Weed control crop rotation

Performance and market share for competitive herbicide products Since the USEPA Special Review was initiated in 1994, acres treated with various corn herbicides have been carefully monitored. No clear alternative has proved to be a possible replacement to atrazine. Several facts are noteworthy. No corn herbicide introduced between 1994 and 2000 reached a 10% market share, nor did the market share increase for putative atrazine replacements like 2,4-D or bromoxynil (Table 13.6). These new products suffered from one or more of the following limitations limited spectrum of weeds controlled, crop injury potential, or rotational restrictions. Meanwhile, atrazine s total market share remained constant at approximately 70%. To date market retention has been poor when new herbicides are used alone. Virtually all com herbicides introduced since 1994 are used with atrazine, and the percentage of acreage treated in combination with atrazine is increasing (Table 13.6). 

Nonchemical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program (458,459). In the context of modern intensive chemical herbicide appHcation, nonchemical practices may even represent an innovative approach to weed management and should receive careful consideration. 

A sound crop rotational system is of basic importance to the organic farmer. It reduces the risk of diseases and pests associated with monoculture, gives better control of weeds, spreads the labour requirements more evenly over the year, reduces the financial risk if one crop yields or sells badly and provides more interest for the farmer. For instance, take-all is a disease that cannot even be controlled chemically. Winter crops are more susceptible to take-all than spring sown crops and wheat is more susceptible than barley, so the safest position for winter wheat is immediately after a break crop. 

Allelopathic interactions of plants can provide weed control by (a) use of allelopathic crops as cover crops, mulches or green manure, (b) use of allelopathic plants in crop rotations, (c) crop mixtures and intercropping, (d) varieties with strong allelopathic potential, and (e) use of allelopathic crop water extracts and other agents. 

Crop Rotation with Allelopathic Crops to Control Weeds... 

Rasmussen IA (2004) The effect of sowing date, stale seedbed, row width and mechanical control on weeds and yields of organic winter wheat. Weed Res 44 12-20 Reddy KC (2001) Effects of cereal and legume cover crop residues on weeds yield, and net return in soybean (Glycine max),. Weed Tech 15 660-668 Reynolds LB, Potter JW, Ball-Coelho BR (2000) Crop rotation with Tagetes sp. is an alternative to chemical fumigation for control of root-lesion nematodes. Agron J 92 957-966 Rice EL (1984) Allelopathy. Academic, New York, p 317... 

In permanent crops a higher species diversity can be attained by applying a cover crop rotation for weed control. This is reported for organie olive production (Kabourakis 1996). 

Allelopathic crop plants have already been used experimentally in weed control. Leather (70) found one of thirteen genotypes of the cultivated sunflower tested to be very allelopathic to several weeds. In a 5-year field study with oats and sunflower grown in rotation, the weed density was significantly less than in controi piots with oats only. 

Different types of management may be developed for weed, insect, and disease control, for crop rotation, and for agroforestry employing allelopathy. In the following paragraphs these are discussed separately. 

The move into continuous cereal production and away from mixed farming with firm rotations meant that farmers could cultivate high value crops on all available land without the need for fallow. This could not have been achieved without the use of chemical herbicides. The movement of people from the land to the urban environment during the industrial revolution and thereafter reduced the available labour for hand weeding and weed control became a real problem for the farmer. The revolution was the introduction of 2,4-D and MCPA that allowed broad-leaved weeds to be controlled selectively in the previously dirty cereal crops. 

Rasmussen, LA., Askegaard, M. and Olesen, J.E. (1999). Weed control in organic crop rotations for grain production. In Gut, D. (ed.) Proceedings 11th EWRS Symposium. Basel, Switzerland. European Weed Research Society (EWRS), Wageningen. p. 98. 

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