Summer Cover Crops Research Articles

No-till with high biomass cover crops and invasive legume mulches increased total soil carbon after three years of collard production

ABSTRACTStudies have shown that conversion to conservation tillage from conventional tillage does not increase total soil organic carbon (SOC) significantly in the short term (<5 yrs). While no-till increases total SOC in the medium to long term, it was hypothesized that the inclusion of high biomass cover crops and organic mulches may increase total SOC in the short term. The use of invasive, perennial leguminous species as mulch may sustainably increase SOC during limited-input fall vegetable production. The objective of this study, conducted in Alabama (USA), was to quantify total SOC changes due to organic mulches and forage soybean (Glycine max (L.) Merr. cv. Derry) as a summer cover crop after conversion to no-till during limited-input fall collard (Brassica oleracea L.) production. Forage soybean as a summer cover crop did not increase SOC. No-till without mulch (control treatment) significantly increased SOC at 0–5 cm from 6.3 to 14.0 g kg−1 soil in 3 yrs, whereas inclusion of cut-and-carry mulches increased SOC to ≥22.6 g kg−1 soil. Treatments did not affect collard yield, which averaged 17,863 kg ha−1 yr−1. This represents a novel, limited-input system that may help control on-farm invasive species while sustainably increasing SOC in the short term.

Summer Cover Crops and Lettuce Planting Time Influence Weed Population, Soil Nitrogen Concentration, and Lettuce Yields

Cover crops can be used as a sustainable weed management tool in crop production systems. Cover crops have the ability to suppress weeds, reduce soil erosion, increase soil organic matter, and improve soil physical, chemical, and biological properties. In the north-central region of the United States, including Iowa, much cover crop research has been conducted in row crop systems, mainly with corn (Zea mays) and soybean (Glycine max) where cover crops are planted at the end of the growing season in September or October. There is little information available on the use of cover crops in vegetable cropping systems, particularly on the use of summer cover crops for fall vegetable production. The choice of the cover crop will significantly impact the entire fall vegetable production enterprise. Vegetable growers need information to identify the right cover crop for a particular slot in the cropping system and to understand how cover crops would affect weed suppression, soil properties, and successive vegetable crop yield. The time interval between cover crop termination and vegetable planting critically affects the growth and successive yield of the vegetable crop. This study investigated how short-duration summer cover crops impact weed suppression, soil properties, and ‘Adriana’ lettuce (Lactuca sativa) yield. The study also examined appropriate planting times of lettuce transplants after soil incorporation of cover crops. The experimental design was a randomized complete block split-plot design with four replications. Whole plots consisted of cover crop treatments: ‘Mancan’ buckwheat (Fagopyrum esculentum), ‘Iron &amp; Clay’ cowpea/southernpea (Vigna unguiculata), black oats (Avena strigosa), ‘Grazex II’ sorghum-sudangrass (Sorghum bicolor ssp. drummondii), and a control (no-cover crop) where weeds were left to grow unchecked. The subplot treatment consisted of two lettuce transplanting times: planted immediately or 8 days after cover crop soil incorporation. Fall-planted butterhead lettuce was used. Data were collected on cover crop biomass, weed biomass, soil nutrient concentration, lettuce growth, and yield. All cover crops significantly reduced weed biomass during the fallow period as compared with the control treatment. Highest degree of weed suppression (90% as compared with the no-cover crop control treatment) was provided by buckwheat. Southernpea, a legume, increased soil nitrogen (N) concentration and contributed to higher lettuce yield and improved quality. Southernpea also enhanced lettuce growth and led to an earlier harvest than other treatments. Sorghum-sudangrass showed evidence of detrimental effects to the marketable lettuce crop. This was not due to N immobilization but presumably due to alleopathic properties. There is no clear pattern within any cover crop treatment that lettuce planting time following cover crop termination affects plant growth; however, planting early or soon after cover crop incorporation ensures more growing degree days and daylight, thus leading to timely harvest of a higher quality product. This study demonstrates that cover crops can successfully be integrated into vegetable cropping systems; however, cover crop selection is critical.

