Agronomy Journal 92:194-199 (2000)
© 2000 American Society of Agronomy
FORAGES
Botanical Composition, Light Interception, and Carbohydrate Reserve Status of Grazed `Florakirk' Bermudagrass
Carlos G.S. Pedreiraa,
Lynn E. Sollenbergerb and
Paul Mislevyc
a Depto. de Producao Animal, Escola Superior de Agricultura "Luiz de Queiroz," Univ. de São Paulo (ESALQ-USP), Caixa Postal 9, Piracicaba, SP 13418-900, São Paulo, Brazil
b Agronomy Dep., P.O. Box 110300, Univ. of Florida, Gainesville, FL 32611-0300 USA
c Range Cattle Res. & Educ. Ctr., Univ. of Florida, Ona, FL, 33865 USA
les{at}gnv.ifas.ufl.edu
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ABSTRACT
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`Florakirk' bermudagrass [Cynodon dactylon (L.) Pers.] is a new cultivar that has persisted well under grazing in the warmest parts of the southeastern USA. Data are lacking on persistence-related responses from areas of the South where frosts and freezes occur more frequently. In 1993 and 1994, the effect of grazing management on pasture botanical composition, light interception, and reserve status was studied on a sandy, siliceous, hyperthermic Ultic Haplaquod. Treatments were replicated twice in a randomized block design and consisted of all combinations of three lengths of rest period (7, 21, and 35 d) and three postgraze stubble heights (8, 16, and 24 cm). Percentage of Florakirk in herbage mass was 96 or greater and was not affected by grazing treatment. Postgraze light interception was affected only by stubble height; it was as low as 22% for stubble height of 8 cm and 78% or greater when stubble height was 24 cm. Close, frequent grazing had no apparent detrimental effect on Florakirk persistence after 2 yr. The total nonstructural carbohydrate (TNC) pool in rhizomes plus stem bases declined with increasing stubble height in both years (96 to 68 g m-2 in 1993, and 44 to 27 g m-2 in 1994). Lower rhizome TNC pools in 1994 than in 1993 were not associated with reduced herbage accumulation or vigor. Results from 2 yr of grazing suggest that Florakirk persists under a range of rotational stocking treatments, so grazing management decisions can be based primarily on productivity and nutritive value considerations.
Abbreviations: OM, organic matter • PAR, photosynthetically active radiation • TNC, total nonstructural carbohydrate(s)
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INTRODUCTION
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Bouton (1992) stated that persistence is the most important trait of forage species. One way to assess persistence is by monitoring regrowth vigor after successive defoliations. Gomide (1989) listed four major factors determining regrowth: (i) shoot apex survival, (ii) residual leaf area, (iii) carbohydrate reserves, and (iv) tillering potential. Studies on regrowth physiology have shown in many cases that dependence of the plant on stored carbohydrates for recovery from defoliation does not last more than a few days (Harris, 1978; Humphreys, 1991). Even if limited to early phases of plant recovery, however, the role of carbohydrate reserves is important in both regrowth and tissue maintenance, especially when a high proportion of photosynthetic tissue is removed by defoliation. From a morphological perspective, Chapman and Lemaire (1993) proposed that phenotypic plasticity (i.e., the ability of a plant to alter its growth habit in response to defoliation) can confer tolerance of defoliation upon plants by enabling them to maintain leaf area and growing points below the grazing height.
Forage plants that store TNC higher in stem bases generally are poorer competitors against weeds and less persistent than those that store TNC in the crown region or in underground organs such as rhizomes. Adjei et al. (1988) measured TNC concentrations in different plant fractions of `UF-5' and `McCaleb' stargrasses (C. aethiopicus Clayton & Harlan) and in `Ona' stargrass (C. nlemfuënsis Vanderyst var. nlemfuënsis). They reported that, in all three grasses, most of the TNC was stored in the roots–crown fraction (top 5 cm of roots + 2.5 cm aboveground stubble) as opposed to the lower stubble (2.5–10 cm) and upper stubble (>10 cm) fractions. In the same study, they reported that increasing stocking rate caused a linear decrease in TNC concentration across grasses and plant fractions. `Transvala' digitgrass (Digitaria eriantha Steud.) stores most of its TNC between 2.5 and 10 cm in the stubble, and was severely weakened by heavy grazing, which resulted in irreversible stand loss due to encroachment by common bermudagrass (Adjei et al., 1988).
