Quantifying the interactions between defoliation interval, defoliation intensity and nitrogen

fertiliser application on the nutritive value of rainfed and irrigated perennial ryegrass

K.G. PembletonA*, R.P. RawnsleyB, L.R. TurnerB, R. CorkreyC, D.J. DonaghyD

AUniversity of Southern Queensland, School of Agricultural, Computational and

Environmental Sciences and Institute for Agriculture and the Environment, Toowoomba

QLD 4350

BTasmanian Institute of Agriculture, University of Tasmania, Burnie, TAS 7320

CTasmanian Institute of Agriculture, University of Tasmania, Hobart, TAS 7320

DMassey University, Palmerston North 4442, New Zealand

*Corresponding Author: Keith.Pembleton@usq.edu.au

Running head: Impact of defoliation management on nutritive value

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Abstract. A key goal of temperate pasture management is to optimise the nutritive value and

production of pastures. Extensive research has examined the influence of single components

such as irrigation, nitrogen (N) fertiliser, and grazing interval and grazing intensity, yet

conjecture remains regarding practices that optimise pasture nutritive value. Much of this

conjecture relates to interactions between inputs and grazing management. A two-year split-

split plot experiment was undertaken to investigate these interactions using a perennial

ryegrass dominant pasture at Elliott, Tasmania. Irrigation treatments (rainfed or irrigated)

were main plots and defoliation intervals (leaf regrowth stages; 1-leaf, 2-leaf or 3-leaf) were

subplots. Defoliation intensity (30, 55 or 80 mm defoliation height) and N fertiliser (0.0, 1.5

or 3.0 kg N/ha/day) were crossed within sub-subplots. Herbage samples were collected from

each plot four times over the experimental period and were analysed for neutral detergent

fibre (NDF), acid detergent fibre (ADF) and crude protein (CP) concentrations (% dry matter

(DM)). Metabolisable energy (ME) concentration (MJ/kg DM) was estimated from these

values. The ME concentration decreased as defoliation height and defoliation interval

increased for all time points except during winter. The CP concentration increased with

increasing N fertiliser applications in the plots defoliated at the 1-leaf stage, but this increase

only occurred as N applications increased from 1.5 to 3.0 kg N/ha/day for the plots defoliated

at the 2-leaf and 3-leaf stages of regrowth. As N application rates increased from 0 to 1.5 kg

N/ha/day, plots defoliated at the 3-leaf stage had greater increases in NDF concentration

compared to plots defoliated at the 1-leaf stage of regrowth, except during spring. As

defoliation height and interval increased ADF concentration increased in both spring and

summer. While defoliating at frequent intervals (1-leaf stage) and lower heights (30 mm)

produced pasture of a marginally higher nutritional value, these benefits are mitigated by the

previously established negative consequences of lower pasture yield and poor pasture

persistence. Consequently, grazing management that maximises pasture productivity and

persistence (defoliated between the 2- and 3-leaf regrowth stages to a height of 55 mm)

should be applied to perennial ryegrass pastures irrespective of input management.

Key words: grazing management, pasture-based dairy systems, intensive pasture production

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Introduction

The most cost effective feed source for dairy cows in the temperate regions of Australia and

New Zealand are perennial ryegrass (Lolium perenne L.) dominant pastures (Chapman et al.

2009). Consequently, mixtures of perennial ryegrass and white clover (Trifolium repens L.)

are the principle feedbase supporting the dairy industry in these regions (Doyle et al. 2000;

Holmes 2007). Grazing management is a key driver of the productivity, nutritive value and

persistence of temperate pastures and has attracted considerable research attention (Graham

et al. 2000; Fulkerson and Donaghy 2001; Lee et al. 2008). Despite this, conjecture remains

around the application of grazing management principles due to genetic gains in pasture

breeding programs (approximately 0.5% per year; Lee et al. 2012), and an intensification of

grazing systems via increased supplementary feeding and water and fertiliser (particularly

nitrogen (N)) inputs.

The two most important aspects to grazing management are grazing interval (rotation

length or when to graze) and grazing intensity (post-grazing residual). In practice, the

number of days between consecutive grazing events, pasture height, or pasture mass are

generally used to schedule grazing intervals on farm. However, Fulkerson and Donaghy

(2001) identified that using day rotations to schedule grazing events fails to consider seasonal

variation in weather, which affects pasture growth. Pasture height and pasture mass, while

reflecting weather conditions, are animal-related indicators for grazing and do not take into

consideration plant-based factors that influence when a pasture is physiologically ready for

grazing. Scheduling grazing interval based on leaf regrowth stage considers the recovery of

plants in terms of energy reserves as well as pasture growth rates and nutritive value

(Fulkerson and Donaghy 2001). Perennial ryegrass pastures achieve their maximum growth

rates between the 2-leaf and 3-leaf stage of regrowth (Rawnsley et al. 2014). Daughter tiller

formation occurs between the 1-leaf and 2-leaf stages of regrowth, while the maximum

accumulation of plant energy reserves occurs around the 3-leaf stage (Fulkerson and

Donaghy 2001). It is now widely accepted that repeatedly grazing a perennial ryegrass

pasture prior to the 2-leaf stage reduces plant persistence and overall yield, while allowing

pasture to grow beyond the 3-leaf regrowth stage reduces nutritive value, with no additional

benefit on yield or persistence (Rawnsley et al. 2007). Consequently, it is well established

that perennial ryegrass pasture should be grazed between the 2-leaf and 3-leaf stages to

optimise yield, nutritive value and persistence (Fulkerson and Donaghy 2001; MacDonald et

al. 2010).

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Grazing intensity is best conceptualised by the height, mass or leaf area of pasture

remaining after grazing (Brougham 1960; Korte et al. 1982). While grazing to a target leaf

area might result in a good outcome in terms of pasture growth, this practice is difficult to

implement beyond single plants. Target residual pasture mass is often used within grazing

systems (Eastwood and Kenny 2009), however, residual height is often quoted in the

literature because it is easy to control in an experimental context (Lee et al. 2008; Brink et al.

