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Published online 20 June 2006
Published in Crop Sci 46:1682-1691 (2006)
© 2006 Crop Science Society of America
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CROP ECOLOGY, MANAGEMENT & QUALITY

Allelopathic Potential of Winter Cereal Cover Crop Mulches on Grass Weed Suppression and Sugarbeet Development

K. V. Dhimaa,*, I. B. Vasilakogloub, I. G. Eleftherohorinosc and A. S. Lithourgidisd

a Agron. Lab., Technol. and Educ. Inst. of Thessaloniki, 541 01 Thessaloniki, Greece
b Weed Science Lab., Technol. and Educ. Inst. of Larissa, 411 10 Larissa, Greece
c Agron. Lab., School of Agric., Aristotle Univ. of Thessaloniki, 541 24 Thessaloniki, Greece
d Agron. Dep., Univ. Farm, Aristotle Univ. of Thessaloniki, 570 01 Thermi, Greece

* Corresponding author (dimas{at}cp.teithe.gr)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Winter cereals are increasingly adopted for their use as cover crops in several cropping systems. This study was conducted to measure the effect of two barley (Hordeum vulgare L.) and six triticale (xTriticosecale) cultivars and three rye (Secale cereale L.) populations, used as cover crops, on the emergence and growth of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], bristly foxtail [Setaria verticillata (L.) P. Beauv.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and sugarbeet (Beta vulgaris subsp. vulgaris). Also, bioassay studies were conducted to assess allelopathic potential of the winter cereal extracts on large crabgrass and sugarbeet. Large crabgrass and sugarbeet growth were reduced more by rye extracts than by triticale or barley. In the field, barnyardgrass, bristly foxtail, and large crabgrass emergence in mulched plots was 39 to 69%, 0 to 34%, and 0 to 78% lower, respectively, as compared with that in mulch-free plots. Sugarbeet yield in no-herbicide and herbicide subplots mulched with barley or rye was greater than that of triticale or mulch-free subplots. In particular, sugar yield in no-herbicide subplots mulched with ‘Athinaida’ barley or rye from Albania were about 7 and 26% less, respectively, than those obtained in the respective herbicide-treated subplots. These results suggest that Athinaida barley and the rye from Albania could be used as cover crops for annual grass weed suppression in sugarbeet.

Abbreviations: WAP, weeks after planting


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
COVER CROPS such as rye and barley have been used to suppress weeds and increase yield in several crops, including soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), and corn (Zea mays L.) (Liebl et al., 1992; Johnson et al., 1993; Moore et al., 1994; Richard et al., 1995; Ateh and Doll, 1996; Nagabhushana et al., 2001; Reddy, 2001, 2003; Kobayashi et al., 2004; Dhima et al., 2006). Petersen and Rover (2005) also reported that rye and barley cover crop mulch provided excellent weed suppression without reducing yield in sugarbeet grown under different cropping systems. In addition, Khanh et al. (2005) found that allelopathic crops, when used as cover crop, mulch, smother crop, green manure, or grown in rotational sequences, were helpful in reducing noxious weeds and plant pathogens and improved soil quality and crop yield.

The fact that cover crops suppress weeds could be attributed to their ability to release toxic substances in the environment and to create an unfavorable environment for weed germination and establishment. Borner (1960) indicated that cold water extracts of barley, rye, and wheat (Triticum aestivum L.) straws as well as alcoholic extracts of roots contain phenolic compounds toxic to plant growth. Some of these compounds were ferulic acid (4-hydroxyl-3-methoxy-cinnamic acid), p-coumaric acid (4-hydroxycinnamic acid), and vanilic acid (hydroxybenzoic acid). Also, Barnes et al. (1987) reported that allelopathy of rye has been attributed to two major compounds: DIBOA [2,4-dihydroxy-1,4-(2H)benzoxazine-3-one] and its breakdown product BOA [(3H)-benzoxazolinone)]. However, Barnes and Putnam (1987) found that DIBOA had greater activity than BOA on monocots such as barnyardgrass, large crabgrass, and proso millet (Panicum miliaceum L.). In addition, Ben-Hammouda et al. (2001) found that barley releases toxic substances in the environment either through root exudation or decay of its plant residues. Lemerle et al. (1996) reported that incorporation of allelopathic traits, together with other plant interference potential (early vigor, leaf size, plant height, tillering) into commercial cultivars could be a major step toward further development of sustainable crop production systems with less reliance on herbicides. Khanh et al. (2005) also reported that the introduction of allelopathic traits from accessions with strong allelopathic potential to the target crops could enhance the efficacy of crop allelopathy in future agricultural production.

