Weeds control

There are no approved chemical methods to control weeds in organic crops. Growers have to rely primarily on preventive or physical measures to maintain weed control. These measures include rotation, timing of cultivation, undersowing and the use of mulches. Physical methods may also be used and these include hand weeding, the use of mechanical techniques such as brush hoes and thermal destruction. 

The impact of weed control in organic production must not be underestimated. In vegetable production particularly, weed control will often pose more of a challenge than pest or disease control. It is the cost of controlling weeds that can have a significant effect on the economics of organic vegetable production. 

The previous crop has a significant effect on weed infestation of strawberry plots. For weed control the soil is either left bare or covered. 

COVERING THE SOIL. Covering the soil with mulch films or organic material is an alternative to leaving the soil bare. In high-rainfall areas with heavy soil, however, this system tends to lead to waterlogging. 

A layer of straw is placed beneath the strawberry plants to make sure that the fruits stay clean and to prevent the spread of grey rot and rhizome rot [Phytophthora cactorum). In addition the straw conserves soil moisture (this can also be a disadvantage), suppresses weeds and makes it easier to move around. The straw (80-100 kg/acre) is put down just before the fruits touch the soil. If this job is carried out before or during flowering, there is an increased risk of damage from late frost (because the soil is 

Straw which is too finely chopped sticks to the berries in wet weather. 

Bees are important for fruit quality in strawberries, raspberries and brambles and for yield in currants and gooseberries. Four to six vigorous bee colonies per hectare are usually sufficient. 

Many growers converting to organic production are often seriously worried about potential weed problems. In practice, there can be problems but there are some very effective methods of control available. 

Variety should not be underestimated as a method of weed control. Many varieties of the same crop have very different growth and/or leaf habits. Varieties should be chosen that produce a vigorous leaf canopy, such as Cara maincrop potato, which will help suppress weeds. Varieties with horizontal (lax) rather than vertical leaves 

High seed rates can help reduce weed competition. In winter cereals drilling 500seeds/m compared with 200seeds/m can halve the weed biomass. In horticultural crops it is not always possible to plant high populations as this technique can affect crop quality and lead to poor marketable yields. 

Depending on the season, crops may be treated several times with the harrow comb. 

As in conventional farming it is important to start the weed control programme in time before the weeds start competing. In many crops, such as beans, carrots, swedes and onions, the first in-crop weeding is the important one. It should be timed when the weeds are small, usually only three to four weeks after 50% crop emergence. This is when the rows are just visible and the crop plants have their first true leaves. In transplanted crops, the first weeding can take place within a 

The most serious disease problems encountered in organic crops are blight in potatoes and seedbome diseases in cereals. Plant breeders are improving blight resistance in potatoes. As organically grown cereal seed cannot be treated, it is very important that it is tested for seed-borne diseases such as fusarium and smut. Foliar diseases in organic 

Bichard, M. (1990) Breeding for Outdoor Pig Production. Outdoor Pigs. 

Coming, S. (1990) Outdoor Pig Production in the UK. Outdoor Pigs. Chalcombe Publications, Maidenhead. 

Lockhart, J.A.R. Wiseman, A.J.L. (1993) Crop Husbandry and Grassland. Pergamon Press, Oxford. 

Lowman, B.G. (1989) Organic beef production. Organic Meat Production in the 90s. Chalcombe Publications, Maidenhead. 

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CgH,3BrN202. A soil-acting herbicide. White crystalline solid, m.p. 158-159" C. It is a non-selective inhibitor of photosynthesis used for weed control In citrus and cane fruit plantations. It is relatively non-toxic to animal life. 

Other Heterocyclic Nitrogen Derivative Herbicides. The herbicides in this group are heterocycHc nitrogen derivatives that do not readily fall into one of the previously discussed groups. They have a wide range of uses and properties. Most of these herbicides are used for selective, pre-and/or post-emergence weed control. Amitrole is used for post-emergence, nonselective weed control in non-croplands and also as an aquatic herbicide (2,296). 

Metal Organics and Inorganics. The metal organic herbicides are arsenicals used for the selective, post-emergence control of grass and broadleaved weeds in cropland and noncroplands. These herbicides are particularly usehil for weed control in cotton and turf crops (2,296,294). CacodyUc acid is a contact herbicide used for nonselective weed control in cropland and noncropland (299). Ammonium sulfamate [7773-06-0] (AMS) is an inorganic herbicide used for control of woody plants and herbaceous perennials (2). 