2013). Since perennial ryegrass stores the majority of energy reserves in the bottom 40 mm

of the tiller, grazing to below this height can limit energy storage capacity and reduce

regrowth and persistence (Fulkerson and Donaghy 2001). On the other hand, grazing much

above 50 to 60 mm means that herbage is not fully utilised (Lee et al. 2008). Perennial

ryegrass exhibits phenotypic plasticity (changes in growth habit) in response to repeated low

or high post-grazing heights. Grazing studies in Ireland using high or low stocking rates

(analogues to low and high post grazing defoliation heights) have identified changes in the

proportion of leaf and stem in ryegrass swards (O'Donovan and Delaby 2005). Such

adaptation by the plant limits its growth potential (Lee et al. 2008) by reducing the radiation

use efficiency of the canopy, through increasing shading of new photosynthetically-efficient

leaves by older leaves, or increasing the amount of light intercepted by the tiller base rather

than leaf.

While defoliation interval and defoliation intensity, as along with fertiliser use and

irrigation management have been studied in isolation (e.g. Lee et al. 2008; Rawnsley et al.

2009; Pembleton et al. 2013), there is a paucity of information regarding how they interact to

influence pasture growth, nutritive value and persistence. Such knowledge is important to

fine tune best-practice pasture management guidelines as the dairy industry increases its use

of irrigation and N fertiliser to drive pasture production and maintain its cost competitiveness.

Past research has highlighted that there is only a slight decline in pasture nutritive

value as perennial ryegrass progresses from the 1-leaf stage to the 3-leaf stage of regrowth

(Turner et al. 2006), with nutritive value decreasing thereafter as leaves senesce and stem

accumulates (Hunt 1965; Davies 1971). However, nutritive value can decline at any

regrowth stage if canopy closure occurs, due to shading inducing stem formation and leaf

death (Rawnsley et al. 2007) along with the relatively poorer nutritional value of stem and

pseudostem compared to leaf (Beecher et al. 2015). There are indications that under higher

levels of inputs (N and water) that typify modern dairy pasture management, pastures are

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more likely to reach canopy closure early in the regrowth cycle, and this is associated with an

earlier decline in nutritive value (McKenzie et al. 2003).

We have previously reported how the interactions between input use (irrigation and

N), defoliation interval (as determined by leaf regrowth stage), and defoliation intensity

(defined by height) affected the yield and growth rates of perennial ryegrass over a 2-year

period (Rawnsley et al. 2014). From this it was concluded that to achieve maximum growth,

perennial ryegrass should be grazed at the 3-leaf stage unless conditions were conducive to

high (>60 kg dry matter (DM)/ha/day) pasture growth rates (achieved in spring or under high

levels of N fertiliser use; Rawnsley et al. 2014). In these situations, perennial ryegrass should

be grazed between the 2-leaf and 3-leaf regrowth stages. To maximise pasture production,

perennial ryegrass should always be grazed to a target post-grazing height of around 50 mm

irrespective of conditions. In this paper we report on how these interactions affect the

nutritive value of pasture over a two year period and discuss grazing management in relation

to optimising both pasture growth and nutritive value.

Methods

Site description

The experiment was undertaken at the Dairy Research Facility of the Tasmanian Institute of

Agriculture at Elliott in North West Tasmania (-41.093o, 145.780o, 155 m a.s.l.). At this

facility the soil is a deep clay loam red ferrosol (Isbell 1996) and the pre-experiment soil test

indicated a baseline soil chemical fertility of 16.0 mg phosphorus (P)/kg (Olsen extraction)

293 mg potassium (K)/kg (Colwell extraction) and 12.6 mg sulfur (S)/kg (potassium chloride

extraction) along with a pH(water) of 6.3. The climate at this location is classified as ‘temperate

moist’ with a winter dominant rainfall pattern under the Köppen climate classification system

(Kelleher 1994). Prior to the experiment, the site was an established perennial ryegrass (cv.

Impact) pasture that was periodically grazed with dairy heifers and non-lactating dairy cows.

Treatments and experimental design

The experiment investigated the response of rainfed and irrigated perennial ryegrass

defoliated at the 1-leaf, 2-leaf or 3-leaf stages of regrowth (the average number of days

between defoliations, the average growth rate and the average pasture yield for each

treatment in autumn, winter, spring and summer are provided in Table 1 and Table 2) to

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heights of 30, 55 or 80 mm and receiving 0, 1.5 or 3 kg/ha/day of N fertiliser (applied

retrospectively after each defoliation). The experiment was arranged as a split-split plot

design. Plot dimensions were 3 × 2 m and were replicated three times. For each season there

were a total of 162 plots within the three blocks. Each block consisted of two main plots

(irrigation-rainfed) each of which contained three subplots (defoliation interval). Each

subplot consisted of 9 plots to which the defoliation height and fertiliser treatments were

randomly assigned. Irrigated and rainfed main plots were separated by a 12 m buffer area of

perennial ryegrass. Defoliation interval and height subplots were separated by a 1 m buffer

of perennial ryegrass. All buffer areas were regularly defoliated (every two to three weeks)

and the cut material removed.

Agronomic practices

The field site was sprayed with RoundUp (360 g/L, glyphosate, Nufarm Australia Ltd.

Laverton North, Vic, Australia) at a rate of 6 L/ha on 19 August 2009. Fourteen days after

spraying, the site was power harrowed and then perennial ryegrass (cv. Arrow; heading date

of +10 days relative to cv. Nui) was immediately sown at 25 kg/ha with an air seeder drill

(Amazone drill, AMAZONEN-Werke H, Dreyer GmbH & Co., Hasbergen, Germany). The

plots were established in the weeks following sowing and the site was fenced to exclude

livestock. All plots were defoliated with a rotary mower (Brigs and Stratton) to a 55 mm

height four times (8 December 2009 and 6 January, 2 February, and 1 March, 2010).

Following the first defoliation, 241.5 kg P/ha and 11.5 kg S/ha was applied by hand to each

plot in the form of triple superphosphate (21% P, 1% S). After the fourth defoliation, the

irrigation, defoliation and N fertiliser treatments commenced. Nitrogen fertiliser treatments

were applied by hand, in the form of urea (46% N) to the plots on the same day as they were

defoliated.

