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Agronomy Journal 93:27-36 (2001)
© 2001 American Society of Agronomy

ALLELOPATHY SYMPOSIUM

The Use of Allelopathic Legume Cover and Mulch Species for Weed Control in Cropping Systems

Jesús Arturo Caamal-Maldonadoa, Juan José Jiménez-Osornioa, Andrea Torres-Barragánb and Ana Luisa Anayab

a Departamento de Manejo y Conservación de los Recursos Naturales Tropicales, Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Yucatán (UADY), Km. 15.5, Carretera Mérida-Xmatkuil, 97000, Mérida, Yucatán, Mexico
b Instituto de Ecología, UNAM, Ciudad Universitaria, AP 70-243, Mexico, 04510, D.F

Corresponding author (aanaya{at}ifisiol.unam.mx)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Traditional weed control practices in Mexico use legumes as cover crops or manures. Legumes used in these practices play a dual role in agroecosystems by protecting the soil from erosion and by enriching it with organic matter and N through Rhizobium symbiosis. Farmers in the tropical regions of Mexico use Mucuna spp., Canavalia spp. and other legumes to control weeds in their fields. We conducted in vitro bioassays and greenhouse experiments to evaluate the toxic effect of four legumes velvetbean [Mucuna deeringiana (Bort) Merr.], jackbean [Canavalia ensiformis (L.) DC.], jumbiebean [Leucaena leucocephala (Lam.) de Wit], and wild tamarind [Lysiloma latisiliquum (L.) Benth.] on weed growth, and on the survival of insects and nematodes. The aqueous leachates (1%) of the four legumes were tested on three test plants by seed germination and radicle growth bioassays in petri dishes. The aqueous leachates of all four legumes exhibited strong phytotoxic effect on the radicle growth of the test plants. The effects of velvetbean and jackbean leachates were also evaluated on the survival of phytopathogenic nematodes. Both leachates had nematotoxic effects. Greenhouse experiments were conducted to evaluate the effect of the four legumes dried leaves incorporated as mulches to potting soil, on the number and biomass of weeds, and on the biomass of tomato (Lycopersicon esculentum L.) plants. The decomposition of velvetbean and jackbean leaves in potting soil significantly reduced (>50%) the development of phytopathogenic nematodes in the roots of tomato. A 5-yr field experiment conducted at the University of Yucatan (UADY) evaluated the effect of velvetbean and jackbean used as living cover crops, and jumbiebean and wild tamarind used as dead mulches incorporated on soil surface, on weed growth, and corn (Zea mays L.) yield. The experimental field was treated with the traditional slash and burn system in February 1994. In July 1994 the experiment was performed using the local agricultural practices in a complete randomized block design with three repetitions. The treatments were: corn+velvetbean, corn+jackbean, corn+jumbiebean, corn+wild tamarind, corn+Paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) herbicide, corn weeded by hand, and plots without corn weeded by hand. The number, biomass, diversity, and relative importance of weeds, as well as corn yield, were evaluated. In addition, taxonomic composition of weeds was determined. All legumes reduced weed growth with velvetbean (as living cover crop) producing the largest weed biomass reduction (68%). These legumes also improved the yield of corn during the first 2 yr of the experiment. For better management of natural resources, the use of legumes as biological tools in agriculture to control weeds and improve soil conditions should be encouraged through coordinated efforts between farmers, academic, and governmental institutions.

Abbreviations: Alelomex, Laboratorio de Alelopatía • UNAM, Universidad Nacional Autónoma de México • UADY, Universidad Autónoma de Yucatán • LD, low dose • RD, recommended dose • GI, gall index • DW, dry weight • RIV, relative index value • SD = maximum standard deviation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
SOIL is a strategic resource for agriculture. The use of organic fertilizers to improve soil is very old. Theophrastus (372–287 BC), Cato (234–149 BC), and Xenophanes (44 BC), pointed out the importance of legumes and grasses as mulches. Other common organic fertilizers included dung of birds and bats (guano), fish fertilizer, dry blood, and dry meat. The Maya people fertilized their crops with dead leaves from the tropical forest, secondary and savanna plant communities, and animal manures. All these organic compounds help improve the soil by increasing water retention capacity, thus impeding nutrient loss by leaching, by decreasing erosion and surface drainage, and by helping control weeds and other pests (Rosado-May, 1986; Anaya et al., 1987).

Ramos et al. (1983) reported the use of various species of Commelinaceae that cover extensively the soil of shaded coffee (Coffea arabica L.) plantations in Coatepec, Veracruz. Coffee farmers manage the growth of these plants and use them as green fertilizers and weed controllers. Similarly, some species of Ipomoea are used as green manures and as a weed controller in some tropical regions of Mexico. In sugarcane (Saccharum officinarum L.) fields of the state of Morelos, Mexico, farmers promote Ipomoea tricolor (Cav.) growth before sugarcane cultivation. Its allelopathic potential was described by Anaya et al. (1990). Pereda-Miranda et al. (1993) identified Tricolorin A from the resin glycoside mixture of the plants as the major phytogrowth inhibitor.

