Published in Agron. J. 97:303-313 (2005).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Production Papers
Allelopathic Potential of Bermudagrass and Johnsongrass and Their Interference with Cotton and Corn
Ioannis Vasilakogloua,*,
Kico Dhimab and
Ilias Eleftherohorinosc
a Technol. and Educ. Inst. of Larissa, Lab. of Weed Sci., Larissa 41110, Greece
b Technol. and Educ. Inst. of Thessaloniki, Lab. of Crop Sci., Thessaloniki 54101, Greece
c Lab. of Agron., Aristotle Univ. of Thessaloniki, Thessaloniki 54124, Greece
* Corresponding author (vasilakoglou{at}teilar.gr)
Received for publication June 7, 2004.
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ABSTRACT
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Field experiments were conducted in northern Greece during the 2002 and 2003 growing seasons to study interference between bermudagrass [Cynodon dactylon (L.) Pers] or johnsongrass [Sorghum halepense (L.) Pers] and cotton (Gossypium hirsutum L.) or corn (Zea mays L.). Moreover, bioassay studies were also conducted to assess allelopathic potential of these two weeds on cotton and corn as well as on barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] and bristly foxtail [Setaria verticillata (L.) P. Beauv.]. The bioassay experiments showed that cotton, barnyardgrass, and bristly foxtail germination, total fresh weight, and root length were inhibited by bermudagrass or johnsongrass extracts more than those of corn. In addition, johnsongrass extracts caused greater germination, fresh weight, and root length inhibition than bermudagrass extracts. In the field, growth and yield of cotton were reduced due to bermudagrass (200 and 400 stems m–2 from planted rhizomes) or johnsongrass (100 and 200 stems m–2 from planted rhizomes) season-long interference 50 and 74% or 64 and 86%, respectively, averaged over the two weed densities. The corresponding corn losses were 46 and 30% or 62 and 41%, respectively. Both stem number and fresh weight of bermudagrass or johnsongrass increased with increasing interference duration, and they were greater where both weeds were grown with cotton than with corn. These results suggest that there is growth inhibition of both crops due to potential allelopathic substances released from the two perennial weeds, but cotton growth was inhibited more than corn. Furthermore, cotton and corn yield were reduced more by the johnsongrass interference compared with that caused by bermudagrass.
Abbreviations: WACE, weeks after completion of emergence • WAP, weeks after planting
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INTRODUCTION
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BERMUDAGRASS AND JOHNSONGRASS are C4 perennial grasses that are considered to be among the world's worst weeds (Holm et al., 1977, p. 54–61). In addition, they belong in the 10 most common and troublesome weeds of Greece (Damanakis, 1983). Bermudagrass propagates mainly vegetatively, through stolon and rhizome fragmentation (Hakansson, 1982), but johnsongrass reproduces both by seed and by rhizomes (Horowitz, 1973). Rhizomes of both weeds are the main reserves of carbohydrates and dormant buds for overwintering. A single plant of johnsongrass can produce 40 to 90 m of rhizomes per growing season while bermudagrass fresh weight of subterranean parts down to 45 cm can range from 420 to 780 g m–2 during one year (Horowitz, 1972, 1973). Also, rhizomes are the primary means of bermudagrass and johnsongrass dispersal in the field because mechanical tillage of weed-infested fields produces fragmentation and dispersal of rhizomes propagules, from which new ramets can be formed (Fernandez, 2003; Mitskas et al., 2003).
Nutrients, water, and in some cases light competition are observed between bermudagrass or johnsongrass and corn or cotton when they grow together. In some cases, growth and yield reduction observed in crops with bermudagrass or johnsongrass were greater than those expected as a result of competition for water and nutrients (Putnam and Weston, 1986; Smith et al., 2001). This fact could possibly be attributed to bermudagrass and johnsongrass production of allelochemical substances that have significant adverse impact on growth and yield of agronomic crops like corn, barley (Hordeum vulgare L.), soybean [Glycine max (L.) Merr.], and wheat (Triticum aestivum L.) (Meissner et al., 1989; Putnam and Weston, 1986; Velu and Rajagopal, 1996).
