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a The Norwegian Crop Research Institute, Plant Protection Centre, Dep. of Herbology, N-1432 Aas, Norway
lars.brandsater{at}planteforsk.no
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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The classical attempts to reduce competition in cover crop systems have focused on chemical or mechanical suppression of mulch growth or screening for less competitive cover crops. Reducing interference between a white clover (Trifolium repens L.) living mulch and sweet corn (Zea mays L. var. saccharata) by chemical suppression or mechanical suppression has been reported by Vrabel (1983) and by Grubinger and Minotti (1990), respectively. Reduced interference by mechanical suppression of white clover and subclover living mulch in white cabbage (Brassica oleracea var. capitata L.) is reported by Brandsæter et al. (1998). Nicholson and Wien (1983) screened turfgrasses and clovers to find less competitive mulches for sweet corn and cabbage.
A third way to avoid or decrease the competition in such systems is to intercrop a main crop and a cover crop with a synchronized onset of maximum vegetative growth. This synchronization of cover crop and main crop could be achieved in different ways. Müller-Schärer and Potter (1991) concluded that cover plants should be seeded to emerge in the middle of the vegetation period of the main crop, e.g., in field-planted leek (Allium porrum L.). De Haan et al. (1994) have studied the opposite way to avoid interference problems in cover crop systems in the north central region of the USA. They tried to develop a spring seeded smother plant that had been selected for its ability to suppress weeds without affecting crop yield. This cover crop flowered 3 wk after emergence and began senescence 5 wk after emergence (D.L.Wyse, personal communication). Another approach suggested by Ilnicki and Enache (1992) was to use winter annual legumes, e.g., subclover as living mulch. Winter annual legumes sown in late summer grow vegetatively during autumn, become "dormant" in winter, and resume vegetative growth the following spring. Later in the spring or early summer the plant flowers, senesces, and dies. Because of this unique life cycle, a main crop transplanted into the senescencing mulch would be able to use all available water and nutrients. Experiments done by Ilnicki and Enache (1992) in New Jersey, USA, with subclover living mulch showed excellent weed control and no yield depression in corn, sweet corn, cabbage, snap beans (Phaseolus vulgaris L.), and tomato (Lycopersicum esculentum Mill.). Subclover has the ability to reestablish naturally from seed produced in the spring. Ilnicki and Enache (1992) introduced the idea of growing subclover as a continuous cover crop without the need for seeding every year. A second approach to use a winter annual legume as a cover crop was suggested by Abdul-Baki and Teasdale (1993). This approach calls for sowing the cover crop, e.g., hairy vetch, in the autumn and mowing it immediately before transplanting the main crop in the spring.
The experiments of Ilnicki and Enache (1992) and Abdul-Baki and Teasdale (1993) have shown that winter annual legumes are very suitable as cover crops. A requirement for developing cover crop systems with winter annual legumes is to find species and cultivars which are adapted for the local climate and latitude. To avoid competition with the main crop in the subclover cropping system suggested by Ilnicki and Enache (1992), it is essential to use species and cultivars which have a low canopy height and terminate vegetative growth early in the summer. Additionally, the ability to reestablish naturally in the autumn from seeds produced in the spring and early summer would be a useful property. In the hairy vetch cropping system suggested by Abdul-Baki and Teasdale, to avoid yield depression it is of vital importance that cover crop regrowth after mowing is limited. In this cropping system, considerable biomass is needed to suppress weeds after mowing. Rapid ground cover growth in the autumn and at the start of vegetative growth in the spring is important to prevent weed infestation in these winter annual legume cropping systems. Therefore, this study was initiated to investigate the following aspects of winter annual legumes: winter hardiness, biomass accumulation and time of termination of vegetative growth, canopy height, regrowth ability after mowing, time of flowering and senescence, ability for rapid ground coverage, and weed suppression.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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s (59°40'N, 10°46'E), at Jæren (58°47'N, 5°41'E), and at Løken Research Station (61°7'N, 9°4'E). The trial locations were similar in a 2-yr running second experimental series (1994-1995 and 1995-1996), except that the latter location was replaced by Apelsvoll Research Centre, Division Landvik, Grimstad (58°20'N, 8°31'E) and Jæren was omitted in the second year. Løken was chosen for its continental climate which was in contrast to those of Jæren and Landvik which have typical coastal climates. s is characterized with a continental climate moderated by its proximity to the Oslofjord. Figure 1 shows monthly average and normal temperatures (averages of 1961–1990) of daily mean temperatures for the experimental locations for each year.