Likewise, some tropical bean plants are used in Mexico, the Caribbean, and Asian countries for the same purposes. These bean plants are annual vines or shrubs that grow vigorously for a short time after being sowed, and cover the soil with a thick layer of growth, thus reducing light, preventing extreme temperature changes, and impairing weed growth (Lanini, 1987; Radosevich and Holt, 1984). Some of these legumes have allelopathic potential that affects the growth of other weeds (Hart, 1986; Vandermeer, 1989; Lathwell, 1990; Trenbath et al., 1990; Fujii, 1999). In tropical American countries, these bean plants are intercropped with corn, sorghum [Sorghum bicolor (L.) Moench], and millet [Pennisetum americanum (L.) Leeke]. Velvetbean (Mucuna deeringiana, M. cochinchinensis, and M. pruriens) are among the important widely used bean plants because of their fast growth and abundant biomass (Buckles and Barreto, 1994). Some are used also as fodder (Chacón and Gliessman, 1982; Anaya et al., 1992). In Tabasco and Veracruz, Mucuna spp. weed control practices helped maintain corn production around 3.0 t/ha (Arévalo and Jiménez-Osornio, 1988). In Yucatan, experimental practices of jumbiebean and wild tamarind leaves as mulches at the UADY fields are promising (Jiménez-Osornio, 1989).

In the Yucatan Peninsula of Mexico, the production of traditional crops is dependent upon the rainy season. Some of these regions are inadequate for conventional agriculture, primarily because of heavy soil erosion, a situation that is common to most tropical regions of the world. The cultivation of henequen (Agave fourcroydes Lem.) at the end of the 19th century to the present was based on the traditional slash and burn system. Today, this system is considered obsolete due to the ecological, social, and economic circumstances of the whole area (Hernández-Xolocotzi, 1985).

In the northern part of the Yucatan peninsula, the soil is very rocky and scarce. Crop production is poor and it is limited only to a 2-yr period. The fallow period is very short (<8 yr). This reduces secondary vegetation growth, and consequently crops are cultivated in poor soil with no organic matter. The average corn yield in Yucatan is approximately 750 kg/ha, much lower than the national average yield of 2 t/ha. In addition, pest problems are severe, particularly in weed-infested areas resulting in the heavy use of herbicides such as Paraquat dichloride (Gramoxone) and 2,4-D (Esteron) (2,4-dichlorophenoxyacetic acid) (Hernández-Xolocotzi, 1985; Levy and Hernández-Xolocotzi, 1992). In southeastern Mexico, this has led to the drastic reduction of companion crop diversity traditionally associated with corn, such as squash (Cucurbita L. spp.), and beans, `ib' (Phaseolus lunatus L.) and `xpelón' [Vigna unguiculata (L.) Walp]. The use of herbicides causes another problem, that of the selective growth of weeds (Caamal et al., 1996).

New resources management initiatives are necessary to restore the soil; increase organic matter and nutrients; control pests; improve crop production; and find adequate techniques to reach a sustainable production (Warren, 1983; Weston, 1996). The Department of Management and Conservation of Natural Tropical Resources (PROTROPICO) of the Facultad de Veterinaria y Zootecnia of UADY has established a program to improve agricultural practices leading to increase crop yield, control pest growth, and limit usage of harmful agrochemicals (Caamal et al., 1996).

The goal of the present study was to evaluate the effects of some legumes used as living cover crops and dead mulches, on the growth of crops and weeds. The objectives were:

  1. To evaluate the effect of velvetbean, jackbean, jumbiebean, and wild tamarind used as dead mulches on the growth of tomato and weeds, and on the survival of nematodes and insects, using greenhouse experiments and in vitro bioassays.
  2. To evaluate the effect of these legumes on weed growth and corn yield in a 5-yr field experiment (1994–1998) in which velvetbean and jackbean were used as living cover crops, and jumbiebean and wild tamarind as dead mulches incorporated to the soil surface.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
In Vitro Bioassays with Seeds
Leaves of the four legumes were collected in the experimental fields of UADY in 1994, air-dried, and transported to Alelomex Laboratory at the Instituto de Ecología, Universidad Nacional Autónoma de México (UNAM). The herbarium samples were kept at the UADY herbarium. Leaves were crushed by hand, and aqueous leachates were prepared by soaking 2 g of each plant in 100 mL of distilled water for 4 h. Each leachate was filtered and osmotic pressure was measured in a freezing-point osmometer (Osmette A. Precision Systems) to prevent the negative effects of a high-concentration solution on the germination and growth of the test plants. Each aqueous leachate (2%) was mixed (50:50) with 1.5% pure agar to obtain a 1% test solution. Pure agar (0.75%) was used as control.