The Influence of Bahiagrass, Tillage, and cover crops on Organic Vegetable Production and Soil Quality in the Southern Coastal Plain

&lt;p&gt;Conventional farming utilizing bahiagrass (Papsalum notatum Flugge) in rotation with crops has been shown to increase yield, improve soil quality, and decrease weed and disease pressure. Organic production systems in the Southern Coastal Plain are challenged with limited soil fertility and a wide array of insect, disease, and weed pests. The purpose of this study was to investigate the influence of sequential years in bahiagrass and tillage (conventional and conservation) on organic vegetable yield and soil indices. After 0-4 years in bahiagrass, a crop rotation of rye and oats (winter cover crop), bush beans (spring vegetable crop), soybean (summer cover crop), and broccoli (fall vegetable crop) was implemented. Vegetable crop yields, plant biomass, plant C and N, and soil C, N, and P were measured for the four crops in the rotation over a three year period. Two years or more of bahiagrass prior to initiating the vegetable crop rotation showed positive effects on vegetable crop yields and soil quality parameters. Tillage treatments did not have a consistent effect on measured parameters. Soil C was not impacted by years in bahiagrass but was influenced by years of crop production. Potential soil N and P mineralization indicated an increase of soil organic fractions with years in bahiagrass. Available N increased after cover crops, and available P decreased with increasing years in bahiagrass.&lt;/p&gt;

Cover Crops, Fertilizer Nitrogen Rates, and Economic Return of Grain Sorghum

Leguminous cover crop systems have been envisaged as a critical component of sustainable agriculture because of their potential to increase soil productivity by cycling C and N in agricultural systems. Our objectives were to: (i) determine the effects of including summer cover crops and N rates in the cropping system on the growth and yield of the succeeding grain sorghum [Sorghum bicolor (L.) Moench] crop, (ii) calculate the N fertilizer replacement value (NFRV) and evaluate economic returns, and (iii) determine the most cost‐effective cropping system. Field experiments were conducted for two seasons in Kansas. Leguminous summer cover crops and double‐cropped soybean [Glycine max (L.) Merr.] residues improved soil N availability; N was subsequently used by the succeeding crop. Across years and cropping systems, the mean increases in grain yield as a result of including cowpea [Vigna unguiculata (L.) Walp.], pigeonpea [Cajanus cajan (L.) Millsp.], sunn hemp (Crotalaria juncea L.), double‐cropped soybean, and double‐cropped grain sorghum in the rotation compared with a fallow system with 0 kg N ha−1 were 56, 62, 43, 32, and 3%, respectively, and NFRVs across the years were 53, 64, 36, 27, and –3 kg N ha−1, respectively. Across years, grain sorghum in a double‐cropped soybean system and a fallow system with 90 kg N ha−1 gave profitable economic net returns. We conclude that including leguminous cover crops in a cropping system has the potential to reduce N requirements and improve the N availability and grain yield of the succeeding grain sorghum crop.

Response of Maize to Cover Crops, Fertilizer Nitrogen Rates, and Economic Return

Leguminous cover crops are considered part of sustainable agricultural systems. With the development of no‐till cropping systems, cover crops have been recognized for their ability to provide N for succeeding crops. The objectives of this study were: (i) to determine the N contribution of summer cover crops and double‐cropped grain crops following winter wheat (Triticum aestivumL.) and N rates to subsequent maize (Zea maysL.) crops’ physiological traits and yield, (ii) to calculate the fertilizer N replacement value, and (iii) to perform economic analyses of the cropping systems. Field experiments were conducted during the 2012–2013 and 2013–2014 growing seasons in Kansas. The grain yield of maize in all the cover crop systems and the double‐cropped soybean [Glycine max(L.) Merr.] system was similar to maize in the fallow system with 45 kg N ha−1. The mean increase in grain yield as a result of including cowpea [Vigna unguiculata(L.) Walp.], pigeonpea [Cajanus cajan(L.) Millsp.], sunn hemp (Crotalaria junceaL.), double‐cropped soybean, and double‐cropped grain sorghum [Sorghum bicolor(L.) Moench] in the rotation over the fallow system with 0 kg N ha−1was 78, 91, 66, 72, and 12%, respectively. Fertilizer N replacement values for cowpea, pigeonpea, sunn hemp, double‐cropped soybean, and double‐cropped grain sorghum were 53, 64, 43, 47, and –5 kg N ha−1, respectively. We conclude that the inclusion of summer leguminous cover crops in a cropping system has the potential to reduce or supplement N requirements and increase the grain yield of subsequent maize crops.