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s and a 3 by 10 factorial at Jæren and Løken. Five sowing dates at s, and three sowing dates at Jæren and Løken, composed the main treatments. Nine subclover cultivars (Daliak, Esperance, Geraldton, Green Range, Karridale, Mount Barker, Nuba, Seaton Park, and Woogenellup) and one red clover (Trifolium pratense L.) cultivar (Molstad) were the subplot treatments. The main plot and subplot dimensions were 6.00 by 6.65 m and 1.33 by 3.00 m, respectively. The cover crops were sown on the following dates: (i) 12 May, (ii) 14 June, (iii) 15 July, (iv) 5 August, and (v) 26 August. The trials located at Jæren and Løken were sown only on the three latter dates.
Seeding rates for the subclover and the red clover were 27 and 20 kg ha-1, respectively. The seeds were mixed with quartz sand to obtain a uniform distribution. The mixture was broadcasted by hand. The seeds were worked into the soil and irrigated, if necessary, to obtain uniform establishment. Plots were hand weeded for weed control. Soil classification and pH were as follow: s, loam-sandy loam (160 g kg-1 clay, 320 g kg-1 silt, and 520 g kg-1 sand) and pH 6.3; Jæren, peat (209 g kg-1 organic matter) and pH 5.4; and Løken, loam (80 g kg-1 clay, 430 g kg-1 silt, and 490 g kg-1 sand) and pH 5.1. Fertilizer rate ha-1 was 25 kg N, 7 kg P, and 20 kg K. Fertilizer was broadcasted one day prior to sowing.
Vegetative Growth during Autumn and Spring.
Biomass of all cover crops was assessed on the following dates in the autumn 1993 and spring 1994, respectively: Løken on 23 to 24 September and 5 May, s on 1 to 5 October, and 15 April, and Jæren on 7 October and 1 April. The biomass measurements were carried out by cutting and collecting all the aerial biomass within two quadrats of 0.25-m2 randomly placed in each subplot. The cutting procedure was done with a scissors and cutting to a stubble height of 2 to 3 cm. The biomass was oven-dried for 72 h at 60°C and then weighed.
Soil Coverage and Canopy Height.
The space occupied by living and dead cover crop mulch, as indicated by percentage ground coverage, was visually estimated on the total sub plot area (1.30 by 3.00 m). The area covered by living and dead mulches and area not covered by mulch, added up to 100%. The same person at all sites and dates estimated the ground coverage. Visual estimation of crop and weed coverage is frequently used in scientific experiments (Miller and Callihan, 1995; Weber et al., 1995; Brandsæter et al., 1998). At s, the assessments were done every second week during the summer and autumn in 1993, at Jæren and Løken only on the dates of biomass measurements. On the same dates, canopy height was measured at 5 different positions in each plot and the means were used in the statistical analyses.
Flowering. For all sowing dates at s, date of flowering onset was assessed on 10 randomly harvested plants from each plot. On the same plants, the number of flowering plants were counted 60, 75, and 90 d after sowing.
Experiment 2: Winter Annual Legumes
General.
The trial design was a randomized complete block design with three replications. The plot dimensions were 1.50 by 5.00 m and 3.50 by 4.00 m for the first and second year, respectively. The cover crops were sown on 15 to 19 Aug. 1994 and 8 to 10 Aug. 1995. The legumes were sown in areas previously used for cereal and vegetable production, except for the Jæren location in which the preceding crop was grass. However, early in the season in the year of sowing the legumes, the area was mechanically fallowed.
