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Notes & Unique Phenomena

Distribution of Water-Soluble Carbohydrate Reserves in the Stubble of Prairie Grass and Orchardgrass Plants

^a Tasmanian Institute of Agricultural Research, Univ. of Tasmania, P.O. Box 3523, Burnie 7320, Tasmania, Australia
^b School of Agricultural Science, Univ. of Tasmania, Private Bag 54, Hobart 7001, Tasmania, Australia

^* Corresponding author (Lydia.Turner{at}utas.edu.au)

Received for publication August 3, 2006.

	ABSTRACT

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A greenhouse study was undertaken to investigate the distribution of water-soluble carbohydrates (WSC) within the lower 100 mm of ‘Kara’ orchardgrass (Dactylis glomerata L.) and ‘Matua’ prairie grass (Bromus willdenowii Kunth.) stubble through four distinct regrowth cycles. Water-soluble carbohydrate levels were consistently higher in prairie grass tillers compared with orchardgrass tillers. A decrease in WSC levels with increasing stubble height was observed for vegetative tillers of both species. However, the WSC concentration gradient was better defined for orchardgrass, with a clear decrease in WSC concentration between the 21- to 30- and 31- to 40-mm segments, and 77% of WSC content contained within the 0- to 30-mm stubble height range (with 0 mm representing the base at ground level). The WSC concentration gradient for prairie grass was less clearly defined, with a relatively high WSC concentration throughout the 0- to 100-mm stubble height range. There was a trend for decreasing WSC concentration between the 31- to 40- and 41- to 50-mm segments, with 62% of WSC content contained within the 0- to 40-mm stubble height range. These results suggest that the previously adopted defoliation stubble height of 45 to 50 mm, which is the optimal defoliation management for perennial ryegrass (Lolium perenne L.), maintains over 60% of stubble WSC reserves and therefore should not be detrimental to the persistence of orchardgrass and prairie grass. While decreasing defoliation height to 30 mm may be acceptable for orchardgrass, prairie grass is more sensitive to defoliation severity, with defoliation below 45 mm not recommended.

Abbreviations: DM, dry matter • WSC, water-soluble carbohydrates

	INTRODUCTION

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WATER-SOLUBLE CARBOHYDRATES located in the lower region of grass tillers (stubble) are utilized as an energy source to initiate new growth of perennial grasses until photosynthesis is sufficient to sustain plant respiration and growth (White, 1973). Water-soluble carbohydrate levels are affected by defoliation severity, as indicated by residual height of stubble (Troughton, 1957; Davidson and Milthorpe, 1966; Wilson and Robson, 1970), as well as the interval between defoliations (Davies, 1965; Bell and Ritchie, 1989; Fulkerson and Slack, 1995).

The recommended defoliation height for perennial ryegrass is 45 to 50 mm, which combined with defoliation between the 2- to 3-leaf regrowth stages, maximizes WSC accumulation and therefore the rate of plant regrowth (Fulkerson and Slack, 1995). The importance of an appropriate defoliation interval to maximize the regrowth and persistence of orchardgrass and prairie grass has been established, with results from recent field and greenhouse studies confirming the 4-leaf stage as the optimal time of defoliation for both species (Fulkerson et al., 2000; Slack et al., 2000; Turner et al., 2006a,b). In these studies, the recommended defoliation stubble height for perennial ryegrass (45–50 mm) has been adopted for defoliation of prairie grass and orchardgrass plants, but the optimal defoliation height for these species is yet to be defined.

Although defoliation interval is generally of primary importance in determining the regrowth of pasture plants, and stubble height is of secondary importance (Bell and Ritchie, 1989; Fulkerson and Donaghy, 2001), stubble height can induce significant effects on plant regrowth and persistence. Fulkerson and Slack (1994b) assessed the effect of cutting height on production and persistence of perennial ryegrass and found that defoliation to 60-mm stubble height, as opposed to 120 mm, yielded 54% more edible dry matter (DM) and resulted in a 65% higher plant survival rate. Fulkerson and Slack (1994b) concluded that the 120-mm defoliation height provided shading that adversely affected perennial ryegrass, while the 60-mm defoliation height allowed for sufficient reserves to support regrowth following defoliation. Detrimental consequences are reported to be associated with defoliation below a 50-mm stubble height, with depletion of stubble WSC reserves following defoliation leading to delayed regrowth and decreased plant persistence (Davidson and Milthorpe, 1966; Wilson and Robson, 1970).

By providing a balance between maintenance of WSC reserve levels and minimization of shading, defoliating grass plants at an optimal defoliation height maximizes the rate of plant regrowth. The aim of the current study was to investigate WSC reserve storage patterns in the stubble of Matua prairie grass and Kara orchardgrass plants through four distinct regrowth cycles.