Bermudagrass and johnsongrass must be controlled in corn and cotton to prevent yield losses. Soil cultivation alone cannot control these weeds effectively and, in some cases, could increase weed densities because it increases the number of shorter rhizomes (McWhorter, 1972). Postemergence application of the sulfonylurea (nicosulfuron, primisulfuron, and rimsulfuron) herbicides provides very good control of johnsongrass in corn (Eleftherohorinos and Kotoula-Syka, 1995; Foy and Witt, 1990). In addition, postemergence application of the aryloxyphenoxypropionate (fluazifop-p-butyl, haloxyfop, propaquizafop, and quizalofop ethyl) and cyclohexanedione (clethodim and cycloxydim) herbicides provides very good control of johnsongrass and bermudagrass in arable crops like cotton (Devine and Shimabukuro, 1994; Haitas et al., 1995).
Competition of johnsongrass and corn has been extensively studied, but published data on interference between johnsongrass and cotton or between bermudagrass and cotton or corn are limited. Therefore, any in-depth studies on interference between bermudagrass or johnsongrass and cotton or corn can provide more knowledge for weed management strategies in these two crops. The objectives of this research were (i) to assess under field conditions the impact of bermudagrass or johnsongrass interference on growth and yield of corn and cotton and (ii) to determine under laboratory conditions their allelopathic potential on these two crops as well as on the annual grass weeds barnyardgrass and bristly foxtail.
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MATERIALS AND METHODS
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Laboratory Experiment
Bermudagrass and johnsongrass plants grown at University Farm of Thessaloniki were harvested in July 2003. Also, purple nutsedge (Cyperus rotundus L.) plants were collected at the same time and they were used as plant control. Weeds that were in a vegetative growth stage were gently washed with deionized water, dried between two paper towels, and separated as aboveground (leaves plus stems) and underground (rhizomes or tubers) parts. The harvested plant components were chopped into 2-cm-long pieces, and they were air-dried for 48 h at room temperature (25 ± 4°C). Then, the dried plant parts were ground in a Wiley mill through a 40-mesh screen. Aqueous extracts (w/v) were prepared by extracting 2 and 4 g of weed samples with 100 mL of deionized water in a horizontal shaker for 12 h at 200 rpm using four 400-mL glass jars with plastic covers. Three jars were used for each weed x plant part x extract concentration. The solutions were filtered through four layers of cheesecloth to remove fiber debris, centrifuged at low speed (3000 rpm) for 1 h, and the supernatants were then filtered through one layer of filter paper (Whatman no. 42). Extracts were stored at less than 5°C until bioassayed.
Petri dish bioassays were performed to compare the germination, total fresh weight, and root length of cotton and corn in perlite treated with bermudagrass, johnsongrass, or purple nutsedge extracts. Ten cotton (‘Hazera Vered 171’) or five corn (F1 hybrid ‘Pioneer Costanza’) seeds were placed on the bottom of 8.5-cm-diam. plastic disposable Petri dishes and were covered with 6 g of perlite. The open Petri dishes were moistened with 15 mL per Petri dish from each of the aboveground or underground weed extract. Deionized water was used as a control. Also, the same bioassay was used to evaluate the adverse effect of bermudagrass or johnsongrass extracts on barnyardgrass and bristly foxtail, which are among the most common annual grass weeds found in corn and cotton fields in Greece. Fifty seeds of each weed were placed on one layer of filter paper in the bottom of 8.5-cm-diam. plastic disposable Petri dishes and were covered with 2 g of perlite. The open Petri dishes were moistened with 8 mL per Petri dish from each of the underground or aboveground weed extract. Afterwards, the Petri dishes stored on shallow trays were placed inside in a plastic bag to retain moisture. The trays were then placed in an illuminated (16 h light/8 h dark) growth chamber at 27 ± 2°C for 8 d. At the end of the incubation period, plants were removed from the Petri dishes, carefully washed free of perlite, and the average germination, total fresh weight, and root length were measured. The inhibitory percentage was calculated by the Eq. [1] used by Chung et al. (2001):
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Two Petri dishes were used for each jar, and Petri dishes were arranged in a completely randomized design. The experiment was repeated in time. Fungal contamination was not observed during these experiments.