Previous studies showed that rye and barley have the ability to release toxic substances in the environment and to suppress weed germination and establishment after their use as cover crop mulches in cotton and corn (Liebl et al., 1992; Johnson et al., 1993; Moore et al., 1994; Richard et al., 1995; Ateh and Doll, 1996; Reddy, 2001, 2003; Dhima et al., 2006). However, the information for rye and barley mulch effect on sugarbeet is limited, while that for triticosecale allelopathic ability on weeds and sugarbeet development is absent.

The objectives of this research were (i) to study, in the laboratory, the dynamics of allelopathic potential of three rye populations and six triticosecale and two barley cultivars against sugarbeet and large crabgrass, and (ii) to determine, under field conditions in northern Greece, their cover crop effect on the emergence and biomass of three annual grass weeds (barnyardgrass, bristly foxtail, and large crabgrass) as well as on sugarbeet development.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Laboratory Experiment
Extract Preparation
Plants of three rye populations originated from Albania, Germany and Greece, as well as six triticale (Thisvi, Niovi, Vronti, Catria, Vrito, and Artemis) and two barley (Athinaida and Thessaloniki) cultivars from Greece were harvested at the beginning of stem elongation and inflorescence emergence (time of their incorporation into the soil in the field experiment). The harvested plants were chopped into 5-cm-long pieces, dried in an oven at 70°C for 48 h, and ground in a Wiley mill through a 1-mm screen. Then, aqueous extracts (w/v) were prepared in 400-mL glass jars by adding 4 or 8 g from each plant sample in 200 mL of deionized water and shaking in a horizontal shaker for 4 h at 200 rpm. The solutions were filtered through four layers of cheesecloth to remove fiber debris, centrifuged at 1750 g in a centrifuge with 30-cm rotor diameter for 1 h, and the supernatants were then filtered through a layer of filter paper (Whatman No. 42). The extracts were stored at <5°C until bioassayed. The two extract concentrations [2 and 4 g 100 mL–1] for each crop cultivar species were chosen in accordance with those studied by Chung et al. (2001) and Ben-Hammouda et al. (2001). There were three replicate glass jars for each plant material by extract concentration treatment [2 and 4 g 100 mL–1].

Bioassay Procedure
Petri dish bioassays were performed to compare the germination and root length of sugarbeet and large crabgrass in Perlite treated with each of the winter cereal extracts. Fifty sugarbeet (cultivar ‘Asso’) or 300 large crabgrass seeds were placed in 8.5-cm-diam. plastic Petri dishes and were covered with 6 g of Perlite. The open Petri dishes were moistened with 10 mL of winter cereal extract per Petri dish from each of the winter cereal extracts. Deionized water was used in control Petri dishes. There were two Petri dishes for each grass jar and Petri dishes were arranged in a completely randomized design. Afterward, the Petri dishes were stored on shallow trays and were placed inside 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 average (mean of the two Petri dishes used for each replicate glass jar) germination and root length (of the germinated seeds only) were measured. Percentage inhibition was calculated by the Eq. [1] used by Chung et al. (2001):

Formula 1[1]
The experimental procedure was repeated twice by using in both bioassay experiments the same crop extracts. Fungal contamination was not observed during these experiments. A factorial arrangement of treatments (winter cereal extracts x extract concentration) was used for the two experiments in three replicates (glass jars) of a completely randomized design.

Field Experiment
Winter Cereal Cultivation
Two field experiments were conducted during the 2001/2002 and 2002/2003 growing seasons at the University Farm of Thessaloniki in northern Greece. The site was located at 22°59' N, 40°32' E. Experiments were established on a calcareous loam soil (Typic Xerorthent) whose physicochemical characteristics were silt (48%), clay (27%), sand (25%), organic matter (1.5%), and pH (1:2 soil to H2O) of 8.3.