Phytogrowth inhibitory activities of the aqueous leachates were determined by seed germination and radicle growth bioassays in petri dishes on three test species: barnyardgrass [Echinochloa crusgalli (L.) P. Beauv. (Poaceae)], alegría or amaranth [Amaranthus hypochondriacus (L.) Amaranthaceae], and tomato.

Treatments were set up in 5.5-cm Petri dishes. Ten seeds randomly selected from a sample of the seed population were sown equally spaced in a circle on the agar of each dish. Four repetitions were used per treatment in a completely randomized block design. Petri dishes were incubated in darkness at 27°C. Germination and radicle growth were measured after 24 h for alegría, 48 h for barnyardgrass, and 72 h for tomato. Data were analyzed with the ANOVA and Tukey's test.

Bioassays with Nematode Larvae
To evaluate the effect of aqueous leachates (1%) of the four legumes on the survival of phytopathogenic nematodes, a bioassay was conducted using the phytopathogenic nematodes Meloidogyne incognita Kofoid & White, and Nacobbus aberrans Thorne as test species. Larvae were hatched from eggs obtained from the roots of infested tomato plants at the Colegio de Posgraduados, in Texcoco, by placing the eggs in water in a petri dish and incubating at 24°C until hatched. Treatments were: leachates (1%) from velvetbean, jackbean, jumbiebean, and wild tamarind leaves, and distilled water was used as control. For bioassay, 10 larvae J2 of M. incognita or N. aberrans were placed in a 3-cm petri dishes containing 3 mL of each treatment including control in a complete randomized design with four repetitions per treatment. After 24 h incubation at room temperature, the petri dishes were examined and the percent nematode survival was calculated as follows: [(No. of live nematode)/10] x 100. Results were analyzed by ANOVA.

Bioassay with Nematode-Infested Seedlings
The treatments used in this experiment were the same as the previous one. Vials containing 20 g of sterile sand were moistened with 2.5 mL of water. Then, 3-mL of solution comprising of 1 mL of each test larvae suspension (1000 larvae/mL) and 2 mL of each leachate treatment, or distilled water (for the control) was added to each vial. The vials were incubated at room temperature for 24 h. Thereafter, a 6-wk old tomato seedling (with two true leaves) was transplanted into each vial and kept at room temperature for 3 wk. Seedlings were moistened when needed and 1 mL of a nutrient solution (Triple 17: N, P, K at 3%) was added weekly. At the end of the experiment, the gall index in the roots of each plant (Bridge and Page, 1980) and their dry weight were determined. The results were analyzed by ANOVA.

Bioassays with Insects
The toxic effect of legumes was evaluated on army worm (Spodoptera frugiperda Smith, Lepidoptera, Noctuidae). Insects were obtained from the Colegio de Posgraduados, in Texcoco. First and second instar larvae were kept in aired jars with a special diet consisting of:

This formula was mixed with warm agar (2.5%) by stirring for 10 min until completely homogenized, and kept at 4°C. When the larvae reached the third stage, they were transferred to individual jars to avoid cannibalism. Adults male (gray with small dark spots) and females (gray) were grouped in 10 to 12 couples and placed in wax paper bags. Egg masses were collected daily and placed in new jars with the diet. The treatments were as follows: Control (distilled water) and aqueous leachates (1 and 2%) of: velvetbean, jackbean, jumbiebean, and wild tamarind leaves. Each treatment was added to a petri dish containing 30 g of the diet. The same amount of treatment sufficient to wet the diet was previously established. The prepared diet was divided in 10 equal parts (3 g approximately) and placed in separate jars. Two first instar larvae were added per jar and maintained at room temperature for 10 d. Survival was recorded after this time. The bioassay was set up in a complete randomized block design with three repetitions. Results were analyzed by ANOVA.

Greenhouse Experiment 1
A greenhouse experiment was conducted to evaluate the effect of the legume dried leaves used as mulches in potting soil, on the number and biomass of weeds growing spontaneously from the seed bank of the soil, and on the biomass of tomato plants. Crushed leaves of each test plant were added (1 and 2%) to soil (300 g) in pots. Vermiculite was added (1 and 2%) to the control pots. Herbicide was added to the soil at the same time as the mulches. Pots were watered to field capacity and kept in the greenhouse for 8 d. A 21 d old tomato seedling was transplanted to each pot. Experimental pots were maintained in the greenhouse for 6 wk and watered when needed. Treatments evaluated were:

  1. Control soil + vermiculite (1%)
  2. Control soil + vermiculite (2%)
  3. Soil + velvetbean (1%)
  4. Soil + velvetbean (2%)
  5. Soil + jackbean (1%)
  6. Soil + jackbean (2%)
  7. Soil + jumbiebean (1%)
  8. Soil + jumbiebean (2%)
  9. Soil + wild tamarind (1%)
  10. Soil + wild tamarind (2%)
  11. Soil + vermiculite (1%) + herbicide (LD)
  12. Soil + vermiculite (2%) + herbicide (RD)

[Herbicide: DACTHAL W-75 (Fermenta ASC, dimethyl tetrachloroterephtalate). Recommended dosage (RD) = 11.2 kg/ha. Low dosage (LD) = 0.5 of RD].

