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a Dep. of Crop and Soils Sciences, Michigan State Univ., East Lansing, MI 48824
b W.K. Kellogg Foundation, 1 Michigan Ave E., Battle Creek, MI 49017
c Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON N1G 2W1, Canada
d L.G. Seeds, 710 N. Main, Suite 201, River Falls, WI 54022
e Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108
Corresponding author (johnfisk1{at}home.com)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: EL, East Lansing • KBS, Kellogg Biological Station • NIS, non-ionic surfactant • COC, crop oil concentrate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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There are a number of mechanisms responsible for the effect of cover crops on weeds. The living cover crop can reduce light (Teasdale and Mohler, 1993) and moisture available to fall germinating seeds. Weeds attempting to establish along with a cover crop would be in competition for resources and may not develop sufficiently to survive the winter. Cover crop residue can modify the conditions under which weeds germinate or regrow in the spring. Such effects could be due to changes in soil temperature, increase in soil moisture, release of allelopathic chemicals, and physical impediments to weed seedlings (Facelli and Pickett, 1991; Teasdale, 1996; Teasdale and Mohler, 1993).
Many legume species that are used as cover crops in no-till corn production are winter annuals or short-lived perennials. In northern regions of the USA, over-wintering species are normally established in the summer or fall and accumulate most of their biomass when they regrow in the spring. Despite the positive effects often produced by winter annual cover crops in corn production, there is also a potential for reduction in corn yield. Spring regrowth of legumes can lower available water in the subsoil creating conditions of moisture stress for corn in years of low precipitation (Badaruddin and Meyer, 1989; Frye et al., 1988; Hesterman et al., 1992; Tiffin and Hesterman, 1998). In addition, winter annuals require some form of control, either chemical or mechanical, before or at the time of corn planting. Contact herbicides are most commonly used; however, these can result in incomplete control of weeds and cover crops (Worsham and White, 1987; Yenish et al., 1996). Although herbicide options for cover crop control have improved, variability of spring conditions can still lower their effectiveness. Field and weather conditions can delay application as well as reduce uptake of herbicides into plant tissue.
In contrast to winter annuals, summer annual legume species will not over-winter in northern regions of the USA or other areas with prolonged freezing temperatures. When these species are fall-planted and allowed to winter-kill, they may be able to provide the benefits of winter annuals without reducing available soil moisture and eliminate the need for chemical or mechanical control in the spring.
Currently, there is interest in annual species of Medicago (annual medics) and other annual legumes for use as cover crops in midwestern grain cropping systems. Originating in North Africa and the Middle East, annual medics have adapted to a range of environmental conditions (Lesins and Lesins, 1979). Annual medics were introduced for grazing purposes into Australia and New Zealand, and are now a common component of sheep pastures. In southern Australia, annual medics are used in ley cropping systems, where they are rotated with cereal crops (Puckridge and French, 1983). In these systems, medics provide high quality forage, contribute N to the soil and nonlegume pasture species and improve physical structure of the soil (Crawford et al., 1989). Annual medics have also been tested as forages in Michigan (Shrestha et al., 1998) and in Minnesota (Zhu et al., 1996). Berseem clover is an annual legume used as a forage plant in India and in areas with Mediterranean climates. It has the potential to produce large amounts of biomass rapidly and can be cut several times a year (Shrestha et al., 1998; Westcott et al., 1995).
Recent investigations have indicated the potential for annual legumes to reduce weed populations. DeHaan et al. (1997) found that weed populations were reduced where annual medics were interseeded with corn. However, in this same study corn yield was also reduced, due to competition for nutrients or moisture when medic and corn were planted at the same time. Annual medics interseeded several weeks after corn planting did not affect corn yield; however, weed dry weight was not reduced compared with a no-cover control either. Moynihan et al. (1996) reported a 65% reduction in fall weed biomass compared with no-cover control following a grain barley (Hordeum vulgare L.) and medic intercrop.
