Tolerant weeds

Others take a more balanced approach, and are quite happy to tolerate "weeds" such as clover and daisies. 

In some orchards where repeated applications of triazine herbicides have been used, there are isolated instances of triazine-resistant weeds. These include common groundsel in the United Kingdom (Holliday and Putwain, 1977) and common lambsquarters and pigweeds in Czechoslovakia, Poland, and Bavaria. Some studies indicated an increased prevalence of some tolerant weeds in orchards when triazine herbicides were used in certain crop weed systems of common vetch (Heeney et al., 1981a), field bindweed (Meith and Connell, 1985), and quackgrass (Hertz and Wildung, 1978). 

Simazine has been widely used in raspberry, boysenberry, and blueberry production (Welker and Brogdon, 1968). When used alone, broadleaf weed control has been excellent, but annual grasses and perennial weeds were not controlled in blueberry (Hertz and Wildung, 1978). Spring-planted raspberry crops do not tolerate weed competition well (Lawson and Wiseman, 1976). Simazine and diuron have been used extensively in many states in new or established cane-planted raspberry. 

TBA is readily absorbed and translocated by plants. It moves both acropetally and basipetally and causes typical auxin effects in sensitive plants. It strongly inhibits apical growth and leaf formation (Zimmermann and Hitchcock, 1951). It does not cause epinasty in the MCPA-tolerant weeds but stops their growth so that they cannot compete with the crop at the later growth stages. 

Since a number of triazine tolerant weed biotypes have been discovered through triazine application to crop lands, development of... 

BRASSICA CAMPESTRIS (BIRDSRAPE MUSTARD) TRIAZINE TOLERANT WEED... 

Most of the triazine tolerant weed biotypes Identified to date have economic relatives but few if any are as closely related to economic crops as the birds rape. To date, this cytoplasm has been transfered to oilseed rape, rutabaga and oriental mustard by several breeding groups. Partial cross-compatibi 1 ity with several other Brassica species including the cole crops, and other more distantly related species (eg, Diplotaxis and Raphanus species) may permit transfer by classical methods (perhaps assisted by embryo rescue techniques). Chloroplast or mitochondrial X nuclear Incompatibilities however, may limit this potential. As other triazine tolerant weed biotypes emerge, opportunities for transfer to closely related economic species by classical methods may evolve. 

Selective toxicity is also important in relation to the development of resistance or tolerance to pollutants from two distinct points of view. On the one hand, there is interest among scientists concerned with crop protection and disease control in mechanisms by which crop pests, vectors of disease, plant pathogens, and weeds develop resistance to pesticides. Understanding the mechanism should point to ways of overcoming resistance, for example, other compounds not affected by resistance mechanisms or synergists to inhibit enzymes that provide a resistance mechanism. On the other hand, the development of resistance can be a useful indication of the environmental impact of pollutants. 

Sulfentrazone is a broad-spectrum, pre-emergent herbicide that provides good control over broadleaf weeds, grasses and sedges in crops and turf. The metabolism of sulfentrazone in animals and plants is similar. The major plant metabolite of sulfentrazone is 3-hydroxymethyl sulfentrazone (HMS). The soybean tolerance of 0.05 mg kg includes residues of sulfentrazone plus its major metabolite, HMS. The rotational crop tolerance includes residues of sulfentrazone and its major metabolites, HMS and... 

Weibel, F.P., Ladner, J., Monney, P. and Sutter, F. (2006a). Improved organic and low-input apple production by weed competition tolerant rootstocks , Acta Horticulturae (.ISHS), Submitted Sept. 2006. 

Faulkner, J.S. and B.M.R. Harvey. 1981. Paraquat tolerant Lolium perenne L. effects of paraquat on germinating seedlings. Weed Res. 21 29-36. 

Molinate (S-ethyl hexahydro-azepine-l-carbothioate) has been widely used for broadleaf and grass weed control in rice culture. The 96-hour LC50 of technical molinate in the common goldfish is 30 ppm (1) and the medium tolerance limits (LTm) for molinate in mosquito fish (Gambusia affinis) are 16.4 ppm for 96 hours (J2). Kawatsu (3) estimated the 20-day lethal concentration of molinate in Japanese carp (Cyprinus carpio) var. Yamato koi at 0.18 ppm. However, there was no toxic effect of molinate on American carp at concentrations of 10 ppm during a two-week observation (40. ... 

Weed Science Society of America. (1994). Herbicide handbook. Champaign, IL Weed Science Society of America. United States Environmental Protection Agency. (1997). Glyphosate Pesticide tolerances Federal Register April 11,1997 (Volume 62, Number 70). 

Table II summarizes these results. Most noticable is the good activity of the m-toluate against wild oats and black grass but poor activity against mustard. On the other hand, mustard is very sensitive to the -toluate, whereas wild oats and black grass are quite tolerant. Careful studies with the more active enantiomer of AC 222,293 showed it to be approximately twice as active as AC 222,293. It should be noted that the selectivity shown by AC 222,293 is dependent on the presence of the methyl ester function. The acids are not selective but studies (2) indicate that it is, in fact, the acid which is the toxicant and this is liberated at different rates from the ester in the sensitive weeds and the crops.

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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