Effect of Defoliation Management on Water-Soluble Carbohydrate Energy Reserves, Dry Matter Yields, and Herbage Quality of Tall Fescue D. J. Donaghy,* L. R. Turner, and K. A. Adamczewski Pasture Management
ABSTRACT
There is limited information on the effect of leaf stage based defoliation management on the regrowth of tall fescue (Festuca arundinacea Schreb.). The aim of this study was to investigate the physiological changes in tall fescue during its regrowth cycle up to the five-leaf stage, and the effect of repeated defoliation at the one-leaf, two-leaf, and four-leaf stages on herbage quality, water-soluble carbohydrate (WSC) energy reserves and the rate of subsequent plant regrowth. Crude protein (CP) and metabolizable energy (ME) concentrations decreased with increased leaf regrowth stage, from 27% and 11.3 MJ kg−1 dry matter (DM) at the one-leaf stage, to 16.1% and 9.2 MJ kg−1 DM at the five-leaf stage, respectively. Acid detergent fiber (ADF) and neutral detergent fiber (NDF) concentrations increased during the regrowth cycle by 16 and 9%, respectively. Frequent defoliation therefore maximized the CP and ME concentrations and minimized the ADF and NDF concentrations of tall fescue, but limited DM yields during regrowth. Defoliation at the four-leaf stage resulted in 30% higher stubble WSC concentration and 20% higher leaf DM yield than defoliation at the two-leaf stage of regrowth, but compromised herbage quality. The stubble was confirmed as the primary storage organ for WSC reserves, while leaf and root growth were found to have an equally high priority for available energy following defoliation.
D
efoliation interval is defined as the time between one defoliation event and the next, and for dairy pastures is commonly based on number of days, pasture sward surface height or herbage mass. However, a defoliation interval based on leaf regrowth stage is a preferable method of managing pastures, as it more readily reflects the extent of plant recovery from grazing in terms of energy reserve levels and herbage quality for ruminant nutrition (Fulkerson and Donaghy, 2001). Leaf stage defoliation management is based on an understanding of the physiological changes that occur in the grass plant throughout the regrowth cycle. Donaghy and Fulkerson (1997) investigated the effect of defoliation on levels of WSC in the stubble (tiller bases below 50 mm height) of perennial ryegrass (Lolium perenne L.) and found that WSC reserve levels and DM yield of leaves, tillers, and roots were optimal between the two-leaf and three-leaf stages of regrowth, with each “leaf stage” defined as the time taken to full emergence of one new leaf. Repeated defoliation before the two-leaf stage resulted in depleted levels of WSC reserves and delayed regrowth of leaves and roots. Poor root recovery following defoliation is of particular concern as it can subsequently lead to poor pasture persistence (Fulkerson and Donaghy, 2001). The herbage quality of perennial rye-
Tasmanian Institute of Agricultural Research, Burnie, Tasmania 7320, Australia. Received 13 Jan. 2007. *Corresponding author (Danny.
[email protected]). Published in Agron. J. 100:122–127 (2008). doi:10.2134/agronj2007.0016 Copyright © 2008 by the American Society of Agronomy, 677 South Segoe Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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grass was optimal for grazing cattle at the three-leaf stage of regrowth, from which point the fiber content increased and DM yields were reduced due to increased leaf senescence. Tall fescue is a widely used dairy pasture species due to its wide range of adaptability (Langer, 1990). It is tolerant of drought, heat, mildly wet or salt soils, and insects, and it displays continued growth during dry, warm conditions (Milne and Johnson, 1997). There is limited information on the effect of leaf stage based defoliation management on the regrowth of tall fescue. To date, studies have investigated the regrowth of tall fescue following defoliation at various heights (Booysen and Nelson, 1975; Virkajarvi, 2003) and current defoliation interval recommendations are based on a combination of number of days and pasture sward surface height (Milne and Johnson, 1997; Callow et al., 2003; Kemp, 2004). Two greenhouse experiments were conducted to investigate physiological changes of tall fescue during its regrowth cycle and the effect of leaf stage based defoliation management on herbage quality, WSC energy reserves, and subsequent DM yields. MATERIALS AND METHODS This study was conducted in the Tasmanian Institute of Agricultural Research greenhouse, University of Tasmania, Burnie, Tasmania (41°8′ S, 145°49′ E, elevation 206 m). The first of two experiments was conducted under natural light and temperature conditions between May and November 2000. The second experiment was conducted under natural light, with temperature conditions controlled to maintain day/night temperatures of 25/15°C, between December 2003 and July 2004. Daylength (hours), radiation (MJ m−2), Abbreviations: ADF, acid detergent fiber; ANOVA, analysis of variance; CP, crude protein; DM, dry matter; ME, metabolizable energy; NDF, neutral detergent fiber; WSC, water-soluble carbohydrates.
