Postemergence activity, herbicides

A variety of herbicides have been used in combination with simazine and other triazines. These herbicides have preemergence and postemergence activity and are primarily effective on grass and perennial weeds. The preemergence herbicides used in combinations with simazine include oryzalin, pendimethalin, prodiamine, norflurazon, oxy-fluorfen, and diuron. Glyphosate, oxyfluorfen, paraquat, and amino triazole (used in some parts of the world) are... 

DSMA is an organic arsenical having contact, postemergence activity. It was first utilized for crab-grass control in turf. It is an effective herbicide in cotton and in citrus trees, but must be used as a directed spray to avoid contact with the crop foliage. 

The most active pyridazine of the group was the 3-methoxy analog, AC 247,909. Like most of the active pyridazine analogs, preemergence application of AC 247,909 caused bleaching. As the most active postemergence pyridazine herbicide, AC 247,909 caused rapid necrosis, suggesting a potential use as a contact-type herbicide. As in the 3-amino series, an extension of the alkyl chain resulted in the loss of activity. Besides methyl, other substituents introduced at the 3-position included alkylsulfonyl, cyano, carboxy, and amido. These pyridazines were inactive at 1 kg/ha and in some cases at 8 kg/ha. 

Although the synthesis program did not result in any commercial herbicides, two types of pyridazines were discovered which produced unexpected results, both in the level and the type of activity. The first series, which includes the 3-methoxy- and the 3-dimethylaminopyridazines, resulted in a high level of postemergence activity not observed in the lead or in the other 3-sub-stituted-pyridazines. Secondly, based on a comparison with the N-oxides of other pyridazines, the N-oxide of the 3-hydropyridazine resulted in unexpectedly high preemergence activity, yet without phytotoxicity to cotton. 

In summary, with the discovery in Dow that a-(2,2,2-trichloroethyl) styrene possessed unique preemergent herbicidal activity and the synthesis and evaluation of close to one thousand related materials, a new product, tridiphane, the active ingredient in TANDEM will enter the marketplace in 1986. Tridiphane will be the first postemergent grass herbicide for use in corn and will be used in combination with triazine herbicides such as atrazine and cyanazine. 

Its held of application is similar to that of 2,4-D. It is used as a postemergence foliage herbicide in cereals, flax and on pasture land at a rate of280 g-2.25 kg active ingredient/ha. It is effective mainly for the control of dicotyledonous weeds, killing both annual and perennial weeds. Its herbicidal spectrum differs somewhat from that of 2,4-D. Cereals are more resistant to MCPA than to 2,4-D, while the reverse is true for maize. 

It is a postemergence systemic herbicide, recommended for the control of some Poligonum spp., Galium aparine. Stellaria media, and Tussilago farfara weeds in cereal crops at a rate of 2.5 kg active ingredient/ha. It is generally used in combination with other herbicides (2,4-D MCPA 2,4,5-T mecoprop ioxynil bromoxynil, bentazon). 

Butachlor is a pre- and early postemergence selective herbicide for the control of grass weeds and a few broad-leaved weeds. Depending on the organic components of the soil, it is used at a rate of 1.5-3 kg active ingredient/ha on seeded and transplanted rice in Asia, and in Central and South America. 

Monalide is a pre- and postemergence contact herbicide absorbed through leaves and roots. At rates of 1-4 kg active ingredient/ha it can be used postemergence in umbelliferous crops, and preemergence in beans and onions. It is rapidly (in 4-6... 

Both compounds are postemergence selective herbicides with short residual activity used in vegetables, strawberries and tomatoes. They are nontoxic to mammals (Moore, 1960 Freund et al., 1%2). Pentanochlor is still used, while NCA has been replaced by more efficient herbicides. 

Prepared by the reaction of 3,5-dibromo-4-hydroxy-benzaldehyde oxime with 1-chloro-2,4-dinitrobenzene (BP 1 096 037), bromofenoxim is a postemergence selective herbicide with strong contact activity. It is used for the control of annual broad-leaved weeds in winter- and spring-sown cereals at a rate of 1-2.5 kg active ingredient/ha. To enhance the effect against grassy weeds, it is also available in combination with terbutylazine in active substance ratios of 3 1 and 1.65 1. 