The experiments included subclover, hairy vetch, crimson clover, black medic, white clover, and red clover. Red clover and white clover, two commonly used legumes in Norway, were included as control species for evaluation of winter hardiness. Cultivar names and the location of the species are given in Table 1 and Table 2 . All of the assessments mentioned, including vegetative growth during autumn, spring and early summer, cover crop coverage, canopy height, time of flowering, and weed growth in the plots were carried out the first year (1994-1995). For the second year (1995-1996), only biomass and cover crop ground coverage in autumn and spring were assessed.
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The soil textures and pH were as follow: s, loam-sandy loam (160 g kg-1 clay, 320 g kg-1 silt, and 520 g kg-1 sand) and pH 6.1; Jæren, texture not determined (73 g kg-1 organic matter) and pH 5.7; Grimstad, loamy sand (100 g kg-1 clay, 150 g kg-1 silt, and 750 g kg-1 sand) and pH 6.0. Only the s location was fertilized. The fertilizer rate was 25 kg N, 7 kg P and 20 kg K ha-1.
Vegetative Growth during Autumn, Spring, and Early Summer.
For the 1994-1995 experiment, cover crop biomass was measured once in autumn and four times during spring and early summer at 3-wk intervals. The spring and early summer regrowth assessments were measured 3 wk later. Dates of the assessments are shown in Fig. 2
. Biomass measurements for the 1995-1996 experiment were carried out in autumn, 21 to 27 September, and spring, 26 to 28 May. The biomass measurements were carried out by cutting and collecting all the aerial biomass above a stubble height of 2 to 3 cm within four rectangles of 0.125 m2 randomly placed in each subplot with a scissors. The biomass was oven-dried for 72 h at 60°C and then weighed.
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To measure the potential for reestablishment from seeds of the winter annual legumes, the ground coverage was, for both living and dead plant material, visually estimated in early August, early September, and early October at the s and Grimstad locations.
Weeds.
In the autumn of 1994, weed emergence was recorded on the same dates and in the same rectangles as for the cover crop biomass assessment. Additionally, weed emergence and total weed dry weight were recorded four times during spring and early summer. These measurements were carried out on the same dates and in the same rectangles as for the cover crop biomass assessments. The weed biomass was oven-dried for 72 h at 60°C and then weighed.
Statistical Considerations
Analysis of variance was carried out in both experiments. The assumptions for normality and variances underlying the statistical analyses were tested for all response variables. To stabilize variance, some response variables were transformed (see Results). If the statistical analyses were carried out with transformed values, it is mentioned in the tables or figures. A testing procedure (SAS Institute Inc., 1992) for detecting outliers was performed. Newman-Keuls multiple tests (SNK multiple-stage test) were used for comparing treatments (SAS Institute Inc., 1988).
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Establishment in the Autumn
In the subclover experiment, no significant differences in ground coverage appeared at any sowing date between the cultivars (data not presented). All cultivars sown on the first three dates completely covered the ground by late autumn. When sown on 5 August, the coverage of the cultivars varied between 70 and 80%. The coverage on plots sown on 26 August was about 10% for all the cultivars. The cultivar biomass production in autumn for each location are presented in Table 3
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s and Løken on 15 July or later, none of the cultivars flowered in late autumn and they appeared as winter annuals. Even then they were not winter hardy enough to stand the winter at these locations, and all cultivars, independent of sowing time, were dead in the spring (Table 3). At Jæren, some flowering plants of Geraldton and Daliak sown on 15 July appeared in late autumn and none of these two cultivars survived winter as expected. The four cultivars Green Range, Nuba, Woogenellup, and Esperance showed some ability to survive (see Table 3). However, even at Jæren the winter hardiness was not very good that year. The statistical analyses of biomass at springtime showed a significantly higher biomass for the red clover cultivar than all the subclover cultivars. Because of a lack of seed, the cultivar Mount Barker, which has been reported to be one of best cultivars for winter-survival in USA (Enache, 1989) was not sown in late July at Jæren.