Volcanic deep clay Feozem soil from a crop field from San Pablo Oztotepec hills, Milpa Alta, Mexico City, was used in this experiment. Physical and chemical characteristics of this soil show that it is a clay-loam soil with a pH slightly acid (6.3), with 2.4% of organic matter, extremely poor in K (1.12 cmolc/kg), poor in available Ca (0.8 cmolc/kg) and Mg (0.20 cmola/kg), and rich in P (500 mg/kg) and N (3010 mg/kg). The soil was air-dried and sieved before use. A completely randomized block design was used with four repetitions per treatment. The results were analyzed by ANOVA and Tukey tests.

Effects of the water collected from pots containing legume mulches on radicle growth. The objective of this experiment was to test the effect of the water collected from mulch-treated pots on the radicle growth of alegría and barnyardgrass. Water was collected five times from pots during the course of the experiment filtered through a filter paper and mixed (1:1) with agar (1.5%) in a petri dish. Control petri dishes contained only distilled water plus agar. Ten seeds of alegría or barnyardgrass were placed in each dish. Bioassays were performed in a randomized complete block design with four repetitions per treatment. Petri dishes were maintained in the dark at 27°C. Radicle growth was measured after 24 h for alegría and 48 h for barnyardgrass. The results were analyzed by ANOVA.

Greenhouse Experiment 2. This experiment evaluated the effects of velvetbean and jackbean used as mulches on the survival of nematodes in tomato-infested plants. In this experiment only velvetbean and jackbean were evaluated because they showed a more toxic effect in vitro on larvae survival. Both plants were dried, crushed by hand, and incorporated (1 and 2%) to potting (300 g). Pots were watered to field capacity and kept 8 d in the greenhouse. Three mL of a J2 larvae suspension (1000 larvae/mL) or distilled water for the control were added to each pot. A 15 d old tomato seedling was transplanted into each pot. Pots were watered as needed. Treatments were as follows:

  1. Control 1 soil + vermiculite (1%)
  2. Control 2 soil + vermiculite (2%)
  3. Soil + velvetbean (1%)
  4. Soil + velvetbean (2%)
  5. Soil + jackbean (1%)
  6. Soil + jackbean (2%)
  7. Soil + vermiculite (1%) + nematicide LD
  8. Soil + vermiculite (2%) + nematicide RD

[Nematicide: NEMACUR 400 CE (Bayer, ethyl-3-methyl-4-(methylthio) phenyl (1-methylethyl) phosphoramide. Recommended dosage (RD) = 17.5 kg/ha. Low dosage (LD) = 0.5 of RD. Nematicide was added 15 min after the inoculation of soil with nematodes].

A complete block design with six repetitions was performed. Two months later, the gall index in the roots of each plant (Bridge and Page, 1980) was determined. The results were analyzed by ANOVA and Tukey tests.

Field Experiment
To evaluate the effect of the four legumes on weed growth and on corn yield, a 5-yr field experiment was conducted (from 1994 to 1998) in an abandoned henequen (Agave fourcroydes Lem.) experimental field of PROTROPICO (Program of Conservation and Management of Tropical Natural Resources, UADY) at Xmatkuil, Merida, Mexico. Velvetbean and jackbean were used as living cover crops, jumbiebean and wild tamarind as soil-incorporated dead mulches or green manures. The climate of the region is warm and subhumid, with a dry season in winter and a shorter dry season in summer. Average annual temperature fluctuates around 27.5°C, and the average rain (May–November) is about 900 mm annually (García, 1973). The soil is heterogeneous, shallow, rocky (limestone), clay-loam, with a pH about 7.7, and 16% organic matter. The experiment depended solely on the rainy season for water. The experimental field (50 by 60 m) subdivided into 5 by 10 m plots was treated with the traditional slash and burn system in February 1994. In July 1994 the experiment was established following a complete block design with three repetitions. Treatments were:

  1. Corn with velvetbean
  2. Corn with jackbean
  3. Corn with jumbiebean
  4. Corn with wild tamarind
  5. Corn with herbicide (Paraquat)
  6. Corn weeded by hand
  7. Plots without corn weeded by hand

Two corn seeds were sowed, each 50 cm apart (60000 plants/ha). Velvetbean or jackbean were sowed, each 50 cm apart, 20 d after the corn sowing between the furrows. Distance between furrows was 1 m. Due to the rocky conditions, jumbiebean and wild tarmarind (mulches) could not be incorporated into the soil, so they were added on the furrows surface (12 t/ha of fresh leaves) 2 d before the sowing of corn. Following local common practices, Paraquat herbicide was applied over weed seedlings (0.6 kg/ha) 8 d after the sowing of corn. All treatments were hand-weeded once 20 d after the sowing of corn. These procedures were repeated each year.