	MATERIALS AND METHODS

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The experiment was conducted in a greenhouse at the Tasmanian Institute of Agricultural Research, Burnie, Australia (41°04' S, 145°53' E; elevation, 206 m), between December 2004 and October 2005. Three seeds were planted in each polyvinyl bag (100-mm diam. x 280-mm depth) on 28 Dec. 2004. The bags had perforated bases and contained a potting mixture composed of 50% Pinus radiata D. bark, 30% sand and 20% Spaghnum sp. moss. The plants were arranged in the greenhouse at a density of 100 plants m^–2 and were watered daily via an underlying geotextile membrane capillary mat, to replace evapotranspiration losses. Greenhouse conditions were controlled to maintain day/night temperatures of 20/10°C. Daylength and radiation (MJ m^–2) values for the experimental period are presented in Fig. 1 . Within 4 wk of germination, the weakest seedlings were removed to allow a single healthy seedling to reach maturity in each bag. Plants were fertilized bimonthly with Osmocote (15–9–12, N–P–K; Scotts Australia Pty. Ltd., New South Wales, Australia) at a rate equivalent to 40 kg N ha^–1.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Monthly means for daylength (hours, closed triangles), and radiation (MJ m–2, open bars) for the experimental period between January and October 2005.

Experimental Design
Plants were arranged in a randomized complete block design, with four replicates each containing eight randomly allocated treatments. Each treatment consisted of a row of nine plants block^–1, resulting in 36 plants in total treatment^–1. The treatments were each characterized by one of two species (Matua prairie grass and Kara orchardgrass) and one of four successive destructive harvests (H₁–H₄). Buffer plants were placed around the minisward to minimize boundary effects. These plants were harvested but otherwise not included in analyses.

Destructive Harvest Regime
On 14 February (48 d following sowing), all plants were defoliated to a stubble height of 100 mm to promote tillering. At the 4-leaf stage of regrowth, plants were again defoliated to 100 mm, an event termed H₁. At H₁, one row of plants for each species per block was destructively harvested to ground level. The stubble was immediately divided into 10-mm increments and samples were combined to produce one sample per row for each of the 10-mm increments. This harvest regime was repeated three times to investigate possible changes in WSC storage patterns through four sequential regrowth cycles (H₁–H₄). Harvest dates were as follows: 22 April (H₁), 16 June (H₂), 11 August (H₃) and 14 October (H₄). Immediately before each harvest event, numbers of vegetative and reproductive tillers plant^–1 were recorded, and reproductive and vegetative tillers were separated at each harvest for individual analysis. Harvests were consistently performed 3 h after sunrise, to negate the confounding effect of diurnal fluctuations in WSC (Fulkerson and Slack, 1994a). All stubble samples were frozen, freeze-dried and weighed.

Determination of Water-Soluble Carbohydrates
Freeze-dried stubble samples were ground to pass through a 1-mm sieve in preparation for analysis of WSC content. The concentration of WSC was determined by cold extraction of plant material in a reciprocal shaker for 1 h using 0.2% benzoic acid–water solution (C₆H₅COOH), and the hydrolysis 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 [K₄Fe(CN)₆3H₂O], again heated at 90°C, and then measured using an autoanalyzer (420 nm) [Technicon Industrial Method no. 302-73A, derived from the method outlined by Smith (1969)].

Statistical Analyses
Water-soluble carbohydrate means were compared using an ANOVA three-way factorial design (segment by species by harvest) with replication, vegetative tiller number plant^–1 means were compared using an ANOVA two-way factorial design (species by harvest) with replication, and reproductive tiller number plant^–1 means were compared using an ANOVA two-way factorial design (harvest by block) without replication. The statistical package SPSS (Version 11.5, SPSS Corp., Illinois, USA) was used for all statistical analyses, and least significant difference (LSD), as defined by Steel and Torrie (1960) was used to separate means.

	RESULTS

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Vegetative Tillers
There were significant segment by species (P < 0.001) and species by harvest (P < 0.001) interactions for stubble WSC concentration (g mg^–1; Table 1). With the exception of the 0- to 10-mm segment, there was a significantly higher (P < 0.001) WSC concentration in the corresponding segments of prairie grass compared with orchardgrass. For prairie grass, there was a significant increase (P < 0.05) in stubble WSC concentration between the 0- to 10- and 11- to 20-mm segments, with no significant difference (P > 0.05) between the 11- to 20- and 31- to 40-mm segments, and a significant decline (P < 0.05) in stubble WSC concentration between each segment in the range 31 to 40 to 51 to 60 mm. For orchardgrass, there was a significant decrease (P < 0.05) in stubble WSC concentration between each segment in the range 11 to 20 to 31 to 40 mm. Levels of WSC in orchardgrass stubble stabilized between 40 and 100 mm. There was no significant (P > 0.05) harvest by segment interaction for WSC concentration in vegetative tillers.

