Jointed goatgrass (Aegilops cylindrica) is a serious problem for winter wheat producers throughout the western U.S. Interference from this weed can severely reduce grain yield and contaminate harvested grain, resulting in dockage losses. There are currently no selective herbicides registered for controlling jointed goatgrass in wheat. Imazamox, an imidazolinone herbicide, was applied to an imidazolinone herbicide-resistant (IMI) wheat mutant of the winter wheat cultivar ‘Fidel.’ Jointed goatgrass control from spring postemergence application of imazamox ranged from 61 to 97% when applied at 36 g/ha. Negligible crop injury from imazamox treatment was observed at 36 g/ha at several locations under dryland environments in the U.S. Pacific Northwest. Wheat yield was increased 19 to 41% by imazamox treatment in three of four experiments. Percent dockage resulting from jointed goatgrass spikelet contamination of harvested wheat grain was eliminated by imazamox treatment. Introduction of the IMI trait into commercial wheat cultivars could provide an effective method for selective control of jointed goatgrass in winter wheat.

Field studies were conducted to examine weed control efficacy and crop injury with SAN 582 (proposed name, dimethenamid) in irrigated ‘Russet Burbank’ potato. SAN 582 applied preemergence (PRE) at 1.1 to 1.7 kg ai/ha controlled light to moderate infestations of redroot pigweed, common lambsquarters, and hairy nightshade > 90%. Season-long control of these species with mixtures of SAN 582 plus metribuzin or rimsulfuron was > 98% and was similar to or better than registered mixtures. Green foxtail control with SAN 582 alone ranged from 83 to 100%, depending on rate and year. SAN 582 mixtures with metribuzin or rimsulfuron usually controlled green foxtail > 90%. In weed-free plots, SAN 582 was applied alone PRE or early postemergence (EPOST) at rates up to 3.4 kg/ha. SAN 582 applied EPOST usually caused 10 to 30% more potato injury than when applied PRE, but U.S. No. 1 yield was not reduced by SAN 582 applied either PRE or EPOST. SAN 582 shows good potential for PRE use in irrigated potato.

Field experiments were conducted to determine the effect of different rates and application times of MON 37500 on downy brome control in winter wheat and on rotational crop injury. In southeastern Washington, MON 37500 applied postemergence at 36 to 72 g ai/ha in the fall and spring controlled downy brome 90 to 97% and 45 to 71%, respectively. MON 37500 was applied postemergence to winter wheat at 18, 36, and 72 g/ha during fall 1995 and spring 1996 near Moscow, ID; Pendleton, OR; and Endicott, WA, to determine its effect on rotational crops planted during spring 1997. At Pendleton, OR, fall applications of MON 37500 at 18, 36, and 72 g/ha applied to actively growing wheat reduced seed yield of spring barley and pea 13 to 59% and 26 to 29%, respectively, when these crops followed winter wheat in the rotation. At this site, spring applications of 36 and 72 g/ha reduced seed yield of spring barley 17 to 34%; when applied in the fall or spring, MON 37500 reduced canola yield 31 to 73%. MON 37500 at 18 g/ha in the spring reduced pea biomass 32% at Endicott. Aboveground biomass and seed yield of barley, pea, lentil, and mustard at Moscow were not affected by MON 37500 carryover. However, using a grain sorghum bioassay, predicted final concentrations of MON 37500 in soil at all three locations 360 d after treatment (DAT) was similar, ranging from 0.5 to 2 ng ai/g. In growth chamber herbicide dose–response studies, grain sorghum shoot height was reduced more at lower doses (0.025 to 5 ng/g) of MON 37500 in Pendleton soil than in soils from the other field locations. Perhaps the greater rotational crop injury observed at the Pendleton location was partly related to greater soil activity of the herbicide.