Field Experiment
Two field experiments were conducted during the 2002 and 2003 growing season at the University Farm of Thessaloniki (northern Greece) to determine the effect of season-long interference between bermudagrass or johnsongrass and cotton or corn. The site was located at 22°59'6.17'' N, 40°32'9.32'' E. Experiments were established on a calcareous loam soil (Typic Xerorthent) whose physicochemical characteristics are presented on Table 1. The experimental area was not infested with natural bermudagrass or johnsongrass populations. Mean monthly temperature and rainfall data recorded near the experimental area are given in Fig. 1.
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Table 1. Selected soil physicochemical characteristics from experimental site on University Farm calcareous loam (0–30 cm depth).
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The land was plowed in the fall after harvest of the proceeding winter cereal crop and left undisturbed during winter. In mid-April of both growing seasons, the experimental area was cultivated with a harrow disk to prepare the soil for crop planting and to incorporate into the soil the 170 kg N ha–1 and 70 kg P ha–1. Then, one- to three-node fragments of freshly collected bermudagrass or johnsongrass rhizomes, of 3 to 5 cm long, were incorporated with a rotovator into the soil of each plot. This was performed 2 d before crop planting with the weed rhizomes collected from a highly infested field located in University Farm of Thessaloniki. Seventy or 140 g m–2 of bermudagrass fragments and 160 or 320 g m–2 of johnsongrass fragments were individually planted in each plot to achieve the desirable weed densities (200 or 400 stems m–2 and 100 or 200 stems m–2 for bermudagrass and johnsongrass, respectively).
Cotton variety (Hazera Vered 171) was planted in a 90-cm row spacing at an approximate density of 220000 seed ha–1. Also, corn F1 hybrid (‘Pioneer Costanza’) was planted in an 80-cm row spacing at an approximate density of 62500 seed ha–1. The planting dates were 24 April 2002 and 29 April 2003. Carbofuran (2,3-dihydro-2, 2-dimethylbenzofuran-7-ylmethylcarbamate) granules were applied (12 kg a.i. ha–1) at the time of planting for insect management. Also, N at 100 kg ha–1 was applied 35 d after corn planting. Broadleaf weeds were controlled by atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] (Gesaprim 50 SC, 50 g a.i. L–1, Syngenta, Basel, Switzerland) applied pre-emergence at 1.5 kg a.i. ha–1 and by prometryn [N,N'-bis(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine] (Gesagard 50 SC, 50 g a.i. L–1, Syngenta, Basel, Switzerland) applied pre-emergence at 1.0 kg a.i. ha–1 on corn and cotton, respectively. In addition, hand weeding was done throughout the growing season. Irrigation and other common cultural practices were conducted as needed during the growing season.
A split-plot design was employed in a randomized complete block design with four replicates. Plot size was 9 by 7 m. In each plot, four subplots of 4.0 by 3.0 m were created (included four rows of cotton or corn), and all subplots were separated by a 1-m-wide alley. The two weed species by two weed densities plus the weed-free control was the main plot factor. For weed data, the crop (cotton or corn) by interference duration [0, 3, 6, or 15 weeks after completion of emergence (WACE)] was the subplot factor. However, for crop data, the crop (cotton or corn) by interference duration (0, 3, 6, or 15 WACE) by weed control [nontreated and herbicide treated at 4 weeks after planting (WAP)] was the subplot factor. Bermudagrass and johnsongrass control in these subplots was achieved with 0.1 kg a.i. ha–1 of quizalofop {(±)-2-[4-[(6-chloro-2-quinoxalinyl)oxy]phenoxy]propanoic acid} (Targa 5 EC, 5 g a.i. L–1, Rhone Poulenc, Lyon, France) and 0.04 kg a.i. ha–1 of nicosulfuron {2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide} (Milagro 4 SC, 4 g a.i. L–1, Syngenta, Basel, Switzerland) applied postemergence on cotton and corn, respectively. Also, in corn subplots, bermudagrass was removed by hand.
Crop and weed densities were determined 3 WAP (completion of weed emergence) when all crop seedlings and most of the weed plants from rhizomes had emerged. Bermudagrass and johnsongrass plants were harvested in a 1-m2 area in the two center rows of each subplot 0, 3, 6, and 15 WACE in both growing seasons. Weed stem number as well as fresh weight were measured at each sampling. The weed-free plots (0 wk interference) were hand-weeded immediately after cotton and corn emergence and kept free of grass and broadleaf weeds throughout the growing season.