Irrigation treatments were applied through a pressurised irrigation system consisting

of 32 micro sprinklers (MP 200-360 rotator, Hunter Industries Inc. San Macros, CA, USA)

arranged on a 4 × 4 m grid pattern in the irrigated main plots. With the use of pressure

regulators, this system achieved a distribution uniformity greater than 80% with a delivery

rate of 5 mm/hour. Irrigation water was applied on a 20 mm rainfall deficit, calculated from

estimated evapotranspiration and rainfall. Rainfall was measured at the experimental site.

Estimated evapotranspiration was calculated by the FAO 56 method (Allen et al. 1998) using

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weather data collected at the experimental site. In total, 180 and 380 mm of irrigation water

were applied in the 2010/11 and 2011/12 irrigation seasons, respectively (Table 3).

Soil tests (to a depth of 75 mm) across the experimental site occurred in July 2010

(28.7 mg P/kg Olsen extraction and 138 mg K/kg Colwell extraction) and March 2011 (26.3

mg P/kg Olsen extraction and 268 mg K/kg Colwell extraction). Based on these results, the

plots received 52.5 kg P/ha and 500 kg K/ha in August 2010 and 52.5 kg P/ha and 250 kg

K/ha in September 2011. These fertilisers were applied in the form of triple superphosphate

along with muriate of potash (50% K).

Plots were harvested with a rotary mower when the perennial ryegrass plants reached

their assigned defoliation interval treatment. The leaf regrowth stage of each main plot by

subplot treatment was assessed twice weekly by sampling 30 random tillers per treatment.

Sample collection

Samples were collected from the harvested material from each plot in spring 2010

(November), winter 2011 (June/July), summer 2011/12 (December) and autumn 2012

(April). Samplings occurred when each defoliation treatment was harvested and occurred

over a maximum length of 21 days (shorter during periods of rapid pasture growth and leaf

emergence) for each period of sampling. Approximately 200 g of fresh herbage material was

collected from each plot and dried at 60oC for 48 hours in a fan-forced drying oven. Dried

herbage samples were ground to pass through a 1 mm screen and then stored in sealed bags

while awaiting analysis of nutritive value.

Nutritive value analysis

All herbage samples were analysed by the DairyOne Forage Laboratory (Ithaca, New York,

USA). Nitrogen concentration was determined via Kjeldahl digestion followed by titration

(Thiex et al. 2002). Crude protein (CP) was calculated by multiplying N concentration by

6.25. The neutral detergent fibre (NDF) and acid detergent fibre (ADF) concentrations were

determined using the methods outlined in van Soest et al. (1991) and AOAC (1990).

Metabolisable energy (ME) at three times maintenance intake was calculated using the

National Research Council 2001 energy model for dairy cattle (National Research Council

2001). These calculations required acid detergent insoluble crude protein (ADICP) and

neutral detergent insoluble crude protein (NDICP) to be estimated from CP and NDF using

the following equations (P. Sirois (DairyOne) pers. comm. February 2014):

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ADICP% DM = CP% DM × 0.07

NDICP% DM = (CP% DM × 0.33) + (NDF% DM × 0.143) – 8.77

In the calculation of ME, standard values for ryegrass forage of 4.4, 7.8 and 2.6% DM were

used for fat, ash and lignin concentrations, respectively. These values were based on the

DairyOne forage database (available online at http://dairyone.com/analytical-services/feed-

and-forage/feed-composition-library/; Accessed: February 2014).

Statistical analysis

Data from each sampling event were analysed separately. All analysis was conducted using

R (R Core Team 2015) assuming a split-split design in which sub-subplots contained a

completely random design. Post-hoc comparisons were calculated using least significant

differences (LSDs). Effects were regarded as significant at the 0.05 level, except where

otherwise indicated. Residuals from the analyses were examined using quantile-quantile

plots to assess outcome data for normality and homogeneity. No data required transformation

prior to analysis.

Results

At each sampling event, there were significant effects of N fertiliser rate (P < 0.01) and

defoliation interval (P < 0.01) on herbage ME concentration (Table 4). For spring 2010,

winter 2011 and autumn 2012 assessments, these factors interacted (P < 0.001, 0.01 and

0.001, respectively). There was a trend for ME concentration to decrease with increasing

defoliation interval, although the maximum difference in ME between the 1-leaf stage and the

3-leaf stage treatments was 0.8 MJ /kg DM (Fig. 1). In spring 2010, the ME concentration of

swards that were defoliated at the 1-leaf stage of regrowth was greater than those defoliated

at the 2-leaf and 3-leaf stages. While the swards defoliated at the 2-leaf stage had greater ME

concentrations than those defoliated at the 3-leaf stage when zero N was applied, when 1.5 or

3 kg N/ha/day was applied, there was no difference in the ME concentration. For the zero N

fertiliser treatment there was no difference in ME concentration observed between defoliation

interval treatments in winter 2011. In summer 2011/2012, the ME concentration of plots

defoliated at the 3-leaf stage was lower than the other defoliation interval treatments,

irrespective of N fertiliser treatment. In autumn 2012, the ME concentration was not affected

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by N fertiliser application rate when plots were defoliated at the 1-leaf stage, but in the

swards defoliated at the 2- or 3- leaf stage was lower with the application of 1.5 kg N/ha/day.

There was a significant effect (P < 0.001) of defoliation height at each sampling

(Table 4), with a trend for ME to decrease slightly with higher defoliation heights across all

defoliation interval treatments at the spring 2010, summer 2011/12 and autumn 2012

samplings (Fig 2). At the winter 2011 sampling, the swards defoliated at the 1-leaf stage did

not show a decrease in ME concentration as defoliation height was increased. The average

decrease in ME concentration with defoliation height did not exceed 0.7 MJ /kg DM.

Leaf regrowth stage and N fertiliser rate both influenced pasture CP concentration (P

< 0.001 and P < 0.001, respectively; Table 4), with CP concentration increasing under more

frequent defoliation (especially the 1-leaf stage) at all sampling events (Fig. 3). Crude

protein concentration generally increased with increasing N applications in spring and winter.

In summer 2011/12 (for all defoliation interval treatments) and autumn 2012 (for the 2-leaf

and 3-leaf stage defoliation interval treatments), CP concentrations initially declined as N

fertiliser application rate increased from 0 to 1.5 kg N/ha/day, and then increased as N

fertiliser application rate further increased to 3.0 kg N/ha/day. In all seasons, the CP

concentration of plots decreased as defoliation height increased from 30 mm to 55 mm (Table

5). In summer 2011/12 and autumn 2012 there was a further decline when defoliation height

increased from 55 to 80 mm. However, these decreases were relatively small with the

greatest decline of 3.6% DM observed for the summer 2011/12 sampling (Table 5).