A 1984–1985 and 1988–1989 field study near Pullman, WA, evaluated the effect of two winter wheat planting geometries on the growth of wheat and competition against jointed goatgrass. Treatments included paired and constant row planting geometries of wheat and locations of jointed goatgrass within each geometry. No planting geometry by weed location interactions occurred at any harvest date for plant height, shoot number, leaf area, plant dry weight, or N uptake for either wheat or jointed goatgrass. During the 1984–1985 growing season, N uptake data indicated that by heading, jointed goatgrass had taken up N that had been deep-banded between wheat rows located 25 cm from the weed. Winter wheat yields were not different in the paired-row and the constant row spacing geometry in a weed-free environment. Within years, for both planting geometries, winter wheat yield reduction from weed competition was similar for the jointed goatgrass locations. In contrast, based on jointed goatgrass spikelets produced, wheat grown in paired-rows was more competitive against jointed goatgrass compared to constant row spacing.

Field studies were conducted to assess weed control and potato (Solanum tuberosum) tolerance to ethalfluralin. Ethalfluralin applied preemergence (PRE) alone at 1.05 kg ai/ha generally did not control weeds adequately. However, ethalfluralin at 1.05 kg/ha combined with either metribuzin at 0.28 kg ai/ha or rimsulfuron at 0.018 kg ai/ha controlled common lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), and green foxtail (Setaria viridis) ≥ 98%, which was similar to control observed with several currently registered herbicide mixtures. Volunteer oat (Avena sativa) control with either ethalfluralin at 1.05 kg/ha plus EPTC at 3.4 kg ai/ha or ethalfluralin plus metribuzin was equal to registered two-way mixtures. Ethalfluralin plus metribuzin did not adequately control hairy nightshade (Solanum sarrachoides), but ethalfluralin mixtures with either rimsulfuron or EPTC controlled hairy nightshade equal to or better than the registered two-way mixtures evaluated. A sequential application of ethalfluralin PRE followed by rimsulfuron or rimsulfuron plus metribuzin postemergence (POST) did not improve hairy nightshade control compared to ethalfluralin plus rimsulfuron applied PRE. Potato tolerance to herbicide treatments applied PRE or POST to potato was evaluated in weed-free studies. Ethalfluralin alone or with metribuzin was compared to mixtures of metribuzin with either pendimethalin or EPTC. Initial visual injury with ethalfluralin PRE was ≤ 4% both years. In 1996, initial injury with ethalfluralin POST was ≤ 4% and U.S. No. 1 and total tuber yields were not affected by herbicide treatment or application timing. However in 1997, initial injury from POST ethalfluralin at 1.05 or 2.1 kg/ha was 2 or 8% and increased to 9 or 17%, respectively, at potato row closure. Averaged over all herbicide treatments, POST applications reduced U.S. No. 1 and total tuber yield 7% relative to PRE applications in 1997.

Field studies assessed weed control and potato injury with rimsulfuron applied postemergence at various rates in combination with various adjuvants. Weed control was influenced by choice of adjuvant and rimsulfuron rate. Rimsulfuron at 0, 9, 18, 26, and 35 g ai/ha was applied with nonionic surfactant (NIS), crop oil concentrate (COC), methylated seed oil (MSO), or silicone-polyether copolymer (SIL). Potato injury was less than 5% for all rimsulfuron rates and adjuvant combinations. Redroot pigweed was controlled greater than or equal to 93% by all treatments except rimsulfuron at 9 g/ha + SIL. Except for redroot pigweed, rimsulfuron treatments with SIL controlled kochia, hairy nightshade, common lambsquarters, and volunteer oats less than with other adjuvants. At lower rimsulfuron rates, weed control with rimsulfuron + MSO tended to be greater than with rimsulfuron + NIS or rimsulfuron + COC. Common lambsquarters control was 75% or less regardless of rimsulfuron rate or adjuvant. Tuber yield generally increased with increasing rimsulfuron rates. Depending on rimsulfuron rate, tuber yield was 10 to 15% lower with rimsulfuron + NIS or rimsulfuron + COC compared to rimsulfuron + MSO, while tuber yield was 18 to 37% lower with rimsulfuron + SIL compared to rimsulfuron + NIS, rimsulfuron + COC, or rimsulfuron + MSO.