Nitrogen as ammonium sulfate and P as superphosphate were applied at 50 and 66 kg ha–1, respectively, and incorporated into the soil before winter cereal planting. The 11 winter cereals mentioned above were planted at a seed rate of 160 kg ha–1 on 25 Nov. 2001 and 18 Nov. 2002. There were four replicates of a randomized complete block design. Plots were 10 by 3 m, and a 2-m-wide alley separated all plots. In both growing seasons, herbicides were not used on crops because of their low weed infestation. However, in plots where winter cereals had not been planted, weeds were controlled with 0.6 kg ha–1 of paraquat (1,1'-dimethyl-4,4'-bipyridinium), which was applied in early spring, about 3 wk before incorporating the winter cereals. This was done to reduce plant residues in control subplots (treatment without any mulch) before seedbed preparation. Other cultural practices were according to recommended production practices for the area.

Sugarbeet Cultivation
In early spring of 2002 and 2003 (at the beginning of stem elongation and inflorescence emergence) the winter cereals were incorporated into the soil (8- to 10-cm depth), and 10 d later the experimental area was infested with weed seed by broadcasting 12 g m–2 barnyardgrass ({approx}2500 seeds m–2 with 26% germinability), 6 g m–2 bristly foxtail ({approx}4000 seeds m–2 with 14% germinability), and 5 g m–2 large crabgrass ({approx}6000 seeds m–2 with 9% germinability). The germinability of these seeds was evaluated before planting by carrying out petri dish experiments in a growth chamber. Then, the weed seeds were incorporated into the soil (5-cm depth) with a rotovator, and sugarbeet was planted. The barnyardgrass, bristly foxtail, and large crabgrass seeds were harvested from a nearby area during the previous year of each experiment.

Before sugarbeet planting, N as ammonium sulfate, P as superphosphate, and K as potassium sulfate at 110, 66, and 125 kg ha–1, respectively, were incorporated into the soil. Asso sugarbeet was planted in 45-cm rows at an approximate density of 85 000 seeds ha–1 on 30 Mar. 2002 and 5 Apr. 2003. A split-plot arrangement of treatments was used in four replicates of a randomized complete block design. Plots were 10 by 3 m. In each plot, two subplots of 4.5 by 3 m were created (included six rows of sugarbeet), and all subplots were separated by a 1-m wide alley. The 11 winter cereals plus the winter cereal-free treatment (control) were the main plot factor and chemical weed control (with or without herbicide application at 4 wk after sugarbeet planting) was the subplot factor. Barnyardgrass, bristly foxtail, and large crabgrass control in these subplots was achieved with 1.5 kg ha–1 of quizalofop-ethyl {2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-N,N-dimethyl-3-pyridinecar-boxamide} applied postemergence on sugarbeet or by hand weeding. Also, broadleaf weeds in no-herbicide subplots were removed by hand during both growing seasons.

Insect management was conducted with 1.2 kg ha–1 of carbofuran (2,3-dihydro-2, 2-dimethylbenzofuran-7-ylmethylcarbamate) applied at the time of sugarbeet planting. Irrigation and other common cultural practices were imposed as needed during the growing season. Mean monthly temperature and rainfall data recorded near the experimental area are given in Fig. 1 .


Figure 1
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  		Fig. 1. Total monthly rainfall and mean monthly temperature during the experiment.

 
Crop and weed densities were determined four weeks after planting (WAP). Also, barnyardgrass, bristly foxtail, and large crabgrass plants were harvested in a 1-m2 area (2.2- by 0.45-m) in two of the four center rows of each subplot at 12 and 16 WAP in both growing seasons. Weed shoot number and fresh weight were determined at each sampling.

At physiological maturity, sugarbeet root from the two, 4.5-m-long center rows (not sampled for weed assessment) of each subplot were hand harvested and root and sugar yield was determined. Samples (six sugarbeet roots) for each subplot were analyzed for sugar concentration by the Greek Sugar Industry. The determination of polarization of sugarbeet was performed by the macerator or cold aqueous digestion method using leaf acetate as clarifying agent (Method GS6–1) (International Commission for Uniform Methods of Sugar Analysis, 2005, p. 341).

Statistical Analysis
For the laboratory study, a combined across time ANOVA was performed for the germination and root length inhibition data of large crabgrass and sugarbeet. Data before the ANOVA were log transformed to reduce their heterogeneity. Means were compared using the Fisher's Protected LSD test (P < 0.05). Because the ANOVA indicated no significant treatment x time interaction, the means of each extract concentration were averaged across the two laboratory experiments. Also, single degree of freedom contrasts were performed comparing the main effects of the three crop species (barley, rye, and triticale) on germination and root length inhibition of sugarbeet and large crabgrass.

