Atrazine crop tolerance

Application versatility, combined with a high level of crop tolerance, led to atrazine being the most widely used corn herbicide in history. In fact, atrazine led the US com herbicide market within several years of its introduction. Illinois corn farmer surveys show a steady increase in the use of atrazine in corn from its debut in 1960, with 75-85% of com being treated with atrazine since 1975. 

Analysis approach - corn and sorghum A two-tiered approach was also used to characterize the benefits associated with uses of atrazine and simazine in corn and sorghum. First, a comparative analysis was made of product labels. The following parameters were considered in this review performance profiles, including efficacy, spectrum, and crop tolerance label comparisons physical and chemical characteristics of the product hazard profiles economic benefits and other relevant issues, such as use restrictions, etc. 

Simazine was the main product studied in early research on the triazines for weed control in tree fruits and vineyards (Doll, 1960 Larson and Ries, 1960). On mature grapevines in a deep, fine, and sandy loam soil, no differences in crop tolerance were observed between simazine and atrazine (Leonard and Lider, 1961). However, subsequent studies indicated that grapevines were more tolerant to simazine than to atrazine (Lange et al., 1969a). Prometryn was intermediate between the two in terms of crop tolerance (Lange et al., 1969a). 

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. 

At this point, evidence that similar molecules acted as effective antidotes by inducing needed metabolic pathways for herbicide detoxication was at most very speculative. However, another hypothesis emerged. Could early herbicide pretreatments increase crop tolerance to these herbicides by elevating the substrates and enzymes needed for detoxication While not a new concept in animal systems, such an idea has received little attention in plant systems and it certainly has not been exploited in any practical way. The whole idea has seemed much more credible with the study by Jacetta and Radosevich (19) of photosynthetic recovery in corn after treatment with atrazine. More specifically, they showed that inhibition of photosynthesis was reduced and the rate of recovery enhanced in corn plants treated for the second or third time with atrazine compared to "first exposed" plants (Figure 2). Furthermore, the faster recovery was related to enhanced rates of atrazine metabolism in the previously treated plants (Table III). 

The metabolism of atrazine and a series of 2-chloro-.v-lriaz.ines were reported by Lamoureux et al. (1972) in excised leaf or shoot tissue of barley, corn, sorghum, and sugarcane. The authors found that the primary route of metabolism was the displacement of the 2-chloro group with glutathione or 7-glutamylcysteine. The overall rate of metabolism in susceptible barley was much slower than in tolerant crops. 

Many triazines have been evaluated for weed control in orchards and vineyards, but primary emphasis will be placed on simazine as the herbicide of preference in most of these crops. In general, simazine is better tolerated by most tree fruits, nuts, and vines than high rates of atrazine, prometryn, propazine, terbutryn, terbuthylazine, or metribuzin. 

Worldwide there have been a total of 20 commercialized triazine herbicides. Of the 20 triazines, 7 are currently registered for land use within the United States ametryn, atrazine, metribuzin, prometryn, simazine, terbutryn, and prometon. For purposes of this discussion, only dietary estimates for the 5 most widely used domestic triazines are presented since the USEPA revoked cyanazine tolerances in 2004, prometon is not used for food crops and terbutryn has very limited use. Additionally, propazine was used under USEPA Section 18 registrations in the 1990s, and in 2007 was registered for weed control in sorghum (USEPA, 2007). 

Further development of herbicides with true physiological tolerance to specific crop plants such as is exhibited by atrazine on corn. 

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