Field studies were conducted at Aberdeen, ID; Ontario, OR; and Paterson, WA, to evaluate potato tolerance to flumioxazin and sulfentrazone. In ‘Russet Burbank’ tolerance trials conducted in 2000 at ID, OR, and WA, sulfentrazone applied preemergence (PRE) at rates ranging from 105 to 280 g ai/ha caused significant injury consisting of stunting, leaf discoloration-blackening, and/or leaf malformation-crinkling at 4 wk after treatment (WAT). By 12 WAT, injury was ≤5%. At 4 WAT, flumioxazin applied PRE at 105 and 140 g ai/ha resulted in injury, whereas 53 g ai/ha did not cause significant injury. At 12 WAT, no visual injury was present at the ID site, whereas flumioxazin at 140 g/ha was still causing injury in WA. Regardless of initial injury, Russet Burbank tuber yields at ID, OR, and WA were not reduced as a result of any flumioxazin or sulfentrazone treatment compared with the nontreated controls. In potato variety tolerance trials conducted at ID in 2000 and at WA in 2002 with Russet Burbank, ‘Ranger Russet’, ‘Russet Norkotah’, and ‘Shepody’ and at ID in 2002 with those varieties plus ‘Alturas’ and ‘Bannock Russet’, early season injury caused by flumioxazin or sulfentrazone applied PRE at rates as high as 210 g ai/ha or 280 g/ha, respectively, occurred, but variety tuber yields were not reduced compared with nontreated control yields. In contrast, at ID in 2001, early injury caused by flumioxazin or sulfentrazone applied PRE at 105 or 210 g/ha translated to tuber yield reductions of all six varieties tested compared with the nontreated controls. At WA in 2001, Ranger Russet tuber yields were reduced by PRE applications of flumioxazin at 53 to 140 g/ha or sulfentrazone at 105 to 280 g/ha, and Shepody total tuber yields were reduced by all rates of PRE-applied sulfentrazone. Russet Burbank and Russet Norkotah tuber yields were unaffected by either herbicide. Unusual heat stress occurring early in the 2001 growing season at both locations may have compounded the effects of herbicide injury and, consequently, tuber yields were reduced in 2001, whereas injury occurring in 2000 or 2002 during relatively normal growing conditions did not translate to yield reductions.

Field studies were conducted to evaluate weed control, tuber yield, gross return, economic return on investment (EROI), and net return in glyphosate-resistant ‘Ranger Russet’ potato in 2000 and 2001 at the University of Idaho Aberdeen Research and Extension Center near Aberdeen, ID. Three types of weed control programs were evaluated: a total glyphosate program of single or sequential applications (TGLY), tank mixtures of glyphosate and residual herbicides applied early postemergence (GLY + RES EPOST), and residual preemergence herbicides followed by (fb) a late postemergence glyphosate application (RES PRE fb LPOST GLY). A standard rimsulfuron + metribuzin + nonionic surfactant EPOST treatment was included for comparison. The standard EPOST treatment and all glyphosate-containing treatments controlled hairy nightshade 88 to 99%. RES PRE fb LPOST GLY treatments improved hairy nightshade control compared with the RES PRE components applied alone. All herbicide treatments controlled kochia 87 to 99% and green foxtail 87 to 100%. Redroot pigweed and common lambsquarters were controlled ≥85 and ≥89%, respectively, by all herbicide treatments except a single EPOST application of glyphosate at 420 g ae/ha. Depending on the year, sequential applications of glyphosate, GLY + RES EPOST, or RES PRE fb GLY LPOST treatments controlled weeds better than single EPOST glyphosate applications. Single LPOST glyphosate applications generally controlled kochia, redroot pigweed, common lambsquarters, and green foxtail better than single EPOST applications. However, single EPOST glyphosate applications controlled hairy nightshade better than a single LPOST application of glyphosate at 420 g/ha. RES PRE fb GLY LPOST treatments improved redroot pigweed, common lambsquarters, and green foxtail control, compared with the RES PRE components applied alone, depending on the RES PRE component and the year. Sequential applications of glyphosate at 840 g ae/ha and the standard nonglyphosate EPOST, GLY + RES EPOST, and RES PRE fb GLY LPOST treatments generally provided similar weed control. No crop injury was observed as a result of any herbicide treatment. Sequential applications of glyphosate at 840 g/ha had better tuber yields and economic returns than a single EPOST or LPOST application of glyphosate at 420 g/ha or a single LPOST application of glyphosate at 840 g/ha. A single EPOST application of glyphosate at 420 g/ha had lower tuber yields and economic returns than a single EPOST application of glyphosate at 840 g/ha. The RES PRE alone treatments, except metribuzin + pendimethalin, had similar tuber yields, EROI, and net returns as sequential applications of glyphosate at 840 g/ha. Glyphosate + rimsulfuron resulted in lower tuber yields than sequential applications of glyphosate at 840 g/ha, whereas EROI and net returns were similar. All other combinations of glyphosate and residual herbicides except glyphosate + pendimethalin EPOST, had similar tuber yields, EROI, and net returns as sequential applications of glyphosate at 840 g/ha.

