This chapter addresses the complexity of managing cover crops in selected growing regions of the world. It focuses on the contributions that cover crops can make to weed management and the trade-offs that may be required between achieving weed management, crop production, and environmental benefits. Since cover crops can play a significant role in mitigating environmental impacts worldwide, interactions between weed management and management to enhance environmental protection are emphasized.

SummaryThe success of weed management aimed at depleting the regenerative structures of perennial weeds depends largely on the sprouting activity of rhizome and root buds. Seasonal variation in sprouting of these buds on Cirsium arvense, Sonchus arvensis and Elymus repens was studied for plants collected from Denmark, Finland, Norway and Sweden. At 2-week intervals from July to October, 5-cm fragments of roots or rhizomes were cut from plants grown in buckets and planted into soil in pots, half of which were placed immediately into growth chambers at 18°C for 4 weeks. The other half of the pots were initially placed in a dark room at 2°C for 4 weeks before being transferred to the same growth chamber, also for 4 weeks. During the growth chamber period, the numbers of emerged shoots in each pot were counted weekly. The sprouting activity of C. arvense and E. repens was relatively uniform during this period and bud dormancy was not apparent. In all ecotypes of S. arvensis, innate bud dormancy developed during the latter part of the growing season. For all three species, differences in sprouting readiness were found among ecotypes. The results imply that C. arvense and E. repens are more likely to be controlled by mechanical measures in autumn than S. arvensis.

and Wien (1983) screened turfgrasses and clovers to find less competitive mulches for sweet corn and cabbage. Weed suppression is one of several benefits achieved by includingA third way to avoid or decrease the competition in a cover crop in a cropping system. A requirement for developing cover crop systems is to find species and cultivars which are adapted such systems is to intercrop a main crop and a cover for the local climate and latitude, prevent weed infestation, and do crop with a synchronized onset of maximum vegetative not compete with the main crop. Two experimental series were estabgrowth. This synchronization of cover crop and main lished at three locations in Norway to evaluate the ability of different crop could be achieved in different ways. Mü ller-Schä rer species for use as cover crops in vegetable production. The first series and Potter (1991) concluded that cover plants should of experiments was with subclover (Trifolium subterraneum L.) cultibe seeded to emerge in the middle of the vegetation vars sown at different dates throughout the summer and the second period of the main crop, e.g., in field-planted leek (Alwith different winter annual legume species sown in late summer. lium porrum L.). De Haan et al. (1994) have studied Winter hardiness, life cycle, growth characteristics, and weed suppresthe opposite way to avoid interference problems in sion ability of winter annual legumes sown in the autumn were studied. The experiments showed the following ranking of winter hardiness: cover crop systems in the north central region of the hairy vetch (Vicia villosa Roth.) (best), black medic (Medicago lupu-USA. They tried to develop a spring seeded smother lina L.), crimson clover (Trifolium incarnatum L.), and subclover plant that had been selected for its ability to suppress (poorest). Time of flowering and senescence, factors related to compeweeds without affecting crop yield. This cover crop flowtition in cover crop systems, showed that subclover was the most ered 3 wk after emergence and began senescence 5 wk promising species followed by crimson clover and hairy vetch-black after emergence (D.L.Wyse, personal communication). medic. The winter annual legume species showed the ability for naturalAnother approach suggested by Ilnicki and Enache reestablishment in the second autumn. Hairy vetch showed the highest (1992) was to use winter annual legumes, e.g., subclover biomass production, the lowest regrowth ability after mowing, and as living mulch. Winter annual legumes sown in late

Weed suppression is one of several benefits achieved by including a cover crop in a cropping system. A requirement for developing cover crop systems is to find species and cultivars which are adapted for the local climate and latitude, prevent weed infestation, and do not compete with the main crop. Two experimental series were established at three locations in Norway to evaluate the ability of different species for use as cover crops in vegetable production. The first series of experiments was with subclover (Trifolium subterraneum L.) cultivars sown at different dates throughout the summer and the second with different winter annual legume species sown in late summer. Winter hardiness, life cycle, growth characteristics, and weed suppression ability of winter annual legumes sown in the autumn were studied. The experiments showed the following ranking of winter hardiness: hairy vetch (Vicia villosa Roth.) (best), black medic (Medicago lupulina L.), crimson clover (Trifolium incarnatum L.), and subclover (poorest). Time of flowering and senescence, factors related to competition in cover crop systems, showed that subclover was the most promising species followed by crimson clover and hairy vetch–black medic. The winter annual legume species showed the ability for natural reestablishment in the second autumn. Hairy vetch showed the highest biomass production, the lowest regrowth ability after mowing, and the best weed suppression. Subclover showed a very promising life cycle for the use as living mulch; however, more winter hardy cultivars or species with similar growth characteristic are needed before common commercial use in northern regions. In conclusion, until more suitable species or cultivars are found or developed through plant breeding, hairy vetch is the most promising winter annual cover crop under Norwegian conditions.

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The purpose of these experiments was to study the effects of various experimental procedures and conditions on bioassay sensitivity in allelopathic studies. The following factors were considered: bioassay methods, choice of test species, the effect of osmotic potential on germination and growth of the test species, growth in dark or in light and the effect of concentrating the extracts in vacuo. Extracts from rape and rye were used in the studies to act as allelopathic agents. Ryegrass appeared as the most sensitive test species when grown horizontally on quartz sand, while radish was the most sensitive test species when grown on filter paper in transparent boxes at a 45° angle. Root length was the most sensitive growth variable measured. Percent germination appeared a more sensitive measure than the speed of germination index, "S", when germinated seeds were counted after one and two days for radish and ryegrass, respectively. A rise in the osmotic potential affected germination more than root length, and radish appeared more sensitive to a rise in osmotic potential than ryegrass. A confounding of germination and root length inhibition may therefore, give misleading results.

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