Field studies were conducted in 2018 and 2019 in Arkansas, Indiana, Illinois, Missouri, and Tennessee to determine if cover-crop residue interfered with herbicides that provide residual control of Palmer amaranth and waterhemp in no-till soybean. The experiments were established in the fall with planting of cover crops (cereal rye + hairy vetch). Herbicide treatments consisted of a nontreated or no residual, acetochlor, dimethenamid-P, flumioxazin, pyroxasulfone + flumioxazin, pendimethalin, metribuzin, pyroxasulfone, and S-metolachlor. Palmer amaranth took 18 d and waterhemp took 24 d in the cover crop–alone (nontreated) treatment to reach a height of 10 cm. Compared with this treatment, all herbicides except metribuzin increased the number of days until 10-cm Palmer amaranth was present. Flumioxazin applied alone or in a mixture with pyroxasulfone were the best at delaying Palmer amaranth growing to a height of 10 cm (35 d and 33 d, respectively). The herbicides that resulted in the lowest Palmer amaranth density (1.5 to 4 times less) integrated with a cover crop were pyroxasulfone + flumioxazin, flumioxazin, pyroxasulfone, and acetochlor. Those four herbicide treatments also delayed Palmer amaranth emergence for the longest period (27 to 34 d). Waterhemp density was 7 to 14 times less with acetochlor than all the other herbicides present. Yield differences were observed for locations with waterhemp. This research supports previous research indicating that utilizing soil-residual herbicides along with cover crops improves control of Palmer amaranth and/or waterhemp.

Henbit is a winter annual weed that is not effectively controlled by spring-applied herbicide applications. Research was conducted to determine henbit’s emergence pattern and whether fall-applied residual herbicides would be effective for henbit control in the spring. Henbit emerges in Louisiana from late October through March, but emergence predominantly occurs in the last week of October through the first week of December. Applying paraquat plus flumioxazin, oxyfluorfen, or rimsulfuron:thifensulfuron November 1 through December 15 provided better than 90% henbit control in March. Applying paraquat plus S-metolachlor on November 15 or December 1 provided 92% henbit control, which was similar to flumioxazin, oxyfluorfen, and rimsulfuron:thifensulfuron. The addition of flumioxazin, oxyfluorfen, or rimsulfuron:thifensulfuron to paraquat reduced the height of henbit plants by 4% to 22% of nontreated plants when applied November 1 through December 15. These studies indicate that crop producers can achieve control of henbit in March following November 1 through December 15 applications of paraquat plus flumioxazin or rimsulfuron:thifensulfuron; however, paraquat plus oxyfluorfen can be applied October 15 through December 15 to achieve similar control.

Dinitroaniline herbicides are major herbicides used to control annual grass weeds in cool-season turfgrasses. At least three herbicides, benefin, pendimethalin, and trifluralin are labeled preemergence to control weeds in established turfgrass. Prodiamine could be a competitive product. Benefin at 2.2 to 3.4 kg ai/ha, pendimethalin at 1.7 to 3.4 kg ai/ha, prodiamine at 0.6 to 1.7 kg ai/ ha, and benefin plus trifluralin at 1.1 plus 0.6 to 2.3 plus 1.1 kg ai/ha effectively control many annual grass weeds, including large and smooth crabgrass, yellow and green foxtail, barnyardgrass, goosegrass, fall panicum, and annual bluegrass. Most dinitroaniline herbicides provide residual preemergence weed control for the entire growing season. In general, Kentucky bluegrass, perennial ryegrass, tall fescue, red fescue, and other cool-season turfgrasses tolerate most of these herbicides, although higher-than-normal rates of any dinitroaniline herbicide could restrict root growth and cause stand thinning in turfgrass species.