In autumn 2012, defoliation height interacted with N fertiliser treatment and also with

irrigation treatment (P < 0.01 and P < 0.001, respectively) to influence CP concentration

(Table 4). At the 55 or 80 mm defoliation heights, plots receiving 3 kg N/ha/day had

between 1.5 to 2.1% DM greater CP concentration compared to those plots receiving 0 or 1.5

kg N/ha/day (Fig. 4a). Under irrigation, CP concentration decreased as defoliation height

increased from 30 to 55 to 80 mm (Fig. 4b). Under rainfed conditions, the CP concentration

decreased between the 30 and 55 mm defoliation height treatments but did not differ between

the 55 and 80 mm defoliation heights.

In summer 2011/12, CP concentration decreased as defoliation interval increased

(Table 6). Under rainfed conditions, this decrease was more apparent between the 2- and 3-

leaf stage intervals compared to the 1- and 2-leaf stage intervals. .

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Apart from swards that received zero N fertiliser and were defoliated at the 3-leaf

stage of regrowth in spring or the swards that were defoliated a the 1-leaf stage in winter,

swards that received zero N fertiliser had a lower NDF concentration than those receiving

either 1.5 or 3.0 kg N/ha/day N fertiliser (Fig. 5). However, there was no difference in NDF

between the swards that received either 1.5 or 3.0 kg N/ha/day. When zero N fertiliser was

applied there were only minimal (<3% of DM) differences in NDF between the defoliation

interval treatments.

The NDF concentration was lower at the 30 mm defoliation height compared to the 80

mm defoliation height (Table 5). In winter and spring, the NDF concentration for swards

defoliated to a height of 55 mm were not significantly different to swards defoliated to a

height of 30 mm. In summer and autumn the NDF concentration of swards defoliated to a

height of 55 mm was intermediate to those defoliated to either 30 or 80 mm, and significantly

different to both.

In winter 2011, NDF concentration was influenced by a three way interaction (P <

0.05) between irrigation, defoliation height and defoliation interval treatments (Table 4).

Under rainfed conditions there was no impact of defoliation height on NDF concentration

when the pastures were defoliated at the 1-leaf or 2-leaf stages of regrowth. However, when

defoliated at the 3-leaf stage, the NDF concentration increased as defoliation height was

raised from 30 to 80 mm (Fig. 6). Under irrigation, there was no difference in NDF

concentration between the defoliation interval treatments when defoliated to a height of 30

mm, but when defoliated to a height of either 55 or 80 mm, the swards defoliated at the 2-leaf

stage had a greater NDF concentration than those defoliated at the 1-leaf stage. When the

irrigated swards were defoliated to a height of 80 mm, those defoliated at the 3-leaf stages

had a greater NDF concentration compared to those defoliated at the 1-leaf stage.

In summer 2011/12, NDF concentration was affected by an interaction (P < 0.01)

between irrigation and defoliation interval treatment (Table 4). While NDF concentration

increased as defoliation interval increased from the 1- to 3-leaf stage under both irrigated and

rainfed conditions, the pattern of increase was different (Table 6). Under rainfed conditions,

the NDF concentration for plots defoliated at the 2 and 3 leaf stage interval were not

significantly different. In contrast under irrigation the NDF concentration for plots defoliated

at the 2-leaf stage were lower than those defoliated at either the 1- or 3- leaf regrowth stage.

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The ADF concentration was affected (P < 0.001) by the defoliation interval treatment

in spring 2010 (Table 4) with the 1-leaf stage resulting in a lower ADF concentration

compared with the 3-leaf stage (Table 7). Defoliation height also influenced ADF

concentration in spring 2010 (P < 0.05) and summer 2011/12 (P < 0.001; Table 4). In spring

2010 the swards defoliated to 55 mm had a lower ADF concentration than those defoliated to

80 mm. In summer 2011/12 the swards defoliated to 30 mm had a lower ADF concentration

compared with those defoliated to 55 or 80 mm. The ADF concentration in summer 2011/12

was affected by an interaction (P < 0.05) between irrigation treatment and defoliation interval

(Table 6). The ADF concentration increased by 2.5% DM between the 2- and 3-leaf

regrowth stage under both rainfed and irrigated conditions and by 2.3% DM and 1.0% DM

between the 1- and 2-leaf regrowth stages under rainfed and irrigated conditions,

respectively.

In winter 2011 defoliation height interacted with defoliation interval (P < 0.001) and

N fertiliser treatment (P < 0.05) to affect ADF concentration (Table 4). When swards were

defoliated to 30 mm there was no difference in the ADF concentration between the

defoliation interval treatments, with an average of 21.7% DM (Fig. 7a). At the 1-leaf

regrowth stage the swards defoliated to 80 mm defoliation height had the same ADF

concentrations as those defoliated to 55 mm while for those defoliated at the 2-leaf and 3-leaf

stages the ADF concentration increased as defoliation height increased from 55 to 80 mm.

There was no effect of N fertiliser application on the ADF concentration of swards defoliated

to 30 mm (Fig. 7b). However, at 55 or 80 mm defoliation height the swards that received

zero N fertiliser had lower ADF concentrations than the swards that received 1.5 kg

N/ha/day. When the swards were defoliated to 80 mm the ADF concentration was reduced

by 1.6% DM under the zero N fertiliser treatment compared with the 3 kg N/ha/day

treatment.

Discussion

Overall the differences in key nutritive value parameters between treatments were relatively

small. This was unexpected, considering the variation between some of the treatments (e.g. 0

vs 3.0 kg N/ha/day of N fertiliser applied or defoliated at the 1-leaf vs 3-leaf regrowth stage).

While greater differences for ME, NDF and ADF may have been observed under more

extreme treatments, the treatment combinations in our experiment reflect a wide range of

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pasture practices applied within pasture based dairy systems in temperate Australia and New

Zealand. This indicates that the potential gains to be made in pasture nutritive value through

a refinement of well-established best management grazing practices will be incremental

rather than transformative. Consequently, grazing management of dairy pastures should be

prioritised to optimise yield and persistence, with nutritive value only considered when larger

effects are observed (i.e. in summer).