At the silage stage (when the kernels began to glaze) of corn hybrid (14 WAP), 20 plants (4 m of row) were harvested from each subplot and the silage yield recorded. The silage stage was determined by breaking the ears of corn and visually evaluating the kernel's stage of development. At grain maturity (19 September of both growing seasons), ears of corn plants (of the two 4-m-long center rows of each subplot) were hand-harvested, and grain yield (adjusted to 15.5% moisture content) was determined. Cotton aboveground fresh weight was determined by removing the plants of one row (4 m long) from each subplot approximately 14 WAP. At seed cotton maturity (30 October of both growing seasons), seed cotton was handpicked from the two center rows of each subplot.
Statistical Analysis
A combined-over-time analysis of variance (ANOVA) was performed for each germination, total fresh weight, and root length inhibitory percentage data of the bioassay experiment by using a factorial approach (crop or annual grass weed x perennial weed species x perennial weed plant part x extract concentration). Differences between treatment means were compared at the 5% level of significance using the LSD test. Because the analysis of variance indicated no significant treatment x repetition time interaction, the means of each extract concentration averaged over the two experiments are presented.
Data for bermudagrass and johnsongrass stem number and fresh weight were regressed against time (WACE). In these regression equations, stem number and fresh weight were the dependent variables (y) and time the independent variable (x). Also, crop yield data were analyzed separately for each crop, and means were compared at the 5% level of significance using the LSD test. The programs MSTAT and SPSS were used to conduct the analysis of variance and the regression analysis, respectively.
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RESULTS AND DISCUSSION
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Laboratory Experiment
Germination, root length, and seedling total fresh weight of cotton, corn, barnyardgrass, and bristly foxtail were significantly affected by crop or annual weed species (P < 0.001), extract concentration (P < 0.001), and perennial weed species (P < 0.001). In particular, johnsongrass extracts inhibited germination of cotton and corn by 68 and 16%, respectively, averaged across perennial weed plant part x extract concentration (Fig. 2 and 3). Johnsongrass extracts also caused greater fresh weight inhibition (47%) of crops than bermudagrass (28%) or purple nutsedge (12%) extracts, averaged across crop species x perennial weed plant part x extract concentration. However, cotton total fresh weight was inhibited by bermudagrass and johnsongrass extracts more than that of corn while bermudagrass and purple nutsedge extracts of 2 g 100 mL–1 deionized water did not significantly affect germination and seedling total fresh weight of corn. Again, cotton root length was inhibited by bermudagrass and johnsongrass extracts more than those of corn. In particular, the higher rate of johnsongrass extract (averaged across perennial weed plant part) reduced cotton and corn root length by 86 and 59%, respectively. In addition, johnsongrass extracts caused greater root length inhibition (46%) than bermudagrass (38%) or purple nutsedge (22%) extracts, averaged across crop species x weed plant part x extract concentration.
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Fig. 2. Inhibitory effect of johnsongrass, bermudagrass, or purple nutsedge extracts on cotton seedling germination, total fresh weight, and root length. Means are averaged over two experiments.
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Fig. 3. Inhibitory effect of johnsongrass, bermudagrass, or purple nutsedge extracts on corn seedling germination, total fresh weight, and root length. Means are averaged over two experiments.
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Johnsongrass or bermudagrass extracts reduced annual grass weed germination by 54 and 39%, respectively, as averaged across annual grass weed x perennial weed plant part x extract concentration (Fig. 4 and 5). In particular, germination of barnyardgrass and bristly foxtail was inhibited by johnsongrass 46 and 63%, respectively, averaged across perennial weed plant part x extract concentration. The corresponding inhibition of barnyardgrass and bristly foxtail germination by bermudagrass extracts was 21 and 57%, respectively. Barnyardgrass and bristly foxtail fresh weight, averaged across johnsongrass plant part x extract concentration, was reduced by 45 and 85%, respectively. The corresponding reduction caused by bermudagrass extracts was 15 and 81%. Barnyardgrass and bristly foxtail root length was reduced by 73 and 66%, respectively, averaged across perennial weed species x weed plant part x extract concentration. In particular, johnsongrass extracts caused 79 and 75% reduction of barnyardgrass and bristly foxtail root length, respectively, while the corresponding reduction caused by bermudagrass extracts was 66 and 57% (Fig. 4 and 5).