The newly released Florakirk bermudagrass is a rhizomatous, productive cultivar that has persisted under clipping in north Florida for 8 yr and has survived winter temperatures of -13°C (Mislevy et al., 1995). The effects of grazing management on persistence-related responses of Florakirk bermudagrass in north Florida have not been determined and are important if persistence and productivity are to be optimized and because a closely related hybrid, `Tifton 78' bermudagrass, was not persistent under grazing in the region. Therefore, research was initiated to evaluate the effects of a range of rotational stocking treatments on determinants of Florakirk persistence. The specific objectives were to quantify the impact of different lengths of rest period and stubble height on pasture persistence by measuring (i) pasture botanical composition, (ii) canopy light interception, and (iii) TNC and N concentrations and pools in selected plant parts.
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Materials and methods
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The study was conducted in 1993 and 1994 at the Forage Evaluation Field Laboratory of the University of Florida Beef Research Unit, 18 km northeast of Gainesville. Length of grazing seasons, soil characteristics, fertilization management, and rainfall data for the experimental site have already been presented (Pedreira et al., 1999). The herbicide 2,4-D (dimethylamine salt of 2,4-dichlorophenoxy acetic acid) was applied to control broadleaf weeds on 31 Mar. 1994 at a rate of 1 kg a.i. ha-1. This was done because broadleaf weed control is a common management practice in bermudagrass swards in Florida, and from both a producer perspective and for our purposes, invasion of pastures by grass weeds is of much greater significance and a more reliable indicator of weakening stands than is occurrence of broadleaf weeds.
Treatments consisted of all possible combinations of three lengths of rest period (7, 21, and 35 d) and three postgraze stubble heights (8, 16, and 24 cm) in two replications of a randomized complete block design. Rest period included a grazing period of 1 to 10 h. Experimental units were 10- by 20-m pastures separated by electric fence. Each year, the first grazing occurred when average sward height was approximately 16 cm above the assigned stubble height. Grazing animals were crossbred (Bos taurus x B. indicus) 350-kg yearling beef heifers. A given grazing event ended when stubble height was reached based on the mean of 15 height measurements taken several times per event as the target stubble was approached.
Pasture botanical composition was determined before the first and last grazing of 1993 and 1994. At the first and last pregraze herbage mass sampling in each season, fresh herbage clipped from three 0.25-m2 sampling sites was hand-separated into Florakirk bermudagrass and a weed fraction that included all other grasses. Broadleaf weeds were not present in measurable amounts in either year. The two fractions were then dried in a forced-air drier at 60°C to constant weight.
Light interception by the grass canopy was measured pre- and postgraze in two consecutive mid-season grazing cycles each year. Before and after grazing, light interception was determined at eight representative sites per experimental unit. For this purpose a 1-m-long line quantum sensor connected to a model LI-188B Li-Cor integrating quantum radiometer/photometer (Li-Cor, Lincoln, NE), was used. At each of the eight sites, the probe was first held horizontally about 1.5 m above the top of the canopy, and the amount of incident photosynthetically active radiation (PAR) was recorded. The probe was then immediately inserted underneath the canopy, on the soil surface, in the same orientation as above the canopy, and the amount of PAR reaching the probe was recorded. Percent light interception at each site was calculated as the amount intercepted by the canopy (total incident PAR minus PAR reaching the soil surface) divided by total incident PAR and multiplied by 100. The light interception for a pasture was calculated as the average of the eight sites and two grazing cycles. Measurements were taken on clear days between 1100 and 1400 h.