Stubble WSC content was generally higher for prairie grass compared with orchardgrass, as evidenced by a significant (P < 0.01) species effect and means of 3.50 and 2.89 mg segment^–1 tiller^–1 for these species, respectively. Significant (P < 0.001) segment and harvest effects represented the trends of decreasing stubble WSC content with increasing segment height, and a higher WSC content at H₄ than at any previous harvest (Table 1).

Reproductive Tillers
Only prairie grass exhibited reproductive growth, and at H₁ and H₄, reproductive tiller samples were of a sufficient size for determination of WSC reserves. There was significantly higher (P < 0.001) stubble WSC content at H₄ compared with H₁ (Table 2). There was a significant (P < 0.01) segment effect on stubble WSC content for reproductive prairie grass tillers, with WSC content decreasing with increasing segment height (Table 2). There was no significant (P > 0.05) harvest by segment interaction for WSC content in reproductive prairie grass tillers. Mean WSC content appeared generally higher in reproductive prairie grass tillers than in vegetative tillers.

	DISCUSSION

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A decrease in WSC levels with increasing stubble height was observed for vegetative tillers of both species. However, the WSC concentration gradient was more clearly defined for orchardgrass, with a clear decrease in WSC concentration between the 21- to 30-mm and 31- to 40-mm segments, and 77% of WSC content contained within the 0- to 30-mm stubble height range. The WSC concentration gradient for prairie grass was not as distinct as for orchardgrass, with a relatively high WSC concentration throughout the 0- to 100-mm stubble height range. There was a trend for decreasing WSC concentration between the 31- to 40-m and 41- to 50-mm segments, with 62% of WSC content contained within the 0- to 40-mm stubble height range. These results suggest that the previously adopted defoliation stubble height of 45 to 50 mm maintains over 60% of stubble WSC reserves and therefore should not be detrimental to the persistence of these species.

The pattern of WSC reserve storage in orchardgrass stubble suggests that more severe defoliation to a height of 30 mm may be acceptable. As plants in this study were defoliated to a height of 100 mm, a further study that implements a range of defoliation heights, combined with the recommended defoliation interval of the 4-leaf stage is required to test the effects of a more severe defoliation regime on orchardgrass regrowth, production and persistence.

In terms of defoliation frequency, the contribution of N reserves to regrowth has been reported by Turner et al. (2006a; Turner et al., 2006, unpublished data) to be minimal for orchardgrass, but considerable for prairie grass. The effect of this defoliation regime on N reserves of prairie grass would therefore also be of interest, given the possibility that proteins may be mobilized for growth and respiration following severe defoliation of pasture plants.

The pattern of WSC reserve storage in prairie grass suggests that this species is sensitive to severe defoliation, possibly due to its upright habit and lower tillering capacity (Hume, 1991a). Prairie grass is also sensitive to frequent defoliation (Hume, 1991b; Turner et al., 2006, unpublished data), and the importance of careful defoliation management to maximize plant persistence must be emphasized. With this in mind, defoliation interval should be regarded as of primary importance, followed by defoliation height, which is of secondary importance (Bell and Ritchie, 1989; Fulkerson and Donaghy, 2001).

The variation in WSC reserve accumulation between harvests may be explained by seasonal changes in light intensity and duration throughout the experimental period. While temperature, moisture and nutrient availability were constant variables, light intensity and duration were lower between H₁ and H₃ (late autumn and winter harvests) than at H₄ (midspring harvest). Increased light intensity and duration would have resulted in a higher photosynthetic capacity, which combined with stable respiration (due to constant temperature and moisture availability), resulted in greater WSC accumulation at H₄ than at previous harvests. Orchardgrass exhibited a more pronounced response to seasonal light intensity than prairie grass.

In conclusion, these results suggest that the previously adopted defoliation stubble height of 45 to 50 mm is suitable to maintain WSC reserves at levels adequate to maintain optimal regrowth and persistence of prairie grass and orchardgrass. While decreasing defoliation height to 30 mm may be acceptable for orchardgrass, prairie grass is more sensitive to defoliation severity, with defoliation below 45 mm not recommended.

	ACKNOWLEDGMENTS

 
The authors would like to acknowledge financial support provided by Dairy Australia and technical assistance provided by Andrew Turner and Karen Christie.

	REFERENCES

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