For the field experiment, data for barnyardgrass, bristly foxtail, and large crabgrass shoot number and fresh weight, as well as sugarbeet root and sugar yield, were analyzed across years. Data before the ANOVA were not log transformed as it was not needed. Single degree of freedom contrasts comparing the main effects of the three crop species (barley, rye, and triticale) on the emergence and growth of the grass weeds and on sugarbeet yield were also performed. The means were compared using Fisher's Protected LSD test (P < 0.05) and are presented across years. The MSTAT program (Michigan State University, 1988) was used to conduct the ANOVA.

Finally, a correlation analysis between the inhibition of large crabgrass germination or root length data collected in the laboratory and the weed shoot number or fresh weight data recorded in the field was performed to examine how well the allelopathy observed in the laboratory explained the results found in the field.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Laboratory Experiment
Large crabgrass germination and root length were significantly affected by winter cereal species (P < 0.001), extract concentrations (P < 0.001), and by the winter cereal species x extract concentration interaction (P < 0.05). In general, large crabgrass germination and root length inhibition increased with increasing extract concentration, but the increase was not proportionally similar for all winter cereal species (Table 1). In particular, the germination inhibition percentage of large crabgrass by the lesser extract concentration ranged from 55 to 100%, while the corresponding inhibition by the greater extract concentration ranged from 82 to 100%. Artemis and Vrito triticale, at both extract concentrations, caused 100% germination inhibition of large crabgrass, while the respective inhibition by Vronti triticale was 71 and 82%. In addition, the rye populations from Albania, Greece, and Germany, at both extract concentrations, caused 96 to 100% germination inhibition of large crabgrass. Athinaida and Thessaloniki barley caused similar (86 and 85%, respectively) germination inhibition of large crabgrass. Concerning the other winter cereals, the greater extract concentration inhibited large crabgrass germination by 98 to 100%, while the inhibition by the lesser extract concentration ranged from 55 to 62%.


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  		Table 1. Inhibitory effect (% of water control) of 11 winter cereal extracts on germination and root length of sugarbeet and large crabgrass. Means are averaged across two experiments.{dagger}

 
Root length inhibition percentage of large crabgrass by the greater extract concentration ranged from 81 to 100%, while the corresponding inhibition by the lesser extract concentration ranged from 57 to 100% (Table 1). Root length inhibition of large crabgrass by Artemis and Vrito triticale, at both extract concentrations, was 100%, while the inhibition caused by Thisvi and Vronti triticale ranged from 74 to 88% and 81 to 84% for the low and high concentrations, respectively. Root length inhibition of large crabgrass by the lesser extract concentration of rye populations from Albania, Germany, and Greece, ranged from 69 to 92%, while the corresponding inhibition of the greater extract concentration ranged from 81 to 100%. The greater extract concentration of Athinaida and Thessaloniki barley caused 96 and 94% root length inhibition of large crabgrass, respectively.

The single degree of freedom comparisons between crop species showed clearly that rye extracts inhibited germination of large crabgrass more than barley or triticale extracts (Fig. 2 ). Furthermore, rye or triticale extracts caused greater root length reduction of large crabgrass compared with that produced by barley extracts.


Figure 2
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  		Fig. 2. Inhibitory effect (% of water control) of the cover crop species on germination and root length of sugarbeet and large crabgrass. Columns within each graph with different letters indicate significant difference at P < 0.05.

 
Sugarbeet germination and root length were also significantly affected by both winter cereal species (P < 0.001), extract concentrations (P < 0.001), and by their winter cereal species x extract concentration interaction (P < 0.05). In general, sugarbeet germination and root length inhibition increased with increasing extract concentration, and, as for large crabgrass, the increase due to increased extract concentration was not proportional for all winter cereal species (Table 1). However, winter cereal extracts at both concentrations inhibited germination and root length of sugarbeet to a lesser degree than that of large crabgrass. In addition, winter cereal extracts, in most cases, caused greater germination than root length inhibition of sugarbeet. In particular, the germination inhibition percentage of sugarbeet by the lower extract concentration ranged from 40 to 74%, while the corresponding root length inhibition ranged from 32 to 65%; the germination inhibition caused by the greater extract concentration ranged from 59 to 100% and that of root length from 45 to 100% (Table 1). Among the cereals tested, rye populations from Albania and Greece at the greater extract concentration caused the greatest germination and root length inhibition (100%) of sugarbeet, while Catria and Vronti triticale and Athinaida barley caused the lowest (45 to 78%). Greater extract concentration of rye populations from Greece and Albania inhibited germination and root length of sugarbeet by 100%, while the inhibition by lesser extract concentration was 64 and 73% for germination and 59 and 49% for root length, respectively. On the contrary, the corresponding germination and root length inhibition by the two extract concentrations of triticale cultivar Catria were 43 and 59%, and 38 and 51%, respectively.