Carbohydrate reserve status was measured in three fractions of the sward immediately after the first and last grazing of the season each year. On each sampling date, four 20- by 20-cm cores per experimental unit were dug (10-cm depth) at sites that represented the treatment stubble height. The core sample was then divided into three fractions. The below-ground fraction was composed of roots and rhizomes to a 10-cm depth (hereafter referred to as rhizome fraction). The fraction between soil level and 5 cm above ground was the base of the stubble. The top of the stubble was all plant material above the 5-cm height. Within an experimental unit, the material pertaining to a fraction was combined across the four sampling sites. The below-ground fraction was washed, and all fractions were oven-dried at 105°C for 1 h and then transferred to a forced-air drier at 60°C until they reached constant weight. All samples were ground to pass through a 1-mm stainless steel screen.
Determination of TNC followed a modification of the procedure of Christiansen et al. (1988) and has been described in detail by Chaparro et al. (1996). 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).
All samples were analyzed for N concentration using a modification of the aluminum block digestion technique (Gallaher et al., 1975). Ammonia in the digestate was determined by semiautomated colorimetry (Hambleton, 1977). Nitrogen concentration was expressed on an organic matter (OM) basis.
Because the effects of rest period and stubble height on botanical composition, light interception (post- and pregraze), and reserve status differed between years, data are reported by year. Within years, data were analyzed using the Response Surface Regression procedure (PROC RSREG) of SAS (SAS Inst., 1989). This procedure tests for fitness of a second-order polynomial regression model of the form
(rest period) + ß2 (stubble height)+ ß3 (rest period)2 + ß4 (stubble height)2 + ß5 (rest period x stubble height) + 
, where y is the response variable, ß0 is the intercept, ß1 is the linear coefficient for rest period, ß2 is the linear coefficient for stubble height, ß3 is the quadratic coefficient for rest period, ß4 is the quadratic coefficient for stubble height, ß5 is the interaction or crossproduct coefficient for rest period and stubble height, and is the experimental error term.
The second-order polynomial model was also tested for lack of fit. Reduced models were fitted with the coefficient estimates that showed a significant effect in the full model (P 
0.10). The PROC GLM procedure (SAS Inst., 1989) was used to test for significance (P 0.10) of the coefficient estimates in the reduced models. Terms that were not significant in the full model were included in the reduced model when higher-order terms were significant. For example, when there was a rest period x stubble height interaction, the linear effects of both rest period and stubble height were included in the model regardless of their level of probability. Likewise, the linear effect of a factor was included when the quadratic effect of that factor was significant. If both rest period and stubble height affected a response, the response surfaces generated are shown as two-dimensional projections on a plane surface (contour plots).
Responses related to reserve status included mass of plant fractions (rhizomes, base of stubble, top of stubble), TNC and N concentrations in each fraction, and pool (mass x concentration) of TNC and N in each fraction. Data relative to these responses were first analyzed using repeated measures analysis of variance (Littel et al., 1991) in space (plant fractions) and time (years). Because there were interactions of treatment and year, data are reported by year. Within years, no differences among treatments were detected using the RSREG procedure for botanical composition and reserve status responses. Thus, for these responses orthogonal contrasts were used to test for both linear and quadratic effects of rest period and stubble height. This analysis assigns to known sources the variation included in the error term for the RSREG analysis, which results in a more powerful test of treatment effects. Polynomial contrasts were considered to be significant when P 0.10.
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Results and discussion
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Botanical Composition
Botanical composition was not affected by treatments (P > 0.10) on any of the four sampling occasions. At the end of the 1993 and 1994 grazing seasons, pasture composition averaged 98 and 99% Florakirk, respectively. Results of the present study were similar to those of Adjei et al. (1989) who found 3% common bermudagrass ground cover in Florakirk pastures grazed every 4 wk for 2 yr; whereas Tifton 78 bermudagrass pastures had an 89% ground cover by common bermudagrass under the same grazing management. Chapman and Lemaire (1993) suggested that grasses with phenotypic plasticity responded to severe defoliation by altering their growth habit and tended to be more competitive than those that were less tolerant of close and frequent defoliation. This has been observed in `Coastal' bermudagrass and was related to its ability to retain photosynthetic surface after defoliation and compete effectively for light (Roth et al., 1990).