Winter wheat–corn is a common rotation in the Midwest USA. The period between wheat harvest and corn planting is an ideal time for establishing a cover crop. Annual medics and berseem clover planted after wheat harvest have been shown to accumulate aboveground biomass of between 2.1 and 5.3 Mg ha-1 and increase no-till corn yields (Fisk, 1997). Our objectives were to investigate the impact of legume cover crops in a winter wheat–legume cover–corn cropping sequence on: (i) winter annual and perennial weed populations prior to no-till corn planting; (ii) summer annual and perennial weeds before application of postemergence herbicides; and (iii) to determine the role of legume residue on summer annual and perennial weeds in this cropping sequence.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Four cover crop treatments and a no-cover control were arranged in a randomized complete block design with four replications at each location. Cover crop treatments included two annual medic species (Santiago burr medic and Mogul barrel medic), Bigbee berseem clover, and medium red clover. The cropping system in this study was a sequence of winter wheat/cover crop/ and no-till corn. Legume cover crops were planted after wheat harvest.
Cover crops were no-till drilled into a wheat stubble field (straw removed) on 8 Aug. and 9 Aug. 1994 at EL and KBS, respectively, and on 9 Aug. and 11 Aug. 1995 at EL and KBS, respectively, with a drill with 20-cm row spacing to a depth of 1 to 2 cm. Santiago burr medic and Mogul barrel medic were seeded at a rate of 269 live seeds m-2 or 13.4 kg ha-1 and 15.7 kg ha-1, respectively. Red clover and berseem clover were planted at 16.8 kg ha-1, which is the common seeding rate in Michigan. Legume seeds were inoculated with the appropriate Rhizobia spp. before planting. Berseem clover did not establish at EL in 1995 because of equipment malfunction and when reseeded several weeks later, it failed to produce consistent stands.
Before cover crop planting, each field received an application of glyphosate (N-(phosphonomethyl)glycine) at 1.68 kg a.i. ha-1 with 0.1% nonionic surfactant (NIS). In addition, sethoxydim (2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclo-hexen-1-one) was applied at a rate of 0.32 kg a.i. ha-1 with 2.31 L ha-1 crop oil concentrate (COC) at EL on 19 Aug. 1994 to control volunteer wheat, large crabgrass (Digitaria sanguinalis L.), and quackgrass [Elytrigia repens (L.) Nevski].
Annual legumes winter-killed and the red clover was killed with herbicide in the spring before planting no-till corn. Each field received an application of glyphosate at 1.68 kg a.i. ha-1 with 2,4-D ester (2,4-dichlorophenoxyacetic acid, butoxyethylester) at 0.532 kg a.i ha-1 and 0.5% NIS prior to corn planting to control established weeds and the red clover cover crop. In 1994 at both locations, red clover was not killed by the herbicides. Therefore, red clover plots were mowed by hand at EL (avoiding the corn seedlings) and followed by an application of dicamba (3,6-dichloro-2-methoxybenzoic acid) at 0.56 kg a.i. ha-1. This resulted in complete control of the red clover. Also in 1994 at EL, bentazon (3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one2,2-dioxide) at 0.84 kg a.i. ha-1 with 2.31 L ha-1 COC was spot-sprayed for purple nutsedge (Cyperus esculentus L.) control on 3 June.
`Pioneer 3751' corn was planted no-till in mid-May of each year at 62220 seeds ha-1 at a row spacing of 76 cm. Fertilizer P and K were broadcast before corn planting according to soil test results. This study was a part of another experiment that measured no-till corn response to N following cover crops; however, in this study weed samples were taken before side dress N application. Postemergence herbicide applications were made 45 and 60 d after chemical kill in 1995 and 1996, respectively. Nicosulfuron (2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino] carbonyl]amino] sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide) at 0.035 kg a.i ha-1, and bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) at 0.28 kg a.i. ha-1 with 0.25% NIS were applied at EL in 1995. At KBS, sethoxydim at 0.21 kg a.i. ha-1 with 2.31 L ha-1 COC was applied in 1995. In 1996 at EL, bromoxynil was applied at 0.28 kg a.i. ha-1 with 0.25% NIS.