Miscellaneous Trifluoromethyl Compounds. The herbicides in this group are used for a wide variety of weed-control purposes. Acifluorfen, lactofen [77501-63-4] and oxyfluorfen are used for selective, pre-, and post-emergence weed control in croplands. Fluorochloridone is used for selective, pre-emergence weed control in cropland, and fluridone, fomesafen, and mefluidide [53780-34-0] are used for post-emergence control (296). Fluridone is also used as an aquatic herbicide (2). 

Miscellaneous Other Herbicides. The herbicides in this group are not readily included in any of the preceding groups. Acrolein [107-02-8] (2-propenal) is used as a contact, aquatic herbicide. Sethoxydim, clethodim, and tridiphane are used for selective, post-emergence weed control. 

Cinmethylin and cloma2one [81777-89-1] are used for selective pre-emergence control and etholumesate [26225-79-6] for selective pre- and post-emergence weed control (2,296). 

Between 1979 and 1991, the amounts of herbicide apphed in the United States have remained constant, but the expenditures on herbicides have increased 54%. Agricultural costs accounted for ah. of this increase and more, since herbicide user expenditures in the government/commercial and home sectors combined dropped 3 to 4% during that period. Increased weed control costs related to crop protection have also contributed to the 37% increase, since 1988, in total annual user expenditures for pesticides in general, ie, herbicides, fungicides, and insecticides. In the United States, agricultural uses (ca 1993) account for more than 67% of total pesticide user expenses and 75% of the quantity used annually. Herbicides are now the lea ding type of pesticides in terms of both user expenditures and volumes used (1). 

Chemical, cultural, and mechanical weed control practices have been relatively successful ia reducing yield losses from weeds (448). However, herbicide-resistant weed populations, soil erosion, pesticide persistence ia the environment, and other problems associated with technologies used (ca 1993) to control weeds have raised concerns for the long-term efficacy and sustainability of herbicide-dependent crop production practices (449). These concerns, coupled with ever-increasing demands for food and fiber, contribute to the need for innovative weed management strategies (450). 

The immediate and total removal of weeds is often recommended. However, this recommendation may be based more on when control methods can most easily be appHed, rather than on considerations of the optimal time for effective weed control (454). Controlling plants that are not actually problems or that are present at noncritical times is cosdy and may not truly benefit the producer. However, weeds that are present initially ia very low numbers may require subsequent eradication if iatroduction of a new noxious species is to be prevented. 

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. 

There are two principal approaches to the biological control of weeds (483—485). The first approach is referred to as classical or inoculative biological weed control. Plants that have been introduced to areas outside of their natural range often encounter fewer growth and seed dissemination... 

Weed Control Manual, Meister Publishing Co., Willoughby, Ohio, 1992 and later updates. 

The encapsulation of herbicides has received much attention. Encapsulated alachlor is a high volume herbicide product generally sold as a Hquid formulation, although a dry granule version is also available. The capsules, produced by interfacial polymeri2ation (11), are reported to be spherical with a diameter of 2—15 p.m (75). Two thiocarbamate herbicides, EPTC and vemolate [1929-77-7], were encapsulated by interfacial polymeri2ation because they are volatile compounds. When appHed in unencapsulated form, they must be incorporated in the soil within two hours in order to provide effective weed control. When appHed as a microencapsulated formulation, the rate of volatili2ation is lower and soil incorporation can be delayed 24 hours (76). 

Plants can also be pests that need to be controlled, particulady noxious weeds infesting food crops. Prior to 1900, inorganic compounds such as sulfuric acid, copper nitrate, sodium nitrate, ammonium sulfate, and potassium salts were used to selectively control mustards and other broadleaved weeds in cereal grains. By the early 1900s, Kainite and calcium cyanamid were also used in monocotyledenous crops, as well as iron sulfate, copper sulfate, and sodium arsenate. Prom 1915 to 1925, acid arsenical sprays, carbon bisulfate, sodium chlorate, and others were introduced for weed control use. Total or nonselective herbicides kill all vegetation, whereas selective compounds control weeds without adversely affecting the growth of the crop (see Herbicides). 

To control weeds, approximately 90% of U.S. agricultural acreage is treated with nonchemical, mechanical weed controls, such as mechanical cultivation and rotary hoes (15), whereas only about 25% receives treatment with herbicides (13). 

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