Table 1 shows that hairy vetch cultivar AU Early Cover was completely dead in the spring of 1994 at all locations. Both subclover cultivars were completely dead in the spring at s. Only some individual plants of black medic were still alive at Landvik and Jæren. Species–cultivars, location, and the interaction between the two factors appeared significant in the statistical analyses of dry weight in spring. Because of the interaction between species–cultivar and location, separate statistical analyses were carried out for each location. At all three locations, hairy vetch Welta in spring showed a significantly higher biomass dry weight than all the other species–cultivars. At s, no significant differences appeared between black medic, white clover, and red clover. At Landvik, crimson clover showed a significantly higher biomass dry weight than subclover Mount Barker. Biomass production of subclover Nuba was higher than for Mount Barker. However, the difference was not significant. At Jæren no significant differences between crimson clover and subclover cultivars appeared. Subclover Nuba exhibited a higher biomass dry weight than crimson clover, contrary to the results at Landvik.
Because of the poor performance of subclover at s, this species was omitted in the experiment in 1995-1996. Table 2 shows that all species survived at s; however, black medic did rather poorly. Redclover showed the highest biomass, followed by hairy vetch, white clover, and black medic. The statistical analyses showed significant differences between all species. Subclover and crimson clover this year were completely dead in the spring at Landvik. Black medic and also, rather surprisingly, hairy vetch showed poor winter survival at Landvik. The difference in hairy vetch in winter survival between Landvik and s showed that winter temperature is not the only important factor for winter survival. White clover was the only species that showed satisfactory survival at Landvik this year.
The overall results showed the following ranking of biomass in spring as an expression of winter hardiness among the winter annual species: hairy vetch Welta (best)>black medic>crimson clover>subclover (poorest). Hairy vetch seemed to be the species best suited for cold climates; its fitness in terms of winter survival seems not to differ much from red clover and white clover. Crimson clover and the most winter hardy cultivars of subclover seemed to have some potential for winter survival; however, they are only suitable for the south and southwest coasts of Norway. It is important to take into consideration that the winter of 1993-1994 was rather normal and the winter of 1994-1995 was warm (see Fig. 1), but the exceptionally cold winter of 1995-1996 killed both species.
The conclusion of winter hardiness is to some degree supported by the results of a growth chamber freezing experiment carried out by Brandsæter (1996). In this experiment the relative root lengths after freezing of black medic, hairy vetch, and crimson clover were generally greater than the subclover cultivars. However, the differences were not significant for all comparisons. Surprisingly, the frost hardiness of hairy vetch AU EarlyCover was as good as for Welta in the growth chamber experiment. This cultivar showed promising winter hardiness, better than subclover and crimson clover, in experiments in North America (J.R. Teasdale, personal communication). A response to day length, as a result of degree of latitude, could be an explanation for the loss of winter hardiness under Norwegian field conditions. That means that this early cultivar may reached flower initiation because of early sowing and thereby may lose its frost hardiness.
Biomass and Ground Coverage Development in Early Summer
Only species–cultivars which exhibited satisfactory winter survival in the 1994-1995 experiment will be further presented (see Fig. 2–5)
and analyzed. In cover crop systems, including mowing the cover crop, it is important to obtain as thick a layer of plant residues as possible. At s, hairy vetch exhibited a significantly higher biomass dry weight than white clover on all sampling dates. The differences between these species were considerable (see Fig. 2a and b). Because of big differences in biomass production at Landvik and Jæren and a significant interaction between these two locations and species, the analyses were carried out separately. At Landvik, the biomass dry weight of hairy vetch was significantly higher than for crimson clover and subclover on the three sampling dates, except for crimson clover on 9 July. Furthermore, the biomass of crimson clover was significantly higher than for subclover on all sampling dates, except 9 July. At Jæren, the biomass of hairy vetch was highest followed by crimson clover and subclover on all sampling dates. However, the difference between hairy vetch and crimson clover was significant only on the first sampling date. Biomass of hairy vetch was significantly higher than for subclover on all sampling dates. The difference between crimson clover and subclover was not significant at any date.
