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a Tasmanian Institute of Agricultural Research, Univ. of Tasmania, Burnie Tasmania, Australia
b Univ. of Tasmania, Hobart Tasmania, Australia
* Corresponding author (wilsonlr{at}utas.edu.au)
Received for publication November 13, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: CP, crude protein • DM, dry mass • ICP-AES, inductively coupled plasma-atomic emission spectrometry • ME, metabolizable energy • NIRS, near-infrared spectrometry • WSC, water-soluble carbohydrate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Although several studies have clearly shown prairie grass is capable of greater herbage production than perennial ryegrass (Vartha, 1977; Fraser, 1982; DeLacy, 1987; Fulkerson et al., 2000), when grazed in the same manner as ryegrass-based pastures (as is common practice), prairie grass is disadvantaged, particularly from late spring to early autumn when the leaf appearance rate of prairie grass is more rapid than for ryegrass (Turner et al., 2006).
For example, in a Tasmanian study, prairie grass was initially shown to be unproductive with poor persistence (Department of Primary Industry, 1987) because existing best management practice for ryegrass-based pastures was applied to both pasture types. In a subsequent study, prairie grass was grazed to leave longer postgrazing residuals and allowed to set seed in spring. Utilization from the prairie grass pasture rose to 2, then 4 Mg DM greater than utilization of ryegrass in successive years (Department of Primary Industry, 1987).
Grazing management based on the physiological status of the prairie grass plant is more efficient than grazing management based on herbage accumulation or on set day rotations (Thom et al., 1990; Fulkerson and Donaghy, 2001). A useful plant-related indicator of optimal defoliation timing is the full expansion of a particular number of leaves per tiller. For perennial ryegrass, the 2- to 3-leaf stage of regrowth has been accepted as the optimal defoliation interval (Fulkerson and Donaghy, 2001).
When perennial ryegrass plants are defoliated before the 2-leaf stage, there is insufficient time for replenishment of WSC reserves, which are depleted immediately following defoliation. Repeated defoliations before plants are physiologically "ready" therefore results in reduced leaf regrowth, tillering, and root growth (Donaghy and Fulkerson, 1997, 1998; Rawnsley et al., 2002). In addition to the changes in stubble WSC concentration during regrowth, Fulkerson et al., (1998) suggest that there may also be an imbalance in herbage mineral levels and the ratio of carbohydrates to protein when perennial ryegrass is grazed before the 2-leaf stage.
When perennial ryegrass plants are defoliated after the 3-leaf stage, leaf senescence and an increase in fiber concentration leads to increased rejection of herbage by grazing stock and therefore lower levels of DM utilization. The optimal grazing intervals defined for perennial ryegrass pastures are the key to grazing management for the maintenance of a balance between pasture growth, persistence, quality and utilization (Fulkerson and Donaghy, 2001). As species-specific grazing management is required to maximize the potential of perennial grass species, it follows that the optimal performance of prairie grass as a dairy pasture species would be similarly reliant on a customized optimal grazing interval.
Fulkerson et al. (2000) found that a grazing rotation based on the 3.5- to 4-leaf stage of regrowth favored prairie grass growth, persistence and herbage feed quality throughout the year. The aim of the present study was to further investigate morphological and physiological changes in prairie grass during regrowth and hence the mechanisms underpinning the existing defoliation management recommendations for prairie grass pastures.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Determination of Leaf Stage
Each leaf regrowth stage was defined as the presence of a particular number of fully expanded leaves per tiller. Prairie grass maintains 4 to 5 live leaves per tiller, with a new leaf commencing growth when the previous leaf has reached approximately 75% of its full length. Thus, at any given time two prairie grass leaves are expanding, and measurement of "leaf stage" is estimated by the amount of growth of the current expanding leaves.
Experimental Design
Plants were arranged in a randomized complete block design, with five blocks each containing seven randomly allocated treatments. Each treatment consisted of a row of nine plants per block, resulting in 45 plants in total per treatment. Treatments consisted of one preliminary harvest (0) and six sequential harvests at each leaf regrowth stage from one to six fully expanded leaves per tiller (1–6L). Buffer plants were placed around the minisward to minimize boundary effects. These plants were harvested but otherwise not included in analyses.
On 30 June (55 d following sowing), all plants were defoliated to a stubble height of 50 mm to promote tillering. At the 4-leaf stage of regrowth, plants were again defoliated to 50 mm in preparation for the destructive harvesting regime.
Destructive Harvest Regime and Determination of Dry Matter Yield
The second defoliation to 50 mm constituted the first pretreatment destructive harvest (0). Sequential harvests then followed at each leaf regrowth stage, up to the 6-leaf stage.