In 1994 and 1995 a local variety of corn (`Criolla') was cropped. Corn production in 1995 was lost because of hurricane Roxana. From 1996 to 1998 an improved variety of corn (V-528) was cropped because its shorter life cycle and lower height were beneficial to avoid damage caused by the hurricane season.

Weeds were sampled at 20 and 60 d during each crop cycle. The plots (5 by 10 m) were subdivided by two sections (5 by 5 m each) and five random samplings were made in each section using 0.25 m2 guide in a 3 by 3 working plot. To avoid border effects, the 2-m outer limits of each half plot were not considered for sampling.

The following parameters were determined for each crop cycle: total weed biomass and weed biomass by species, total weed number and diversity, and weed relative abundance and frequency. These data were analyzed by ANOVA using log + 1 biomass and arc-sine cover. Means were compared using orthogonal contrasts (SAS Inst., 1985). Relative importance values (RIV%) of weeds were calculated:




Yield of corn was estimated by harvesting grains inside a 3 by 3 m area plot in each 5 by 5 m section. Results were expressed in kg/ha. These data were analyzed by ANOVA using as covariable the number of plants by plot. Orthogonal contrasts were used to compare means (SAS Inst., 1985).


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
In Vitro Bioassays with Seeds
The results of these bioassays are shown in Table 1.


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  		Table 1 Effects of aqueous aerial part leachates (1%) of velvetbean, jackbean, jumbiebean, and wild tamarind on the radicle growth of three test species

 
Almost all the leachates had a significant (p < 0.05) inhibitory effect on the radicle growth of test plants. Velvetbean and jackbean caused the highest inhibition on alegría radicle growth. Tomato was strongly inhibited by jackbean and by other treatments an indication that the use of these legumes as mulches in tomato crop should be avoided. Barnyardgrass was most resistant to the effect of aqueous leachates.

Bioassays with Nematode Larvae
The results are summarized in Table 2. All leachate treatments had a significant nematotoxic effect on both larvae species.


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  		Table 2 Effect of aqueous aerial part leachates (1%) of the four legumes on the survival of J2 larvae of two phytopathogenic nematodes

 
Bioassay with Nematode-Infested Seedlings
The aqueous leachates of both velvetbean and jackbean reduced the gall index of M. incognita (Table 3) in the roots of tomato. The nematotoxic effect of velvetbean was stronger than that of jackbean, but there was no significant difference between the tomato biomass of the two treatments.


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  		Table 3 Effect of aqueous aerial part leachates of velvetbean and jackbean on Gall Index (GI) of Meloidogyne incognita in the roots of tomato, and on the dry weight of the infested tomato seedlings

 
Bioassays with Insects
Table 4 summarized the effects of leaves leachates of the four legumes on the survival of Spodoptera frugiperda larvae. Velvetbean and jumbiebean had some toxic effect on Spodoptera frugiperda larvae, whereas jackbean and wild tamarind had no effect at all.


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  		Table 4 Effects of aqueous aerial part leachates (1 and 2%) of the four legumes on the survival of Spodoptera frugiperda larvae

 
Greenhouse Experiment 1
We only present the results of the effects of the four mulch legumes at 2% proportion on the number of weeds (Fig. 1) , because this proportion showed clearly the influence of each treatment on weed dynamics during this 6-wk greenhouse experiment. During the first week of treatment, only velvetbean and the herbicide treatment were significantly different from each other. The 2nd week revealed three significantly different groups of treatment: a = velvetbean and jackbean exhibiting the lowest number of weed, ab = jumbiebean and control with a medium weed number, and b = herbicide and wild tamarind, with the highest weed number. Third and 4th weeks exhibited the same significantly different two groups of treatments with velvetbean, jackbean, and jumbiebean treatments, showing the least amount of weeds compared with the control, herbicide, and wild tamarind treatments group. In the 5th and 6th weeks, the number of weeds in the control treatment was significantly higher than those in remaining treatments, with herbicide treatment consistently showing the lowest number of weeds.