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s and the statistical analyses of ground coverage of the interaction between species–cultivars and location gave a significant difference, the results from the two former locations were analyzed together in the 1994-1995 experiment. Separate analysis of the results from s (Fig. 3) showed that the ground cover in late autumn was significantly better in plots with the hairy vetch Welta than in plots with white clover. Hairy vetch also developed a significantly better ground cover than all the other species–cultivars at Landvik and Jæren (Fig. 3). No significant differences appeared between subclover Nuba and crimson clover. The cover crops covered the ground significantly more rapidly at Landvik than at Jæren. At s, the ground cover in early May of hairy vetch was significantly higher than for white clover. At Landvik and Jæren, hairy vetch performed significantly better than the two other species. Figure 3 shows that hairy vetch had covered the ground by late May at all locations. Complete coverage for white clover at s was attained about 30 d later. Crimson clover and subclover Nuba at Landvik developed quite similar to or a little later than hairy vetch. At Jæren, these two species showed less ability for rapid ground cover development. This was probably because of competition from Poa annua L. (discussed later). Later in the summer, the growth habit of crimson clover at both locations showed a more open canopy than the other cover crops, and the ground cover decreased.
Cover Crop Regrowth
The effect on regrowth of cutting–mowing the cover crops is presented in Fig. 5. From these data, it can be concluded that cutting date was of vital importance in relation to regrowth ability of subclover and crimson clover. Hairy vetch had generally very little or no regrowth, regardless of mowing time. At Landvik, the ability for regrowth decreased for crimson clover with delayed mowing in spring–early summer. Subclover showed a peak of regrowth when mowed on 21 May. At Jæren, subclover and crimson clover showed a similar peak of regrowth ability when mowed in late May. At Landvik, the regrowth of hairy vetch was significantly lower than for crimson clover and subclover at all cutting dates, except for crimson clover on 14 June. No significant difference appeared between crimson clover and subclover, except on 21 May (see Fig. 5d and e). At Jæren, the regrowth of subclover was significantly higher than for the two other species at the two first dates. Regrowth of hairy vetch at s was significantly lower than for white clover on the last two dates. White clover clearly showed an increased ability for regrowth in summer compared to spring.
All the species in the experiments, except black medic and particularly subclover, have high canopies and must be mowed before transplanting crop plants. To avoid competition, it is important that the species have very little or no ability for regrowth after mowing. Hairy vetch showed very little regrowth ability, especially after mid-May. Hence, this species is a very promising cover crop. Subclover and crimson clover showed more regrowth, but also for these species the ability decreased in mid-June. White clover showed significantly higher regrowth during summer than the winter annual species and would consequently compete heavily with the main crop.
Termination of Vegetative Growth
An early termination of vegetative growth could be important in cover crop systems to avoid competition. In the experiments, three different assessments or estimates were made for the determination of termination of vegetative growth: (i) date of flowering, (ii) biomass accumulation between the last two sampling dates, and (iii) ground coverage of dead cover crop material.
In 1994-1995, subclover (both cultivars) started flowering in mid-May at Landvik and Jæren. Crimson clover and hairy vetch flowered a little later, in the beginning of June and mid-June, respectively, at these locations. Hairy vetch and black medic flowered in late June at s. The latter species flowered throughout the summer (data not shown).
The biomass accumulations (i.e., differences between biomass in mid-June and mid-July) averaged for the Landvik and Jæren locations were crimson clover 176 g m-2, subclover 157 g m-2, and hairy vetch 110 g m-2. However, only differences between locations, which indicated that biomass accumulation was greater at Landvik than at Jæren, appeared significant in the statistical analyses. Significant differences were found between hairy vetch (565 g m-2) and white clover (213 g m-2) at s.
