However, the rhythm of change

depended mainly on nitrogen supply, as observed

previously.36 Nitrogen de®ciency reduced or delayed

greatly the number of tillers which reached the R2

stage. The effect of nitrogen de®ciency was greatest at

the R10 stage, because it has an effect on stem length

in addition to that on apex development.

The stem proportion in the above-ground herbage

yield was closely linked to the proportion of repro-

ductive tillers, as observed on several occasions.3

Nitrogen supply increased the proportion of repro-

ductive tillers, as observed previously on cocksfoot37

and on perennial ryegrass.38 Conversely, cutting in

winter sometimes decreased it, as Gillet12 observed. In

addition to these general trends, comparisons of total

tiller numbers and tillers reaching the R1 stage have

shown that the stem or the reproductive tiller propor-

tions depended on tiller mortality. In 1996, tiller

mortality preferentially affected vegetative tillers, while

it concerned mainly reproductive tillers in 1997. It is

known that tiller mortality affects mainly the smallest

tillers. which are usually the youngest.19 This is

consistent with the 1996 data. However, Gillet and

Breisch39 show that in spring, tiller mortality could

affect up to 50% of the reproductive tillers as a result of

competition. This is probably what happened in 1997.

Tiller mortality must also be in¯uenced by tiller

competition. Gillet et al40 hypothesised that it could

result from a lack of carbon. Carbon demand should

depend on organogenesis activity, which depends on

temperature. In consequence, the competition was

highest when temperature was high and radiation was

low. Detailed observations show that when the

number of tillers passing the R1 stage increased and

decreased in 1996 and 1997 respectively (the last

10days in April and the ®rst 20days in May), climatic

conditions became very different. In 1996 the radia-

tion:temperature ratio increased by 30% concomi-

tantly with an increase in the tiller number passing the

R1 stage. Conversely, this ratio decreased to the same

extent in 1997 while tillers were dying.

The proportion of reproductive tillers was not

in¯uenced by the appearance of new tillers in spring.

We only noticed an increase in tiller density during a

10-day period in some cases when the competition for

light was limited:16 NC0 and CC0 treatments in 1996

and 1997 respectively. However, this proportion was

not greatly affected by the appearance of tillers in

winter. Thus the proportion of reproductive tillers

depended mainly on tiller induction in winter on the

one hand and tiller mortality on the other. In short, the

stem proportion depended not only on the number of

tillers which were induced, but also on the tiller

dynamics.

A framework to predict the effect of swardcomponents or structure on herbage digestibilityUsually, herbage digestibility is predicted from her-

bage age or development stage (eg heading, etc).4,41 In

this way it is easy to take into account the main factor

affecting change in herbage digestibility (aging), but it

is not possible to consider the effects of management.

Moreover, it was not possible, as was done for

lucerne,34 to express herbage digestibility as a function

of the structural component weight (the stem) and its

digestibility. Firstly, the lamina digestibility of grasses

decreased differently from that of lucerne; secondly,

the stem proportion varied greatly for a given herbage

yield. However, it turns out that the stem length was a

satisfactory indicator of the whole herbage digestibil-

ity, because it was correlated both to the proportion of

stems (affected mainly by nitrogen and cutting date)

and to their digestibility. The relationship could be

improved by taking into account the herbage nitrogen

status or the stem mass per unit length, probably

because it changes the tissue anatomy or its biochem-

ical composition.

CONCLUSIONFor yield stages close to or later than the heading stage

the in vitro herbage digestibility of a cocksfoot sward

was signi®cantly higher when no nitrogen fertiliser was

supplied. This difference resulted from a decrease in

the proportion of stem and a higher digestibility of

these stems. Winter cutting sometimes increased the

herbage digestibility, but its effect, which resulted

mainly in a smaller proportion of stem, was less than

that of nitrogen fertiliser. The experimental data

allowed us to establish an empirical relationship

between herbage digestibility and stem length for any

growing season or cutting time. To build a mechanistic

model, it would be necessary to study the co-

ordination between the meristematic activity and the

cambial activity, particularly by using different nitro-

gen fertiliser rates.

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