Several herbicides were applied in January, February, and March with a comparison of the intervals of treatment of 2 and 4 weeks after the initial treatment each month for postemergence control of winter annual weeds in bermudagrass [Cynodon dactylon (L.) Pers.] turf. Glyphosphate [N-(phosphonomethyl)glycine] treatments applied at 2-week intervals with the initial treatment made in January or February controlled a higher percentage of annual bluegrass (Poa annua L.) than when applied in March. Hop clover (Trifolium agrarium L.) control was also higher when glyphosate was initially applied in January or February than when applied in March regardless of time interval between first and second treatment. Combination treatments of (a) 2,4-D [(2,4-dichlorophenoxy)acetic acid] + dicamba (3,6-dichloro-o-anisic acid) and (b) 2,4-D + mecoprop {2-[(4-chloro-o-tolyl)oxy] propionic acid} + dicamba applied at 2-week intervals with the initial treatment made in January or February controlled more corn speedwell (Veronica arvensis L.) and hop clover than when applied in March. Highest henbit (Lamium amplexicaule L.) control was obtained from the combination 2,4-D treatments made at 4-week intervals when initial treatment was made in February and March. Weed control was not influenced by dates and interval of repeated treatments with either paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) or atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] treatments. Germination and regrowth of weeds were greater in plots treated with glyphosate and paraquat initially in January or February than other herbicide-treated plots. Weeds were not reestablished in any of the atrazine-treated plots. Paraquat and combinations of 2,4-D + dicamba or 2,4-D + mecoprop + dicamba injured bermudagrass when applied initially in January and February even though they were applied to turf that appeared dormant. All herbicides injured bermudagrass more when applied to semi-dormant turf in March than to dormant turf in January or February. Atrazine affected bermudagrass less than any of the other herbicides tested when initial treatment was applied in March to semi-dormant turf.

Greenhouse and field experiments were conducted to evaluate the efficacy of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} on Russian thistle (Salsola iberica Sennen and Pau # SASKR) in a spring wheat (Triticum aestivum L.) summer-fallow rotation. In the greenhouse, when chlorsulfuron was applied to Russian thistle at three stages of growth, shoot dry weight was reduced similarly with either 4.4 g ai/ha plus surfactant or 140 g ai/ha plus surfactant. In the field, postharvest control (1981) of Russian thistle with 17.5 to 140 g/ha of chlorsulfuron ranged from 35 to 60% without surfactant and from 76 to 93% with surfactant. During the summer-fallow year (1982), Russian thistle treated in the preflowering stage of growth was not effectively controlled with 17.5 or 35 g/ha. In contrast, residual control from postharvest treatments 1 yr after application ranged from 90 to 100%. Chlorsulfuron applied in the crop (1983) controlled Russian thistle throughout the growing season. All treatments of chlorsulfuron applied in 1981, 1982, and 1983 increased wheat yield at least 75%.

A postemergence (POST) timing study was conducted on established populations of burcucumber (Sicyos angulatus) in corn (Zea mays), and a second study examined the residual activity of several herbicides for burcucumber control under greenhouse conditions. In the field study, flumiclorac, halosulfuron, primisulfuron, CGA 152005, and CGA 152005 + primisulfuron (45, 71, 40, 40, and 20 + 20 g ai/ha, respectively) were applied at two POST timings. CGA 152005, primisulfuron, and the combination provided greater than 85% control of burcucumber 14 wk after planting (WAP). Flumiclorac and halosulfuron provided 60% control or less by 8 WAP. Timing of the POST applications did not influence burcucumber control by 11 WAP with any herbicide. In the greenhouse, germinated burcucumber seeds were placed in soil treated with atrazine, chlorimuron, primisulfuron, or CGA 152005 at normal field use rates. All treatments provided similar residual control early; however by 4 wk after treatment (WAT), control from atrazine was less than 10% compared to 69% for chlorimuron and about 50% for primisulfuron and CGA 152005. This research suggests that CGA 152005 and primisulfuron can both be effective for managing burcucumber in corn, whereas flumiclorac and halosulfuron proved ineffective.

