Greenhouse and field experiments were conducted from 1987 to 1990 at the Lacombe Research Station to determine the influence of ammonium sulfate (AS) on various grass control herbicides. In field studies, AS had slight or no effects on the phytotoxicity of aryloxyphenoxypropanoate (APP) herbicides (fenoxaprop, fluazifop, haloxyfop, and quizalofop). The largest AS-mediated increase in APP herbicide phytotoxicity was 19% (based on fresh weight reduction) for wild oat with haloxyfop at 50 g/ha. AS consistently mediated increases in cyclohexanedione (CHD) herbicide phytotoxicity. With added AS, barley fresh weight was reduced 75% (1988) with BAS 517 at 50 g/ha, and 100% (1990) with clethodim at 25 g/ha. Greenhouse studies confirmed field studies, but differences were less substantial and consistent. It is suggested that APP herbicides are either less susceptible to UV degradation than CHD herbicides, and/or that APP herbicides may penetrate plant cuticles quickly enough to nullify any protection from UV degradation that AS might provide via rapid absorption.

Field experiments were conducted at Kalispell, MT, to determine the optimum adjuvant combination for wild oat control in peppermint with quizalofop. Quizalofop was applied to four- and eight-leaf wild oat plants at 20 and 50 g ai/ha with either a nonionic surfactant (NIS) or methylated seed oil (MSO) alone or in combination with 28% urea ammonium nitrate (UAN) liquid fertilizer. Differences among adjuvants were most apparent when quizalofop was applied at the lowest rate. MSO was more effective than NIS for enhancing quizalofop activity. Quizalofop efficacy with both adjuvants increased when applied with UAN. Greater than 90% wild oat control was obtained with the lowest rate when applied with MSO plus UAN to four-leaf wild oat plants. These results demonstrate the potential to improve the consistency of weed control as well as reduce postemergence herbicide rates when applied with the proper adjuvant combination.

A diagnostic test (Syngenta Quick-Test, QT) used for testing grass weed survivors to herbicides in the field for resistance was evaluated. Cuttings from grass weeds were transplanted into pots to regenerate new leaves, then treated with herbicide. In greenhouse experiments, resistance of known herbicide-resistant blackgrass biotypes to the aryloxyphenoxypropanoate herbicides CGA 184927 and fenoxaprop-ethyl and to the phenylurea herbicide isoproturon was verified by the QT. The findings were similar to those for seedlings grown from seed. Rigid ryegrass from suspect resistant fields in South Australia was sampled and sent by post to Switzerland for QT analysis. Resistance was confirmed in less than 4 wk, which verified resistance as responsible for the field failures. The added features of the QT over current resistance tests suggest a likely fit for in-season testing of surviving weeds and possible follow-up action.

A survey conducted across agricultural ecoregions of Saskatchewan in 1996 revealed that wild oat (Avena fatua) populations resistant to acetyl-CoA carboxylase (ACCase) inhibitors were present in approximately 10% of Saskatchewan fields (2.4 million ha). In the Aspen Parkland and Boreal Transition ecoregions, this increased to 17%. The objective of this study was to determine if agronomic practices promoted or delayed resistance and to assess producer awareness of herbicide resistance. Weed resistance and management questionnaire data from the 1996 resistance survey and management questionnaire data from the 1995 Saskatchewan weed survey were submitted to multiway frequency analysis. The frequency of occurrence of herbicide-resistant wild oat was related directly to ACCase inhibitor use. Resistance to cyclohexanedione (CHD) herbicides was not related to CHD use but to frequency of ACCase inhibitor use (i.e., CHD + aryloxyphenoxypropanoate [AOPP]), suggesting that the pressure imposed by AOPPs contributed to the selection of CHD resistance in wild oat. ACCase inhibitor use was more extensive in the Aspen Parkland and Boreal Transition ecoregions than in the Mixed and Moist Mixed Grassland ecoregions. Crop rotations were not conducive to rotation of herbicides with different sites of action. Frequency of ACCase inhibitor use increased with frequency of annual crops, in spite of the inclusion of cereal and dicot crops in the rotation. Producers utilizing conservation tillage practices in the Grassland ecoregions used proportionally more ACCase inhibitors than those using conventional tillage practices. This increase in ACCase use in conservation tillage systems did not result in an increased incidence of wild oat populations resistant to ACCase inhibitors. Producers reporting troublesome wild oat populations tended to have proportionally more ACCase-resistant wild oat. Producers who reported practicing weed sanitation were less likely to have resistant wild oat than those who were less careful. Increased awareness and implementation of management practices that will reduce the dependency on ACCase herbicides are required to better enable producers to prevent, delay, or manage herbicide-resistant wild oat populations.