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Agronomy Dep., P.O. Box 110300, Univ. of Florida, Gainesville, FL 32611-0300
* Corresponding author (les{at}ifas.ufl.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: DM, dry matter • TNC, total nonstructural carbohydrate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Vaseygrass has long been considered a weed in pastures and in harvested forage crops (Blaser et al., 1945; Hanson, 1965), but despite early recognition of its prolific seed production and rapid establishment (Blaser et al., 1945), studies on control of vaseygrass in pastures are limited. As limpograss has been increasingly adopted by producers in Florida (Adjei et al., 1998), the awareness of vaseygrass as a potential invader of limpograss pastures has increased (Chambliss et al., 1999).
Grazing management is a tool that can be used to influence pasture botanical composition (Humphreys, 1991). Close, continuous stocking (Blaser et al., 1945; Hanson, 1965) and high rates of N fertilization (Ehara and Tanaka, 1972) negatively affected vaseygrass stands. A more recent study showed that maintaining a 20-cm stubble height by continuous stocking reduced vaseygrass plant density and percentage cover in limpograss pastures, but common bermudagrass [Cynodon dactylon (L.) Pers.] cover increased (Newman et al., 2003).
Responses to defoliation may be a function of plant morphology. Many upright-growing grasses have growing points within the grazed portion of the canopy and usually store reserves in stem bases making them vulnerable to heavy grazing (Adjei et al., 1988). Gates et al. (1999) reported that bahiagrass (Paspalum notatum Flügge) varieties with a more upright growth habit and smaller basal diameter were less persistent under continuous stocking than more decumbent types. Chaparro et al. (1996), working with ‘Mott’ dwarf elephantgrass (Pennisetum purpureum Schumach.), observed that frequent, close defoliation of this upright-growing plant resulted in depletion of reserves with a concomitant decline in pasture stand. In general, plants that are upright growing, as is the case with vaseygrass, are likely to be less persistent under intense defoliation than stoloniferous plants such as limpograss.
Due to the anecdotal nature of most observations on control of vaseygrass, research is needed to determine if management practices can be used to reduce vaseygrass proportion in limpograss pastures. The objectives of this study were to assess the effects of different management practices including continuous vs. rotational stocking, short vs. taller pasture stubble height, and pasture N fertilization rate on (i) changes in vaseygrass plant density and cover and (ii) nonstructural carbohydrate and N content in vaseygrass plant parts.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Experimental Design and Treatments
Treatments were arranged as a split-plot experiment in two replicates of a completely randomized design. There were a total of 16 experimental units; each was 250 m2 (10 by 25 m) and was fenced and grazed separately. Main plots (500 m2) were the combination of one grazing method (continuous or rotational) and one stubble height (15 or 30 cm). Subplots (250 m2) within each main plot were N fertilization rates (50 or 150 kg N ha–1). The experimental units were the 250-m2 pastures, and sampling units were the entire experimental unit for measure of cover, 1-m2 quadrats (five per pasture) for measure of vaseygrass density, and individual vaseygrass plants (three per pasture) for measures of vaseygrass diameter, mass, TNC, and N.
Before initiation of the experiment, early spring growth of limpograss and vaseygrass was grazed during the last week of April 1994. After this grazing, the pastures were mowed to a 10-cm stubble height. In May, all pastures were fertilized with 19 kg P ha–1 and 74 kg K ha–1. Nitrogen was applied according to treatment as ammonium nitrate in four equal applications. The first application occurred on 15 May, 21 d before grazing treatments were imposed, and subsequent applications occurred after the first, second, and third grazing cycles of the rotational treatments.