Buckwheat Species as Summer Cover Crops for Weed Suppression in No-Tillage Vegetable Cropping Systems

Buckwheat is a broadleaved annual species that is often used as a summer cover crop for its quick growth, weed suppressive ability, and ease of management. Tartary buckwheat is a species related to buckwheat, with many of the same traits valued in buckwheat as a cover crop. However, Tartary buckwheat has been reported to grow more vigorously than buckwheat, especially in cool conditions, which might fill a unique niche for vegetable farmers in Wisconsin and other northcentral states. Our research objectives were to determine the effectiveness of Tartary buckwheat relative to buckwheat for weed suppression, both during the cover-cropping phase and after cover-crop termination during cabbage production, and quantify weed suppression, soil compaction, soil nitrogen availability, and cabbage yield in no-tillage (roller-crimped or sickle-bar mowed) and conventional-tillage (rototilled) systems. Across three site-years, we found that buckwheat emerged earlier and produced 64% more shoot dry biomass than Tartary buckwheat. Pretermination weed shoot biomass (predominantlyAmaranthusandSetariaspp.) in Tartary buckwheat treatments was approximately twice that of buckwheat, and did not differ from weed shoot biomass in a control fallow treatment. Cabbage yield did not differ between cover crop species nor did yield differ between conventional-tillage cover cropped and control fallow treatments. However, weed biomass was greater, and cabbage yield was reduced, in no-tillage compared to conventional-tillage treatments. We also found evidence of greater soil compaction and less nitrate–nitrogen (NO3–N) availability in no-tillage than conventional-tillage treatments. These results suggest that Tartary buckwheat is not a suitable summer cover crop alternative to buckwheat for weed suppression prior to cabbage production.

Economic Viability and Environmental Impact Assessment of Three Different Strawberry Production Systems in the Southeastern United States

In this study, we investigate the economic viability and environmental impact of three different soil management systems used for strawberry (Fragaria ×ananassa) production in the southeastern United States: 1) a conventional production system that is based on the current production practices implemented by growers, 2) a nonfumigated compost system with summer cover crop rotations and beneficial soil inoculants, and 3) an organic production system that includes practices approved for use under the National Organic Program (NOP). Under our assumptions, all three systems resulted in positive net returns estimated at $14,979, $11,100, and $19,394 per acre, respectively. The nonfumigated compost system and organic system also both resulted in considerable reductions in negative environmental and human health impacts measured by a set of selected indicators. For example, the total number of lethal doses (LD50) applied per acre from all chemicals used in each system and measuring acute human risk associated with each system declined from 118,000 doses/acre in the conventional system to 6649 doses/acre in the compost system and to 0 doses/acre in the organic system. Chronic human health risk, groundwater pollution risk, and fertilizer use declined as well in the compost and organic systems as compared with the conventional system.

Introduction of various cover crop species to improve soil biological P parameters and P uptake of the following crops

Field experiments were conducted to clarify whether the introduction of several cover crop species increases P uptake of the following wheat and soybean. Four summer cover crops (sorghum, buckwheat, groundnut and crotalaria) and four winter cover crops (oat, rye, vetch and lupin) were tested. Growth and P uptake of succeeding wheat were significantly increased by P fertilizer application and tended to be increased by sorghum, groundnut or crotalaria incorporation, whereas buckwheat did not show positive effects. Growth and P uptake of succeeding soybean were significantly increased by oat or vetch incorporation and tended to be increased by P fertilizer or other cover crop incorporation. These positive effects of cover crops were attributed to the large amount of P incorporation, increase in the P-solubilizing fungal population and/or biomass P in soil. Sorghum, oat, rye and vetch were thought to be suitable cover crops to accelerate P uptake of the following crops since a large amount of P would be incorporated. Sorghum, groundnut and lupin were thought to be suitable cover crops because they increased the indigenous P-solubilizing fungal population in soil. Soil biomass P correlated with P uptake of wheat. Incorporation of suitable cover crops as a P source and activation of indigenous soil microorganisms by the carbon supply were thought to have accelerated P uptake of the following wheat and soybean. It is therefore thought that introduction of suitable cover crops could be an effective means to reduce P fertilizer application for the following crops.