Across all the treatments, the CP concentration was in excess of what is required for

high levels of milk production (National Research Council 2001). Excess CP (>17% DM for

high producing cows, National Research Council 2001) in the diet is metabolised into

ammonium in the rumen, then converted to urea in the liver, and excreted in the urine. This

has two consequences within pasture-based dairy systems. First, the process requires energy

to be expended (Oldham 1984) (between 0.035 and 0.050 MJ/g N; National Research

Council 2001), reducing the amount of energy available for milk production, and second, it

leads to an increase in urinary N output which increases N losses to the environment (either

through leaching, volatilisation or de-nitrification from urine patches) (Dijkstra et al. 2013;

Pacheco and Waghorn 2008). These consequences decrease the N use efficiency of dairy

systems (de Klein and Ledgard 2001; Eckard et al. 2004). By far the greatest influence on

CP concentration that we observed was the leaf regrowth stage at defoliation. In this study

the pastures defoliated at the 3-leaf regrowth stage maintained the lowest CP concentration at

all samplings, between 14.5 and 31.5% DM. Allowing pastures to complete their regrowth

cycle (for ryegrass, to the 3-leaf stage) not only positively impacts on pasture yield and

persistence (Fulkerson and Donaghy 2001), but should help reduce excess CP intake, and

maintain a desirable WSC:CP ratio (Turner et al. 2014). Therefore, grazing rotation is a

powerful tool in pasture-based dairy systems, to reduce N lost to the environment.

Across the experiment, the greatest impact on pasture ME was defoliation interval, as

determined by the leaf regrowth stage. We observed a general decline in pasture ME

concentration and an increase in NDF concentration as defoliation interval was extended

from the 1-leaf to the 2-leaf then to the 3-leaf stage, which is consistent with previous

findings (Fulkerson and Donaghy 2001; Turner et al. 2006). For the summer sampling we

also observed some evidence of a decrease in ME concentration as N fertiliser increased or

defoliation height was increased. Interestingly, these declines occurred irrespective of the

leaf regrowth stage for all but one of the sampling events. Potentially the increase in plant

size driven by the higher N fertiliser applications increased the relative proportion of stem

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and pseudostem and hence increasing the structural (cellulose, hemicellulose and lignin)

components of the plant (Beecher et al. 2015). In winter, the ME concentration of swards

defoliated at the 3-leaf stage decreased with increased N fertiliser. However, this did not

occur for the swards defoliated at the 1- or 2-leaf stage. During spring, increasing N

application rate arrested both the decline in ME concentration and the increase in NDF

concentration with increasing defoliation interval. This seems counter-intuitive as higher

rates of N fertiliser use during periods of rapid growth should increase the prevalence of

canopy closure. However, the N deficit stress under the zero N fertiliser treatments during

this period of rapid growth (Rawnsley et al. 2014), coupled with the longer defoliation

interval of the 3-leaf stage defoliation treatment, would have enhanced the onset of

reproductive development (and hence stem growth) in this treatment, leading to higher

concentrations of fibre and a concomitant lower ME concentration.

Defoliation height had a relatively consistent impact on the nutritive value of the

pasture over the experiment, with pastures defoliated to 80 mm having lower CP, greater

fibre, and lower ME concentrations than the pastures defoliated to 30 mm. Interestingly,

there were often only minimal differences in the NDF concentration between the plants

defoliated at 30 mm and 55 mm. Under grazing O'Donovan and Delaby (2005) found only a

small (but statistically significant) effect from different grazing intensities on the NDF

concentrations across a number of ryegrass cultivars. Lee et al. (2008) identified a similar

result for perennial ryegrass pastures defoliated across five different defoliation heights that

ranged from 20 mm to 100 mm. These authors concluded that a post-grazing height of

between 40 and 80 mm should be targeted to optimise nutritive value and production. Our

results suggest that this recommendation could be refined to a target post-grazing pasture

height of around 55 mm, although the authors recognise the practical difficulty of

consistently achieving this target on-farm. While for one season there was an interaction

between leaf regrowth stage and defoliation height, this effect only influenced the relative

ME difference between the different defoliation heights.

Irrigation influenced the CP, NDF and ADF concentrations in the pasture during

summer. However, this impact was mediated by the leaf regrowth stage at which the pastures

were defoliated. For the rainfed pastures NDF and ADF concentrations tended to increase

between the 1-leaf and 2-leaf regrowth stages, while for the irrigated pastures this nutritive

value decline occurred between the 2-leaf and 3-leaf stages. Despite these different patterns

of decline, there were minimal differences in the CP, NDF and ADF concentrations between

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irrigated and rainfed pastures at the 3-leaf regrowth stage. The environment our experiment

was undertaken in may be partly responsible for such a result, with an average annual rainfall

of 1200 mm and above average summer rainfall received for the duration of the experiment.

Greater differences in nutritive value between dryland and fully irrigated pastures were

observed by Jensen et al. (2003) in a drier environment (470 mm annual rainfall).

In our previous paper (Rawnsley et al. 2014) we showed that defoliating at the 1-leaf

and 2-leaf regrowth stages resulted in 25% and 6% lower yields, respectively, compared with

defoliating at the 3-leaf stage. We have also shown a decreases in perennial ryegrass

persistence associated with such management (Turner et al. 2013). The results presented

from the present paper show that there is little justification to modify the recommendations

made by Rawnsley et al. (2014) with regard to the grazing management required to maximise

perennial ryegrass productivity (i.e. defoliated between the 2- and 3-leaf regrowth stage to a

height of 55 mm), when considering nutritive value. While the nutritive value (in terms of

ME) of perennial ryegrass could be marginally increased by defoliating at an earlier leaf stage

and to a lower height, the negative consequences to pasture productivity do not justify the

small gain in nutritive value. Such practices will also result in an excess CP concentration in

the cow’s diet with negative consequences to production and the environment.

Acknowledgments

The authors gratefully acknowledge the financial support provided by Dairy Australia Ltd.

and the technical support provided by Mr Peter Chamberlain.