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Fig. 2. Monthly means for (a) radiation (MJ m –2 , square) and daylength (hours, triangle), and (b) maximum (square) and minimum relative humidity (%, triangle) for the experimental period between December 2003 and July 2004.
mize boundary effects. These plants were defoliated but otherwise not included in analyses. Defoliation treatments commenced when plants had grown at least 20 tillers plant−1, between 10 and 12 wk from sowing. Each leaf regrowth stage was defined as the time taken for the production of one fully expanded leaf tiller −1. The leaf appearance interval, or time to emergence of one new leaf, was approximately 16 d (187 ddegrees). Plant material above 50 mm was termed “leaf,” below this to ground level was termed “stubble” and below ground level was termed “root.”
Fig. 1. Monthly means for (a) radiation (MJ m –2 , square) and daylength (hours, triangle); (b) maximum (square) and minimum relative humidity (%, triangle); and (c) maximum (square), and minimum temperature (°C, triangle) for the experimental period between May and November 2000.
Experimental Design Plants in both experiments were arranged in a randomized complete block design with five blocks each containing six randomly allocated treatments. Each treatment consisted of a row of seven plants per block, resulting in 42 plants in total per treatment.
minimum and maximum relative humidity (%), and maximum and minimum temperature (°C; Exp. 1 only) values for the experimental periods are presented in Fig. 1 and 2. For both experiments, tall fescue (‘Advance’) was sown in polyvinyl bags (100 mm diam. × 230 mm depth), containing potting mixture composed of 50% Pinus radiata D. bark, 30% sand, 20% Sphagnum spp. moss and fertilizers (dolomite, superphosphate, Osmocote [Scotts Australia Pty. Ltd., NSW, Australia], blood and bone, potash, lime, and Saturaid [Debco Pty Ltd., Mount Waverley, VIC, Australia]). Three seeds were sown in each pot and once established (i.e., two to three tillers plant−1) the weakest seedlings were removed to allow a single healthy seedling to reach maturity in each bag. Plants were watered daily to replace evapotranspiration losses and were fertilized with Nutricote (N12:P5.2:K12 plus micronutrients; Yates Ltd., NSW, Australia) and Osmocote (N15:P9:K12 plus micronutrients) at monthly intervals from sowing to maintain a standard rate of 40 kg N ha−1. Plants were arranged in a mini-sward at a density of 100 plants m−2 and were surrounded by a row of buffer plants to miniAgronomy Journal
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Treatments included one preliminary destructive harvest at the four-leaf regrowth stage (leaf regrowth stage 0) and five sequential destructive harvests when each new leaf had regrown, up to the five-leaf stage (1–5L). Experiment 2
Treatments comprised three defoliation intervals (defoliation at the one-leaf, two-leaf or four-leaf stage). When the one-leaf defoliation interval (1L) had been completed four times, the two-leaf defoliation interval (2L) twice, and the four-leaf defoliation interval (4L) once (i.e., all plants defoliated), three rows of plants per 2008
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Table 1. Stubble water-soluble carbohydrate (WSC) content (mg plant−1 and mg tiller−1) and concentration (% of dry matter, DM) and root WSC content (mg plant−1) of tall fescue plants before defoliation and at each corresponding leaf regrowth stage, under greenhouse conditions. Exp. 1 Leaf regrowth stage 0 1L 2L 3L 4L 5L LSD (0.05)
WSC Root mg 31.6 8.1 39.5 30.9 44.9 35.9 17.5
Stubble plant −1 51.2 25.3 57.4 80.8 121.0 92.7 34.3
Stubble
Stubble
tiller−1
% of DM 5.99 4.04 6.50 9.14 10.16 6.50 2.58
mg
2.61 1.17 2.72 3.21 4.84 4.13 1.34