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Fig. 4. Inhibitory effect of johnsongrass or bermudagrass extracts on barnyardgrass seedling germination, total fresh weight, and root length. Means are averaged over two experiments.
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Fig. 5. Inhibitory effect of johnsongrass or bermudagrass extracts on bristly foxtail seedling germination, total fresh weight, and root length. Means are averaged over two experiments.
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The inhibition of cotton, corn, barnyardgrass, and bristly foxtail seed germination, seedling total fresh weight, and root length by the extracts originated from the three perennial weed species could be attributed to their potential allelopathic abilities. Czarnota et al. (2001) working with Sorghum spp. found that sorgoleone {2-hydroxy-5-methoxy-3-[(8'Z,11'Z)-8',11',14'-pentadecatriene]-p-benzoquinone}, a potential inhibitor of chlorophyll formation, was detected in root extracts. Also, the Sorghum species, including johnsongrass, grain sorghum [Sorghum bicolor (L.) Moench], and sudangrass [Sorghum x drummondii (Steud.) Millsp. & Chase], were found to produce and release cyanogenic glycosides (dhurrin and taxiphyllin) and phenolic acids (p-hydroxybenzoic acid and p-coumaric acid) that can contribute to suppression of plant growth (Nicollier et al., 1983; Sene et al., 2001). In addition, Velu and Rajagopal (1996) reported that bermudagrass and purple nutsedge subterranean organs contain allelopathic substances such as caffeic acid, chlorogenic acid, cinnamic, p-coumaric acid, and ferulic acid. The differences in response of the two crops and the two annual grass weeds could be attributed to chemical differences among the perennial weed extract since the extracts were used at the same concentration. This result is similar to those found by others (Chung et al., 2001) who worked with allelopathic rice cultivars.
The greater inhibition of corn germination, seedling total fresh weight, and root length caused by purple nutsedge and johnsongrass underground extracts than by bermudagrass ones is in agreement with results reported by Horowitz and Friedman (1971), who found that purple nutsedge and johnsongrass underground extracts caused greater inhibition of barley root growth than bermudagrass. The similar growth inhibition, in most cases, of both crops and annual grass weeds by either underground or aboveground perennial weed plant part extracts could have resulted from the possible similar concentration and type of allelopathic constituents present in both underground and aboveground plant part extracts of bermudagrass or johnsongrass. Similar studies conducted by Friebe et al. (1995) indicated that the allelopathic constituents DIMBOA (2,4-dihydroxy-7methoxy-2H-1,4-benzoxazin-3-one) and DIBOA (2,4-dihydroxy-2H-1,4-benzoxazin-3-one) were present in shoot extracts and root exudates of quackgrass (Agropyron repens L.).
The increased inhibition caused by each perennial weed species extract with increasing extract concentration (Fig. 2, 3, 4, and 5) is in agreement with results reported by Einhellig and Souza (1992) and Nimbal et al. (1996) who found that root and shoot growth of barnyardgrass [Echinochloa crus-galli (L.) Beauv.], crabgrass [Digitaria sanguinalis (L.) Scop.], velvetleaf (Abutilon theophrasti Medic.), jimsonweed (Datura stramonium L.), and redroot pigweed (Amaranthus retroflexus L.) were reduced with the increase of sorgoleone concentration.
Field Experiment
Bermudagrass and johnsongrass, in both growing seasons, emerged at the same time with cotton and corn, and their presence did not affect emergence of either crop (data not shown). Most of the bermudagrass and johnsongrass plants had completed growth and reached the flowering stage within the first 9 WAP (data not shown). These results are in agreement with those of Mitskas et al. (2003) who studied johnsongrass from seed and rhizomes interference with corn.
Cotton fresh weight was significantly affected by weed species (planted in two densities and weed-free control), weed control (nontreated and herbicide treated), and their interaction (Table 2). Cotton seed yield and corn silage yield were significantly affected by growing season, weed species (planted in two densities and weed-free control), and by weed control (nontreated and herbicide treated). Also, cotton seed yield and corn grain yield were affected by growing season, weed species (planted in two densities and weed-free control), weed control (nontreated and herbicide treated), and by their interactions (Table 2). The growing season x weed species (planted in two densities and weed-free control) x weed control (nontreated and herbicide treated) interaction means are presented (Fig. 6 and 7).