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List of figures

Fig. 1. The estimated metabolisable energy (ME) concentration (MJ/ kgDM) of perennial

ryegrass pastures defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) stages of regrowth when

receiving nitrogen (N) fertiliser application rates of either 0, 1.5 or 3 kg N/ha/day. On each

panel the left error bar represents the LSD (P = 0.05) for comparisons within the defoliation

interval treatments, and the right error bar represents the LSD (P = 0.05) for the comparisons

between the defoliation interval treatments.

Fig. 2. The estimated metabolisable energy (ME) concentration (MJ/ kgDM) of perennial

ryegrass pastures defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) stages of regrowth when

defoliated to a height of 30, 55 or 80 mm. On each panel the left error bar represents the LSD

(P = 0.05) for comparisons within the defoliation interval treatments, and the right error bar

represents the LSD (P = 0.05) for the comparisons between the defoliation interval

treatments.

Fig. 3. The crude protein (CP) concentration (%DM) of perennial ryegrass pastures

defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) stages of regrowth when receiving

nitrogen (N) fertiliser application rates of either 0, 1.5 or 3 kg N/ha/day. On each panel the

left error bar represents the LSD (P = 0.05) for comparisons within the defoliation interval

treatments, and the right error bar represents the LSD (P = 0.05) for the comparisons between

the defoliation interval treatments.

Fig. 4. The crude protein (CP) concentration (%DM) of perennial ryegrass pastures during

autumn 2012 when defoliated to heights of 30, 55 or 80 mm and receiving 0, (○), 1.5 (□) or

3.0 (∆) kg N/ha/day (panel a) or grown under rainfed (○) or irrigated (●) conditions (panel b).

On panel a, the error bar represents the LSD (P = 0.05). On panel b, the left error bar

represents the LSD (P = 0.05) for comparisons within irrigation treatments and the right error

bar represents the LSD (P = 0.05) for comparisons between irrigation treatments.

Fig. 5. The neutral detergent fibre (NDF) concentration (%DM) of perennial ryegrass

pastures defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) stages of regrowth when

receiving nitrogen (N) fertiliser application rates of either 0, 1.5 or 3 kg N/ha/day. On each

19

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8

9101112131415161718192021

22

23242526272829303132

panel the left error bar represents the LSD (P = 0.05) for comparisons within the defoliation

interval treatments, and the right error bar represents the LSD (P = 0.05) for the comparisons

between the defoliation interval treatments.

Fig. 6. The neutral detergent fibre (NDF) concentration (%DM) of perennial ryegrass

pastures during winter 2011 when grown under rainfed (○) or irrigated (●) conditions and

defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) leaf regrowth stages and defoliated to

heights of 30, 55 or 80 mm. The left error bar represents the LSD (P = 0.05) for comparisons

within the irrigation and defoliation interval treatments, the centre error bar represents the

LSD (P = 0.05) for comparisons within the irrigation and defoliation heights treatments and

the right error bar represents the LSD (P = 0.05) for the comparisons within the defoliation

intervals and defoliation height treatments.

Fig. 7. The acid detergent fibre (ADF) concentration (%DM) of perennial ryegrass pastures

during winter 2011, when defoliated to a height of 30, 55 or 80 mm and defoliated at the 1-

leaf (○), 2-leaf (□) or 3-leaf (∆) leaf stages of regrowth (panel a) or receiving nitrogen

fertiliser application rates of either 0 (●), 1.5 (■) or 3 (▲) kg N/ha/day (panel b). The left

error bar on panel a represents the LSD (P = 0.05) for comparison between defoliation

heights within each leaf stage defoliation interval and the right error bar represents the LSD

(P = 0.05) for the comparisons between each leaf stage defoliation interval. The error bar on

panel b represents the LSD (P = 0.05) for comparison between the nitrogen and defoliation

height treatments.

List of Tables

Table 1. The average number of days between each leaf stage defoliation in autumn, winter,

spring and summer.

Table 2. The average growth rate (kgDM/ha/day) and pasture yield (above defoliation height;

kgDM/ha) of each treatment (Dryland and Irrigated, defoliated at leaf stage 1, 2 or 3,

defoliated to 30, 55 or 80 mm height, and receiving 0, 1.5 or 3.0 kg N/ha/day) in autumn,

winter, spring and summer.

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141516171819202122

23

24

2526

27282930

Table 3. The rainfall (mm) received and the irrigation (mm) applied to the irrigated treatment

over the 2010/11 and 2011/12 irrigation seasons.

Table 4. Summary of the P values from the ANOVA of the nutritive value parameters of

Metabolisable energy (ME), Crude protein (CP), Neutral detergent fibre (NDF) and Acid

detergent fibre (ADF) from each sampling event. Significant effects are in bold for clarity.

Table 5. The crude protein (CP) and neutral detergent fibre (NDF) concentration (%DM) of

perennial ryegrass pastures when defoliated to heights of 30, 55 or 80 mm.

Table 6. The crude protein (CP) neutral detergent fibre (NDF) and acid detergent fibre

(ADF) concentration (%DM) of perennial ryegrass pastures in summer 2011/12 when grown

under irrigated or rainfed conditions and defoliated at the 1-, 2- or 3-leaf stage of regrowth

(defoliation interval).

Table 7. The effect of irrigation treatment, defoliation interval, defoliation height and

nitrogen (N) fertiliser application rate on the acid detergent fibre (ADF) concentration

(%DM) of perennial ryegrass pastures.

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10111213

14

151617

Spring 2010

N rate (kgN/ha/day)

ME

(MJ/

kg/D

M)

0.0 1.5 3.0

10.5

11.0

11.5

12.0

12.5

a

Winter 2011

N rate (kgN/ha/day)

0.0 1.5 3.0

b

Summer 2011/12

N rate (kgN/ha/day)

0.0 1.5 3.0

c

Autumn 2012

N rate (kgN/ha/day)

0.0 1.5 3.0

d

Fig. 1. The estimated metabolisable energy (ME) concentration (MJ/ kgDM) of perennial ryegrass pastures defoliated at the 1-leaf (○), 2-leaf

(□) or 3-leaf (∆) stages of regrowth when receiving nitrogen (N) fertiliser application rates of either 0, 1.5 or 3 kg N/ha/day. On each panel the

left error bar represents the LSD (P = 0.05) for comparisons within the defoliation interval treatments, and the right error bar represents the LSD

(P = 0.05) for the comparisons between the defoliation interval treatments.