Can conservation trump impacts of climate change on soil erosion? An assessment from winter wheat cropland in the Southern Great Plains of the United States

With the need to increase crop production to meet the needs of a growing population, protecting the productivity of our soil resource is essential. However, conservationists are concerned that conservation practices that were effective in the past may no longer be effective in the future under projected climate change. In winter wheat cropland in the Southern Great Plains of the U.S., increased precipitation intensity and increased aridity associated with warmer temperatures may pose increased risks of soil erosion from vulnerable soils and landscapes. This investigation was undertaken to determine which conservation practices would be necessary and sufficient to hold annual soil erosion by water under a high greenhouse gas emission scenario at or below the present soil erosion levels. Advances in and benefits of agricultural soil and water conservation over the last century in the United States are briefly reviewed, and challenges and climate uncertainties confronting resource conservation in this century are addressed. The Water Erosion Prediction Project (WEPP) computer model was used to estimate future soil erosion by water from winter wheat cropland in Central Oklahoma and for 10 projected climates and 7 alternative conservation practices. A comparison with soil erosion values under current climate conditions and conventional tillage operations showed that, on average, a switch from conventional to conservation tillage would be sufficient to offset the average increase in soil erosion by water under most projected climates. More effective conservation practices, such as conservation tillage with a summer cover crop would be required to control soil erosion associated with the most severe climate projections. It was concluded that a broad range of conservation tools are available to agriculture to offset projected future increases in soil erosion by water even under assumed worst case climate change scenarios in Central Oklahoma. The problem is not one of a lack of effective conservation tools, but one of adoption and implementation. Increasing the implementation of today’s conservation programs to address current soil erosion problems associated with the large year-to-year climate variability in the Southern Great Plains would greatly contribute towards mitigation of projected future increases in soil erosion due to climate change.

Eficiência de espécies de adubos verdes sobre a população do nematoide reniforme

The objective of this study was to evaluate the growing of soil improving crops on the population of Rotylenchulus reniformis in naturally infested soil. It was evaluated the effect of 6 species of plants as cover crops in winter and 13 summer species and a fallow treatment on the nematode population under greenhouse. After 60 days, the root system was collected. Then, a sample of soil was taken in order to extract juveniles from the soil and quantification the final population of the pathogen in each pot for determining of the reproduction factor (RF). Fallow and all winter species of green manure, except hairy vetch, reduced the population of R. reniformis after cultivation in infested soil, in comparison to the control. Regarding summer cover crops, it was observed that sorghum ‘SI03204’ (Sorghum vulgare), millet ‘BRS1501’ (Pennisetum glaucum), Brachiaria ruziziensis, finger millet (Eleusine coracana), estylo ‘Campo Grande’ (Stylosanthes capitata x S. macrocephala), peanut ‘IAC Tatu ST’ (Arachis hypogaea) and dwarf velvet bean (Mucuna deeringiana) reduced the population of R. reniformis, when compared to the control, could be used in the management of this nematode.

US-1136, US-1137, and US-1138 Cowpea Lines for Cover Crop Use

US-1136, US-1137, and US-1138 are new cowpea lines with vigorous, indeterminate growth habits that were released by the Agricultural Research Service of the U.S. Department of Agriculture in 2010 and are recommended for use as a cover crop. Cowpeas are valued as a summer cover crop (Clark, 2007), because they thrive in hot moist environments, grow well in low-fertility soils, and their vigorous growth smothers weeds. They can fix up to 220 kg·ha nitrogen and are an excellent source of nitrogen for fallplanted crops. Resistance of cover crop cultivars to soilborne diseases is necessary to avoid increasing populations of pathogens and plant–parasitic nematodes that induce disease in rotational crops. Many indeterminate cowpea genotypes produce seeds with impermeable seedcoats (hard seeds). The major attributes of lines US-1136, US-1137, and US-1138 have rapid growth, high biomass production, a long vegetative growth period, and southern root knot nematode [Meloidogyne incognita (Chitwood) Kofoid and White] resistance, and they do not produce seeds with impermeable seedcoats.

Extreme Grain-Based Cropping Systems: When Herbicide-Free Weed Management Meets Conservation Tillage in Northern Climates

