Weeds

Due to its ability to outcompete weeds, Jerusalem artichoke was once advocated as a form of weed control in soils given no other form of cultivation. Two years of Jerusalem artichoke cultivation, for example, appeared to clear soil of quack grass (Shoemaker, 1927). However, Jerusalem artichoke is more commonly known as a weed itself, especially in cereals in the U.S. The tubers are difficult to harvest completely and volunteers result. If Jerusalem artichoke is cultivated on the same piece of ground for a number of years, volunteer plants developing among newly planted tubers pose little problem. If the following crop in a rotation is maize/com, Jerusalem artichoke can become a significant weed problem. Chemical, mechanical, and control approaches are used to control Jerusalem artichoke as a weed (see Section 12.3.2). 

Winter barley is an ideal entry for oilseed rape because the earlier harvest allows early drilling of rape. It has been shown on organic farms that building up fertility by using a green manure crop, such as red clover, has paid off financially, with the subsequent boost in yield of the following cereal crop. 

Varieties are very different in their susceptibility to disease, and this is of great importance to the organic farmer. It makes sense to consider a varietal diversification scheme and to select at least three varieties for every farm, so that each has a different resistance factor. These different varieties can be sown in adjacent fields. 

In organic systems the number of hours of hand weeding needs to be reduced, as labour, particularly in the developed countries, is expensive. This can be achieved by competition, the timing of cultivations, adaptation of the rotation nutrient management system and by crop density (Kropff, et al., 2000). Barberi, et al. (2002) experimented with a system of soil disinfection using hot steam to sterilise weed seeds. 

The use of field margins around organic crops to increase the 

In Europe in 1996, the largest area devoted to organic arable crops was in Denmark (91000 ha), with Italy next (62000 ha) and then France (34000 ha). This compares with 4800 ha devoted to organic arable crops in Great Britain (Foster Lampkin, 1999). However, by 2003 this had increased dramatically to 44413 ha (Soil Association, 2003). Changes in crop areas are shown in Table 5.1. Of the total crop area of 4 515 000 ha in 1996, wheat occupied 44% of the area. The area of spring barley had decreased by nearly 50% from the 1988 figure. 

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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. 

CgHjClaOj. M.p. 155°C. Used as a selective herbicide. It is made from 2,4,5-trichloro-pheno) and sodium chloroacetate. Ester sprays and combined ester sprays with 2,4-D are available. 2,4,5-T products are of particular value in that they control many woody species, and eradicate perennial weeds such as nettles in pastures. 

The hormone weed-kiUer MQPA (TM 160) is needed m large quantities. Suggest an economical synthesis for it. 

These programs systematically determine which bonds could be broken or formed in order to obtain the desired product. This results in generating a very large number of possible synthesis paths, many of which may be impossible or impractical. Much work has been done to weed out the unwanted synthesis routes. One major strength of this technique is that it has the capacity to indicate previously unexplored reactions. 

Some programs incorporate very simple energy calculations. These are very quickly calculated estimations of electronegativity and other properties. Although not accurate enough to be completely reliable, these techniques are useful in weeding out undesirable reaction routes. 

The sassafras tree itself grows like a weed in the wilds and along disturbed fence lines throughout the south, southeast, eastern... 

A new source of safrole that Strike has just stumbled upon is in Strike s back yard. Momma comes home from the local nursery with two plants that have these enormous leaves. They are succulents and grow like crazy. Strike What ya got there Momma Momma Don t think I haven t noticed those filthy drug books you write, you sick little monkey Strike Aw c mon. Mom. Let s not get into this again. Strike doesn t... Momma Well I needed some new ground cover plants, and...sigh...I know you re into this twisted licorice thing...so I bought these weeds that are called Licorice Plants. ... 

H Br Effective non 776 selective herbicide in controlling weeds in orchards and forests ... 

A great number of various substituted aminophenyl derivatives of thiazolium, and their organic or metallic complexes, have been patented as weed-killers or regulating growth factors of plants (135-138). 

Weber numbers Weber s law Web formation Web knitting Web offset inks Webster hardness gauge Weed killers... 

Herbicide Handbook, 5th ed.. Weed Science Society of America, Champaign, HI., 1983, p. 128. 

Acrolein at a concentration of <500 ppm is also used to protect Hquid fuels against microorganisms. The dialkyl acetals of acrolein are also useful in this apphcation. In addition, the growth of algae, aquatic weeds, and moUusks in recirculating process water systems is also controlled by acrolein. 

GibberelHc acid is also used successfliUy in rice culture to promote seed germination and the growth of semidwarf varieties. The treated seed can be planted deeper than normal. In addition, the sprouting seedlings are much taller than the untreated ones they compete weU against weeds. The material is sold under the trade name Release. 

The commercial use of 2,4-D has decreased substantially and (ca 1993) it has general use for home lawns to control broadleaved weeds it also is used on a limited basis to control broadleaved weeds in commercial moncotyledonous crops, eg, sugarcane. 2,4-D is used on citms when the fmit is 1/3 to 1 inch in diameter to increase fmit size and to limit fmit drop on trees more than six years old. It should not be appHed to trees that are in fliU flush. A further use includes treatment of harvested lemons at 500 mg/L to improve storage properties and to delay yeUowing (23). It is used in certain parts of the world to increase latex flow in old mbber tree plantations. 

Cell Division Inhibitors. The most common mode of action of soil-appHed herbicides is growth inhibition, primarily through dkect or indkect interference with cell division (163). Such growth inhibitory activity is the basis for most pre- or post-emergent herbicides intended to control germinating weed seeds. In germinating seeds, cell division occurs in the meristems of the root and the shoot. Meristematic cells go through a cycle... 

Degradation or Transformation. Degradation or transformation of a herbicide by soil microbes or by abiotic means has a significant influence not only on the herbicide s fate in the environment but also on the compound s efficacy. Herbicides that are readily degraded by soil microbes or other means may have a reduced environmental impact but may not be efficacious. Consider the phenomenon of herbicide-resistant soils. In these cases, repeated application of a given herbicide has led to a microbial population with an enhanced ability to degrade that herbicide (252,253). This results in a decrease or total loss of the ability of the herbicide to control the weed species in question in a cost-effective manner. 

Phenoxyalkanoics. The phenoxyalkanoic herbicide grouping is composed of two subgroups, the phenoxyacetic acids and the phenoxypropionic acids. The phenoxyacetic acid herbicides include some of the first commercially successhil herbicides, eg, 2,4-D. They continue to be widely used for foUar control of broadleaf weeds. The more heavily functionalized phenoxypropionic acid herbicides are relatively new herbicides compared to the phenoxyacetic acids and are used primarily for selective control of grassy weeds in broadleaf crops (2,296,297). 

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