In both years, simulated continuous and rotational stocking treatments were imposed starting on 6 June, and the height of pastures at grazing initiation was approximately 50 cm. The grazing cycle for rotationally stocked pastures was 4 wk, with 26- and 2-d rest and grazing periods, respectively. After the initial grazing, four grazing cycles were completed each year. The final grazing occurred on 26 September. Depending on herbage mass, three to six 350-kg yearling beef steers were assigned to each rotational pasture at the beginning of a grazing period. Pasture height was measured at 20 sites per pasture using a 1-m ruler, and animals were removed when average rule height of the unextended canopy reached the treatment height. Because of limited land area, simulated continuous treatments were unable to support constant presence of animals. Depending on pasture growth rates, yearling steers grazed these pastures for 2 to 6 h during each of 2 or 3 d per week to maintain the target stubble height. This allowed for frequent visits by cattle to a given feeding station, and pastures developed the spot-grazed appearance that is characteristic of continuously stocked swards. From this point forward the simulated continuous treatment will be referred to as "continuous."
Plant Density and Cover
Pasture characterization occurred before grazing treatments were imposed and after grazing ended in 1994 (3 June and 1 November, respectively). In 1995, characterization occurred only at the end of the grazing season on 1 November.
The effects of treatments were measured on vaseygrass plant density and on percent cover of vaseygrass, limpograss, and other plants. Because the objective was to measure changes in plant density over time, quadrats were selected systematically so that observations could always be made at the same sites. This was accomplished by establishing a transect that bisected the length of the plots from north to south. Then, five 1.0-m2 quadrats per pasture were placed at prescribed locations that were 4, 8, 12, 16, and 20 m from the northern end of the experimental unit. Vaseygrass plant density within each experimental unit was measured by counting plants in these five quadrats. Cover of vaseygrass, limpograss, and other plants was estimated visually by three observers who characterized the entire experimental unit (250-m2 pastures). Cover data reported are the average of the three observers, and observers trained by estimating cover in 1.0-m2 quadrats that were divided into 10 by 10 cm grids.
Vaseygrass Characteristics
Vaseygrass plant characteristics were measured before initiation of the experiment (3 June 1994) and at the end of the experiment (1 Nov. 1995). Data reported are from 1 Nov. 1995 after two seasons of grazing.
A total of three representative plants per pasture were chosen by three observers. Diameter at the base was determined for these plants. Two measures were taken per plant (along perpendicular lines bisecting the center of the base of the plant) at 3 cm above the soil surface, and the average of the two was considered to be basal diameter. Next, these plants were dug to assess dry matter (DM) mass and TNC and N concentration. Plants were fractionated into top, which included all herbage from 5 cm above the soil surface to the tip of the plant (data not reported on this fraction); stem base, which included plant material from 5 cm above soil level to the root; and root, which included all root to 10 cm below soil level. Stem-base and root fractions were washed to remove soil before each plant was separated into various parts. After separation, plant fractions were dried at 100°C for 1 h and 60°C to constant weight.
Material within a plant fraction was composited for the three plants per plot. Plant fractions were then ground utilizing a Wiley mill and analyzed for N and TNC concentrations. Concentrations of TNC were determined by a modification of the procedure described by Christiansen et al. (1988). This procedure combines an enzymatic digestion phase (Smith, 1981) for conversion of starch and oligosaccharides into monosaccharides with a photometric copper reduction method for reducing sugars (Nelson, 1944). For each sample of 0.2 g, 1 mL of an enzyme mixture was used. The enzyme mixture was prepared using 40.5 mL of deionized water, 2.25 mL of acetate buffer (0.2 M), 2.25 mL of invertase concentrate (G. Schlesinger Industries, Inc., Karle Place, NY) and 0.6 g of amyloglucosidase (Boehringer Mannheim Corp., Indianapolis, IN). Concentrations of N were determined using a micro-Kjeldahl method; samples were digested using a modification of the aluminum block digestion technique described by Gallaher et al. (1975), and analysis for ammonia in digestate was done by semi-automated colorimetry (Hambleton, 1977).
