“Wither'd hogs … on ragweed nags, They skim the muirs [moors].” Address to the Diel, 1785, Scottish poet Robert Burns (1759–1796).

Tolerance of direct-seeded pepper under clear plastic mulch to herbicides applied pre- and post-emergence was investigated in 1991 and 1992. Weed control of herbicides applied preemergence was also studied. Paprika cultivar ‘Buketen’ and canning cultivars ‘Piquillo de Lodosa’ and ‘Pico de Mendavia’ were seeded in the field in Zaragoza, Spain, in a loamy soil. Percent emergence, plant mortality after the postemergence treatment, dry weight of plants, and fruit weight were recorded. Pepper was tolerant to diphenamid and napropamide applied preemergence but these herbicides did not control broadleaf weeds. Clomazone, when applied preemergence resulted in chlorotic seedlings and at 2 kg ai/ha reduced the final stand, but was selective when applied at the 6 to 8-leaf stage of the crop. Pepper was tolerant to diethatyl-ethyl when applied preemergence. However, diethatyl-ethyl applied postemergence caused severe phytotoxicity symptoms, and when mixed with diphenamid, fruit ripeness in 1992 was delayed. Weed control during the crop emergence period was better when diethatyl-ethyl was used mixed with diphenamid or napropamide.

The comparison of incorporation vs no incorporation on a mid-October application of the granular formulation of trifluralin to control infestations of wild oat and green foxtail was evaluated in spring barley, wheat, and canola in west central Saskatchewan over a 3 yr period. Incorporation treatments consisted of: no incorporation, one incorporation in fall, one incorporation in fall and a second in spring, one incorporation in fall and two additional incorporations in spring. All treatments eliminated green foxtail. In all three crops wild oat panicle counts were equivalent from incorporated and non-incorporated trifluralin. Wild oat fresh weights in crops grown on stubble were similar for incorporated and non-incorporated trifluralin. In fallow crops, wild oat fresh weight reductions were greater in three of nine site years with incorporation. There was little difference in crop yields from incorporated and non-incorporated trifluralin.

Greenhouse and laboratory studies were conducted to determine the effect of 22 postemergence corn broadleaf herbicide combinations on the efficacy of sethoxydim applied to giant foxtail, large crabgrass, and shattercane. Eighteen combinations caused a reduction in sethoxydim efficacy on at least one grass species. Dicamba, atrazine plus dicamba, atrazine plus bentazon, bromoxynil, primisulfuron, CGA-152005 plus primisulfuron, MON 12000, and flumetsulam plus clopyralid plus 2,4-D (NAF-73) were evaluated further on the efficacy and foliar absorption of sethoxydim applied to giant foxtail, large crabgrass, and shattercane. In timing studies, applying all herbicide combinations at 7 or 3 d before sethoxydim application eliminated significant antagonistic interactions. However, applying NAF-73, primisulfuron, or CGA-152005 plus primisulfuron 1 d prior resulted in a reduction in sethoxydim efficacy on at least one grass species. Dicamba, atrazine plus dicamba, and atrazine plus bentazon decreased 14C-sethoxydim absorption 9 to 63% across grass species. Replacing crop oil concentrate (COC) with DASH increased sethoxydim absorption when applied with these herbicides but not to the full extent of sethoxydim applied alone with DASH. Sethoxydim efficacy was retained or improved with DASH when applied with dicamba, atrazine plus dicamba, atrazine plus bentazon, or bromoxynil. When primisulfuron, CGA-152005 plus primisulfuron, MON 12000, or NAF-73 was applied with 14C-sethoxydim no effect on sethoxydim absorption was observed. DASH was less effective than COC at restoring sethoxydim efficacy when applied with these herbicides.

A field study was conducted from 1987 to 1992 to determine the effectiveness of several treatments consisting of combinations of glyphosate and/or quizalofop with tillage to control quackgrass in legume plowdown and continuous annual crop rotations. The treatments were imposed on the quackgrass in 1987 and/or 1988, evaluated in 1989 (cycle 1), reimposed on the same plots in 1990 and/or 1991 and re-evaluated in 1992 (cycle 2). Where continuous barley was grown, glyphosate applied at 0.45 kg/ha before seeding in 1987 (cycle 1) and 1990 (cycle 2), and combined with fall tillage in the first two years of each cycle, reduced quackgrass shoot density and rhizome dry weight by more than 96%. In a legume plowdown rotation, consisting of barley underseeded to red clover followed by plowdown and barley in subsequent years, quackgrass was reduced by a similar amount by glyphosate at 0.45 kg/ha applied in 1987 and 1990 before seeding or in 1988 and 1991 at 5 d before or 6 wk after red clover plowdown. Quackgrass reduction from all of these treatments was as effective as the labelled rate of glyphosate (0.90 kg/ha) applied at the same times, and also as effective as glyphosate applied at 0.45 kg/ha in combination with tillage in a year of fallow. Without fall tillage the efficacy of glyphosate applied before seeding was reduced. Quizalofop at 0.20 kg/ha applied to canola in a rotation of canola followed by two years of barley, reduced quackgrass, but was less effective than glyphosate treatments with fall tillage. Reducing quackgrass populations resulted in significant increases in crop yields.