Nonsheath leaf tissue (>50 mm) was removed at each harvest event, and leaf DM yield plant–1 determined after drying samples for at least 24 h at 60°C in a forced draft oven. Stubble tissue below 50 mm and roots were collected at each harvest event. Stubble and washed roots were dried for at least 24 h at 80°C in a forced draft oven then weighed. Tiller DM and number per plant were also determined at each harvest interval. Harvests were consistently performed at 3 h after sunrise, to negate the confounding effect of diurnal fluctuations in WSC (Fulkerson and Slack, 1994). Dried leaf, root and stubble samples were ground through a 1-mm sieve before chemical analyses.
Differentiation in Leaf Material
Leaf and tiller senescence as well as reproductive development proceeded as plants matured, and measures were taken to define differentiation in leaf material with plant growth. At the 5-leaf and 6-leaf stages, two subsamples from the harvested leaf material within each block were taken and one was separated into live and dead material, while the other was sorted into leaf and stem material, with stem determined as elongated internode.
Water-Soluble Carbohydrate Analysis
Root, stubble and leaf samples were analyzed for WSC. The concentration of WSC was determined by cold extraction of plant material in a reciprocal shaker for 1 h using 0.2% benzoic acid C6H5CO2H)–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 ferrocyanide [K4Fe(CN)3H20], again heated at 90°C, and then measured at 420 nm using an autoanalyser (Technicon Industrial Method no. 302-73A, derived from the method outlined by Smith, 1969).
Herbage Quality Analysis
Metabolizable Energy
Leaf samples were analyzed for ME concentration using NIRS at Hamilton FeedTEST Laboratories (Victoria Department of Primary Industries).
Minerals
Phosphorus, K, Na, Mg, and Ca concentrations of leaf samples were determined by digestion of finely ground plant material in nitric acid (HNO3) using a Milestone microwave (Milestone, Connecticut, USA) and measurement by ICP-AES (McQuaker et al., 1979).
Nitrogen
Finely ground leaf material was combusted at 950°C in O2 using a Leco FP-428 Nitrogen Analyzer (LECO Instruments, St. Joseph, MI, USA). The released N from the sample was measured as it passed through a thermal conductivity cell (Sweeney and Rexroad, 1987). The CP content was calculated as N concentration (g kg–1 DM) x 6.25.
Statistical Analyses
Pretreatment DM values from defoliation Event 0 were used as a covariate. Differences between treatment (leaf regrowth stage) means were tested for the following variables: leaf DM, root DM, individual tiller DM, WSC, ME, CP, Ca, P, Ca/P, K, Na and Mg. Means were compared by ANOVA using the statistical package SPSS, and least significant difference (LSD) when F tests were significant at P = 0.05, as defined by Steel and Torrie (1960), Regression (r2) between WSC level and plant regrowth was tested using the statistical functions of EXCEL.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A positive correlation was found between stubble DM and root WSC concentration (g kg–1 DM), although the proportion of experimental variation (r2) was only 0.40. No such relationship was found between root DM, leaf DM or tiller number and root WSC concentration, or between tiller number and stubble WSC concentration or content.
Changes in Tiller per Plant with Regrowth
Before defoliation, there were 6.7 ± 0.4 tillers plant–1 and this remained relatively constant (P > 0.05) until the 3-leaf stage, after which tiller numbers increased (P < 0.05) to 8.0 ± 0.4 at the 4-leaf and 5-leaf stages, and 9.4 ± 0.4 at the 6-leaf stage of regrowth.
Changes in Dry Matter Yield with Regrowth
Stubble DM (mg tiller–1) decreased (P < 0.05) following defoliation and increased (P < 0.05) at a constant rate until the 3-leaf stage of regrowth. Stubble DM was higher (P < 0.05) at the 5-leaf and 6-leaf stages than at any previous regrowth stage (Table 2). There was no difference (P > 0.05) in root DM (mg plant–1) from defoliation until the 3-leaf stage, followed by an increase (P < 0.001) from the 4-leaf to the 6-leaf stage of regrowth (Table 2). Following defoliation, there was a steady increase (P < 0.05) in leaf DM (mg plant–1) with regrowth (Table 2). Leaf senescence was first measured at the 5-leaf stage, with 0.05 of leaf DM senescent, increasing to 0.09 at the 6-leaf stage of regrowth (Fig. 2
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There was a general decline in leaf CP, Ca, Mg, P, and K concentrations with regrowth following defoliation. However, the decline was not constant, with a plateau effect evident between the 2-leaf or 3-leaf and 5-leaf stages (Table 3). The concentration of Na in the leaf decreased (P < 0.05) between the 1-leaf and 2-leaf stages, and increased (P < 0.05) between the 3-leaf and 5-leaf stages of regrowth (Table 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Both the concentration of WSC and the level of replenishment following defoliation were greater in the stubble than in the root system, confirming that, as in ryegrass (Danckwerts and Gordon, 1987; Fulkerson and Slack, 1994) and orchardgrass (Dactylis glomerata L.) (Davidson and Milthorpe, 1966a; Rawnsley et al., 2002), the stubble is the major storage site for WSC in prairie grass. Immediately following defoliation, WSC replenishment commenced, resulting in significantly higher WSC levels at the 5-leaf and 6-leaf stages than at any previous regrowth stage. In contrast to orchardgrass plants, in which WSC reserves are replenished to predefoliation levels by the 4-leaf stage (Rawnsley et al., 2002), prairie grass WSC reserves in this study were replenished to predefoliation levels by the 2-leaf stage of regrowth.