The challenges associated with the adoption of conservation tillage and/or low-input cropping systems, whether organic or herbicide-free, across Canada are shaped by scale, environment, and local practices. A study in eastern Canada captured the challenges of introducing low-input cropping systems in mature (20+ yr) tillage treatments in a barley/red clover/corn/soybean rotation. Each mature tillage system came with its own weed problems, but this did not affect crops such as barley and red clover, which produced similar yields across high and low input systems. However, some form of primary tillage was needed to achieve adequate weed control and yield in organic (ORG) and herbicide-free (HF) systems. The HF and ORG systems with no-till actually failed to produce a corn crop but produced soybean yields that were half or less than that for other treatments. The successful combination of conservation tillage practices and low-input systems in eastern Canada would thus appear to be crop-specific, being easier to achieve in competitive cereal crops. In western Canadian organic agriculture, tillage is practiced with low-disturbance chisel plows instead of inversion plows. However, green manure crops (summer cover crops) are often terminated with tandem discs. Both roller crimpers and mowing can successfully kill annual green manure crops such as field pea and rye, and usually result in reduced weed growth following termination. However, the lack of tillage can result in lower crop yields in wheat following green manure terminated by roller crimping compared to tillage. Challenges for no-till organic practices include perennial weed control and soil fertility. Overall, flexible crop production programs such as the former Manitoba Pesticide Free Production program and the “Agriculture raisonnéeTM” program in Québec are more likely to promote sustained environmental, economic, and social prosperity than rigid adherence to organic or no-till practices.

Soil microbial communities associated with the rhizosphere of cucumber under different summer cover crops and residue management: A 4-year field experiment

Continuous cropping has led to the predominant decline of soil productivity and crop yields in Chinese vegetable production systems. Recently, the use of summer cover crops to restore degraded soil during vegetable production has attracted attention in China. A 4-year field experiment on a greenhouse cucumber double-cropping system was therefore conducted to understand how different summer cover crops affect soil microbial communities associated with the crop rhizosphere. The treatments included sweet corn with residue removal after harvest (SR), sweet corn with residue incorporation after harvest (SI), common bean with residue removal after harvest (CR), common bean with residue incorporation after harvest (CI), Garland chrysanthemum and edible amaranth as summer cover crops (GR), and bare fallow during the summer period (Control). Soil microbial communities were evaluated with a combination of sole-carbon-source utilization capacities, the population of microorganisms and enzymes. Generally, sole-carbon utilization capabilities and enzyme activities were higher in the summer cover crop-related soils compared to control soil in each cropping season. Multivariate analysis indicated that microbial communities differed among the cropping seasons, and significantly separated summer cover crop treatments from the control. Treatments SR, SI and GR with relatively high cucumber productivity generally showed relatively high urease activity and relatively low Fusarium oxysporum population. We concluded that summer cover crops generally had a greater effect on soil microbial communities associated with crop rhizosphere compared to the traditional bare fallow. Soil microorganisms associated with crop rhizosphere played an important role during the phytoremediation processes of summer cover crops.

The effect of total carbon on microscopic soil properties and implications for crop production

Soil structure is a dynamic property affected by physical, chemical, and microbiological processes. Addition of organic matter to soils and the use of different management practices have been reported to impact soil structure and crop production. Moderation in soil temperature and increases in microbial activity and soil water retention are often suggested as reasons for the rise in crop yield when organic matter is added to the soil. Less is known about the direct effect of changes in soil structure on crop production. A field experiment was conducted to study the effect of summer cover crop and in-season management system on soil structure. The experiment was a nested design with summer cover crop as the main plot and management system as the subplot. Summer cover crop treatments included cowpea (Vigna unguiculata L. Walp.) incorporated into the soil in the fall (CI), cowpea used as mulch in the fall (CM), sudangrass (Sorghum vulgare) incorporated into the soil in the fall (S), and dry fallow or bare ground (B). Management systems were organic (ORG) and conventional (CNV) systems. Lettuce (Lactuca sativa L.) and cantaloupes (Cucumis melo L.) were cultivated in rotation in the plots for three consecutive years using the same cover crops and management systems for each plot. Disturbed and undisturbed soil cores were collected at the end of the third year and used for laboratory experiments to measure physical, chemical, and hy- draulic properties. Image analysis was used to quantify soil structure properties using a scanning electron micro- scope on thin sections prepared from the undisturbed soil cores. We found that total soil carbon was correlated with porosity, saturation percentage, and pore roughness. Pore roughness was correlated with crop production in gen- eral and with marketable production in particular. We found that the higher the complexity of the pore space, the more water retained in the soil, which may increase soil water residence and reduce plant water stress.