block were destructively harvested. This event was termed H1. The remaining three rows of plants per block were destructively harvested when they had reached the four-leaf stage of regrowth, an event termed H2. Determination of Dry Matter and Tillers Per Plant At each defoliation and harvest event in both experiments, leaf material was removed and leaf DM yield plant−1 determined following drying of samples for 24 h at 60°C in a forced draft oven (Reuter et al., 1997). At each destructive harvest event, root samples were collected and stored at 2°C for up to 48 h until they could be washed free of potting mix by hand. Stubble and washed root samples were dried at 80°C for 24 h (Reuter et al., 1997) and weighed to determine root and stubble DM yield plant−1. Dried leaf, stubble, and root samples were ground through a 1-mm sieve. Number of tillers plant−1 was recorded at each destructive harvest in Exp. 1. Determination of Water Soluble Carbohydrate The concentration of WSC in dried stubble and root samples was determined at the Wagga Wagga Agricultural Institute Laboratories (New South Wales Department of Primary Industries) by cold extraction of plant material in a reciprocal shaker for 1 h using 0.2% benzoic acid-water solution, and the hydrolyzation of the cold water carbohydrates to invert sugar by 1 mol L −1 HCl. This was heated at 90°C, and the sugar was dialyzed into an alkaline stream of potassium ferricyanide, again heated at 90°C, and then measured using an autoanalyzer (420 nm) (Technicon Industrial Method number 302–73A, derived from the method outlined by Smith, 1969). Herbage Quality Analyses Leaf samples were analyzed for ADF, NDF, dry matter digestibility (DMD), and N concentrations using near-infrared spectrometry at Hamilton FeedTEST Laboratories (Victoria Department of Primary Industries, Hamilton, Victoria). Crude protein and ME concentrations were subsequently calculated using the following equations: 124
CP = N % × 6.25
[1]
ME = (0.17 × DDM) − 2 [2] (Standing Committee on Agriculture, 1990) Statistical Analysis In both experiments, each row of seven plants was combined for WSC and herbage quality analysis as one sample. In Exp. 1, differences between leaf regrowth stage treatment means were tested for the following variables: leaf and root DM yields, WSC, ADF, NDF, DMD, ME, and CP concentrations. Means were compared by two-way ANOVA without replication using the statistical package SPSS (Version 11.5, SPSS Corp., Chicago, IL). Regression (r 2) between WSC level and plant regrowth was tested using the statistical functions of EXCEL (Microsoft Corp., Redmond, WA). In Exp. 2, means of DM yields and WSC were compared using an ANOVA split plot design using SPSS, while means of herbage quality variables were compared at H1 only by two-way ANOVA without replication using SPSS. For both experiments least significant difference (LSD), as defined by Steel and Torrie (1960), was used to separate means. RESULTS Experiment 1 Changes in Stubble and Root Water-soluble Carbohydrates with Leaf Regrowth
At defoliation (leaf regrowth stage 0) and throughout the subsequent regrowth cycle, the mean WSC content (mg plant−1) in the stubble was significantly greater (P < 0.001) than in the roots (Table 1). Following defoliation, there was a significant decline (P < 0.05) in root WSC content. There was a significant increase (P = 0.001) in root WSC content to pre-defoliation levels between the one-leaf and two-leaf stages, at which point WSC levels in tall fescue roots stabilized (Table 1). Stubble WSC content (mg tiller −1) significantly decreased (P < 0.05) following defoliation, returned to pre-defoliation levels by the two-leaf stage of regrowth and was significantly higher (P < 0.05) at the four-leaf and five-leaf stages than at any previous regrowth stage. Stubble WSC concentration (% of DM) remained stable following defoliation, before significantly increasing (P < 0.05) between the two-leaf and three-leaf stages and decreasing (P < 0.01) between the four-leaf and five-leaf stages of regrowth (Table 1). Relationship between Plant Water-soluble Carbohydrate Levels and Regrowth
There was a positive linear relationship between leaf DM yield at each regrowth stage and stubble WSC content (mg tiller −1 and mg plant−1), and between root DM yield and stubble and root WSC content (mg plant−1), as follows: Agronomy Journal
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Table 4. Mean stubble, root and leaf water-soluble carbohydrate (WSC) content (mg plant−1) and stubble WSC concentration (% of DM) of tall fescue, for plants defoliated four times at the one-leaf stage (1L), twice at the two-leaf stage (2L), or once at the four-leaf stage (4L), at cessation of defoliation treatments (H1), and at the four-leaf stage of regrowth following H1 (H2), under greenhouse conditions.