The single degree of freedom comparisons between crop species indicated again that rye extracts caused greater germination and root length inhibition of sugarbeet compared with those of barley or triticale (Fig. 2). Furthermore, triticale extracts reduced sugarbeet root length more than barley extracts.

The different inhibition effect of winter cereal extracts on large crabgrass and sugarbeet germination and root length could be explained by the differences in amount and characteristics of physicochemicals, which possibly include allelochemicals produced by these winter cereals. Similar results were reported by Hanson et al. (1981), Lemerle et al. (1996), Burgos et al. (1999), Chung et al. (2001), and Dhima et al. (2006), who worked with rye, barley, wheat, and rice (Oryza sativa L.) extracts. Also, Peters and Zam (1981) found that various cultivars of tall fescue (Festuca arundinacea Schreb.) exhibited different levels of large crabgrass suppression.

The greater germination and root length inhibition of large crabgrass compared with that of sugarbeet is in agreement with results reported by Burgos and Talbert (2000), who found that small-seeded crops such as tomato (Lycopersicon esculentum Mill.) were affected by rye extracts more than large-seeded crops (e.g., corn). Also, Dhima et al. (2006) found that winter cereal extracts caused greater growth inhibition of bristly foxtail than that of barnyardgrass, while corn germination and root length were not significantly affected by winter cereal extracts. However, Ben-Hammouda et al. (2001) reported that extracts of barley plants did not significantly affect seed germination of either durum (Triticum durum L.) or bread wheat cultivars.

The lower root length inhibition of sugarbeet by the winter cereal extracts in comparison with that of germination is in contrast with the findings of Hedge and Miller (1992), Chung and Miller (1995), and Chon et al. (2000). The increased sugarbeet emergence and root length inhibition with increasing extract concentration agrees with results reported by Burgos and Talbert (2000), Chon and Kim (2004), and Dhima et al. (2006).

Field Experiment
The ANOVA of the weed data indicated that, in most cases, barnyardgrass, bristly foxtail, and large crabgrass shoot number and fresh weight were significantly affected by winter cereal cover crop (P < 0.001). However, because the ANOVA, in most cases, indicated no significant treatment x time interaction, the means are presented across years (Tables 2, 3, 4).


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  		Table 2. Effect of 11 winter cereal cover crop mulches on shoot number and fresh weight of barnyardgrass grown in sugarbeet. Means are averaged across 2002 and 2003 growing seasons.

 

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  		Table 3. Effect of 11 winter cereal cover crop mulches on shoot number and fresh weight of bristly foxtail grown in sugarbeet. Means are averaged across 2002 and 2003 growing seasons.

 

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  		Table 4. Effect of 11 winter cereal cover crop mulches on shoot number and fresh weight of large crabgrass grown in sugarbeet. Means are averaged over 2002 and 2003 growing seasons.

 
Barnyardgrass, bristly foxtail, and large crabgrass shoot number at 4, 12, and 16 WAP and fresh weight at 12 and 16 WAP, in most cases, were less in plots where winter cereals had been incorporated compared with those of winter cereal cover crop mulch-free plots (control) (Tables 2, 3, 4). In particular, 4 WAP in plots where barley cultivar ‘Athinaida’ had been incorporated, barnyardgrass, bristly foxtail, and large crabgrass shoot number was reduced by 69, 60, and 78%, respectively, compared with winter cereal cover crop mulch-free plots (control). However, in plots where the other winter cereals had been incorporated, barnyardgrass, bristly foxtail, and large crabgrass shoot number reduction ranged from 38 to 59%, 0 to 17%, and 0 to 56%, respectively (Tables 2, 3, 4). However, large crabgrass shoot number, in plots where the rye population from Greece had been incorporated, was 22% greater compared with that of winter cereal cover crop mulch-free plots. At 12 WAP, Athinaida barley cover crop mulches caused the greatest barnyardgrass and bristly foxtail shoot number reduction (76 and 80%, respectively) compared with that caused by the other winter cereals studied (Tables 2, 3). On the contrary, large crabgrass shoot number was decreased more (90%) in plots where Vrito triticale had been incorporated (Table 4). At 16 WAP, Athinaida barley cover crop mulches caused barnyardgrass and bristly foxtail shoot number reduction by 71 and 59%, respectively, while in plots where the other winter cereals had been incorporated, barnyardgrass and bristly foxtail shoot number reduction ranged from 48 to 64% and 22 to 62%, respectively (Tables 2, 3). However, at the same sampling (16 WAP), Athinaida barley and Vrito triticale cover crop mulches caused the same shoot number reduction (about 79%) of large crabgrass, while the shoot number reduction by the rye population from Greece and Thisvi triticale cover crop mulches was 0 and 28%, respectively (Table 4). In general, the greatest barnyardgrass and bristly foxtail fresh weight reduction was caused by barley cultivars, while the lowest by triticale. However, for large crabgrass, the greatest fresh weight reduction resulted from barley or triticale cultivars, while the lowest was from rye populations.