Cover Crops in Mono- and Biculture for Accumulation of Biomass and Soil Organic Carbon

Plant biomass production associated with soil organic carbon (C) accumulation is a critical challenge for sustainable agriculture development because soil quality degradation and organic carbon pool depletion have become a concern under some circumstances. To elucidate cover crops and their synergetic effects in biomass production and soil organic C improvement, legume and non-legume winter/summer cover crops, in mono- and biculture (mixture of legume/non-legume) were evaluated in the field and controlled (growth chamber) conditions. Under field conditions, biculture of sunn hemp and sorghum sudangrass produced 24.1 Mg ha−1 in contrast to 20.1 and 2.9 Mg ha−1 for each one alone; and biculture of okra and cowpea reached as much as 11.5 Mg ha−1 in contrast to 2.0 and 5.3 Mg ha−1 for each one in monoculture. After the growth of winter followed by summer cover crops, the soil organic C content increased with substantial quantities of plant biomass returned to the soil. The results suggest that both summer and winter cover crops demonstrate a promising potential in biomass C accumulation, thereby, can play an important role in soil fertility improvement to benefit the sustainable development of agriculture when appropriate types and combinations are selected.

Summer Cover Crops Fix Nitrogen, Increase Crop Yield, and Improve Soil–Crop Relationships

Impact of cover crops (CCs) on winter wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] yields is not well understood. We assessed crop yield and its relationships with CC‐induced changes in soil properties for a 15‐yr CC experiment in wheat–sorghum rotation at 0, 33, 66, and 100 kg ha−1 of N application in south central Kansas. Hairy vetch (Vicia villosa Roth) was used as a winter CC from 1995 to 2000, while sunn hemp (SH; Crotalaria juncea L.) and late‐maturing soybean [LMS; Glycine max (L.) Merr.] were used as summer CCs in no‐till from 2002 to 2008. Summer CCs increased crop yields particularly at low rates of N application. At 0 kg N ha−1, SH increased sorghum yield by 1.18 to 1.54 times, while wheat yield increased by 1.60 times in the first year (2004) after CC establishment relative to non‐CC plots. At 66 kg N ha−1, SH had no effects on sorghum yield, but it increased wheat yield in 2 of 3 yr. Cover crops increased soil total N pool by 270 kg ha−1 for the 0‐ to 7.5‐cm depth. Crop yield increased with the CC‐induced decrease in soil maximum compactibility (soil's susceptibility to compaction) and soil temperature, and increase in soil aggregate stability, soil organic carbon (SOC) and total N concentration, and soil water content, particularly at 0 kg N ha−1. Principal component analysis (PCA) selected soil compactibility and total N as the best yield predictors. Inclusion of summer legume CCs in no‐till fixes N, increases crop yield, and improves soil–crop relationships.

Allelopathic Effects of Sunnhemp (Crotalaria juncea L.) on Germination of Vegetables and Weeds

Sunnhemp (Crotalaria juncea L.) is a tropical legume that could be an important summer cover crop in the southeastern United States, but it has the potential for suppressing both crops and weeds. Allelopathic effects of sunnhemp on weeds, vegetable crops, and cover crops were evaluated in greenhouse and growth chamber experiments. In the greenhouse, ground dried sunnhemp residues (applied mixed with the soil at 1.6% w/w) reduced percent germination of lettuce (Lactuca sativa L.) and smooth pigweed (Amaranthus hybridus L.) to a similar degree as that caused by cereal rye (Secale cereale L. subsp. cereale) residues (applied at 1.5% w/w). The allelopathic activity of sunnhemp was greater in the leaves than in the roots or stems. In growth chamber studies, the mean reduction in germination (relative to the control) caused by sunnhemp leaf aqueous extracts was: bell pepper (100%), tomato (100%), onion (95%), turnip (69%), okra (49%), cowpea (39%), collard (34%), cereal rye (22%), sweet corn (14%), Austrian winter pea (10%), crimson clover (8%), cucumber (2%), and winter wheat (2%). In lettuce, carrot, smooth pigweed, and annual ryegrass, sunnhemp aqueous leaf extract reduced seedling length to a degree similar as that produced by rye aqueous leaf extract. Sicklepod [Senna obtusifolia (L.) H.S. Irwin &amp; Barneby CA] germination was not inhibited by any of the sunnhemp or rye aqueous extracts. In conclusion, sunnhemp reduced the germination percentage and seedling growth of various crop species. The allelochemical activity in sunnhemp was primarily in the leaves and remained active at least 16 d after harvest under dry conditions. Sunnhemp's allelochemical effect may be a useful attribute for weed management in sustainable production systems. However, plant growth in the field in crops such as bell pepper, tomato, onion, and turnip may be impacted as a result of allelopathic activity of sunnhemp residues. Thus, weed management may be more effective when sunnhemp is grown in rotation with crops that tolerate the allelochemicals from sunnhemp, resulting in optimization of the rotation effects.

