Barnyardgrass and many broadleaf weeds were severely stunted by POST applications of isoxaben. Most grasses were only moderately affected by isoxaben at rates up to 1 kg/ha. POST activity was enhanced by soil interception, but evident with foliar interception alone. Cell swelling in stems and in interveinal leaf lamina was observed after foliar application. A radiotracer study in redroot pigweed showed that only a small amount (3.3% of applied) of isoxaben entered the leaf following foliar application, and that little (0.08% of applied) isoxaben migrated beyond the point of application. The potential of isoxaben as a POST herbicide may be limited, in part, by poor absorption and translocation.

Three triazine-resistant biotypes of grass weeds, hood canarygrass (Phalaris paradoxa L. # PHAPA), ryegrass (Lolium rigidum Gaud.), and slender foxtail (Alopecurus myosuroides Huds. # ALOMY) were collected along roadsides on the coastal plain of Israel that had been treated repeatedly with s-triazine herbicides. Resistant biotypes (R) survived up to 4 kg ai/ha of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] applied pre-and postemergence, while susceptible (S) biotypes were killed by 0.25 kg/ha. R and S biotypes were equally sensitive to diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea]. Electron transport in chloroplasts isolated from R biotypes was not affected by atrazine, whereas in S biotypes electron transport was inhibited 50% by 0.4 to 1.0 μM atrazine. Chloroplasts from both biotypes were equally sensitive to diuron. These data indicate that the R biotypes have a plastidic mode of resistance to atrazine. In addition, seedlings of R biotypes exhibited resistance to triazinone herbicides. The R biotype of hood canarygrass was more tolerant to postemergence application of diclofop {(±)-2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}.

Investigations were made under controlled environmental conditions of isoproturon [N-(4-isopropylphenyl)-N’,N′-dimethylurea] absorption and translocation in wheat (Triticum aestivum L.) (tolerant) and slender foxtail (Alopecurus myosuroides Huds. ♯ ALOMY) (sensitive). Effects of isoproturon on 14CO2 fixation and 14C-assimilate translocation were also examined in the test species. The amount of 14C-isoproturon translocated by slender foxtail was about four times greater than by wheat. Foliar application of isoproturon resulted in greater inhibition of photosynthetic 14CO2 fixation in slender foxtail than in wheat. The reduction in 14C-assimilate translocation was also significantly greater in slender foxtail than in wheat.

The resistance of four populations of slender foxtail from England to chlorotoluron, pendimethalin, and the methyl ester of diclofop was studied in whole-plant spray assays. Two populations showed cross-resistance to all three herbicides while a third showed resistance to chlorotoluron only. Resistance, although not absolute, was sufficient to cause substantial reductions in herbicide performance at recommended field doses. Resistant (Peldon) and susceptible (Rothamsted) populations also were assessed for resistance to six dinitroaniline herbicides in a petri dish test in which the effects on shoot development were measured. The degree of resistance of the Peldon population varied considerably among herbicides, with greatest resistance to pendimethalin, whereas sensitivity to ethalfluralin, isopropalin, and trifluralin was similar to that of the Rothamsted standard. Differential response to butralin and oryzalin was smaller than to pendimethalin. The large difference between populations in their response to pendimethalin and trifluralin was confirmed in a pot experiment.

The effect of chlorotoluron on different slender foxtail populations that survived normal agricultural rates of this herbicide (2.5 to 3.5 kg ai ha–1) was investigated under controlled laboratory conditions. Among five populations tested, two had tolerance for the herbicide. The ED50 values for these biotypes ranged from 0.33 to 2.43 kg ha–1. Assays with radiolabeled 14C-chlorotoluron indicated similar absorption and translocation of the herbicide between susceptible and resistant biotypes. Chlorophyll fluorescence and Hill reaction analysis seem to support the view that the mechanism of resistance of one biotype to the herbicide is due to degradation/detoxification rather than to modification of the target site, although compartmentation processes cannot be ruled out. This resistant biotype had high tolerance to other photosynthetic inhibiting phenylureas, including linuron, chlorbromuron, diuron, monolinuron, and neburon.

A method of calculating the cost to farmers of herbicide resistance in weeds is described along with examples to demonstrate the inherent difficulties and benefits. The main problem is defining cropping systems/practices and herbicide strategies that prevent the development of resistance. The benefits include focusing advisory and research efforts and helping to persuade farmers to adopt practices that prevent the development of resistance.

The first significant cases of herbicide-resistant weed populations were to the triazines in the 1970s. In the last 10 years there has been an increase in the number of weed populations that have become resistant to an array of herbicides. In some of these cases, like rigid ryegrass in Australia, a multitude of resistant biotypes has evolved with different mechanisms of resistance. If the present trend continues, the number of herbicides effective on certain weed species may diminish rapidly. To counteract this trend, industry has organized a number of intercompany working groups to specifically address the development of resistance and to implement plans to manage resistance. University and extension along with industry personnel across the world have begun educating growers on resistance management. However, this effort needs to be intensified to find new solutions for controlling weeds through the use of integrated weed management practices that incorporate new and established herbicides with cultural, mechanical, and biological control methods. The challenge is to develop cost effective, environmentally sustainable programs for weed control while maintaining the present efficiency in food and fiber production so that needs of an ever expanding human population can be met.

The metabolism of the herbicide 14C-glufosinate (GA) was studied in excised shoots and leaves of 20 weed and nonweed species; GA was fed through the xylem. After 24 or 48 h of incubation, the plant material was examined for phytotoxic symptoms, analyzed by autoradiography, and extracted. The extract was cleaned up and analyzed by high-performance liquid chromatography. GA and its metabolites were identified by cochromatography with authentic 14C-labelled reference compounds. 14C-GA was rapidly absorbed by the excised plant parts. Most of the radioactivity (91.3 to 99.7%) in the shoots and leaves was extractable. The main metabolite observed with all species was 3-(hydroxymethylphosphinyl)propionic acid (MPP); lower amounts of 2-hydroxy-4-(hydroxymethylphosphinyl)butanoic acid (MHB) were also found in 14 species. Metabolic rates of GA varied considerably ranging between 13.1 and 2,836.5 ng GA h−1 mg−1 protein. The species with the highest rates of metabolism of GA were Galium verum (2,836.5 ng GA h−1 mg−1 protein), Lythrum hyssopifolia (846.0 ng GA h−1 mg−1 protein), and Digitalis purpurea (494.8 ng GA h−1 mg−1 protein). The mean value across all species was 275.9 ng GA h−1 mg−1 protein.