Fresh weight of barnyardgrass and bristly foxtail 12 WAP were decreased more (77 and 79%, respectively) in plots where Athinaida barley had been incorporated. However, the fresh weight reduction by the other cereal mulches ranged from 21 to 63% and 30 to 67%, respectively (Tables 2, 3). On the contrary, at the same sampling (12 WAP) the greatest (78%) large crabgrass shoot number reduction was caused by Vrito triticale, while the respective reduction by Athinaida barley cover crop mulches was 65%. However, the fresh weight reduction caused by the rye population from Germany and Thessaloniki barley was only 2 and 12%, respectively (Table 4). At 16 WAP, the greatest barnyardgrass fresh weight reduction (72%) was caused again by Athinaida barley, while Vrito triticale caused the greatest (60%) reduction of bristly foxtail fresh weight. In relation to other cereal mulches, fresh weight of barnyardgrass and bristly foxtail reduction ranged from 39 to 67% and 0 to 59%, respectively (Tables 2, 3). However, bristly foxtail fresh weight in Thisvi triticale plots was 18% greater compared with that of winter cereal cover crop mulch-free plots (Table 3). At the same sampling (16 WAP), the greatest (91%) large crabgrass fresh weight reduction was caused again by Vrito triticale and the lowest (24%) by Vronti triticale. However, large crabgrass fresh weight in rye population from Greece plots was 27% greater compared with that of winter cereal cover crop mulch-free plots. In general, barley cultivars caused again, as they did for fresh weight, the greatest barnyardgrass and bristly foxtail shoot number reduction, while triticale caused the lowest. Concerning large crabgrass, barley or triticale cultivars caused the greatest and rye the least shoot number reduction.

The single degree of freedom contrasts showed clearly, as reported in general earlier, that barnyardgrass and bristly foxtail shoot number and fresh weight were decreased more in subplots where barley cultivars had been incorporated, with a lesser reduction observed in triticale mulch-treated plots (Fig. 3 , 4) . However, large crabgrass shoot number and fresh weight decreased more in plots where barley or triticale cultivars had been incorporated than in rye mulch-treated subplots (Fig. 5 ).


Figure 3
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  		Fig. 3. Inhibitory effect of cover crop species on shoot number and fresh weight of barnyardgrass. Columns within each graph and each sampling with different letters indicate significant difference at P < 0.05. WAP = weeks after planting.

 

Figure 4
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  		Fig. 4. Inhibitory effect of cover crop species on shoot number and fresh weight of bristly foxtail. Columns within each graph and each sampling with different letters indicate significant difference at P < 0.05. WAP = weeks after planting.

 

Figure 5
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  		Fig. 5. Inhibitory effect of cover crop species on shoot number and fresh weight of large crabgrass. Columns within each graph and each sampling with different letters indicate significant difference at P < 0.05. WAP = weeks after planting.

 
The correlation between the inhibition of large crabgrass germination or root length data in the laboratory and those of shoot number or fresh weight in the field indicated very low correlation coefficients (less than 0.1 for the samplings made at 4, 12, and 16 WAP).