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  		Fig. 1 Total number of weeds in the pots with the different incorporated mulch treatments (2%) during the 6-wk greenhouse experiment (Herbicide at RD) (SD = max. SD = 10) (P < 0.05). (1st wk, velvetbean and herbicide are significantly different; 2nd wk, there are three groups: (a) = velvetbean and jackbean, (ab) = jumbiebean and Control; (b) = Herbicide and wild tamarind; 3rd and 4th wk, there are two groups, (a) = velvetbean, jackbean, and jumbiebean, (b) = Control, Herbicide and wild tamarind; 5th wk velvetbean, jackbean, wild tamarind, and Herbicide are significantly different from Control; 6th wk, Herbicide, wild tamarind, and jackbean are significantly different from Control)

 
These results suggest that velvetbean and jackbean were the more effective weed-suppressing mulches. Wild tamarind could act as a weed suppressor the last 2 wk of the experiment. The herbicide had its best effect on weeds in these last 2 wk. However, in the 2nd and 3rd wk wild tamarind and herbicide stimulated the number of weeds, but this effect was reverted the last 2 wk. On the other hand, jumbiebean had a moderate control effect on the number of weeds. However, data on weed biomass (Fig. 2) show that jumbiebean had a stimulatory effect on weed biomass and also on tomato biomass at the end of the experiment. Both wild tamarind and jackbean strongly decreased the biomass of weeds and tomato. The biomass of tomato was similar in the control, herbicide- and velvetbean-treated pots. The biomass of weeds was similar in velvetbean and control treated pots, in spite of the lower number of weeds in velvetbean pots compared with the control pots. Phytotoxic effects of all the legumes were observed, but velvetbean, affected the growth of weeds more than their germination (Fig. 1). The above data indicated that velvetbean could be used as a mulch or living cover crop in tomato field because of its phytotoxic effect on weeds and the fact it does not damage tomato crop.



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  		Fig. 2 Mean of tomato and weeds biomass in pots with the incorporated mulch treatments (2%) at the end of the greenhouse experiment (SD = max. SD tomato = 33.1, weeds = 65). Different letters mean significant differences between treatments for tomato biomass. Different numbers mean significant differences between treatments for weed biomass

 
Effects of the drain water collected from mulch-containing pots on the radicle growth of two test seeds. The results of the bioassays are summarized in Table 5. Alegría was more sensitive to all treatments compared with barnyardgrass response. Water from velvetbean and wild tamarind-containing pots strongly inhibited the growth of the test plants in the first collect (3 d). The inhibitory effect of drain water from velvetbean containing pots on the radicle growth of alegría persisted for 1 mo. Water from wild tamarind containing pots also had a strong inhibitory effect, but only during the first two collections. Water from jackbean- and jumbiebean-containing pots had an inhibitory effect on alegría growth until 22 d. Drain water from all legumes lost their phytotoxicity at 60 d. We have to remember that the pots were watered to field capacity and kept in the greenhouse 8 d before the tomato seedlings were transplanted to each pot. This is probably the reason why drain water from pots with the legumes showed a strong inhibitory effect at the first collection (3 d), which corresponds to 11 d of the decomposition period.


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  		Table 5 Effect of drain water of incorporated mulch{dagger} pots collected at different times during the greenhouse experiment on the radicle growth of Amaranth and Barnyardgrass (petri dishes bioassays)

 
Drain water from velvetbean-containing pots had the stronger inhibition on barnyardgrass radicle growth. This inhibitory effect persisted until 22 d. Water from jackbean-containing pots also inhibited the growth of barnyardgrass, but only the two first collections. Water from jumbiebean- and wild tamarind-containing pots had an inhibitory effect on the radicle growth of barnyardgrass only in the second collection period.

Greenhouse Experiment 2
The effects of velvetbean and jackbean used in potting soil on survival of nematodes of infested tomato plants are shown in Table 6. The nematicidal effect of velvetbean and jackbean on M. incognita was evident and were in agreement with the results of bioassays performed in vials with sterile sand. Gall index was strongly reduced by the commercial nematicide (Nemacur) at the low dosage and totally inhibited at the recommended dosage. The nematicidal effect of the two legumes is similar. Although we did not observe a total disappearance of nematodes during the decomposition of velvetbean and jackbean in the soil, the significant decrease of these pathogens by the effect of both legume mulches could be beneficial for a tomato crop in natural conditions.


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  		Table 6 Gall Index of Meloidogyne incognita in the roots of tomato plants with the different treatments

 
The results obtained in the various bioassays with legume mulches open a promising area of research that offers multiple alternatives to control different agricultural pests. The management of legumes as mulches or cover crops in agrosystems shows a more ecological way to obtain a sustainable agriculture without a negative impact to the environment and biodiversity that commercial pesticides sometimes cause.

Field Experiment
The effects of the different treatments during the field experiment (1994–1998) on the mean biomass of weeds (g/m2) are shown in Fig. 3 . Results of corn yield in 1995 are not shown because of damage caused by Hurricane Roxana. Weed biomass was not determined in 1997 because of problems with field workers; only the weed cover (%) in the plots was determined that year (Fig. 4) .