Table 2. Stubble, root, and leaf dry matter (DM, g plant−1) of tall fescue plants before defoliation and at each corresponding leaf regrowth stage, under greenhouse conditions. Exp. 1 Leaf regrowth stage
Root
Leaf
Stubble
0 1L 2L 3L 4L 5L LSD (0.05)
1.50 0.80 1.29 1.65 2.19 2.09 0.62
g plant −1 3.94 0.56 2.48 4.37 6.75 7.11 0.91
0.87 0.69 0.89 0.89 1.17 1.44 0.29
DM
Exp. 2
Table 3. Mean leaf crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), and dry matter digestibility (DMD) concentrations (% of DM), and metabolizable energy (ME, MJ kg−1) concentration of tall fescue before defoliation and at each corresponding leaf regrowth stage, under greenhouse conditions. Exp. 1 Leaf regrowth stage 0 1L 2L 3L 4L 5L LSD (0.05)
CP 19.6 27.0 21.1 17.9 15.5 16.1 1.4
ADF NDF % of DM 30.5 58.1 20.9 50.1 26.9 53.9 29.6 57.5 32.0 59.2 34.8 62.1 1.2 1.2
DMD 74.6 78.0 76.1 73.0 71.3 65.7 1.3
H1 H2 LSD (0.05)
18.6 22.9 2.6
WSC Stubble Root 58.5 189.3 394.8 53.3
mg plant −1 157.1 220.9 233.2 ns†
173.2 255.3 35.6
112.9 294.5 151.4
Leaf 107.3 232.7 374.4 57.7 191.2 285.0 78.0
† ns, not significant at P ≤ 0.05.
Changes in Herbage Quality with Regrowth
The DMD and ME concentrations of the leaf significantly decreased (P < 0.001) with increasing leaf stage, while the ADF and NDF concentrations of the leaf significantly increased (P < 0.001) throughout the leaf regrowth cycle (Table 3). The CP concentration of the leaf significantly decreased (P < 0.05) between the one-leaf and four-leaf stages of regrowth (Table 3).
ME MJ kg −1 DM 10.7 11.3 10.9 10.4 10.1 9.2 0.2
Experiment 2 Effect of Defoliation Interval on Water-Soluble Carbohydrate Levels
Leaf DM plant−1 (g)
[3]
= 0.510 stubble WSC (mg tiller−1) + 1.010 (r2 = 0.61) = 12.809 stubble WSC (mg plant−1) + 18.515 (r2 = 0.61) Root DM plant−1 (g) = 1.980 stubble WSC (mg tiller−1) [4] + 0.0305 (r 2 = 0.67) = 50.760 stubble WSC (mg plant−1) + 9.187 (r 2 = 0.70) = 20.387 root WSC (mg plant−1) + 0.565 (r 2 = 0.62) Changes in Dry Matter Yield with Regrowth
Following defoliation, there was a steady and significant increase (P < 0.001) in leaf DM yield (g plant−1) up to and including the four-leaf stage of regrowth (Table 2). Leaf DM yield was significantly higher (P < 0.001) at the four-leaf and five-leaf stages than at any previous regrowth stage. There was an initial significant decrease (P < 0.05) in root growth following defoliation, with root DM yield (g plant−1) returning to pre-defoliation levels between the two-leaf and three-leaf stages of regrowth (Table 2). Stubble DM yield (g plant−1) had an initial decrease (P < 0.05) after defoliation, returned to pre-defoliation levels at the two-leaf stage and this increased to be significantly higher (P < 0.05) at the four-leaf and five-leaf stages than at any previous regrowth stage. Agronomy Journal
1L 2L 4L LSD (0.05)
Stubble % of DM 10.4 21.3 30.6 3.1
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There was a significant (P < 0.001) defoliation interval effect on stubble and leaf WSC content (mg plant−1) and stubble WSC concentration (% of DM), with WSC increasing with increasing defoliation interval (Table 4). Watersoluble carbohydrates in the stubble and leaves of plants defoliated at the four-leaf stage were significantly higher (P < 0.001) than in plants defoliated at the two-leaf stage, and WSC content in plants defoliated at the two-leaf stage was significantly higher (P < 0.001) than in plants defoliated at the one-leaf stage. Defoliation interval did not have a significant (P > 0.05) effect on root WSC content. There was a significantly higher (P < 0.01) mean stubble WSC concentration and WSC content in stubble, leaves, and roots of fescue plants at H2 (after the recovery period that followed treatments, with all plants at the four-leaf stage of regrowth) than at H1 (immediately following treatments; Table 4). Effect of Defoliation Interval on Dry Matter Yield
Leaf and root DM yields following H1 were closely related to defoliation interval before H1, with yield increasing with leaf regrowth stage (Table 5). There were significant differences in leaf growth between plants defoliated at the four-leaf and two-leaf stages (P = 0.001) and the two-leaf and one-leaf stages (P < 0.001). Leaf DM yield was significantly higher (P < 0.001) at H2 than at H1. Root DM yield of plants defoliated at the one-leaf and two-leaf stages was significantly lower (P < 0.05) compared with plants defoliated at the four-leaf stage. There was no significant difference (P > 0.05) in root DM yield between the one-leaf and two-leaf stage 125
Table 5. Mean root and leaf dry matter (DM) yield (g plant− for tall fescue plants defoliated four times at the one-leaf stage (1L), twice at the two-leaf stage (2L) or once at the fourleaf stage (4L), at cessation of defoliation treatments (H1), and at the four-leaf stage of regrowth following H1 (H2), under greenhouse conditions.