In Fig. 3, the appearance of dead plant material originating from the winter annual legumes is also shown. Analysis of differences between species and locations was based on the ground cover assessments in mid-July. Hairy vetch, which was the only winter annual legume at s, had not begun senescence at this time. At Landvik and Jæren differences between species and locations appeared significant. There was no interaction between the two factors. The ground cover of dead plant material averaged over locations varied from 37% (subclover Nuba) to 15% (hairy vetch). Subclover Nuba was not significantly different from crimson clover (31%). Subclover and crimson clover were significantly different from hairy vetch. Subclover and crimson clover were completely dead, and hairy vetch was almost dead in early August at Landvik (see Fig. 3). Because black medic did not show senescence during summer at all (data not presented), it is reasonable to expect competition problems.
Canopy Height
In the subclover experiment, the early flowering cultivars, Daliak and Geraldton, generally exhibited the lowest biomass production and the lowest canopy height (data not presented). The maximum canopy heights, averaged over location in the 1994-1995 experiment, during summer were hairy vetch, 53 cm; crimson clover, 55 cm; subclover, 18 cm; and white clover, 37 cm.
Reestablishment
The winter annual species showed a very interesting ability for natural reestablishment from seeds in the second autumn. Exploiting this ability is probably most interesting in the use of subclover. Figure 3 shows that all these species, especially crimson clover and subclover, were reestablished in the autumn of 1995 at Landvik. Hairy vetch also showed some ability for reestablishment at s.
Weed Suppression
In the 1994-1995 experiment, the cover crop weed suppression ability was analyzed by measuring weed biomass dry weight in mid-July. The biomass of the dominant and minor weed species in hairy vetch, crimson clover, and subclover Nuba at Landvik and Jæren, and in hairy vetch and white clover at s, is presented in Fig. 4. Chamomilla suaveolens (Pursh) Rydb. and Matricaria perforata Merat, were the dominant weed species at s. For Landvik and Jæren, respectively, C. suaveolens and P. annua were the dominant species.
At Landvik and Jæren, differences between species, location, and the interaction between the factors appeared significant in the statistical analyses of total weed biomass (dry weight). At Landvik, the total weed biomass was significantly higher in plots with subclover than in plots with crimson clover.
At Jæren, there was significantly less total weed biomass in plots with hairy vetch than in plots with subclover or crimson clover. No significant difference in total weed biomass was found between subclover and crimson clover.
Figure 4 shows that C. suaveolens and M. perforate were abundant in plots with white clover at s. Only negligible amounts of weeds were growing in plots with hairy vetch. Total weed biomass was significantly higher in plots with white clover than in plots with hairy vetch at s.
The ranking of the species–cultivars in ability to cover the ground in the 1994-1995 experiment was as follows: hairy vetch (most rapid), subclover and crimson clover, and white clover (slowest). Weeds emerging in plots with hairy vetch showed very limited survival. Even winter annual weed species, germinating at the same time as the cover crops, developed very poorly in hairy vetch plots. Also at Jæren, the weed suppression by hairy vetch was better than with subclover and crimson clover, but even hairy vetch did not suppress a dense stand of P. annua effectively. Because undisturbed plants of hairy vetch and crimson clover have high canopies, they must be mowed. If an early mowing is required, the weed suppression ability of these species could be reduced. At all locations, there was very little or no germination of weed species in the springtime. However, winter annual weed species growing in plots with subclover and crimson clover, and particularly white clover, must be mowed in springtime.
Suitability as Cover Crops
The main obstacle for the utilization of subclover as a cover crop in Norway is the lack of winter hardiness. Even the most winter hardy cultivars in these experiments can only survive near the south and southwest coasts of Norway. More experiments are needed to clarify the winter hardiness of this species, but utilization in the southern coastal area of Norway should not be completely excluded. To avoid competition in cropping systems necessitating a directly transplanted crop into a living mulch, an early termination of vegetative growth and low canopy height are important. Subclover is the only species among those used in these experiments which has a promising potential in such a cropping system. However, the use of subclover as a living mulch is not without problems. As Ilnicki and Enache (1992) concluded, it may be necessary to mow excessive growth of the subclover to reduce early competition with some crops. However, the second field experiment showed that the regrowth of subclover, after mowing, can be relatively large. Another technique may be to strip till or chemically kill or suppress vegetative growth in the row, prior to, or after, transplanting the crop. Strip tilling in the row would probably cause weed problems in the row. In cabbage, in-row weeds could be controlled by selective flaming (Netland et al., 1994). As the experiments have shown, it may also be necessary to mow weeds growing between rows. The need for mowing to suppress weeds in subclover is also pointed out by other authors (e.g., Lanini et al., 1989). This mowing could be carried out once or twice prior to transplanting the crop. Weed suppressing effects of subclover are probably insufficient if perennials and grass weeds dominate the weed flora.