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  		Fig. 3 Mean biomass of weeds from 1994 to 1998 in the plots with the different treatments. Different letters mean significant differences between treatments within years (a,ab,b = 1994; A,B,C = 1995; x,y,z = 1996; a,b,c,d = 1998) p < 0.05

 


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  		Fig. 4 Percent of weed cover in the plots with different mulch and living crop treatments for the 1997 crop cycle. Different letters mean significant differences between treatments

 
The weed composition in experimental plots from 1994 to 1998 are listed in Table 7 and is comprised of 18 families—Asteraceae is the most common with 9 species, followed by Poaceae with 7 and Acanthaceae and Convolvulaceae with 4.


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  		Table 7 Weed species that grow in the plots during the crop cycles from 1994 to 1998

 
The data in Fig. 3 show that velvetbean (cover crop treatment) was most effective at controlling weeds. The mean weed biomass in the plots with velvetbean was the lowest during all years of the experiment (21.82–58.96 g/m2). In addition, velvetbean covered 95 to 100% of plot soil at the end of the crop cycles. This extensive covering means an important contribution of organic matter to the soil. The biomass of weeds was higher in plots with jackbean cover crop. The two species used as mulch, jumbiebean and wild tamarind, also resulted in the reduction of weed biomass during the study period, except in 1998 (Fig. 3). This same tendency was stronger in the plots with herbicide where the weed biomass in 1998 was 195.47 g/m2. Overall, jackbean, jumbiebean, wild tamarind, and the herbicide treatments resulted in weed control, compared with the common practice of the one-time hand-weeded corn plot. In this last treatment, from 1994 to 1996, corn contributed in reducing the growth of weeds. In 1998, however, biomass of weeds reached a high value in this particular treatment. Plots without corn were weeded once each year, but still showed the highest weed biomass from 1995 to 1998, primarily due to the lack of competition with corn.

Weed biomass was not determined in 1997 because of problems with field workers. We report here only data on weed cover (%). Velvetbean-treated field had the lowest weed cover (Fig. 4). This was followed by jumbiebean, jackbean, and the herbicide-treated fields with similar weed cover. The wild tamarind-treated field and the hand-weeded plots had the third highest and similar weed cover. The highest weed cover was obtained in plots without corn.

The field experiment has clearly established three groups of treatment based on their inhibitory effect on weed growth: (i) velvetbean was the best weed controller, (ii) the remaining legumes, the herbicide, and the hand-weeded plots were good weed controllers, and (iii) hand-weeded plots without corn was the least weed control treatment and contained the highest biomass of weeds (665.2 g/m2 in 1998, Fig. 3).

When the canopy of velvetbean is completely developed, there is a significant decrease in light reaching the soil. This light reduction contributes to the inhibition of other weeds because all of them are heliophytes. In addition we have to consider that velvetbean has a rapid and extensive growth, so it can compete well with other weeds. Finally, it is important to point out its allelopathic potential that was clearly shown in all in vitro, greenhouse, and field experiments.

Velvetbean gave good control of spiny amaranth (Amaranthus spinosus L.), smooth pigweed (A. hybridus L.), field sandbar (Cenchrus insertus M.A. (Curtis), and bitterweed (Parthenium hysterophorus L.). This last species reached a high abundance in the herbicide plots. The weed RIV (Table 8, Fig. 5 and 6) showed that the herbicide and velvetbean had selective effects on the species of weeds growing in their plots. Acanthaceae, Poaceae, and Asteraceae were the main families in velvetbean plots, Asteraceae and Acanthaceae in herbicide plots.


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  		Table 8 Main weed species, with their respective RIV (%), in the plots with herbicide and velvetbean

 


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  		Fig. 5 Relative importance values (%) of main weeds in velvetbean-treated plots in 1998 (from left to right: southern sida (Sida acuta Burm. f.), Rhynchosia longiracemosa M. Martens & Galeotti, tall morning glory [Ipomoea purpurea (L.) Roth], Chenopodium sp., Waltheria americana, Acalypha aff. procumbens, Tridax sp., Elytraria imbricata R. Br., Tridax aff. procumbens, milha [Brachiaria fasciculata (Sw.) R.D. Webster], Ruellia nudiflora

 


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  		Fig. 6 Relative importance values (%) of main weeds in the herbicide treated plots in 1998 (from left to right: lavender-cotton (Sanvitalia procumbens Lam.), Cynodon sp., Neomillspaughia emarginata, Cenchrus insertus, Ruellia nudiflora, southern sida, Elytraria imbricata R. Br. Unknown Asteraceae, bitterweed (Parthenium hysterophorus L.)