Postgraze Light Interception
Canopy light interception was affected by stubble height in both years. Postgraze light interception ranged from 41 to 72% in 1993 and from 31 to 79% in 1994. The response was curvilinear in both years (Fig. 1)
. In 1993, the rate of increase in light interception was greater as stubble height increased; in 1994 the rate of increase in light interception was greater at lower stubble height. It was observed that grass grazed to 8 cm every 7 d assumed a prostrate, almost turf-like growth habit, with short leaves and high percent cover. As a result, postgraze light interception was expected to be greater for this treatment than for those with the same rest period and taller stubble height. This was not the case, however, and we propose that placement of the quantum sensor may have affected this response. Because the sensor was 25 mm tall and rested on the soil surface, it may not have accounted for light interception by plant tissue near soil level, and postgraze light interception may have been underestimated in closely, frequently grazed pastures.
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Fig. 1 Postgraze canopy light interception (%) in Florakirk bermudagrass pastures in 1993 and 1994 as affected by stubble height (SH). In 1993, postgraze light interception = 48 - 1.85 (SH) + 0.12 (SH)2; R2 = 0.80. In 1994, postgraze light interception = -23.6 + 8.12 (SH) - 0.16 (SH)2; R2 = 0.90
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Pregraze Light Interception
Pregraze light interception ranged from 52 to 91% in 1993 and from 51 to 94% in 1994 and was affected by rest period (linear), stubble height (linear and quadratic), and their interaction in both years (Fig. 2)
. Lowest pregraze light interception was observed in pastures that were grazed frequently and closely (under 60% light interception in both years). In 1993, 90% light interception occurred at a rest period of approximately 28 d for pastures grazed to 24 cm. In 1994, long rest periods allowed for light interception of greater than 90% even at the intermediate (16 cm) stubble height (Fig. 2). During the 1994 grazing season, pastures grazed to 8 cm achieved 85% light interception after 35 d of regrowth, while pastures grazed to 24 cm required approximately 10 d to attain 85% light interception.
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Fig. 2 Pregraze canopy light interception (%) in Florakirk bermudagrass pastures in 1993 and 1994 as affected by length of rest period (RP) and postgraze stubble height (SH). In 1993, pregraze light interception = 22 + 2.48 (RP) + 2.09 (SH) - 0.03 (RP)2 - 0.04 (RP x SH); R2 = 0.84. In 1994, pregraze light interception = 12.3 + 1.74 (RP) + 4.09 (SH) - 0.05 (SH)2 - 0.06 (RP x SH); R2 = 0.96
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Rhizome and Stubble Base Mass and Chemical Composition
Mass of rhizomes declined linearly with increasing stubble height in both years (Table 1)
. This response differs from that reported for several other forages. For example, Ortega-S. et al. (1992) observed a decline in rhizome mass of `Florigraze' rhizoma peanut (Arachis glabrata Benth.) as pastures were grazed frequently to a low residual dry matter. Additionally, a decline in root mass of Caucasian bluestem [Bothriochloa bladhii (Retz.) S.T. Blake] due to close and frequent grazing was reported by Svejcar and Christiansen (1987). In the current study, greater rhizome weights with shorter stubble heights suggest that Florakirk bermudagrass responded to close grazing through a tolerance mechanism (Vallentine, 1990); that is, by modifying its growth habit under close and continuous defoliation. When this mechanism is acting, leaf area from closely grazed swards develops at high light intensities and is likely more photosynthetically efficient than that of the taller swards. In taller swards, assimilate from underground parts such as rhizomes may be exported for tissue maintenance if shaded leaves become too strong a sink for photosynthate.