Sampling for weed density and dry weight was done twice: before corn planting and before postemergence herbicide application. The effect of cover crops on winter annual and existing perennial weed density and dry weight was determined at first sampling. In early May, just prior to spring herbicide application, the area within four randomly placed 0.25 m2 quadrats was sampled for weed density and dry weight. These samples were removed, separated by species, dried at 60°C for 72 h, and weighed. Common chickweed (Stellaria media L.) was sampled only for dry weight, since it was difficult to determine actual plant density because of its growth habit.
The effect of cover crops on summer annual and perennial weed populations and growth (weeds that germinated or otherwise initiated growth after application of the herbicide) was determined at second sampling. This sampling was done as described above on spots adjacent to the initial sampling within the treatment plots approximately 45 d after chemical kill of cover crops in 1995. In 1996, weed populations were slow to initiate growth because of dry soil surface conditions. As a result, sampling was delayed until approximately 60 d after chemical kill.
Also, at this time the effect of the cover crop residue on weed populations and growth was measured. This was done by comparing data from plots in which cover crop residue had been removed to data from plots in which residue had not been removed. After sampling in early May, the legume residue was removed from the soil surface in the 0.25 m2 quadrats, leaving legume roots behind, and the spots were marked. Approximately 45 or 60 d later, weeds were sampled in these areas for density and dry weight and separated by species. These results were compared to measurements taken at the same time in adjacent, but previously unsampled, quadrates in the same cover treatment.
Soil temperatures were measured on selected dates in 1995 and 1996 to a 5-cm depth with a soil thermometer. The thermometer was inserted into five randomly chosen spots in each treatment and these values were averaged.
Data were combined across years and locations and subject to analysis of variance using a randomized complete block model with a split-split-plot treatment arrangment. The main plot was the random effect of years, the subplot was location, and the sub-subplots were the five cover crop treatments. Where interactions with year and location were significant (P < 0.05), data were separated accordingly and reanalyzed.
Weed density and dry weight data taken after chemical kill of cover crops was transformed by taking the square root and log, respectively, to correct for heterogeneity of variance. Nontransformed data are presented with statistical interpretation based on transformed data. Means of each cover crop treatment were compared with the no-cover control with a single degree of freedom F test. Data describing the residue effect within cover crop treatment were compared using a t test.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Winter annual weeds germinate and begin growth in the fall, reinitiate growth in the spring, and complete their life cycle by midsummer (Stubbendieck et al., 1995). However, some winter annual species may also germinate in the spring. The observed reduction in weed density could have resulted from the effect of the cover crop on weed seedlings in the fall. Such effects could have been due to a multitude of factors, which include reduction in light interception, changes in soil temperature, increase in soil moisture, release of allelopathic chemicals, and physical impedance to weed seedlings. In our study, soil temperature was not affected by annual legume residue (Table 2). Therefore, weed density and dry weight reduction resulted from factors other than soil temperature.
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0.05). Cover crop biomass at KBS in 1994 for Santiago, Mogul, Berseem clover, and red clover was 2.0, 2.3, 2.2, and 1.4 Mg ha-1, respectively. In 1995, at KBS biomass was 2.8, 2.4, 1.6, and 1.7 Mg ha-1, for Santiago, Mogul, Berseem clover, and red clover, respectively. Except for Santiago medic, which had greater biomass in 1995 than in 1994 (P 0.05), there was no difference in the amount of cover crop biomass at KBS between 1994 and 1995. This may indicate that greater levels of cover crop biomass reduce germination or establishment of winter annual weeds. The growth habit of red clover differs from that of other cover crops used in this study in that it reinitiates growth in the spring from crowns established the previous year. At the time of sampling for winter annual weeds, red clover had grown to between 16 and 24 cm in height, while the annual legumes had left a desiccated residue on the soil surface. Differences in soil temperature, however, were observed in the treatments with a red clover cover crop (Table 2). Soil temperature at seven different sampling events was lower in the red clover treatments than in the no-cover control. This may have influenced weed density and weed dry weight. Also, competition for light and nutrients by the red clover regrowth may have further influenced weed density.