 
During the crop cycles, the treatments resulted in selective pressure on weed populations. Some of these treatments favored the appearance of species that were not observed at the beginning of the experiment. As an example, bitterweed did not appear frequently in the first cycles, but was the dominant weed in herbicide plots at the end of 1998 cycle (Fig. 6). Weeds RIV (Fig. 5) showed that in velvetbean plots, percentage values were similar for various weed species. Velvetbean plots also showed a higher weed species diversity, but none of these weeds had a RIV >20%. On the other hand, herbicide plots (Fig. 6) had lower weed diversity. This treatment favored dominance of some weed species such as bitterweed and other Asteraceae. This fact represents a risk in a long-term period, because herbicide-resistant weeds could increase and become difficult to eliminate if only one herbicide were used. The same would be true for using one legume continuously, except that weed shifts would probably occur more slowly.

The use of velvetbean and other legumes, particularly jackbean, as living cover crops or dead mulches could contribute to the reduction of the weed seed bank in soils and in the improvement of corn production, delaying weed appearance. This can be obtained if interference with the legumes used as living cover crops and corn can be avoided (Dominguez and de la Cruz, 1990). It is known that the main allelopathic agent of velvetbean is L-DOPA, and also in velvetbean and other legume species used as cover crops, various unusual aminoacids have been found (Fujii, 1999).

Figure 7 shows the corn yield in the 5-yr field experiment. As mentioned before, in 1994 and 1995 a local variety of corn (Criolla) was cropped. But because of Hurricane Roxana in 1995, an improved variety of corn (V-528) was cropped in the following years. In 1994 only jackbean and jumbiebean increased corn yield >1000 kg/ha. In 1996 all treatments resulted in increased corn yield. These are relevant results, considering the average corn yield in the region in the second crop cycle is 700 kg/ha. In 1997 corn yield was lower than in the previous cycle. The best treatments were jackbean, velvetbean, and the herbicide. In the 1998 cycle, corn yield was improved by jumbiebean, wild tamarind, and particularly the herbicide. That year, corn had to be sown twice because of the delayed rainy season. The strong reduction of corn yield in the plots with the living cover crops, velvetbean and jackbean, can be explained because the second corn sowing was made at the same time these two legumes were sowed. This circumstance caused a high interference between legumes and corn, resulting in reduction of corn yield. In the previous cycles, legumes were sown 20 d after the corn. Currently we are evaluating the 1999 crop cycle to confirm that interference between velvetbean and jackbean with corn was the reason of the reduction of corn yield in 1998.



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  		Fig. 7 Yield (kg/ha) of corn from 1994 to 1998. Different letters mean significant differences between treatments within years (a,b = 1994; x = 1996; A,AB,B = 1997; a,b = 1998) p < 0.05

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The analysis of the effect of the treatments tested in this long-term study on weeds and corn production is very important to determine the benefits of legume management and to gain the potential sustainability of this alternative agricultural system. The four legume species studied contribute to increased corn yield, a beneficial result for the management of resources considering the ecological, social, and economical deterioration of this region of Yucatan. In addition, in crop fields, these legumes help to control weed growth, and may reduce other harmful pests, such as nematodes and probably some insects. Furthermore, they provide significant amounts of organic matter to the soil, particularly the living cover crops (velvetbean and jackbean) and add N from their symbiosis with Rhizobium. To obtain the best results from the use of cover crops, it is necessary to avoid interference (competence and allelopathy) of the living cover crops and the companion crops. This could be achieved by using the right density of legumes in the crop field, and sowing them at least 15 to 20 d after the corn has been sown.

We need to follow the effects of legumes as living cover crops and mulches for a longer period of time, and to evaluate their impact on other organisms in the agroecosystem (small animals and microorganisms in the soil). It is also important to test the effects of these legumes on other crops in similar field experiments and in other types of agroecosystems and soils. All these studies require a multidisciplinary collaboration to reach an appropriate management of biotic resources in agroecosystems. The main goal is to get a true ecological and sustainable agriculture, to preserve natural resources, to avoid harmful effects of the over use of agrochemicals, and essentially to conserve worldwide biodiversity.


   ACKNOWLEDGMENTS
 
We thank Consejo Nacional de Ciencia y Tecnología (CONACyT), México, for the support given to the projects 1179-N9202, 400346-5-4260N9407, and 498100-5-0306PB. The 5 yr field research was made possible by the generous support of the Rockefeller Foundation. We acknowledge Dr. Sira M. Dabo of Oklahoma State University, and Dr. Rocio Cruz Ortega from Instituto de Ecología of UNAM for their valuable help in the revision and correction of the manuscript. We also acknowledge Rosa María Canul, Laura Meneses, José Castillo, and all the field workers for their support and valuable help in the field experiment.

Received for publication November 29, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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