Germinated seeds of wild oat populations that were susceptible (S) or resistant (R) to triallate at the recommended soil-applied rate (1.7 kg/ha) were treated with six triallate concentrations on filter paper in petri dishes. Measurement of shoot length 8 d after treatment provided an accurate indication of differences among populations, and was more reliable than determining shoot fresh weight. ED50 values (herbicide concentrations that reduced shoot length by 50% relative to untreated controls), derived from nonlinear regression analysis, indicated four and five levels of response to triallate among eight S and seven R populations, respectively. The ED50 values varied from 0.11 to 11 ppm a.i. triallate for the most susceptible to the most resistant populations, respectively. Routine testing of wild oat samples suspected of resistance, at triallate concentrations of 0.5 or 1 ppm in the petri dish bioassay, effectively identified populations that had become resistant to the recommended soil-applied rate.

An in vivo ALS assay was used to differentiate between susceptible and resistant kochia biotypes from Idaho and Montana. Experiments were also conducted using this assay to determine the effect of the timing of herbicide application on ALS activity in resistant and susceptible cocklebur biotypes from Mississippi. When treated with chlorsulfuron, resistant kochia biotypes demonstrated 160-and 170-fold resistance at the whole plant level for the Idaho and Montana biotypes, respectively, compared to susceptible biotypes. Using the in vivo ALS assay, the Idaho and Montana biotypes showed 70- and 490-fold resistance, respectively, based on I50 values. The biotypes also demonstrated a low level of cross-resistance to imazethapyr using the in vivo technique. Resistant cocklebur biotypes showed approximately 200-fold resistance to imazaquin compared with susceptible biotypes. This research demonstrated that the in vivo assay can be used effectively to identify resistant biotypes 6 and 24 h after treatment, and to determine the extent of cross-resistance.

Two sulfometuron-resistant biotypes of Italian ryegrass were identified from rights-of-way of U.S. Highway 49E in Holmes County, MS, and State Highway 14 in Humphreys County, MS. Sulfometuron at 5.2 g ai/ha applied PRE or POST reduced shoot fresh weight of these selections 20% or less at 4 wk after treatment. A susceptible ‘Gulf’ Italian ryegrass was reduced 52% or more in shoot fresh weight by 0.5 g/ha sulfometuron applied PRE or POST The biotype from Highway 49E was reduced 47% or less in shoot fresh weight by 520 g/ha sulfometuron applied PRE or POST.

A study was conducted to evaluate effects of application timing and sulfometuron rate on weed control and bermudagrass tolerance. Sulfometuron was applied at rates from 13 to 105 g ai/ha and compared to 1120 g ae/ha 2,4-D amine. Control of Italian ryegrass and common vetch was not affected by October, November, December, or March application dates and 39 to 105 g/ha sulfometuron provided equivalent control of both species 6 mo after the initial treatment. Percent green cover of bermudagrass at 6 mo after the initial treatment was 22% or less following March applications compared to 36% or greater when applied in October, November, or December. More bermudagrass injury occurred following March treatments of 52 g/ha or greater sulfometuron where green cover was 14% and less compared to 22 and 30% green cover following 13 g/ha sulfometuron or 1120 g/ha 2,4-D amine, respectively.

‘Lemberger’ wine grape response was evaluated when chlorsulfuron, tribenuron, thifensulfuron, chlorsulfuron plus metsulfuron, thifensulfuron plus tribenuron, and 2,4-D were applied up to three times at weekly intervals in 1992 and 1993 at rates simulating drift. All herbicides injured grapevines visibly. Symptoms increased and total leaf area and grape pruning weight decreased as the herbicide rate and number of applications increased. Grapevines generally recovered within 45 to 60 d from symptoms caused by single exposures to low levels of sulfonylurea herbicides. Multiple exposures of grapevines to 2,4-D, tribenuron, and chlorsulfuron plus metsulfuron at 1/100 of the maximum use rate for wheat caused the greatest injury, which persisted throughout the entire growing season and reduced pruning weight. Based on potential use rate, the order of herbicide phytotoxicity was 2,4-D > tribenuron > chlorsulfuron plus metsulfuron > thifensulfuron plus tribenuron > chlorsulfuron > thifensulfuron. Rates that slightly injured grapevines did not reduce growth as measured by pruning weight. However, multiple exposure to high rates of 2,4-D, tribenuron, and chlorsulfuron plus metsulfuron adversely affected growth.

Field experiments were conducted from 1993 to 1995 to evaluate control of annual weeds with glyphosate applied at rates of 560 to 2800 g ai/ha in spray volumes of 93 and 187 L/ha. Glyphosate controlled 100% of giant foxtail, fall panicum, redroot pigweed, and velvetleaf (in 1993) regardless of rate, spray volume, or application timing. Ivyleaf morningglory, velvetleaf, and common lambsquarters control was greater with glyphosate applied early POST than late POST. Ivyleaf morningglory, velvetleaf, and common lambsquarters control increased as glyphosate rate increased with both spray volumes. Velvetleaf control with glyphosate applied late POST was greater with glyphosate in 187 L/ha water compared with 93 L/ha water.