The strong positive linear relationship between stubble WSC levels and the regrowth capacity of prairie grass confirms that WSC reserves play an important role in the entire plant regrowth cycle following defoliation. The importance of WSC reserves in the regrowth of orchardgrass (Davidson and Milthorpe, 1966a; Rawnsley et al., 2002), perennial ryegrass (Davies, 1965; Fulkerson and Slack, 1994; 1995; Donaghy and Fulkerson, 1997, 1998), tall fescue (Festuca arundinacea Schreb.) (Booysen and Nelson, 1975; Volenec, 1986) and timothy (Phleum pratense L.) (Smith, 1974) has been previously reported. However, the extent of reliance of ryegrass (Danckwerts and Gordon, 1987; Fulkerson and Slack, 1994; Donaghy and Fulkerson, 1997) and orchardgrass (Davidson and Milthorpe, 1966a,b) plants on reserves in such studies has often been shown to be limited to several days postdefoliation.
While there was little difference in the strength of linear relationships between regrowth of leaves and stubble WSC content (mg tiller–1) compared with WSC concentration (g kg–1 DM), there were closer relationships between stubble DM and regrowth of roots with stubble WSC content compared with concentration. With prairie grass tillers weighing up to three times that of perennial ryegrass tillers (Turner et al., 2006), the regenerative capacity of prairie grass is not only correlated to WSC content but also the size of the storage organ. Based on the figures from the current study, for any given WSC concentration, there is a significantly higher WSC content (mg tiller–1) in prairie grass compared with perennial ryegrass.
Leaf growth had a higher priority for available energy than root growth and tiller initiation in the current study. This was evidenced by immediate leaf regrowth following defoliation, compared with significant root regrowth not until the 3-leaf stage, and significant increases in tiller number not until the 4-leaf stage of regrowth. This time sequence and overall priority for allocation of WSC reserves is almost identical to that found for perennial ryegrass (root regrowth occurring at the 1-leaf and tillering at the 2-leaf stage; Donaghy and Fulkerson, 1998) and orchardgrass (root regrowth occurring at the 4-leaf and tillering at the 5-leaf stage; Rawnsley et al., 2002). Thus it appears evident that for a range of pasture grasses, a similar priority for energy reserve allocation exists, and this has ramifications for grazing management.
Despite the capability of prairie grass to immediately resume replenishment of energy reserves following defoliation, it is likely that defoliation before the 4-leaf stage of regrowth will not allow tiller initiation to occur. As perenniality of grasses depends on their capacity to replace dying tillers (Colvill and Marshall, 1984; Marshall, 1987), defoliation before the 4-leaf stage of regrowth may result in reduced persistence of prairie grass plants. The detrimental effect of frequent defoliation on the persistence of prairie grass is further exacerbated by the relatively low tiller density of this species compared with perennial ryegrass (Turner et al., 2006). It is also important to note that the plants in the current study were defoliated at the 4-leaf stage of regrowth before treatments commencing—had they been defoliated at an earlier leaf stage, the pattern of WSC replenishment during the subsequent regrowth cycle may have differed considerably.
A reduction in root growth following defoliation has been previously reported for orchardgrass (Jacques and Edmond, 1952; Davidson and Milthorpe, 1966b; Evans, 1973; Rawnsley et al., 2002) and perennial ryegrass (Jacques and Edmond, 1952; Evans, 1971, 1972, 1973; Fulkerson et al., 1993; Donaghy et al., 1997, Donaghy and Fulkerson, 1997, 1998). In the current study, defoliation almost halved the root DM of prairie grass during the time it took to regrow one new leaf, supporting the findings of Evans (1972) and Donaghy and Fulkerson (1998) that root regrowth is highly sensitive to changes in WSC concentrations. Prairie grass resumed significant root growth at the 3-leaf stage of regrowth. Management that restricts root growth affects plant growth in general, due to the limitation placed on water and nutrient uptake (Davidson and Milthorpe, 1966b; Clement et al., 1978), and therefore plant survival. From a root system perspective, defoliation of prairie grass before the 3-leaf stage of regrowth would therefore have a detrimental effect on plant survival.