22

1

234567

Spring 2010

Defoliation height (mm)

ME

(MJ/

kg/D

M)

30 55 80

10.5

11.0

11.5

12.0

12.5

a

Winter 2011

Defoliation height (mm)

30 55 80

b

Summer 2011/12

Defoliation height (mm)

30 55 80

c

Autumn 2012

Defoliation height (mm)

30 55 80

d

Fig. 2. The estimated metabolisable energy (ME) concentration (MJ/ kgDM) of perennial ryegrass pastures defoliated at the 1-leaf (○), 2-leaf

(□) or 3-leaf (∆) stages of regrowth when defoliated to a height of 30, 55 or 80 mm. On each panel the left error bar represents the LSD (P =

0.05) for comparisons within the defoliation interval treatments, and the right error bar represents the LSD (P = 0.05) for the comparisons

between the defoliation interval treatments.

23

1

2

345678

Spring 2010

N rate (kgN/ha/day)

CP

(%D

M)

0.0 1.5 3.0

1020

3040 a

Winter 2011

N rate (kgN/ha/day)

0.0 1.5 3.0

b

Summer 2011/12

N rate (kgN/ha/day)

0.0 1.5 3.0

c

Autumn 2012

N rate (kgN/ha/day)

0.0 1.5 3.0

d

Fig. 3. The crude protein (CP) concentration (%DM) of perennial ryegrass pastures defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) stages of

regrowth when receiving nitrogen (N) fertiliser application rates of either 0, 1.5 or 3 kg N/ha/day. On each panel the left error bar represents the

LSD (P = 0.05) for comparisons within the defoliation interval treatments, and the right error bar represents the LSD (P = 0.05) for the

comparisons between the defoliation interval treatments.

24

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234

56789

Defoliation height (mm)

CP

(%

DM

)

30 55 80

2530

3540

a

Defoliation height (mm)

30 55 80

2530

3540

b

Fig. 4. The crude protein (CP) concentration (%DM) of perennial ryegrass pastures during

autumn 2012 when defoliated to heights of 30, 55 or 80 mm and receiving 0, (○), 1.5 (□) or

3.0 (∆) kg N/ha/day (panel a) or grown under rainfed (○) or irrigated (●) conditions (panel b).

On panel a, the error bar represents the LSD (P = 0.05). On panel b, the left error bar

represents the LSD (P = 0.05) for comparisons within irrigation treatments and the right error

bar represents the LSD (P = 0.05) for comparisons between irrigation treatments.

25

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Spring 2010

N rate (kgN/ha/day)

ND

F (%

DM

)

0.0 1.5 3.0

3040

5060

a

Winter 2011

N rate (kgN/ha/day)

0.0 1.5 3.0

b

Summer 2011/12

N rate (kgN/ha/day)

0.0 1.5 3.0

c

Autumn 2012

N rate (kgN/ha/day)

0.0 1.5 3.0

d

Fig. 5. The neutral detergent fibre (NDF) concentration (%DM) of perennial ryegrass pastures defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf

(∆) stages of regrowth when receiving nitrogen (N) fertiliser application rates of either 0, 1.5 or 3 kg N/ha/day. On each panel the left error bar

represents the LSD (P = 0.05) for comparisons within the defoliation interval treatments, and the right error bar represents the LSD (P = 0.05) for

the comparisons between the defoliation interval treatments.

26

1

2

3

456789

10

Irrigated

Defoliation height (mm)

ND

F (%

DM

)

30 55 80

3035

4045

50 a

Rainfed

Defoliation height (mm)

30 55 80

b

Fig. 6. The neutral detergent fibre (NDF) concentration (%DM) of perennial ryegrass

pastures during winter 2011 when grown under rainfed (○) or irrigated (●) conditions and

defoliated at the 1-leaf (○), 2-leaf (□) or 3-leaf (∆) leaf regrowth stages and defoliated to

heights of 30, 55 or 80 mm. The left error bar represents the LSD (P = 0.05) for comparisons

within the irrigation and defoliation interval treatments, the centre error bar represents the

LSD (P = 0.05) for comparisons within the irrigation and defoliation heights treatments and

the right error bar represents the LSD (P = 0.05) for the comparisons within the defoliation

intervals and defoliation height treatments.

27

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2

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10

11

Defoliation height (mm)

AD

F (%

DM

)

30 55 80

1015

2025

30 a

Defoliation height (mm)

30 55 80

b

Fig. 7. The acid detergent fibre (ADF) concentration (%DM) of perennial ryegrass pastures

during winter 2011, when defoliated to a height of 30, 55 or 80 mm and defoliated at the 1-

leaf (○), 2-leaf (□) or 3-leaf (∆) leaf stages of regrowth (panel a) or receiving nitrogen

fertiliser application rates of either 0 (●), 1.5 (■) or 3 (▲) kg N/ha/day (panel b). The left

error bar on panel a represents the LSD (P = 0.05) for comparison between defoliation

heights within each leaf stage defoliation interval and the right error bar represents the LSD

(P = 0.05) for the comparisons between each leaf stage defoliation interval. The error bar on

panel b represents the LSD (P = 0.05) for comparison between the nitrogen and defoliation

height treatments.

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1011

12

Table 1. The average number of days between each leaf stage defoliation in autumn,

winter, spring and summer.

Rainfed Irrigated1 leaf stage 2 leaf stage 3 leaf stage 1 leaf stage 2 leaf stage 3 leaf stage

Autumn 17.0 27.5 38.8 15.6 25.3 36.8Winter 21.0 36.7 55.5 20.6 36.7 55.5Spring 13.0 20.0 35.3 13.0 20.0 35.3Summer 11.1 21.8 30.3 11.1 20.6 30.3

29

12

3

Table 2. The rainfall (mm) received and the irrigation (mm) applied to the irrigated

treatment over the 2010/11 and 2011/12 irrigation seasons.