Hairy vetch is too tall and terminates vegetative growth too late in the summer to be utilized as a living mulch directly. Hairy vetch has to be converted, e.g., by mowing, into an organic mulch, before transplanting the crop. To prevent competition, it is important that the plants do not continue growth after mowing and the present results showed that regrowth of hairy vetch was negligible and less than for the other species. Curran et al. (1994) found in a 2-yr experiment that the control of hairy vetch after mowing was excellent when the vetch flowered early and the weather was warm and dry. However, in the second year, in which the spring was cool and wet, and the hairy vetch was in the late vegetative stage of development, the vetch resumed growth after mowing. After mowing the cover crop, it is important to obtain as thick a layer of plant residues as possible. Almeida de. (1985) found that cover crops producing more than 450 g (dw) m-2 prior to mowing or a chemical treatment were likely to be the most effective for reducing the weed infestation. In the experiment presented here, hairy vetch exceeded this production in early summer and showed the biggest biomass accumulation of the species compared. To obtain satisfactory amounts of plant residues, it could be necessary to delay the date of transplanting or sowing the main crop. Hairy vetch Welta showed the best winter hardiness of the winter annual legume species screened in the experiments. However, even for the cultivars of this species, more experiments are needed to clarify the winter hardiness characteristics.
Because of canopy height and time of the end of vegetative growth, it seems adequate to use crimson clover in the same manner as hairy vetch. However, this species has some drawbacks relative to hairy vetch. First, this species covered the ground later after sowing and the growth habit was more open. Second, the biomass accumulation throughout the whole life cycle seemed to be less than for hairy vetch. Consequently, the weed suppression effect before mowing was not as good as for hairy vetch. Because of lower rates of organic mulch, it seems reasonable to assume that weed suppression after mowing would not be as good as for hairy vetch. Differences between the species in allelopathic potential or speed of degradation of residues may modify this aspect, however. Third, a large degree of regrowth after mowing in the spring and early summer is likely to cause more competition problems. Fourth, winter hardiness is poorer and the use of this species is probably limited to locations with mild winter climates.
Because black medic showed rather poor winter survival this species is not discussed here.
The present results have shown that white clover covered the ground more slowly than the winter annual legumes. Additionally, vegetative growth continued through the whole summer. Numerous studies have shown that yield depression caused by competition is a serious problem in living mulch systems with perennial legumes (Brandsæter et al., 1998; Nicholson and Wien, 1983; Vrabel, 1983). The need has been reported for reducing interference in such cropping systems by chemical suppression (Vrabel, 1983) and by mechanical suppression (Brandsæter et al., 1998; Grubinger and Minotti, 1990).Almeida de. 1985
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ACKNOWLEDGMENTS |
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Received for publication June 19, 1998.
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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s: Agricultural University of Norway, 1996.This article has been cited by other articles:
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L. O. Brandsæter, T. Smeby, A. M. Tronsmo, and J. Netland Winter Annual Legumes for Use as Cover Crops in Row Crops in Northern Regions: II. Frost Resistance Study Crop Sci., January 1, 2000; 40(1): 175 - 181. [Abstract] [Full Text]
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Vadose Zone Journal | |||
| Journal of Plant Registrations | Soil Science Society of America Journal | ||||
| Journal of Natural Resources and Life Sciences Education |
Journal of Environmental Quality |
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