Annual weeds

Atrazine, used as a selective pre- and post-emergence herbicide to control annual weeds in several crops, is the most representative compound of this group. It is also used as a non-selective herbicide in non-crop areas. After absorption, the compound is metabolized to dealkylated and deisopropy-lated derivatives. The unchanged compound and its metabolites are excreted in urine, where they can be detected by chromatography or enzyme-linked immunosorbent assay (Lucas et al., 1993). A mercapturic acid conjugate of atrazine has also been found in urine samples of workers spraying this herbicide (Lucas et al., 1993) (Table 6). 

O Donovan, J.T. and P.A. O Sullivan. 1986. Annual weed control with paraquat in combination with other herbicides. Can. Jour. Plant Sci. 66 153-160. 

Pollen allelopathy can find utilization in field cultivations that could contain pollen of allelopathic crops or weeds. Pollen allelopathy could be an effective method for annual weed control that reproduce, at least in part, via wind pollination and flower concurrently with the allelopathic species. The effects of allelopathy should result in the loss of genetic variation and so in reduction of reproductive ability, but some plants are probably able to detoxify the pollen allelochemicals (Murphy and Aarssen 1995a, b). Murphy and Aarssen (1989) suggested possible delaying of weed flowering at later, less favorable times of the season or diurnal period, so decrease in weed pressure. However, infestation by perennial weeds can worsen due to compensation of pollen allelopathy through increase in the formation of rhizomes. 

Newly investigated pollen allelopathy could effectively reduce the reproductive ability of wind pollination of annual weeds. Pollen of allelopathic species could be artificially dusted on the stigmatic surface of other plants. This phenomenon needs study and field testing yet. 

Diuron is used for the control of annual weeds in front crops and total weed control in non-crop situations. 

Dacthal is a widely used pre-emergent herbicide that is applied to many crops for the control of annual weeds. Dacthal is typically applied to agricultural soils at 6-14kg ha [214], In the soil environment, Dacthal transforms to mono- and diacid-metabolites that are more water soluble than the parent herbicide [215-217]. In eastern Oregon, where Dacthal is applied to onions, the diacid metabolite is the principal form of Dacthal detected in groundwater obtained from domestic wells [218, 219],... 

The uracils with herbicidal activity do not necessarily contain 5-halo substituents. 3-Cyclohexyl-5-methyluracil [354] (XLIV), l,3-di-isopropyl-6-methyl-uracil [352] (XLV) and 3-s-butyl-5-thiocyanato-6-methyluracil [353] (XLVI), for example, are cited as having this type of activity. 3-Butyl-6-methyluracil (XLlllc) possesses interesting selective activities. For instance, this pyrimidine kills many annual weed species without damage to peas and peanuts, even when applied at twice the concentration needed to kill the weeds [346]. On the other hand, the related 5-bromo derivatives, such as (XLlIlb), are useful as industrial herbicides where it is desirable to kill all plants [346]. 

Several phenolic acids and many nonspecific allelopathic conditions have been shown to alter the mineral content of plants, and certainly phenolic allelochemicals may perturb cellular functions in a number of ways that are of importance to plant nutrition (34,35). However, raising fertility does not always suppress allelopathic inhibition, and the interrelationships between these two factors are still not clear. Bhowmik and Doll (36) showed that allelopathic inhibition of corn and soybeans by residues of five annual weeds was not alleviated by supplemental nitrogen or phosphorus. Similarly, an increase in fertilizer did not overcome inhibition of corn by quackgrass or circumvent the autotoxicity of berseem clover Trifolivm alexandrium L.) (37,38). Even when raising nutrient levels releases Inhibition, it does not mean that allelopathy was inoperative under the original conditions. These instances simply illustrate the importance of the Interaction of the two stress conditions. 

Fogelberg, F. and Dock Gustavsson, A.M. 1999. Mechanical damage to annual weeds and carrots by in-row brush weeding. Weed Research 39, 469-479. 

Rasmussen, J. 1992. Testing harrows for mechanical control of annual weeds in agricultural... 

Desmetryn. Desmetryn proved to be selective in Brassica crops (except cauliflower) at rates as low as 375-500ga.i./ha. It was first launched in England toward the end of the 1960s under the trade name Semeron for control of annual weeds in narrow-stem kale. Semeron is important for use in cabbage in Eastern Europe. 

Synthetic, organic, selective herbicides first appeared in France in 1932 with the patenting of dinitro-o-cresol (DNOC) for the selective control of annual weeds in cereals. Dinitro-cresols and dinitro-phenols soon appeared, but these compounds had variable effectiveness and could kill animals as well as plants. 

Atrazine controls most species of annual weeds of the area when used in preemergence or very early postemergence applications. ... 

Wheat stubble may physically intercept up to half of the atrazine as it is applied (Ghadiri et al., 1984). Subsequent rains wash atrazine off the stubble and bring it into contact with the soil. In Nebraska, suppression of summer annual weeds in wheat stubble was proportional to the amount of stubble. Crutchfield et al. (1985) showed that even though heavier wheat stubble intercepted more metolachlor applied in spring prior to ecofallow com planting and reduced the amount of herbicide actually reaching the soil, weed suppression by the heavier stubble more than offset the interception effect. 

Atrazine plays a central role in ecofallow because of its low cost, effective weed control, and extended soil activity. Atrazine controls volunteer wheat and most of the winter annual weed complex - including cheat, downy brome, wild mustards, and henbit, plus many spring annuals. No alternative herbicide has similar characteristics. Repeated applications of nonresidual, foliar-applied herbicides such as glyphosate or paraquat are not as economical. 

Simazine was among the first herbicides registered for use in Florida citrus in 1962 and was recommended by the University of Florida as an excellent weed control tool for citrus (Kretchman and McCown, 1962). This early registration of simazine was for the control of annual weeds, while today s registration includes broadleaf weeds, annual vines, and annual grasses (Singh and Tucker, 1987). 

California trees, citrus tree trunk and canopy growth, leaf nitrogen level, fruit yield, and fruit quality were decreased by competition from annual weeds and bermudagrass (Jordan, 1981). Suzuki (1981) reported that in Japan, weeds in summer absorb and transpire large amounts of water from the soil and compete with citrus trees. Moisture and nitrogen levels in the soil decreased particularly where large crabgrass and tufted knotweed were present (Ito and Ukei, 1981). 

Kretchman, D.W. (1960). Control of annual weeds in nonbearing citrus groves. Proc. South. Weed Conf, 13 62-66. 

Triazine herbicides, principally simazine, have been an integral part of this change in management practice. Simazine is used alone and in combination with other preemergence herbicides for weed control in the plant row. It has low water solubility, low volatility, long residual activity, and gives a broad spectrum of annual weed control. 

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