The differential response to cover crop treatments among weed species was small. At EL in 1995, the density and dry weight of shepherd's-purse was lower in all cover crop treatments compared with the no cover control (P 0.05). This was the case for volunteer wheat as well, except in the Santiago medic treatment where populations were no different than the control. Dry weight of chickweed was lower following all cover crops in 1995 at EL (P 0.05) and at EL and KBS in 1996 (P 0.1 or 0.05). Following red and berseem clover, dry weights of field pennycress were lower at EL in 1995 (P 0.05). In 1996 at KBS, density of henbit (Lamium amplexicaule L.) was lower in all cover treatments except berseem clover (P 0.05). In 1996 at KBS, dry weights of volunteer wheat in Santiago medic and red clover treatments were lower than in the control (P 0.05); however, they were not lower in mogul medic or berseem clover treatments.
The effects of legume cover crops on weed populations that are present before crop planting have not been adequately documented. Most cover crop species used in north central USA are still alive prior to corn planting and require either mechanical or chemical control. However, annual legume cover crops may permit the reduction of herbicide use before crop planting and enable a shift toward postemergence herbicide options (Teasdale, 1996). Our study indicated this may be possible because cover crops reduced winter annual and early spring germinating weeds.
Summer Annual Weed Density and Dry Weight
Cover crops had inconsistent effects on summer annual weeds. There was a year x location x cover interaction for summer annual weed density and a year x cover interaction for weed dry weight (Table 3). Weed density was reduced following Santiago medic compared with the no-cover treatment in 1995 at EL, but there was no effect in other site years. Weed density following red clover was reduced only in 1996 at EL, whereas berseem clover had no effect on summer annual weed density or dry weight.
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The effect of cover crops on summer annual weeds was more pronounced on dry weight than on density. Dry weight of summer annual weeds following annual medics was reduced in 1995 compared with the no cover treatment (Table 3). However, this effect was not observed in 1996. Aboveground growth of medics was greater in 1994–1995 at EL than in 1995–1996, and this may have contributed to the difference between years (Fisk, 1997). Dominant weeds in 1995 included common lambsquarters (Chenopodium album L.), redroot pigweed (Amaranthus retroflexus L.), giant foxtail (Setaria faberi Herrm.), large crabgrass [Digitaria sanguinalis (L.) Scop.], and smooth crabgrass [Digitaria ischaemum (Schreb. ex Schweig.) Schreb]. In 1996, however, the dominant weed species were common purslane (Portulaca oleracea L.), Pennsylvania smartweed (Polygonum pensylvanicum L.), and barnyardgrass [Echinochloa crusgalli (L.) Beauv.]. In 1995, the dry weight of pigweed was lower in Santiago medic and Mogul medic treatments (P 0.05). Our results are similar to the finding of Yenish et al. (1996), who reported that biomass of weeds sampled 45 d after corn planting was reduced by winter annual legume cover crops.
It has been suggested that weed biomass may be less influenced than weed density by the residue of over-wintering cover crops (Teasdale, 1996), because weeds will compensate for lower density by increasing biomass. In our study, such effects were not observed. This may be because of the mechanisms of annual legumes to limit weed growth, which at this point are unknown. However, reduction in weed growth may have resulted from allelopathic chemicals released by the legumes or from microbial metabolic activity on the residue (Worsham, 1991).
Effect of Cover Crop Residue on Summer Annual Weeds
Annual medic residues reduced the density and dry weight of summer annual weeds (Table 4). Data for all cover crop species were combined over year and location because there was no year x location by cover interaction. Summer annual weed density and dry weight were consistently reduced from 27 to 60% following annual medics and red clover (Table 4). Berseem clover residue, however, had no effect on weed density or dry weight. Some differential response of weed among cover species was observed. Density of common lambsquarters was lower in Santiago and Mogul medic treatments (P 0.1). Dry weight of common purslane (P 0.05), Pennsylvania smartweed, and barnyardgrass (P 0.1) was lower in the Santiago medic treatment. Pennsylvania smartweed density was lower in the Mogul medic treatment (P 0.05) and the Santiago treatment (P 0.01). Density of giant foxtail was lower in the red clover treatment (P 0.05).
