Post-emergent herbicides

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... 

In addition to the use of 2,4-dichlorophenol in the synthesis of 2,4-D herbicides (acid 2,4-D, acid 2,4-DP, acid 2,4-DB), it is also found in the selective post-emergence herbicide, diclofop-methyl [51338-27-3] (61) and as a selective pre-emergence herbicide, oxadia2on [19666-30-9] (62). A postemergence herbicide is appHed between the emergence of a seedling and the maturity of a crop plant. 

Acylanilides (abbreviated as anilides in this article) are generally used as selective pre- and/or post-emergence herbicides in paddy rice fields. The herbicidal activity of the anilides is similar to those exhibited by the auxin-like plant growth regulators. 

Sulfonylurea herbicides are generally applied to crops as an early post-emergent herbicide. Crops that are tolerant to these herbicides quickly metabolize them to innocuous compounds. At maturity, residues of the parent compound in food and feed commodities are nondetectable. Metabolites are not considered to be of concern, and their levels are usually nondetectable also. For this reason, the residue definition only includes the parent compound. Tolerances [or maximum residue limits (MRLs)] are based on the LOQ of the method submitted for enforcement purposes and usually range from 0.01 to 0.05 mg kg (ppm) for food items and up to O.lmgkg" for feed items. There is no practical need for residue methods for animal tissues or animal-derived products such as milk, meat, and eggs. Sulfonylurea herbicides are not found in animal feed items, as mentioned above. Furthermore, sulfonylurea herbicides intentionally dosed to rats and goats are mostly excreted in the urine and feces, and the traces that are absorbed are rapidly metabolized to nontoxic compounds. For this reason, no descriptions of methods for animal-derived matrices are given here. 

Stable in water and under light Bispyribac-sodium is a systemic post-emergence herbicide, used to control a broad range of weeds in rice... 

Pyrithiobac-sodium is a pre- and post-emergence herbicide to control broad-leaved weed and grasses The residue definition is for the parent, pyrithiobac-sodium, only. 

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). 

Diclofop-methyl is an early post-emergence herbicide applied to cereals, oil seeds and a variety of other crops for the control of annual grasses. Both Diclofop-methyl and its hydrolysis product, diclofop, are herbicidal [200] therefore, analysis for Diclofop-methyl in soil includes the determination of both chemicals. 

Cyperquat, a post-emergence herbicide has been determined in surface soil by a method involving catalytic hydrogenation to l-methyl-4-cyclohexypiperdine and analysis by gas chromatography-mass spectrometry. Recovery of cyperquat from fortified soil samples was 77% at the 0.5mg kgy1 level and 85% at the lmg kgy1 level [213]. 

In the search for more effective post-emergent herbicides, many laboratories have measured the inhibition of photosystem II in chloroplasts i.e., the Hill reaction. In a continuing investigation of this system, ( ) Corwin Hansch s group at Pomona College, in cooperation with BASF in Germany, analyzed two sets of phenyl substituted ureas 17 1,1-dimethyl-3-phenyl, and 38... 

Dialkyl- and 2,5,7-trialkyl-pyrimido[4,5-d]pyrimidin-4(3H)-ones have been found to possess pre- and post-emergence herbicidal activity (74USP3830812). 2,4,7-Triamino-5-phenylpyrimido[4,5-if]pyrimidine has diuretic activity similar to triamterene (6-phenyl-2,4,7-pteridinetriamine) (68JMC573). 

Dimethyl 3-phenylisothiazole-4,5-dicarboxylate (4) has been claimed to be an effective broad-leaf post-emergent herbicide.534... 

Uses selective pre-emergence and post-emergence herbicide to effectively control a wide variety of broadleaf weeds in corn, grain, sorghum, maize, rice, and soybeans. 

Uses pre-emergent and post-emergent herbicide used to regulate plant growth and control weeds in carrot, onion, garlic, and other crops. 

Uses systemic post-emergence herbicide to control annual and perennial weeds in cereals, rice, flax, vines, peas,... 

Uses nonselective pre-emergence and post-emergence herbicide to control most annual and broadleaf weeds, grasses, and brush weeds on noncropland. 

Uses as pre- and post-emergence herbicide to control of annual broadleaf weeds in sugar-beet, fodder beet and beet root and also used in combination with other herbicides, etc. 

Recently, herbicides which formerly showed little or no foliar activity (e.g., atrazine and diuron) have been used effectively with surfactants and/or oils as post-emergence herbicides (3, 17, 36, 39, 42, 43). This approach offers promise in that lower rates may be required for equal weed control and at the same time residue hazards may be reduced. 

Post-emergence herbicide with mainly foliar uptake, translocated in xylem and phloem... 

Pre- or early post-emergence herbicides to control annual grasses and broad-leaved weeds 111, 3512... 

Post-emergence herbicides to control broad-leaved annual and perennial weeds in variety of agricultural crops 111,3512... 

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