All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. F orage radish is a new winter annual cover crop in the Mid-Atlantic region. Early work with forage radish as a cover crop in this region included observations that it could provide dramatic fall and spring weed suppression (Weil and Kremen, 2007). However, questions remain about the repeatability, amount, and duration of this suppression as well as the diversity of weed species aff ected. Answering these questions could lead to the development of integrated weed management strategies that reduce the use of herbicides while providing other soil and environmental benefi ts. Radish cover crops are members of the Brassicaceae family and behave diff erently than cover crops currently grown in the Mid-Atlantic region, such as rye (Secale cereale L.), oat (Avena sativa L.), winter wheat (Triticum aestivum L.), crimson clover (Trifolium incarnatum L.), and hairy vetch (Vicia villosa Roth) (Holderbaum et al., 1990; Maryland Department of Agriculture, 2009; Weil and Kremen, 2007). Current cover crop species are winter annuals that grow more slowly in the fall and produce most of their biomass in the spring. Th ey must be terminated in the spring before subsequent crop planting and develop higher C/N ratios when left to grow longer in the spring. Th us they require timely management to reduce the potential for delayed planting and immobilization of N needed by the subsequent crop. Unlike the above-mentioned cover crops, forage radish emerges quickly and grows rapidly in the fall (Weil et al., 2009). Th is may help inhibit undesirable winter annual weed growth. It also has a large white fl eshy tap root that may protrude aboveground as much as 10 to 15 cm. Forage radish is sensitive to frost and typically winter-kills with prolonged exposure to temperatures below-4ºC (Weil et al., 2009). Forage radish roots and shoots have a low C/N ratio. Its residues decompose rapidly during the freeze-thaw cycles that characterize winters in the Mid-Atlantic region and leave little residue on the soil surface the following spring. As a result, a unique low residue and weed-free seed bed is created for planting in the early spring following forage radish cover crops. Th ese characteristics could make forage radish cover crops useful for farmers who are interested in the benefi ts of fall cover crops but want to avoid excessive spring crop residues or for organic farmers who wish to reduce preplant tillage without the use of prohibited herbicides. Few studies have described weed suppression by radish cover crops. In the Netherlands, fodder radish (Raphanus sativus L. cultivar Brutus) suppressed the growth of weeds while it grew in the fall (Kruidhof et al., 2008). Oilseed radish (Raphanus sativus L. var. oleiformis) suppressed the fall growth of volunteer winter wheat in Ont...

Little is known about the mechanism of winter annual weed suppression by forage radish (Raphanus sativus L. variety longipinnatus) winter cover crops. Previous studies suggest that allelopathy from decomposing residue and competition due to rapid canopy development contribute to weed suppression by other Brassica cover crops. Four contrasting experimental approaches were used to identify the mechanism of weed suppression by forage radish cover crops. Results of a fi eld based cover crop residue-transfer experiment supported the hypothesis that fall cover crop weed competition is the dominant mechanism of weed suppression following forage radish cover crops. A high level of early spring weed suppression was observed where forage radish grew in the fall regardless of whether residues were left in place or removed. In contrast, there was limited weed suppression in bare soil treatments that received additions of forage radish tissues. Bioassays using cover crop amended soil or aqueous extracts of cover crop tissues and amended soil did not reveal any allelopathic activity limiting seed germination or seedling establishment. In a fi eld-based weed seed bioassay, forage radish cover crops did not inhibit emergence of winter-planted weed seeds relative to a no cover crop control. Forage radish amended soils stimulated seedling growth of lettuce (Lactuca sativa L.) in all types of bioassays. Th e results of the four experiments in this study point to a common conclusion that fall weed competition is the dominant mechanism for early spring weed suppression following forage radish winter cover crops.

The use of killed cover crop mulch for weed suppression, soil erosion prevention and many other soil and crop benefits has been demonstrated in organic no-till or zero-till farming systems in eastern US regions and in Canada. Implements have been developed to make this system possible by terminating cover crops mechanically with little, if any, soil disturbance. Ongoing research in the US northern Great Plains is being conducted to identify cover crop species and termination methods for use in organic zero-till (OZ) systems that are adapted to the crop rotations and climate of this semi-arid region. Current termination strategies must be improved so that cover crop species are killed consistently and early enough in the growing season so that subsequent cash crops can be grown and harvested successfully. Delaying termination until advanced growth stages improves killing efficacy of cover crops and may provide weed-suppressive mulch for the remainder of the growing season, allowing no-till spring seeding of cash crops during the next growing season. Excessive water use by cover crops, inability of legume cover crops to supply adequate amounts of N for subsequent cash crops and failure of cover crops to suppress perennial weeds are additional obstacles that must be overcome before the use of killed cover crop mulch can be promoted as a weed control alternative to tillage in the US northern Great Plains. Use of vegetative mulch produced by killed cover crops will not be a panacea for the weed control challenges faced by organic growers, but rather one tool along with crop rotation, novel grazing strategies, the judicious use of high-residue cultivation equipment, such as the blade plow, and the use of approved herbicides with systemic activity in some instances, to provide organic farmers with new opportunities to incorporate OZ practices into their cropping systems. Emerging crop rotation designs for organic no-till systems may provide for more efficient use of nutrient and water resources, opportunities for livestock grazing before, during or after cash crop phases and improved integrated weed management strategies on organic farms.

For more than a decade, studies have aimed to adapt the agronomy of organic no-till systems for the environmental conditions of Eastern Canada. Most research on organic no-till practices in Eastern Canada has been conducted in the province of Québec, where 4% of farms are certified organic, and results from these trials have been published in technical reports available in French. The objective of this review was to revisit previous research work on organic farming in Eastern Canada-the majority of which has been published as technical reports in the French language-in order to highlight important findings and to identify information gaps. Cover crop-based rotational no-till systems for organic grain and horticultural cropping systems will be the main focus of this review. Overall, a few trials have demonstrated that organic rotational no-till can be successful and profitable in warmer and more productive regions of Eastern Canada, but its success can vary over years. The variability in the success of organic rotational no-till systems is the reason for the slow adoption of the system by organic farmers. On-going research focuses on breeding early-maturing fall rye, and terminating cover crops and weeds with the use of bioherbicides.

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