夏作カバークロップの生育と多変量解析による特性分類の検討

本研究は，9種類のカバークロップの生育，有機物供給量及び，養分吸収量，有機栽培との適応性を明らかにすることで多変量解析によるカバークロップの総合評価を行った．得られた結果は以下のとおりである．1）カバークロップの乾物重をみると，6月ではエンバク，シロガラシの生育量が大きく，エンバクが 7.5t/ha, シロガラシが 8.5t/ha の乾物重を示し，その他のカバークロップと比べて有意差がみられた．また，7月以降ではソルガムの乾物重が著しく増加し，8月には，31.0t/ha の乾物収量を得た．2）カバークロップの吸収養分特性をみると，多くのカバークロップで有機物生産量が増加するにしたがって養分吸収量も増加することが認められた．3）カバークロップ後作のブッロコリの地上部収量をみるとクロタラリアで 33t/ha の最も高い値を得たのに対し，ワイルドフラワーの混播ではクロタラリアの 6分の 1の収量にとどまった．4）カバークロップのもつ特性について，多変量解析のうちの主成分分析とクラスター分析を適用し客観的な評価を行った結果，カバークロップの特性は生育時期によって大きく異なり，バイオマス増加に伴う養分吸収とカバークロップの CN 比の変化によって主成分分析の結果は変化し，農業者がカバークロップを導入する際の最適な作物の選択と栽培期間の決定するための基礎的なデータとなることが示唆された．

Influence of Summer Cover Crops and Mycorrhizal Fungi on Strawberry Production in the Southeastern United States

The effects of eight summer cover crop treatments combined with two arbuscular mycorrhizal (AM) fungal inoculants on strawberry growth and yields were examined in a 2-year field experiment. Cover crop treatments included 1) sudangrass [Sorghum bicolor (L.) Moench cv. Piper]; 2) pearl millet [Pennisetum glaucum (L.) R.Br. cv. 102 M Hybrid]; 3) soybean [Glycine max (L.) Merrill cv. Laredo]; 4) velvetbean [Mucuna deeringiana (Bort) Merr. cv. Georgia Bush]; 5) sudangrass/velvetbean combination; 6) pearl millet/soybean combination; 7) a non-mycorrhizal host consisting of rape (Brassica napus L. var. napus cv. Dwarf Essex) and buckwheat (Fagopyrum esculentum Moench) in Year 1 and Year 2, respectively; and 8) no cover crop control. Strawberry tips were inoculated with either a native mixture of several AM fungal species or a single species sold commercially, Glomus intraradices. Cover crop treatments were assessed for their aboveground biomass and nutrient uptake as well as their impacts on weed abundance and diversity, soil nutrients, and parasitic nematode populations. Cover crop and AM treatments were assessed for their impact on strawberry growth, yields, AM root colonization, and nutrient uptake. Grass-based cover crop treatments, particularly pearl millet, produced the most aboveground biomass. In both years, all cover crop treatments reduced summer weed biomass compared with the control. Neither cover crop nor AM treatments had an effect on overall strawberry plant growth or yields in either year, although some differences existed at specific growth periods. The results suggest that cover crops are a viable strategy for reducing summertime weeds and that background, native populations of AM fungi in the soil may be just as effective as a commercially available species. It is likely that no overall yield benefit was found among treatments for two reasons: 1) nutrients, especially nitrogen, were not limiting; and 2) the cover crop growth window may have been too short for a significant impact on strawberries over two seasons.

Cover Crop Residue and Organic Mulches Provide Weed Control during Limited-Input No-Till Collard Production

Limited input producers may adopt no-till production if sufficient weed suppression can be achieved. High-biomass producing cover crops used in conjunction with organic mulches may provide sufficient weed control in no-till vegetable production. Our objective was to quantify weed suppression from a forage soybean summer cover crop and three types of organic mulches applied after collard (Brassica oleracea L.) planting. Forage soybean residue did not suppress weeds, but mulches were generally effective. Broadleaf and sedge weeds decreased in population size over the three-year period, but grass weed management remained problematic until three years after conversion to no-till. Grass suppression was greater when mulches were applied after the first year. Collard yield, averaging 17,863 kg ha−1, was not affected by any cover crop or mulch treatment.