Month 2009/10 2010/11 2011/12Rainfall (mm)

Rainfall (mm) Irrigation (mm) Rainfall (mm)

Irrigation (mm)

July . 110 0 169 0August 289 195 0 193 0September 129 164 0 85 0October 69 97 0 102 40November 74 120 20 150 40December 37 174 20 50 100January 5.8 269 80 67 80February 65 81 40 74 60March 98 73 20 167 40April 103 78 0 98 20May 68 32 0 123 0June 103 129 0 134 0Total 1040.8 1522 180 1412 380

30

12

3

4

5

6

Table 3. Summary of the P values from the ANOVA of the nutritive value parameters of Metabolisable energy (ME), Crude protein

(CP), Neutral detergent fibre (NDF) and Acid detergent fibre (ADF) from each sampling event. Significant effects are in bold for

clarity.

Source of variation Degrees of

freedom

  ME   CP   NDF   ADF

Spring Winter Summer Autumn Spring Winter Summer Autumn Spring Winter Summer Autumn Spring WinterSumme

    2010 2011 2011/12 2012   2010 2011 2011/12 2012   2010 2011 2011/12 2012   2010 2011 2011/12Irrigation-rainfed (Irr) 1 0.363 0.566 0.180 0.110 0.810 0.986 0.216 0.305 0.271 0.024 0.289 0.098 0.263 0.565 0.542Defoliation interval (DI) 2 <0.001 0.004 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.082 0.002 0.141 <0.001 0.112 <0.001Irr × DI 2 0.200 0.952 0.665 0.057 0.194 0.061 <0.001 0.096 0.310 0.091 0.003 0.139 0.655 0.183 0.025Nitrogen fertiliser (N) 2 <0.001 0.005 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.015 <0.001 0.130Defoliation height (Ht) 2 <0.001 <0.001 <0.001 <0.001 <0.001 0.013 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.027 <0.001 <0.001Irr × N 2 0.987 0.293 0.895 0.060 0.953 0.005 0.859 0.015 0.943 0.121 0.824 0.632 0.850 0.427 0.204DI × N 4 <0.001 0.006 0.231 <0.001 <0.001 0.044 0.274 <0.001 <0.001 0.002 0.447 0.115 0.144 0.161 0.192Irr × Ht 2 0.278 0.191 0.634 <0.001 0.314 0.109 0.460 <0.001 0.325 0.496 0.893 0.141 0.234 0.097 0.087DI × Ht 4 0.840 0.006 0.433 0.199 0.927 0.272 0.633 0.140 0.711 <0.001 0.563 0.768 0.550 <0.001 0.777N × Ht 4 0.430 0.218 0.359 0.005 0.685 0.216 0.242 0.007 0.197 0.204 0.205 0.060 0.081 0.045 0.365Irr × DI × N 4 0.400 0.419 0.974 0.585 0.262 0.125 0.329 0.196 0.829 0.822 0.055 0.782 0.938 0.940 0.802Irr × DI × Ht 4 0.551 0.045 0.618 0.262 0.426 0.284 0.878 0.538 0.940 0.020 0.328 0.395 0.147 0.077 0.949Irr × N × Ht 4 0.338 0.829 0.263 0.239 0.281 0.682 0.470 0.621 0.733 0.883 0.383 0.077 0.794 0.475 0.931DI × N × Ht 8 0.841 0.994 0.767 0.650 0.843 0.968 0.781 0.667 0.642 0.976 0.812 0.873 0.956 0.718 0.854Irr × DI × N × Ht 8   0.515 0.805 0.193 0.494   0.223 0.581 0.221 0.948   0.954 0.953 0.242 0.268 0.793 0.337 0.699

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Table 4. The crude protein (CP) and neutral detergent fibre (NDF) concentration

(%DM) of perennial ryegrass pastures when defoliated to heights of 30, 55 or 80 mm. Defoliation height (mm) Spring 2010 Winter 2011 Summer 2011/12 Autumn 2012

CP (%DM)

30 25.7 30.4 29.3 32.8

55 24.6 29.7 26.5 31.4

80 24.3 29.8 25.7 30.9

P value <0.001 <0.05 <0.001 <0.001

LSD (P = 0.05) 0.67 0.49 0.48 0.41

NDF (%DM)

30 45.1 38.1 38.6 42.8

55 45.1 37.9 41.6 45.6

80 47.2 40.9 43.1 47.2

P value <0.001 <0.001 <0.001 <0.001

LSD (P = 0.05) 0.90 0.97 0.97 1.24

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Table 5. The crude protein (CP) neutral detergent fibre (NDF) and acid detergent fibre

(ADF) concentration (%DM) of perennial ryegrass pastures in summer 2011/12 when

grown under irrigated or rainfed conditions and defoliated at the 1-, 2- or 3-leaf stage of

regrowth (defoliation interval).

Irrigation treatment1-leaf stage

2-leaf stage

3-leaf stage

P value (Irr × defoliation interval)

ALSD (P = 0.05)

BLSD (P = 0.05)

CP(%DM)

Rainfed 29.8 28.3 21.9 <0.001 0.89 2.08

Irrigated 32.5 27.6 23.2

NDF (%DM)

Rainfed 38.8 42.6 43.1 <0.01 2.11 2.43

Irrigated 40.9 38.0 43.1

ADF (%DM)

Rainfed 19.4 21.7 24.2 <0.05 0.73 2.55

Irrigated 19.7 20.7 23.1

AWithin irrigation treatmentsBBetween irrigation treatments

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Table 6. The effect of defoliation interval, defoliation height and nitrogen (N) fertiliser

application rate on the acid detergent fibre (ADF) concentration (%DM) of perennial

ryegrass pastures.

  Spring 2010 Winter 2011 Summer 2011/12 Autumn 2012

Defoliation interval

1-leaf 23.3 20.3 19.5 21.8

2-leaf 25.4 21.3 21.2 22.5

3-leaf 26.4 21.6 23.7 24.1

P value <0.001 NS <0.001 <0.05

LSD (P = 0.05) 3.08 . 1.54 4.11

Defoliation height (mm)

30 24.9 21.7 20.4 22.3

55 24.6 19.8 21.5 22.9

80 25.7 21.7 22.4 23.2

P value <0.05 <0.001 <0.001 NS

LSD (P = 0.05) 0.82 0.74 0.93 .

N fertiliser application rate (kg N/ha/day)

0.0 25.4 20.0 21.0 22.6

1.5 25.4 21.8 21.9 23.1

3.0 24.3 21.4 21.5 22.7

P value <0.05 <0.001 NS NS

LSD (P = 0.05) 0.82 0.74 . .

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