Treatments of dimethenamid-p at 0.7 kg ai/ha applied PRE in tank mixtures with EPTC (3.4 kg ai/ha), metribuzin (560 g ai/ha), pendimethalin (1.1 kg ai/ha), or rimsulfuron (26 g ai/ha) were compared with the same herbicides applied PRE alone in field efficacy trials in Idaho, Oregon, and Washington. Common lambsquarters, kochia, and redroot pigweed control was generally improved with dimethenamid-p tank mixtures compared with control by any herbicide applied alone except metribuzin. Hairy nightshade control at two locations was generally improved with tank mixtures compared with control by any herbicide applied alone. At Washington, where row spacing was narrower than at other locations, dimethenamid-p alone or in tank mixtures provided similar hairy nightshade control, and this control was greater than control by EPTC, metribuzin, or pendimethalin applied alone. ‘Alturas’, ‘Bannock Russet’, ‘Ranger Russet’, ‘Russet Burbank’, ‘Russet Norkotah’, and ‘Shepody’ potato tolerance to dimethenamid-p PRE at 0, 0.7, or 1.4 kg ai/ha was assessed in weed-free field trials conducted at Aberdeen, ID, in 2002 and 2003. Little or no crop injury was evident during the growing seasons and there were no reductions in U.S. No. 1 and total tuber yields regardless of dimethenamid-p rate or potato variety.

Imazamox is an imidazolinone herbicide being developed for weed control in imidazolinone-resistant wheat (IMI-wheat) cultivars and various legume crops. In a series of studies conducted under a range of dryland cropping environments in the Pacific Northwest United States, imazamox applied to IMI-wheat or pea injured barley and canola grown 1 yr after imazamox treatment in low-rainfall, low–soil pH locations of Oregon. Injury was not observed in higher rainfall locations near Pullman, WA. Non–herbicide-resistant wheat planted 1 yr after IMI-wheat treated with imazamox was not injured. Of particular concern for imazamox carryover are low-rainfall areas with low-pH soils. Reduced soil moisture appears to limit imazamox degradation. Imazamox sorption is reduced in low-pH soils, which increases its bioavailability, thereby increasing the potential for injury to rotational crops such as barley, canola, and spring wheat.

BAY MKH 6561, a sulfonylaminocarbonyl-triazolinone herbicide for postemergence control of annual grasses and selected broadleaf weeds in wheat, was evaluated for weed control and rotational crop (barley, pea, lentil, and mustard) injury in the Pacific Northwest. BAY MKH 6561 was applied postemergence in winter wheat at 22, 45, and 90 g ai/ha during fall 1997 and spring 1998 near Moscow, ID, Pendleton, OR, and Wilcox, WA, to determine its effect on barley, pea, lentil, and mustard planted during spring 1999. At Pendleton, BAY MKH 6561 reduced barley height 6% and grain yield 11%, when applied in the spring at 90 g/ha, and visibly injured mustard 4 to 19% when applied at 45 or 90 g/ha. All BAY MKH 6561 treatments reduced mustard seed yield 47 to 54% at Moscow and 38 to 48% at Wilcox. Pea and lentil seed yields were not affected by herbicide treatments at all locations, whereas barley was not affected at Moscow and Wilcox. In growth chamber soil bioassay experiments, fall-applied BAY MKH 6561 dissipated 10 to 48% faster at Moscow compared to Pendleton, and the predicted half-life ranged from about 68 (Moscow) to 79 d (Pendleton). Dissipation of spring-applied BAY MKH 6561 at 45 and 90 g/ha was 17 to 21% slower at Moscow than Wilcox, and the predicted half-life ranged from 60 (Wilcox) to 69 d (Moscow).