Molinate is a selective pre- and postemergence soil herbicide mainly for the control of grass weeds and particularly for Echinocloa spp. (Smith and Fox, 1965). It is applied either before planting to water-seeded or shallow soil-seeded rice, or post flood on other types of rice culture. Applied at its recommended rate of 2-4 kg active ingredient/ha, its action lasts over the whole crop period (Swain, 1974). 

Simeton, prometon and atraton are compounds used primarily as total herbicides, although atraton can be applied in flax, cotton and maize as a pre- and postemergent selective herbicide with good immediate effect and short duration of action. Simeton and prometon remain active in the soil for several months. 

MSMA and DSMA are selective postemergence contact herbicides with some systemic action. A surfactant must be added to the nonformulated active substances, complete coverage by the spray being vital for adequate herbicidal action. 

Iwataki and Hirono (1978) found that the substituents of the cyclohexane ring and of the side-chain strongly influence herbicidal activity and selectivity. The cyclohexane ring is responsible for the postemergence activity of the molecule, the Rj-Rg alkyl substituents for its selectivity. 

Cross, B., Hoffman, P. P., Santora, G. T., Spatz, D. M. and Templeton, A. R. (1983) The design of postemergence phenylurea herbicides using physicochemical parameters and structure-activity analyses. J. Agric. Food Chem., 31, 260-4. 

Sulfonylureas in which a methylene linkage separates the orr/K>-carboxylate from the phenyl ring, such as phenylacetic ester 1, have also proven to be highly active herbicides, especially against broadleaf weeds in postemergence applications Q). 

The heterocyclic analogs of EL-107 showed interesting herbicidal activity as preplant incorporated (PPI) and surface applied (SA) materials (Table). At high rates (8 Ib/acre) they showed postemergence activity. When preplant incorporated and surface applied, these compounds were best described as broadleaf herbicides most effective on redroot pigweed, jimsonweed, nightshade, and mustard. Some degree of foxtail millet control was seen at 0.5 to 1.0 Ib/acre. Crop safety was seen on the cereals and corn. 

The herbicidal activity and corp selectivity of IG-21 (code number for development HWS) were further examined at 1,200-50 g ai/ha. IG-21 was tested for postemergence activity against a range of plant species including purslane (Portulaca oleracea), slender amaranth (Amaranthus ascendens), goosefoot (Chenopodium album), spiny amaranth Amaranthus spinosus), sugarbeet Beta vulgaris), maize Zea mays) and rice Oryza sativa). Results are shown in Table 2.50. 

Although the antibacterial and antifungal activities of bialaphos and phosphinothricin were not found to be usehil, the two agents were later used as biodegradeable, relatively nonselective, postemergent herbicides. Glutamine synthetase inhibition is toxic to plants because the enzyme is key to ammonia assimilation. There is some selectivity for individual plant species as shown by the LD for bialaphos ranging from 0.125 to 8.5 kg/ha (301—303). 

The sulfonylureas described here are new herbicides with unprecedented activity. Activity at extraordinarily low rates of application has been demonstrated by both preemergence and postemergence applications (Ref. 1). These rates are best describee in grams per hectare compared to kilograms per hectare for most herbicides currently marketed. Furthermore, the combination of low application rates, half lives of generally less than two months in the soil (Ref. 2) and excellent safety to mammals [LD5o s of generally >5000 mg/kg for technical material in male rats] (Ref. IC) makes these compounds attractive products from an environmental viewpoint. 

From the earliest greenhouse studies, it was obvious that AC 252,214 was an interesting compound. Remarkably, it was well tolerated by legumes, especially soybeans, yet showed a broad-spectrum of herbicidal activity, killing broad-leaved, grassy and perennial weeds. Further it was shown that AC 252,214 could be used both as a preemergent and postemergent herbicide. Rates for field application are 125-250 g/ha (12). 

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