Sugarbeet emergence was not significantly affected by the presence of winter cereal cover crop mulches (data not shown). These findings are in contrast with results reported by Petersen and Rover (2005), who found that field emergence of sugarbeet decreased where a winter cover crop had been used. Differences in allelopathic potential of the crop cultivars used and the different environmental conditions prevailing during their experiments could account for these differences. Sugarbeet root and sugar yield were significantly affected by winter cereal cover crop (P < 0.001), herbicide application (P < 0.001), and by the winter cereal cover crop x herbicide application interaction (P < 0.001). Therefore, the simple effects means are presented (Fig. 6 ). In general, sugarbeet root yield in no-herbicide subplots where most of the barley cultivars and rye populations had been incorporated was greater than that of the corresponding subplots where cereals had not been planted or the triticale cultivars had been incorporated. Also, in subplots where most of the barley cultivars and rye populations had been incorporated and the three grass weeds had been controlled by quizalofop-ethyl applied postemergence, sugarbeet root yield was greater than that of the corresponding subplots where cereals had not been planted or the triticale cultivars had been incorporated. However, in subplots where triticale had been incorporated, sugarbeet root yield was similar with that in subplots where cereals had not been planted. In particular, sugarbeet root yield reduction, averaged across growing seasons, was only 6% in no-herbicide subplots where Athinaida barley had been incorporated. On the contrary, the corresponding sugarbeet root yield in no-herbicide subplots where the other winter cereals had been incorporated was decreased by 25 to 62%, as compared with the corresponding herbicide-treated subplots. In addition, sugarbeet root yield in no-herbicide subplots where Athinaida barley and rye population from Albania had been incorporated was significantly higher than that of no-herbicide subplots where winter cereals had not been planted (control), but only 6 and 26% less, respectively, than those obtained in the respective herbicide-treated subplots. However, in no-herbicide subplots where the other winter cereals had been incorporated, sugarbeet root yield increased by 6 to 41%.


Figure 6
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  		Fig. 6. Effect of 11 winter cereal cover crop mulches and three grasses (barnyardgrass, bristly foxtail, and large crabgrass) on sugarbeet yield. BAth = Athinaida barley; BThe = Thessaloniki barley; RAlb = rye Albania; RGre = rye Greece, RGer = rye Germany; TNio = Niovi triticale; TThi = Thisvi triticale; TVro = Vronti triticale; TVri = Vrito triticale, TArt = Artemis triticale, TCat = Catria triticale.

 
The single degree of freedom contrasts indicated that barley cultivars in no-herbicide subplots produced the greatest sugarbeet root yield while triticale cultivars and the crop mulch-free treatment (control) the lowest (Fig. 7 ); the rye populations provided intermediate sugarbeet root yield. However, in herbicide-treated subplots, rye populations produced the greatest sugarbeet root yield, while triticale cultivars and crop mulch-free treatment (control) the lowest (Fig. 7); the barley cultivars provided intermediate sugarbeet root yield.


Figure 7
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  		Fig. 7. Inhibitory effect of cover crop species on sugarbeet yield. Columns within each graph with different letters indicate significant difference at P < 0.05.

 
The sugarbeet yield in sugar content, averaged across growing seasons, in subplots where Athinaida barley and rye population from Albania had been incorporated and the three grass weeds had been controlled by quizalofop-ethyl applied postemergence was 14 and 11%, respectively, greater than that of the corresponding subplots where cereals had not been planted (Fig. 6). On the contrary, in no-herbicide subplots where Athinaida barley and rye population from Albania had been incorporated, sugar yield was significantly higher than that of no-herbicide subplots where winter cereals had not been planted (control), but only 8 and 27% less, respectively, than those obtained in the respective herbicide-treated subplots. On the contrary, in no-herbicide subplots where Thisvi triticale, Thessaloniki barley, and rye population from Greece and Germany had been incorporated, sugar yield was decreased by 50, 54, 53, and 61%, respectively, compared with the corresponding herbicide-treated subplots.

The single degree of freedom contrasts showed that barley cultivars, in no-herbicide subplots, produced the greatest sugar yield, while crop mulch-free treatment (control) the lowest (Fig. 7); rye populations and triticale cultivars provided intermediate sugar yield, but the yield produced in rye was greater than that in triticale. Concerning the herbicide-treated subplots, barley and rye produced the greatest sugar yield, while triticale and crop mulch-free treatment (control) the lowest (Fig. 7).