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Table 2. Analysis of variance of cotton fresh weight and seed yield as well as corn silage and grain yield data as affected by growing season, weed species (two weed densities and weed-free control), and grass control (nontreated and herbicide treated).
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Fig. 6. Total fresh weight (14 wk after planting) and seed yield of cotton grown with and without bermudagrass or johnsongrass interference.
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Fig. 7. Silage yield (14 wk after planting) and grain yield of corn grown with and without bermudagrass or johnsongrass interference.
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The fresh weight of cotton, grown on subplots where bermudagrass and johnsongrass were controlled by herbicides applied at 4 WAP, was similar to that of cotton grown without weed interference (weed-free control subplots) (Fig. 6). On the contrary, in the same subplots, cotton seed yield was reduced from 24 to 44% during 2002 growing season, but it was not reduced during the 2003 growing season. These results are in agreement with those reported by Bridges and Chandler (1987), who found that 3 to 4 wk of rhizome johnsongrass interference during 1983 growing season resulted in significant cotton yield losses, but in the 1984 growing season, the 4 wk of johnsongrass interference was not enough to prevent cotton yield loss. The greater rainfall and temperature recorded during March 2002 compared with that of 2003 may have resulted in better bermudagrass and johnsongrass establishment and stronger interference with cotton during the first 4 WAP in 2002 growing season.
Cotton fresh weight and seed yield were affected more by johnsongrass interference compared with that of bermudagrass (Fig. 6). Averaged over growing season and weed density, cotton fresh weight reduction due to bermudagrass and johnsongrass interference was 50 and 64%, respectively, while the corresponding seed yield reduction was 74 and 86%, respectively. Similar results were reported by Jordan et al. (1978), who found cotton yield reductions up to 84% as the level of johnsongrass control decreased. In addition, Bridges and Chandler (1987) found that cotton yield was reduced 70% with interference of 32 plants of johnsongrass per 9.8 m of row.
Silage and grain yield of corn grown on subplots where bermudagrass and johnsongrass were controlled by herbicides were similar to those of corn grown without weed interference (weed-free control subplots) (Fig. 7). The lack of differences could be attributed to the absence of strong competition between plant species and probably to both soil abiotic (like organic matter content, ion exchange capacity, and inorganic ions) and biotic (like bacteria, fungi, and actinomyctes) factors that may significantly influence interference ability (Inderjit, 2001).
Corn silage and grain yield reduction caused by bermudagrass interference was lower than that caused by johnsongrass (Fig. 7). In particular, averaged over growing season and weed density, corn silage yield reduction due to bermudagrass and johnsongrass interference was 46 and 62%, respectively, while the corresponding grain yield reduction was 30 and 41%.
The greater interference caused by johnsongrass than that of bermudagrass could explain these differences among cotton or corn yield reduction. Greater johnsongrass interference is in agreement with the results reported by Mitskas et al. (2003) who found that corn-silage yield and corn grain yield reduced by 83 and 88%, respectively, with season-long interference by johnsongrass plants from rhizomes. This great reduction could be attributed to the strong competition of johnsongrass with corn for light, soil nutrients, and moisture (Ghosheh et al., 1996; Mitskas et al., 2003; Mueller et al., 1993). Corn yield in the United States was reduced by 33% because of johnsongrass interference (Holm et al., 1977, p. 54–61) while in southeastern Europe, the johnsongrass interference has been associated with corn grain yield loss greater than 35% (Warwick and Black, 1984). On the contrary, bermudagrass lacks some specific attributes to compete against tall crops because it is a low-growing weed and highly sensitive to shading (Burton et al., 1988; Fernandez et al., 2002; Guglielmini and Satorre, 2002). Also, light competition caused by corn severely reduces bermudagrass biomass production when water or N are not limited (Guglielmini and Satorre, 2002).
The greater reduction of cotton fresh weight and seed yield as well as corn silage and grain yield by the higher bermudagrass and johnsongrass density could be attributed to their higher competitive ability as a result of their increased density, which is in agreement with results published by Bridges and Chandler (1987), who found that cotton yield decline was most rapid as johnsongrass density increased from two to eight plants per 9.8 m of row.