Statistical Analyses
Magnitude of change in plant cover and vaseygrass density during the first grazing season and from the beginning of the trial to the end of the second grazing season were assessed. Change was defined as the difference between initial and final measurements (i.e., final minus initial). Data were analyzed in two steps. First, an analysis of covariance was conducted and initial measurements were used as the covariate. Then, data were analyzed by using general linear model methodology through PROC GLM (SAS Institute, Inc., 1996). Vaseygrass plant characteristics (diameter, fraction mass, fraction TNC, and N concentration and content) after the second grazing season were also compared using general linear model methodology. All means reported in the text are least squares means. Because of the variability associated with grazing experiments, measurement of response variables in treatments are considered different when P 
0.10. Trends that are mentioned in the text are accompanied by their P value.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Change in Vaseygrass Plant Density
Stubble height and grazing method affected change in vaseygrass plant density after the first and second years of grazing (Fig. 1 and 2), but there was no height x method interaction, N rate effect, or interaction of N rate with other factors. Averaged across all treatments, vaseygrass initial plant density was 3.3 m–2.
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Change in Percentage Cover of Vaseygrass, Limpograss, and Other Plants
Vaseygrass
Initial vaseygrass cover across all experimental units was 20%. The continuous stocking treatment caused a decrease in vaseygrass cover after first and second years of grazing. After the first grazing season, vaseygrass cover had decreased 16 percentage units for pastures that were continuously stocked but only 3 percentage units when they were rotationally stocked (Fig. 3A). These numbers remained virtually unchanged after the second season of grazing (Fig. 3B).
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In addition, there was a stubble height by N rate effect after the first and second years of the study. This response resembles that of vaseygrass cover. In instances where vaseygrass cover decreased most it was observed that limpograss cover increased to the greatest extent. Specifically, limpograss cover increased or tended to increase when both N rate and stubble height were low (Table 2). Change in limpograss cover was the same regardless of stubble height when N rate was 150 kg ha–1, and it was the same when stubble height was 30 cm regardless of N rate (Table 2).
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Vaseygrass Plant Characteristics
Basal Diameter
Average basal diameter of vaseygrass plants before initiation of the experiment was 17.4 cm. After 2 yr, there was a trend (P = 0.118) toward smaller diameter for plants grazed to 15 cm (9.9 cm) vs. 30 cm (14.4 cm). Vaseygrass plants from continuously stocked pastures had basal diameter of 11 cm while those from rotational pastures had an average diameter of 14 cm (P = 0.26). Plants from the 15-cm stubble height and continuous stocking treatment had a smaller basal diameter (7.8 cm) than those from the 30-cm stubble height and rotationally stocked treatment (15.2 cm). There was no effect of fertilizer N on basal diameter of vaseygrass.
Stem-base and Root Mass
There was no N fertilizer effect on these responses, nor were there interactions of N fertilization rate with other experimental variables. After 2 yr, grazing method and stubble height affected stem-base mass and there was a trend toward an effect of both on root mass (Table 3; P = 0.139 and 0.185, respectively). Continuously stocked pastures had lesser stem-base mass compared to rotationally stocked pastures (12 vs. 23 g plant–1) and plants grazed to 15-cm stubble height had less base mass than those grazed to 30-cm stubble height (12 vs. 23 g plant–1). Magnitude and direction of the response was similar for root mass, but there was greater variability associated with measurement of root mass and statistical differences could not be detected.