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Table 1 Rhizome mass and total nonstructural carbohydrate (TNC) concentration at the end of the 1993 and 1994 grazing seasons, as affected by postgraze stubble height and rest period
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Concentration of TNC in rhizomes was affected only by stubble height in 1993 and only by rest period in 1994 (Table 1). In 1993, TNC concentration increased with increasing stubble height up to 16 cm, but decreased thereafter. Rhizome TNC concentrations in 1994 averaged 43 g kg-1 OM and were less than half those in 1993. In 1994, an increase in rest period from 7 to 21 d resulted in a decrease in rhizome TNC concentration from 41 to 33 g kg-1, but TNC concentration increased to 57 g kg-1 as rest period increased from 21 to 35 d (Table 1).
In 1993, there was a quadratic effect (P < 0.01) of stubble height on TNC in the base of the stubble. Concentration of TNC peaked at 79 g kg-1 when stubble height was 16 cm, and was approximately 65 g kg-1 for the shortest and tallest stubble heights. No effect of rest period was observed in 1993. In 1994, there was a rest period x stubble height interaction (P = 0.063) affecting TNC concentration in the base of the stubble (Table 2)
. At the two lower levels of stubble height, TNC concentration increased linearly with increasing rest period, but at the 24-cm stubble height there was no effect of rest period. Within rest periods, TNC concentration declined as stubble height increased for all but the 7-d rest period.
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Table 2 Interaction effects of length of rest period and postgraze stubble height on total nonstructural carbohydrate (TNC) concentration in the base of the stubble at the end of the 1994 grazing season
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The size of TNC pool (the product of TNC concentration and the mass of a plant fraction) has sometimes been associated with persistence of forage plants under defoliation. Here, the TNC pool in the base of the stubble plus rhizome fractions decreased linearly as stubble height increased in both years (Table 3)
. This response was due in large part to the effect of stubble height on rhizome mass (Table 1). There was no response of TNC pool to rest period in 1993, but there was a linear increase with increasing rest period in 1994 (Table 3). Chaparro et al. (1996) and Spitaleri et al. (1994) found that TNC pool in rhizomes of Pennisetum spp. was reduced strongly by severe defoliation. For these bunchgrasses they found a close positive relationship between TNC pool and persistence.
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Table 3 The total nonstructural carbohydrate (TNC) pool in the base of stubble plus rhizome fractions and the proportion of TNC pool located in these fractions at the end of the 1993 and 1994 grazing seasons, as affected by postgraze stubble height and length of rest period
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In the current study, more than 50% of the TNC was found below ground. There was a rest period x stubble height interaction (P < 0.05) effect on the proportion of total TNC pool in the rhizome fraction in 1993 (Table 4)
. Within each rest period, the proportion of TNC pool in rhizomes declined linearly as stubble height increased. There was no effect of rest period when stubble height was 16 cm. For the 8-cm stubble height, the effect of rest period was quadratic, peaking at the 21-d rest period for which 82% of the TNC pool was found in rhizomes. Increasing the level of rest period from 7 to 35 d when stubble height was 24 cm increased linearly the proportion of the total TNC pool that was in rhizomes. This response was not affected by treatments in 1994.
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Table 4 Length of rest period and postgraze stubble height interaction effects on the proportion of total nonstructural carbohydrate (TNC) pool located in the rhizome fraction at the end of the 1993 grazing season
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When the TNC pool in the base of the stubble was added to that in the rhizomes, these two fractions contained as much as 93% of the total TNC pool (Table 3). The proportion of total TNC pool in this combined fraction was affected by both rest period and stubble height in each year. Increasing stubble height caused a linear reduction in the proportion of TNC pool found in rhizomes plus base of stubble (Table 3). This response was due in part to the greater herbage mass and TNC above 5 cm when stubble height was 16 or 24 cm than when stubble height was 8 cm. Increasing rest period caused a linear increase in the response in both years (Table 3), although the magnitude of this increase was never greater than 10 percentage points. Although the base of Florakirk stubble may store significant amounts of TNC (Adjei et al., 1988), the most important storage organ sites are below the soil surface.