Both stem number and fresh weight of either weed species were significantly affected by growing season, weed species (planted in two densities), crop species, duration of interference, and their interactions (Table 3). Therefore, the growing season x weed species (planted in two densities) x crop species x duration of interference interaction means are presented (Fig. 8–11). Regression equations of bermudagrass and johnsongrass stem number and fresh weight on time (WACE) indicated that the quadratic equation (y = a + bx + cx2) provided the best fit (Tables 3 and 4). Estimated values of b (growth rate) for stem number of bermudagrass were greater than those of johnsongrass when both weeds were planted at the same density (200 stems m–2). In contrast, johnsongrass-estimated values of b and a (intercept) for fresh weight were greater than those of bermudagrass. Both weed stem number and fresh weight increased with increasing interference duration, except for johnsongrass shoot number during 2002 growing season, which was reduced with increasing interference duration (Fig. 9). Also, bermudagrass and johnsongrass b and a for both stem number and fresh weight were greater when weeds grew with cotton than those with corn (Tables 4 and 5).
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Table 3. Analysis of variance of bermudagrass and johnsongrass stem number and fresh weight data as affected by growing season, weed species (two densities), crop species, and duration of interference.
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Fig. 8. Temporal pattern in shoot number of bermudagrass or johnsongrass grown with corn or cotton during the 2002 growing season. Lines describe quadratic regression equations.
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Fig. 11. Temporal pattern in fresh weight of bermudagrass or johnsongrass grown with corn or cotton during 2003 growing season. Lines describe quadratic regression equations.
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Fig. 9. Temporal pattern in shoot number of bermudagrass or johnsongrass grown with corn or cotton during the 2003 growing season. Lines describe quadratic regression equations.
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Fig. 10. Temporal pattern in fresh weight of bermudagrass or johnsongrass grown with corn or cotton during the 2002 growing season. Lines describe quadratic regression equations.
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Table 4. Quadratic regression equations (y = a + bx + cx2) and coefficients of determination (R2) for the relationship between shoot number of bermudagrass or johnsongrass grown with cotton or corn and time (weeks after completion of emergence).
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Table 5. Quadratic regression equations (y = a + bx + cx2) and coefficients of determination (R2) for the relationship between fresh weight of bermudagrass or johnsongrass grown with cotton or corn and time (weeks after completion of emergence).
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The reduction of johnsongrass shoot number with increasing interference duration during 2002 growing season could be related to the greater johnsongrass stem emergence as a result of the higher temperature and rainfall recorded before its establishment (Fig. 1) and the possible intraspecific competition occurring at high densities. Similar results were reported by Bridges and Chandler (1987), who found that the increase of johnsongrass density resulted in less increase in fresh weight because intraspecific competition had occurred.
The great growth rate (values of a and b) for fresh weight of johnsongrass is in agreement with results reported by Ghosheh et al. (1996), Mitskas et al. (2003), and Mueller et al. (1993), who found that johnsongrass had very high ability to utilize light, soil nutrients, and moisture when it was grown with cotton or corn.
The greater bermudagrass and johnsongrass stem number and fresh weight with greater interference duration from 0 to 15 WACE is in agreement with the results reported by Bridges and Chandler (1987) and Mitskas et al. (2003).
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CONCLUSIONS
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The results of the laboratory experiment of this study indicate clearly that inhibitory substances are present in extracts of bermudagrass or johnsongrass rhizomes and foliage. These substances of either weedy species, in addition to their strong potential for resource-based competition on cotton or corn grown under field conditions, could potentially influence initial growth and yield of both crops. Also, the resource-based interference during the first 4 wk after crop planting is able to reduce significantly the yield of corn or cotton. Therefore, control of these perennial weed species should be done within this time to avoid their competition or possible release of their allelopathic substances. Moreover, the substances present in the extracts originated from either of the two perennial weeds could possibly be explored in the future as naturally occurring herbicides or extracts designed to suppress the growth of susceptible weeds like barnyardgrass and bristly foxtail grown in moderately tolerant crops like corn.
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ACKNOWLEDGMENTS
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The authors thank Dr. A. Lithourgidis and the staff of the University Farm of Thessaloniki for their cooperation.
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