Herbicidal activity enhancement

Only the D-isomer of these compounds shows strong herbicidal activity, while the L-isomer is inactive, and any additional substituents on the benzene ring significantly decrease the activity (6). Thus we speculated that the phenoxypropanoic acid part of the structure is the essential moiety, which interacts with a receptor in plant tissues. And if this is so, the pyridinyloxy moiety may serve to optimize the hydrophobicity of the whole molecule for maximum herbicidal activity, enhancing the translocation of the molecule through the plant. 

Toxic equivalence factors (TEFs), 13 140 Toxic free radical activity, herbicides that enhance, 13 297... 

Interactions between surfactants and herbicides in spray solutions are known to occur (57), and those studied either have no effect or are detrimental to herbicide activity. No definite evidence exists that surfactant-herbicide interactions in solution cause enhanced activity of herbicides. 

Hansch analyses have shown that the surfactant Tween 80, when used in herbicidal formulations of TFMS derivatives at the 0.1% (w/v) concentration level, can produce one or more of the following effects in pre-emergence herbicidal tests, depending on the weed type and/or TFMS derivative under evaluation (a) no effect, (b) enhancement of herbicidal activity, (c) inhibition of herbicidal activity, or (d) shifting of optimum herbicidal activity from one TFMS derivative to another within each series. Mathematical equations relating structure to activity have been derived for the TFMS compounds both for the surfactant present and surfactant absent cases. 

In general, substituents characterized by large, positive Hammett <7 constants (electron-attracting substituents) exhibit enhanced pre-emergence herbicidal activity on all weed types tested (Foxtail, Cheat Grass,... 

Foy and Smith (4) recently reviewed the role of surfactants in modifying the activity of herbicidal sprays (66 references). These authors studied the adjuvant effects of a surfactant based on nonyl phenol with varying amounts of ethylene oxide appended. Activity was enhanced at an optimum ethylene oxide chain length. This is another way of stating that there is an optimum surfactant HLB for enhancing herbicidal activity on a given species of weed. 

This conforms with earlier observations that compact size and planarity of a molecule often enhance its herbicidal activities [56N(L)1042 63MI1 76T615]. 

The mixed-function oxidase inhibitors aminobenzotriazole and piperonyl butoxide can synergize herbicide activity in resistant Lolium growing in a hydroponic system. This indicates that at least one aspect of cross-resistance in Lolium rigidum may be related to enhanced metabolic activity of mixed-function oxidazes acting to detoxify herbicides. We are now concentrating on direct studies of herbicide metabolism in resistant biotypes. 

Between 1976 and 1978 Stauffer Chemical Company researchers identified fields where EPTC failed to give expected herbicidal activity. The fields had a history of repeated annual applications of EPTC. Greenhouse bioassays with EPTC demonstrated reduced persistence as the cause of reduced weed control observed in the field. Sterilization of the soil with heat or chemicals restored herbicidal activity. This indicated that, in some soils, enhanced microbial degradation might be associated with the observed reduction in herbicidal activity (D.L. Hyzak, personal communication). 

Swithenbank et al. (1971) synthesised forty related benzamide derivatives to study the relationship between chemical structure and action. The herbicidal activity of N-alkyl-3,S-dichlorobenzamides was enhanced by, y-unsaturation, and activity was further improved by a,a-dimethyl groups. In the case of dimethylpro-pynyl nzamides, derivatives with halogen substituents on the aromatic ring are the most active, substitution in positions 3 and 5 being optimal. 

Noruron is a selective preemergence herbicide used at rates of 0.75-4 kg of active ingredient/ha on cotton, sugar cane, soybeans and potato. In combination with other herbicides, it enhances their grass weed killing effect. 

Nontarget-site resistance is caused by mechanisms that reduce the amount of herbicidally active compound reaching the target site. An important mechanism is enhanced metabolic detoxification of the herbicide in the weed, with the effect that only insufficient amounts of herbicidally active substance will reach the target site. Furthermore, reduced uptake and translocation or sequestration of the herbicide may lead to insufficient herbicide transport to the target site. 

In contrast to this observation the introduction of monocyclic triazolinones dramatically enhanced the herbicidal activity. In a systematic manner hundreds of previously known and new intermediates were prepared and transformed into SACTs. Many new synthetic procedures have been developed. To date, derivatives of the type shown in Table 2.6.2 have been synthesized and published. 

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