1)
Exp. 2
DM Root
Leaf
1L 2L 4L LSD (0.05)
1.68 2.82 3.54 0.40
g 1.91 3.87 6.54 2.18
H1 H2 LSD (0.05)
2.46 2.91 0.32
3.31 4.89 ns†
Stubble
plant −1 0.49 0.88 1.31 0.09 0.74 1.04 0.08
† ns, not significant at P ≤ 0.05.
defoliation treatments or in mean root DM yield between harvests. There was a significant (P < 0.05) defoliation interval by harvest interaction for stubble DM yield. At both harvests, stubble DM yield significantly increased (P < 0.001) with increasing defoliation interval. Stubble DM yield was significantly higher (P < 0.001) for plants at H2 compared with H1. Effect of Defoliation Interval on Herbage Quality
Defoliation interval had a significant (P ≤ 0.001) effect on all herbage quality variables at H1 (Table 6). The DMD and ME concentrations in plants defoliated at the one-leaf stage were significantly higher (P < 0.01) than in plants defoliated at the two-leaf and four-leaf stages of regrowth. There was no significant (P > 0.05) difference in DMD or ME concentration between the two-leaf and four-leaf stage defoliation treatments (Table 6). There was a significant (P < 0.001) decline in CP concentration between the one-leaf and two-leaf stage defoliation intervals, and between the two-leaf and four-leaf stage defoliation intervals (Table 6). The ADF and NDF concentrations in plants defoliated at the one-leaf stage were significantly lower (P < 0.001) than in plants defoliated less frequently. There was no significant (P > 0.05) difference in ADF or NDF concentration between the two-leaf and four-leaf stage defoliation treatments. DISCUSSION The current greenhouse study highlighted the effects of defoliation frequency on the growth and quality of tall fescue plants. Infrequent defoliation maximized WSC reserve accumulation and therefore the regrowth of leaves, stubble, and roots of tall fescue, but compromised herbage quality. Frequent defoliation maximized the herbage quality of tall fescue, but resulted in delayed regrowth and could potentially threaten plant persistence. Both the WSC content and the rate of replenishment throughout the regrowth cycle were greater in the stubble than in the root system, confirming that, as in ryegrass (Danckwerts and Gordon, 1987; Fulkerson and Slack, 1994), orchardgrass (Dactylis glomerata L; Davidson and Milthorpe, 1966; Rawnsley et al., 2002; Turner et al., 2006a) and prairie grass (Bromus willdenowii Kunth.; Turner et al., 2006b), the 126
Table 6. Mean crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), and dry matter digestibility (DMD) concentrations (% of DM), and metabolizable energy (ME) concentration (MJ kg−1 DM) for tall fescue plants defoliated four times at the one-leaf stage (1L), twice at the twoleaf stage (2L), or once at the four-leaf stage (4L), at cessation of defoliation treatments, under greenhouse conditions. Exp. 2 Leaf regrowth stage 1L 2L 4L LSD (0.05)
CP 22.3 13.3 8.7 1.2
ADF NDF % of DM 28.4 56.2 33.5 62.2 33.9 63.1 1.2 1.8
DMD 74.9 72.3 71.6 1.3
ME MJ kg −1 DM 10.7 10.3 10.2 0.2
stubble is the major storage site for WSC in tall fescue plants. Replenishment of WSC in tall fescue stubble commenced immediately following defoliation in Exp. 1, reaching predefoliation levels by the two-leaf stage of regrowth and stabilizing at significantly higher WSC levels at the four-leaf and five-leaf stages than at any previous regrowth stage; following the same leaf stage sequence that was reported for prairie grass by Turner et al. (2006b). The strong positive linear relationship between stubble WSC levels and the regrowth capacity of tall fescue confirms that WSC reserves play an important role throughout the entire plant regrowth cycle. Experiment 1 showed that leaf and root regrowth commence at a similar time in tall fescue plants following defoliation, with both leaf and root DM yields reaching their pre-defoliation levels between the two-leaf and three-leaf stages. Root growth increased until the four-leaf stage and then stabilized, while leaf growth continued to increase until the five-leaf stage of regrowth. For perennial ryegrass (Donaghy and Fulkerson, 1998), orchardgrass (Rawnsley et al., 2002; Turner et al., 2006a) and prairie grass (Turner et al., 2006b) leaf regrowth has a higher priority for allocation of WSC reserves following defoliation compared with the roots. This finding is in agreement with the work of Kemp et al. (2001), who found that tall fescue allocated more of its biomass to roots and pseudostem compared with perennial