The significant inhibition of barnyardgrass, bristly foxtail, and large crabgrass emergence and growth by the winter cereal cover crop mulches agrees with the results reported by Barnes and Putnam (1983), Weston (1990), Teasdale et al. (1991), Reddy (2001), Nagabhushana et al. (2001), Kobayashi et al. (2004), and Dhima et al. (2006). In particular, Barnes and Putnam (1983) and Weston (1990) found that cover crops such as rye, barley, and wheat reduced early season biomass of various weeds by 48 to 98%, compared with no cover crop controls. Teasdale et al. (1991) also reported that carpetweed (Mollugo verticillata L.) and common lambsquarters (Chenopodium album L.) densities were suppressed by rye and hairy vetch (Vicia villosa Roth) cover crop residues. However, Reddy (2001) found that rye, oat, wheat, and hairy vetch cover crop residues suppressed browntop millet [Brachiaria ramosa (L.) Stapf.] in soybean. Also, Nagabhushana et al. (2001) reported that rye is the most weed-suppressing cover crop among several small grains, and subterranean clover (Trifolium subterraneum L.) and crimson clover (T. incarnatum L.) the most suppressive legumes. In addition, Kobayashi et al. (2004) reported that winter barley suppressed emergence of summer annual weeds in soybean, especially during the first 3 yr. However, Dhima et al. (2006) found that barnyardgrass and bristly foxtail growth was less in plots where winter cereals had been incorporated compared with that of winter cereal cover crop mulch-free plots. In contrast, Reddy (2001) reported that large crabgrass density was not affected by rye residue compared with no cover crop. In addition, Teasdale et al. (1991) found that large crabgrass density was not affected by rye and hairy vetch cover crop residues and Moore et al. (1994) reported that redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters emergence were not affected by rye and wheat residues compared with the no cover crop treatment.

The greater sugarbeet root and sugar yield in herbicide-treated rye or barley mulched subplots than that of triticale or crop mulch-free ones could be attributed to the reduced crop competition from the weeds during the early stages, which may result from the lower barnyardgrass and bristly foxtail shoot number recorded in these subplots 4 WAP and reported earlier. These results are in contrast with those reported by Reddy (2001), who found that cover crop residues did not affect significantly soybean yield in plots where weeds had been controlled by postapplied herbicides. Dhima et al. (2006) also found that corn silage yield in subplots where winter cereals had been incorporated and grass weeds had been controlled by postemergence applied herbicides was similar to that of the corresponding subplots where cereals had not been planted.

The sugarbeet root and sugar yield increase in herbicide untreated subplots where Athinaida barley and the rye population from Albania had been incorporated, is in agreement with the findings of Shrestha et al. (2002), who reported that soybean yield was increased by the presence of rye and corn used as cover crops. Also, Moore et al. (1994) reported that soybean yield of the rye and triticale mulch treatments were 69 and 91% greater, respectively, than that obtained in the bare soil treatment. In addition, Swanton et al. (1999) found that corn yield was increased by a rye cover crop.

The different effect of Athinaida barley and rye population from Albania on sugarbeet root and sugar yield, compared with that of the other winter cereals tested, could be attributed to their stronger effect on the emergence of grass weeds and their further growth. Greater amounts and higher phytotoxicity of the allelochemicals produced by these winter cereals could account for these differences (Burgos et al., 1999; Burgos and Talbert, 2000).

The very low correlation coefficients between the inhibition of large crabgrass germination or root length data in laboratory and those of shoot number or fresh weight reduction in the field show clearly that the strong crop allelopathic potential on large crabgrass growth recorded in laboratory was not confirmed in the field. These results are in agreement with those reported by Inderjit and Dakshini (1995) and Blum et al. (1999), who expressed concern that laboratory bioassays did not predict field patterns well. However, Bertholdsson (2005) in a 4-yr study found significant correlation for some years between the laboratory determined potential allopathic activity of some barley cultivars and their weed suppressive activity under field conditions. These differences could be attributed to cultivar and environmental condition differences which affect the amount of the allelochemicals released and their phytotoxic levels (Cheng, 1995; Inderjit and Keating, 1999).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of this study indicate clearly that Athinaida barley and the rye population from Albania could be used along with or instead of conventional chemical and mechanical control methods for suppression of barnyardgrass, bristly foxtail, and large crabgrass in sugarbeet. These findings also show that potential exists for more allelopathic cultivars to be found among those grown commercially worldwide or for development of new ones by introduction of allelopathic traits from accessions with confirmed strong allelopathic potential. The success of such efforts will be a major step toward further development of sustainable crop production systems with less reliance on herbicides.

Received for publication September 15, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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