The ME value of prairie grass remained above 11.80 MJ kg–1 DM until (and including) the 4-leaf stage, after which increased stem and senescent material combined to reduce the ME concentration with regrowth. Given the increased leaf DM at the 5-leaf and 6-leaf stages, total ME yield (MJ ha–1) would be substantially greater at the longer regrowth interval, but may not result in increased pasture utilization in a grazing situation due to increased rejection by stock because of the increased stem and senescent material.
The decrease in CP concentration with regrowth 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. The CP concentration of prairie grass met the requirements of a high-producing dairy cow until (and including) the 4-leaf stage, after which point CP concentrations fell below the requirements, possibly due to the increased stem/leaf ratio and increased levels of WSC (Beever et al., 2000).
A decrease in the concentration of Ca in the leaf with increasing maturity is unusual and in contrast to that reported for perennial ryegrass (Wilman et al., 1994; Fulkerson et al., 1998), white clover (Wilman et al., 1994) and orchardgrass (Rawnsley et al., 2002). Combined with the more rapid decline in the concentration of P in the leaf, there was a 1.5-fold increase in the ratio of Ca/P concentration in the leaf from the 1-leaf to the 6-leaf stage of regrowth. At no stage of regrowth did the Ca/P approach the ratio of 1.6 recommended by National Research Council (1989) for a high-producing dairy cow. The concentration of Ca in the leaf only met these requirements at the 1-leaf stage of regrowth, while the concentration of P in the leaf was in excess of these requirements throughout the regrowth cycle. The low Ca concentrations observed in this study may not translate to Ca deficiencies for grazing animals in the field, as prairie grass would be sown with a companion legume (legumes generally contain more Ca than grasses; Whitehead, 1972) and regular applications of superphosphate and lime to the pasture should maintain Ca levels (Grace, 1989).
The decline in the concentration of Mg in the leaf was in contrast to the pattern of Mg accumulation reported for perennial ryegrass (Wilman et al., 1994; Fulkerson et al., 1998; Fulkerson and Donaghy, 2001) but in agreement with that reported for orchardgrass (Rawnsley et al., 2002). The concentration of Mg in the leaf of prairie grass did not approach the recommended 2.0 g kg–1 DM (National Research Council, 1989) at any stage of the regrowth cycle. A low Mg concentration is one of the previously reported nutritional limitations of prairie grass (Thom et al., 1990; Fulkerson et al., 2000), however the effect of low herbage Mg on milk production has been shown to be insignificant (Wilson and Grace, 1978).
Leaf K concentrations were adequate until (and including) the 4-leaf stage of regrowth, after which point the continuing decline in K concentrations with regrowth failed to meet the National Research Council (1989) recommendation of 54.0 g kg–1 DM. The pattern of Na accumulation in the leaf was somewhat unpredictable, but significantly increased between the 1-leaf and 6-leaf stages and remained in excess of the requirements of a high-producing dairy cow for the duration of the regrowth cycle. The high concentrations of P and Na in the current study contrast with a previous study (Crush et al., 1989), in which these minerals were identified as potential nutritional limitations for prairie grass compared with perennial ryegrass.
In conclusion, a defoliation interval based on the 4-leaf stage of regrowth enables adequate time for prairie grass to replenish WSC reserves, resume root growth and initiate new tillers, before the onset of significant leaf senescence, stem production and consequent reduction in feed quality. These results confirmed that the tiller base is the primary storage organ for WSC (Fulkerson and Slack, 1994) and that leaf growth has the highest priority for available energy after defoliation, followed by root growth and tiller initiation, as is the case for perennial ryegrass (Donaghy and Fulkerson, 1998) and orchardgrass (Rawnsley et al., 2002).
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  D. J. Donaghy, L. R. Turner, and K. A. Adamczewski Effect of Defoliation Management on Water-Soluble Carbohydrate Energy Reserves, Dry Matter Yields, and Herbage Quality of Tall Fescue Agron. J., January 11, 2008; 100(1): 122 - 127. [Abstract] [Full Text] [PDF]
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) and root (
) WSC (g kg–1 DM) concentration with regrowth of prairie grass after defoliation. LSD (P = 0.05) shown as vertical bars.
) and stem (