Selectivity triazine-tolerant crops

Triazine Selectivity in Crops and Weeds Triazine-Tolerant Crops... 

Triazine herbicides provide selective weed control in crops such as corn, sorghum, and sugarcane. In addition, some members of the triazine family are used for weed control in orchards, horticultural, and perennial crops, etc. A unique selective use of triazine herbicides is in triazine-tolerant rapeseed. Although triazine herbicides provide control of a wide variety of grass and broadleaf weeds, the long-term, widespread, and repetitive use of triazine herbicides in crop and noncrop situations has led to the selection of many triazine-resistant weeds. The physiological and biochemical basis of triazine selectivity between crops and weeds and of resistance to triazine herbicides in weeds is well understood. 

C-Alkyl hydroxylations are of very common occurrence in plants notable species differences exist, however, which are of significance with respect to selectivity. The tolerance of certain broadleaf species to chlorsul-furon (5) results from hydroxylation of the 4-methyl substituent on the triazine ring to produce 6, followed by glycosylation (Figure 10.4), rather than 5 ring hydroxylation, as in the case of graminaceous crops. The fact that 6 retained herbicidal activity implies that for this metabolic route the... 

Finally, certain triazine herbicides can be used selectively in orchards and in some horticultural crops. In this case, selectivity is not based only on physiological differences between species, but on physical selectivity associated with the location of the herbicide and the roots of the crop and weed species in the soil. Triazine herbicides such as simazine, which has very low solubility in water, remain close to the soil surface in most mineral soils. Careful application of simazine in horticultural or fruit crops can result in the herbicide being available to control shallow-rooted weed species without harming the deeper-rooted perennial species. The success of this use is dependent not only on the relative rooting depths of the tolerant and susceptible species, but also on soil conditions and other factors that may affect herbicide fate and movement. 

Beginning in the 1950s, when triazines such as simazine, atrazine, prometryn, and ametryn were first synthesized and tested as selective herbicides in the Geigy laboratories in Basel, Switzerland (Gast et al 1955), massive research efforts have focused on the transformation and use of these compounds in the environment. The -triazines represent one of the most widely used and probably the most extensively studied family of herbicides. One of the driving forces for this research was the outstanding performance of triazines with respect to their selective herbicidal effects and crop tolerance. 

Millet and woody crop plants tolerate terbuthylazine and sebuthylazine. The methylmercapto and methoxy derivatives of terbuthylazine (terbuthyrene, or Caragard , Geigy) and the methoxy derivative of sebuthylazine (Etazin , Geigy) are special selective herbicides. From the standpoint of residual effect, these three triazine derivatives are highly persistent herbicides. After a dosage of 10 kg/ha, crop plants sensitive to triazines (particularly cereals) cannot be sown for several years. 

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