One striking aspect of the TNC data was the marked decline in concentration and pool size in 1994 compared with 1993 (Table 3). Studies conducted by Spitaleri et al. (1994) showed that intensive defoliation resulted in reduced rhizome TNC concentration and pool and that these changes were associated with a decline in winter survival and persistence of Pennisetum spp. Ortega-S. et al. (1992) reported lowest rhizome mass and TNC concentration for rhizoma peanut pastures grazed frequently (every 7 d) to low residual dry matter (500 kg ha-1). Rhizoma peanut percentage in these pastures decreased from 90 to 27% in the first grazing season, and from 27 to 9% in the second grazing season. In the current study, the decline in TNC concentration and pool observed at the end of the second grazing season was not accompanied by stand loss or reduced plant vigor. Additionally, herbage accumulation across all treatments in the second year was markedly greater than in the first year (Pedreira et al., 1999). Greater second-year yields were attributable in part to earlier onset of spring rains and a longer growing season that year, but even under these environmental conditions herbage accumulation would not be expected to increase if stand vigor had been compromised. The lack of association of year-to-year changes in TNC concentration and pool with Florakirk percentage in the sward and with herbage accumulation suggests that quantification of carbohydrate reserve status of Florakirk may not be critical to understanding persistence or proper grazing management practice.
Weed encroachment and stand loss were noticed in a companion study where continuously stocked Florakirk pastures were maintained at a 20-cm height (Pedreira et al., 1996, unpublished data). Stand loss occurred when leaf spot (Helminthosporium spp.) and the twolined spittle bug (Prosapia bicincta) were present, but neither was conclusively linked with the stand loss. Results from the current study suggest that close and frequent grazing were not detrimental to short-term Florakirk persistence. At the end of the second grazing season, pastures with a 7-d rest period and an 8-cm stubble height (and all other treatments where stubble height was 8 cm) were 100% Florakirk.
The pool of N in the stubble plus rhizome fractions was not affected by treatments in either year and averaged 6 g m-2 at season end in 1993 and 7 g m-2 in 1994. Although an association between stored carbohydrates and N compounds is accepted in the dynamics of organic reserves of forage plants (Perry and Moser, 1974), nitrogenous compounds do not seem to be stored and utilized in the same way as the carbon compounds (White, 1973). This may be the reason why TNC responses to grazing treatments were more evident than N responses.
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Summary and conclusion
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During a 2-yr period, botanical composition of Florakirk pastures was not affected by rotational stocking treatment and was never below 96%. Treatments with low pregraze or postgraze light interception did not reduce the percentage of Florakirk in the sward. Florakirk responded to close, frequent grazing by assuming a prostrate growth habit and tillering profusely, probably allowing significant leaf area to remain after grazing and reducing the need for reserve storage and mobilization for regrowth. Rhizome mass declined linearly with increasing stubble height in both years, and pool of TNC in rhizomes plus stem bases declined with increasing stubble height, primarily resulting from similar changes in rhizome mass. Lower rhizome TNC concentrations and pools in 1994 than in 1993 were not associated with stand loss or decreased plant survival, supporting the concept that TNC status may not be a critical determinant of Florakirk persistence. Although two years is a relatively short time to assess persistence of a perennial pasture species, Florakirk herbage accumulation increased across all treatments in the second year compared with the first (Pedreira et al., 1999) and pasture composition was 96% or more Florakirk in all treatments in both years. These responses suggest that Florakirk will persist under rotational stocking in regions where it is adapted in the Southeast USA. Because persistence was not greatly affected by choice of rotational stocking treatment, management recommendations should be based in large part on productivity and nutritive value responses. Pedreira et al. (1999) reported that grazing to a stubble height of approximately 20 cm every 14 d was predicted to result in near maximum levels of both herbage accumulation and nutritive value.Littell Freund Spector 1991; Ortega-S. Sollenberger Bennett Cornell 1992; SAS Institute 1989
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NOTES
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Florida Agric. Exp. Stn. Journal Series No. R-06893.
Received for publication April 29, 1999.
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TOP
ABSTRACT
INTRODUCTION