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There were no main effects or interactions on N content of vaseygrass stem-base and root fractions. Content of TNC in the base of vaseygrass plants was affected by grazing method, and stubble height, but root TNC content was affected only by grazing method (Table 3). When pastures were continuously stocked, stem-base and root contents of TNC were lower (0.40 and 0.46 g plant–1, respectively) than when pastures were rotationally stocked (0.78 and 0.91 g plant–1 for base and root, respectively). Stem-base TNC content was less when pastures were grazed to 15-cm (0.44 g plant–1) compared to 30-cm stubble height (0.73 g plant–1). A similar trend (P = 0.186) was observed for root TNC response to stubble height (Table 3).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In the current study, vaseygrass density increased by 3.0 plants m–2 during the first year when stubble height was 15 cm, but decreased by 0.6 plants m–2 when height was 30 cm. This difference can be attributed primarily to greater light penetration to the base of the more closely grazed grass canopies (Pedreira et al., 2000). This is thought to have caused greater germination (not quantified) of the vaseygrass seed reserve in the 15-cm pastures and resulted in a greater increase in short-term stand density than in 30-cm swards. Numerous small plants were noted during stand counts at the end of the first year of grazing. When limpograss pastures were continuously stocked over two grazing seasons in another study (Newman et al., 2003), there was an increase in vaseygrass frequency of occurrence when pastures were grazed to 20 cm but a decrease when height was 40 or 60 cm. The authors attributed this response to greater light penetration to the base of the canopy and less competition from limpograss in the shorter than the taller swards. In the second year of the current study, reduction of vaseygrass density continued in the 30-cm swards, but in the 15-cm swards the trend toward increasing density was reversed and there was a large decrease (2.2 plants m–2) in vaseygrass stand. The response in the second year follows more closely the pattern that was observed by Newman et al. (2003), who reported that close grazing and continuous stocking for 2 yr caused a reduction in vaseygrass density of more than four plants m–2.
Some species respond to close grazing by increasing tillering or altering tiller angle (Pedreira et al., 2000), but for many bunchgrasses phenotypic plasticity is limited and close, frequent grazing compromises persistence (Spitaleri et al., 1994; Chaparro et al., 1996). Thus, in the current study it seems likely that the first year increase in vaseygrass density for closely grazed stands was due to seedling recruitment. Close grazing limited further seed production, however, and fewer seedlings likely were established in Year 2. The decline in vaseygrass stand in 15-cm pastures in the second year appears to be due to the weakening and eventual death of existing plants and limited seedling recruitment. This hypothesis is supported by the reduction in vaseygrass plant diameter, stem-base mass, and contents of TNC in the stem-base and root for plants grazed to the low stubble height.
The change in percentage cover response was more consistent after 1 or 2 yr than was vaseygrass density. Vaseygrass cover declined more than 15 percentage units with continuous stocking and approximately three units with rotational stocking. Vaseygrass cover also decreased markedly in pastures grazed to a 15-cm stubble height and fertilized at the low N rate. In pastures where vaseygrass cover decreased most, limpograss cover increased. The increase in limpograss cover, however, was less than half the amount of the decrease in vaseygrass cover, so clearly other species capitalized on the newly open areas. This highlights a dilemma facing the grazier managing limpograss pastures in which vaseygrass is a weed. Newman et al. (2003) reported that continuous stocking to a 20-cm stubble height severely reduced vaseygrass stands in limpograss, but this decrease was associated with an increase in common bermudagrass. Once it colonizes a pasture, common bermudagrass is very difficult to eradicate. Thus, Newman et al. (2003) recommended a taller stubble height (40 cm) that would decrease vaseygrass in the sward, although not as much as grazing to 20 cm, but without allowing large increases in common bermudagrass.
In summary, continuous stocking appears to be an important tool in reducing vaseygrass cover in limpograss pastures. Based on our data, this change may be due more to a decrease in diameter and mass of existing plants than to a decrease in plant density, although vaseygrass density did decline by one plant m–2 during the two years of continuous stocking in this study. Close grazing in association with low N fertilizer rates also appear to favor the rapid loss of vaseygrass cover. It is prudent to note, however, that frequent close grazing will negatively affect limpograss stands and allow for invasion by common bermudagrass (Newman et al., 2003). The best management for reducing vaseygrass, but not having a large adverse effect on limpograss, is likely continuous stocking to a moderate stubble height (40-cm stubble height; Newman et al., 2003). It may be possible to use this type of management for one or two grazing seasons to reduce vaseygrass and then revert back to rotational stocking until vaseygrass becomes a significant problem again.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication December 17, 2004.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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