Field experiments were conducted on a silty clay loam in Corvallis, OR during the summers of 1995 and 1996 to study the effects of soil solarization, spring-planted green manure crops, fumigation with metham, and combinations of these treatments on annual bluegrass seed survival. Annual bluegrass seeds were incorporated into the soil as a bioassay species and soil samples extracted to a depth of 15 cm to determine effects on seed survival. Soil solarization was applied over a 53- or 59-d period using a 0.6-mil clear polyethylene film. Soil samples were collected from four depths after the solarization period in both solarized and nonsolarized plots and surviving seeds germinated in a greenhouse. Maximum soil temperatures recorded at 5-, 10-, and 20-cm depths were 52, 47, and 33 C in solarized soil, respectively. Solarization reduced annual bluegrass seed survival from 89 to 100% in the upper 5 cm of soil, but did not reduce survival below 5 cm. Solarization may have enhanced seed survival below 5 cm. Cover crops of barley, rapeseed, and sudangrass generally increased survival of annual bluegrass seeds buried 2.5 to 15 cm deep in the soil. Green manure cover crops plus solarization did not improve the efficacy of solarization alone and in some cases diminished the effectiveness of solarization. Solarization significantly improved the efficacy of one-quarter rates of metham (230 L/ha) in the top 5 cm of soil, reducing overall annual bluegrass seed survival in the soil by 40% compared with metham alone (230 L/ha) but only 30% compared with solarization alone. The conventional rate of metham (930 L/ha) was the most effective and consistent treatment across all depths.

The transfer of herbicide resistance genes from crops to related species is one of the greatest risks of growing herbicide-resistant crops. The recent introductions of imidazolinone-resistant wheat in the Great Plains and Pacific Northwest regions of the United States and research on transgenic glyphosate-resistant wheat have raised concerns about the transfer of herbicide resistance from wheat to jointed goatgrass via introgressive hybridization. Field experiments were conducted from 2000 to 2003 at three locations in Washington and Idaho to determine the frequency and distance that imidazolinone-resistant wheat can pollinate jointed goatgrass and produce resistant F1 hybrids. Each experiment was designed as a Nelder wheel with 16 equally spaced rays extending away from a central pollen source of ‘Fidel-FS4’ imidazolinone-resistant wheat. Each ray was 46 m long and contained three rows of jointed goatgrass. Spikelets were collected at maturity at 1.8-m intervals along each ray and subjected to an imazamox screening test. The majority of all jointed goatgrass seeds tested were not resistant to imazamox; however, 5 and 15 resistant hybrids were found at the Pullman, WA, and Lewiston, ID, locations, respectively. The resistant plants were identified at a maximum distance of 40.2 m from the pollen source. The overall frequency of imazamox-resistant hybrids was similar to the predicted frequency of naturally occurring acetolactate synthase resistance in weeds; however, traits with a lower frequency of spontaneous mutations may have a relatively greater risk for gene escape via introgressive hybridization.

‘Russet Burbank’ potato tolerance to dimethenamid-p applied preemergence at 0.7, 1.4, or 2.9 kg ai/ha was assessed in field studies conducted at Aberdeen, ID and Ontario, OR. Although crop injury was evident approximately 2 wk after treatment, most injury had diminished by row closure 2- to 3-wk later. Initial injury did not translate to yield loss and U.S. No. 1 and total tuber yields in dimethenamid-p–treated plots were similar to yields in the untreated, weed-free control.