ryegrass. In the current study, leaf and root growth appeared to be assigned equal priority for energy allocation, as evidenced by the patterns of WSC accumulation in the leaves and roots. The slower leaf appearance rate of tall fescue compared with perennial ryegrass observed by Kemp et al. (2001) is possibly a consequence of this contrasting pattern of WSC allocation. Results of Exp. 2 showed that stubble and leaf WSC content, as well as leaf, root, and stubble DM were significantly higher following defoliation at the four-leaf stage compared with more frequent defoliations. The priority for energy reserve allocation and regrowth patterns of tall fescue following defoliation have significant ramifications for grazing management. Despite the capability of tall fescue to immediately resume replenishment of energy reserves following defoliation, it is likely that repeated defoliation at the twoleaf stage will limit WSC reserve replenishment and therefore subsequent regrowth and plant persistence. The ME concentration of tall fescue decreased with increasing leaf stage in both experiments, ranging from 10.7 to 11.3 MJ kg−1 DM in early regrowth to 9.2 to 10.2 MJ kg−1 DM in Agronomy Journal
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later stages of regrowth. This was expected, since digestibility declines with plant age at a rate that is influenced by the extent of change in cell wall carbohydrate and lignin concentrations (Sinclair et al., 2006). There was a considerable difference in fiber (approximately 8%) and energy (approximately 4%) levels of plants between experiments. This was most likely due to the temperature variation between experiments, with the warmer conditions of Exp. 2 acting to accelerate plant development through increasing conversion of carbohydrates to lignin (Van Soest 1996), therefore reducing plant quality. Other studies have found that tall fescue has lower herbage quality compared to species such as perennial ryegrass and prairie grass (Callow et al., 2003; Sinclair et al., 2006). The lower digestibility of tall fescue is largely due to the lower proportion of cell content and lower cell wall digestibility than other species such as perennial ryegrass (Wilman et al., 1996). The current results emphasize the dichotomy of this species, with relatively frequent defoliation at the two-leaf stage required to maintain an ME concentration above 10.3 MJ kg −1 DM, contrasting with relatively infrequent defoliation at the four-leaf stage required to maximize pasture production and persistence. Crude protein concentration decreased with increasing leaf stage in Exp. 1, and more than halved between the one-leaf and four-leaf stages in Exp. 2 (22.2–8.7%). The decrease in CP concentration with regrowth in both experiments was expected; Minson (1990) investigated the CP levels of a range of grass species and reported an average rate of decline of 0.22% per day. Similar results were obtained from Exp. 1, with an average decline in CP levels between the one-leaf and four-leaf stages of 0.24% per day. However, this rate of decline was much faster in early regrowth at 0.37% per day between the one-leaf and two-leaf stages, then slowed to an average of 0.18% per day between the two-leaf and four-leaf stages. The minimum defoliation interval for tall fescue was identified as the two-leaf stage of regrowth, provided that an adequate recovery period of regrowth is subsequently allowed. Defoliation at the two-leaf stage of regrowth maximized herbage quality, allowed WSC replenishment to pre-defoliation levels and resulted in a satisfactory rate of regrowth. With the poorest herbage quality measured at the five-leaf stage of regrowth, the maximum defoliation interval for tall fescue was identified as the four-leaf stage of regrowth, which resulted in significantly higher WSC reserves and subsequent leaf, stubble, and root DM yields than more frequent defoliation. The stubble was confirmed as the primary storage organ for WSC reserves, while leaf and root growth were found to have an equally high priority for available energy following defoliation. A field study investigating the rotational grazing of tall fescue at different leaf regrowth stages would be valuable to confirm the most effective range of grazing intervals in the field. ACKNOWLEDGMENTS The authors would like to acknowledge the assistance of Dr. Richard Rawnsley with statistical analyses.
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