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Effect of Cover Crops on Perennial Weeds
Cover crops had no effect on density but did affect dry weight of perennial weeds before corn planting (Table 5). For the first sampling date, data for weed density were combined across all site years since no interaction was found. However, an interaction occurred for weed dry weight so data are presented by site year. The failure of cover crop residues to affect weed density was expected because most of the weeds present before corn planting probably established in the fall when the cover crops were just beginning to grow. However, dry weight of perennial weeds was reduced by 30 to 75% following most of the cover crop treatments when compared with the no-cover control (Table 5). Dominant perennial weeds included broadleaf plantain (Plantago major L.), dandelion (Taraxacum officinale Weber in Wiggers), white clover (Trifolium repens L.), and quackgrass [Elytrigia repens (L.) Nevski]). Among perennial weed species, dandelion dry weight was most consistently impacted. Dandelion dry weight was lower in Mogul medic and red clover treatments in 1995 at EL (P 0.05), in Santiago (P 0.01), and red clover (P 0.05) at EL in 1996, and in Santiago (P 0.05), red clover and berseem clover (P 0.01) at KBS in 1996. Reduced dry weights of perennial weeds may have resulted from the competition for resources during the establishment of cover crops or allelopathic chemicals released from cover crop residues.
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0.05). In 1996 at EL dry weight of dandelion was less in the Santiago treatment than in the control (P 0.01). Previous research has found little effect by hairy vetch residue, at common planting rates, on the density of perennial weeds such as dandelion, curly dock (Rumex crispus L.), and quackgrass (Mohler and Teasdale, 1993; Curran et al., 1994). Our results, however, indicated that a potential of annual medics to reduce perennial weed dry weight prior to corn planting existed and an ability to reduce weed density during corn growth existed.
Perennial weed density and dry weight were 35 and 75% lower, respectively, following cover crops when residue was left on the soil surface compared with where it had been removed (Table 7). Data were combined over site years for both weed density and dry weight since no interactions were observed. Similar to annual weeds, the impact of the cover crops on perennial weeds was more pronounced when measured in adjacent plots within treatments than by comparing cover treatment with a no cover control.
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In our study, annual legumes reduced weed density and dry weight in many cases. However, any effect this may have had on corn grain yield was not assessed because of the application of a postemergence herbicide after the final sampling. Studies on weed–crop competition have demonstrated that the relative time of weed emergence with respect to the crop is as, or more, important than weed density in predicting the impact on corn yield (Knezevic et al., 1994; Bosnic and Swanton, 1997). Bosnic and Swanton (1997) reported that barnyardgrass at 39 plants m-2 reduced corn yield by 14% when they emerged at the 3-leaf corn stage as compared with 4% at the 7-leaf stage. In Michigan, Fausey et al. (1997) reported that corn grain yields were reduced by up to 14% by 13 giant foxtail plants m-2 germinating 2 d after corn emergence.
We cannot use these examples to predict the effect of weeds in our study because we do not have data on time of emergence. However, weed densities in this study were in the same range as those in the cited studies where yields were reduced when germination was close to corn emergence. Other researchers have found cover crops reduced weeds, but not enough to eliminate the need for chemical control (Yenish et al., 1996; Teasdale, 1996; Curran et al., 1994; Johnson et al., 1993).
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Dry weight of perennial weeds before corn planting was consistently reduced by both annual legumes and red clover; however, density was unaffected. The annual medics suppressed perennial weeds 45 to 60 d after chemical kill, but this effect was not as strong as the effect observed before corn planting. Residue of all legumes reduced both density and dry weight of perennial weeds. This study indicated an excellent potential for annual medics to reduce weed density and growth in no-till corn grain systems. Further research is needed to quantify if chemical control can be reduced or eliminated by the use of annual legume cover crops.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication June 9, 2000.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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