The effects of postemergence rimsulfuron, metribuzin, and adjuvant combinations on potato crop safety and weed control were evaluated in field studies conducted at the University of Idaho Aberdeen Research and Extension Center in 1999 and 2000. Rimsulfuron at 26 g ai/ha plus metribuzin at 0, 140, or 280 g ai/ha was combined with nonionic surfactant (NIS), crop oil concentrate (COC), or methylated seed oil (MSO) in a 3 by 3 factorial with two controls. Under cool, cloudy conditions in 1999, initial ‘Russet Burbank’ potato injury was greater when metribuzin was included in the tank mixture than when rimsulfuron was applied alone, regardless of adjuvant. Under warmer conditions in 2000, however, adding MSO or COC to the tank mixture caused more injury than adding NIS. Rimsulfuron did not provide acceptable season-long common lambsquarters control in 1999 (76%) or in 2000 (88%), regardless of adjuvant. Rimsulfuron combined with metribuzin at 140 or 280 g/ha provided ≥95% common lambsquarters control both years, regardless of adjuvant. Among adjuvants, using MSO (1999 and 2000) or COC (2000) in the spray mixture improved common lambsquarters control compared with using NIS. Tuber yield and quality were not reduced as a result of metribuzin rate or adjuvant treatments either year compared with the weed-free control.

Volunteer crops resistant to glyphosate and other herbicides pose a potential problem for Pacific Northwest (PNW) growers that rely on glyphosate for control of volunteer crops and weeds during fallow and before planting. Herbicides for control of volunteer herbicide-resistant wheat and canola in PNW conservation tillage systems were evaluated during 2000 and 2001 near Ralston, WA, and Moscow, ID. Paraquat + diuron controlled glyphosate- and imidazolinone-resistant wheat ≥90%, and glyphosate controlled imidazolinone-resistant wheat 88 to 96% 14 d after treatment (DAT). Glyphosate- and imidazolinone-resistant wheat were controlled only 58 to 85% with quizalofop-P and clethodim 14 DAT. By 21 DAT, imidazolinone-resistant wheat control with clethodim and quizalofop-P was ≥93%, but the longer time period required for control to reach an acceptable level could increase disease and insect problems associated with volunteer wheat. Volunteer glyphosate-resistant canola was controlled 92 and 97% 14 DAT and 76 and 98% 21 DAT with paraquat and paraquat + diuron, respectively. Treatments that contained glyphosate controlled imidazolinone- and glufosinate-resistant canola >84% 14 DAT. By 21 DAT, control of imidazolinone- and glufosinate-resistant canola was 94 to 98% with paraquat + diuron and all glyphosate treatments, except glyphosate–isopropylamine salt (IPA) + glufosinate (88 to 93%) and glyphosate-IPA + paraquat (67 to 85%). In these studies, paraquat + diuron was the best alternative to glyphosate for controlling volunteer herbicide-resistant wheat and canola.

A general life cycle model was modified to demonstrate how agronomic practices and weed biology factors affect the rate of appearance of herbicide-resistant downy brome, jointed goatgrass, and wild oat in Pacific Northwest wheat cropping systems. The model suggests herbicide rotation strategies for cropping systems that include imidazolinone-resistant wheat as a weed management tool. Simulation of continuous annual imidazolinone-resistant winter wheat and imazamox herbicide use resulted in the resistant soil seed banks of downy brome, jointed goatgrass, and wild oat surpassing their susceptible soil seed banks in 5, 7, and 10 yr, respectively. Reducing the initial seed bank density of downy brome before beginning a rotation that includes imidazolinone-resistant winter wheat reduces the likelihood of selecting for herbicide-resistant biotypes. The best simulated management option for reducing the total jointed goatgrass soil seed bank in low-precipitation areas is an imidazolinone-resistant winter wheat–fallow rotation. Rotations that include winter and spring crops and rotations that include non–group 2 herbicides minimize herbicide resistance selection pressure and reduce the wild oat soil seed bank.