Field trials were conducted in East Lansing, MI in 2004 and 2005 and in St. Charles, MI in 2004, 2005, and 2006 to compare weed control and sugarbeet tolerance from microrate herbicide treatments that included s-metolachlor and dimethenamid-P. Treatments included the base microrate treatment alone and with full- and split-application rates of s-metolachlor at 1.4 kg/ha or dimethenamid-P at 0.84 kg/ha at the various microrate application timings. All treatments injured sugarbeet. In 2004 and 2006, full rates of both s-metolachlor and dimethenamid-P applied PRE or in the first microrate application injured sugarbeet more than the base microrate treatment. When s-metolachlor or dimethenamid-P were split-applied between PRE and the third microrate application or between the first and the third microrate applications, injury was still greater than from the base microrate treatment. Furthermore, applying dimethenamid-P at one-fourth the full rate in all four microrate applications injured sugarbeet more than the base microrate treatment. A full rate of s-metolachlor or dimethenamid-P applied in either the third or fourth microrate applications or splitting the applications between the second and fourth microrate treatments did not increase sugarbeet injury compared with the base microrate treatment. Control of common lambsquarters and giant foxtail from all treatments containing s-metolachlor or dimethenamid-P, regardless of the time of application, was greater than from the base microrate treatment at all locations. Pigweed spp. control was 94% or greater from all treatments. In 2004, late-season control of giant foxtail was greater from all treatments that included s-metolachlor or dimethenamid-P compared with the base microrate treatment. In 2005, the only treatments that did not improve late-season giant foxtail control compared with the base microrate treatment were the treatments that included a full rate of s-metolachlor or dimethenamid-P applied in the fourth microrate application. Even though some herbicide treatments that included s-metolachlor or dimethenamid-P injured sugarbeet more than the base microrate treatment, recoverable sucrose per hectare was similar among treatments.

Field studies were conducted during 1999 and 2000 to compare weed control after fall and early-preplant (EPP) herbicide applications in no-till soybean. Three residual treatments (chlorimuron plus metribuzin, chlorimuron plus sulfentrazone, and metribuzin) were applied at two rates and timings (fall and 30 d EPP) either alone or in combination with glyphosate and 2,4-D. The addition of glyphosate and 2,4-D to fall-applied residual herbicides significantly increased control of common chickweed, annual bluegrass, cressleaf groundsel, and shepherd's-purse. The effect of application rate on weed control was species dependent. Fall-applied residual herbicides were comparable with EPP treatments with respect to winter annual weed control; however, at planting control of summer annual weed species with fall treatments was less consistent compared with EPP residual herbicides.

Field experiments were conducted in 2011 through 2013 at the MSU Southern Agricultural Research Center near Huntley, MT, to evaluate the effectiveness of various PRE and POST herbicide programs for kochia control in the absence of a crop. PRE herbicides labeled for corn, grain sorghum, soybean, wheat/barley, and/or in chemical fallow were applied at recommended field-use rates. Acetochlor + atrazine, S-metolachlor + atrazine + mesotrione, and sulfentrazone applied PRE provided ≥91% control of kochia at 12 wk after treatment (WAT). Metribuzin, metribuzin + linuron, and pyroxasulfone + atrazine PRE provided 82% control at 12 WAT. PRE control with acetochlor + flumetsulam + clopyralid, pyroxasulfone alone, and saflufenacil + 2,4-D was ≤23% at 12 WAT. Paraquat + atrazine, paraquat + linuron, and paraquat + metribuzin controlled kochia ≥98% at 5 WAT. POST control with bromoxynil + fluroxypyr, paraquat, tembotrione + atrazine, and topramezone + atrazine treatments averaged 84% at 5 WAT, and did not differ from glyphosate. Control with POST-applied bromoxynil + pyrasulfotole, dicamba, diflufenzopyr + dicamba + 2,4-D, saflufenacil, saflufenacil + 2,4-D, saflufenacil + linuron was 67 to 78% at 5 WAT. Because of the presence of kochia resistant to acetolactate synthase-inhibiting herbicides at the test site, cloransulam-methyl was not a viable option for kochia control. In a separate greenhouse study, kochia accessions showed differential response to the POST herbicides (labeled for corn or soybean) tested. Tembotrione + atrazine, topramezone + atrazine, lactofen, or fomesafen effectively controlled the glyphosate-resistant kochia accession tested. Growers should utilize these effective PRE- or POST-applied herbicide premixes or tank mixtures (multiple modes of action) to control herbicide-resistant kochia accessions in the field. PRE herbicides with 8 wk of soil-residual activity on kochia would be acceptable if crop competition were present; however, a follow-up herbicide application may be needed to obtain season-long kochia control in the absence of crop competition.

Weed management in furrow-irrigated corn is challenging because of weed emergence associated with each irrigation event. Residual herbicides that provide extended in-season control of a broad spectrum of weeds would be beneficial to producers in this system. Field experiments were conducted in 2005 and 2006 in Yellowstone County, Montana, to evaluate KIH-485 for the control of velvetleaf, kochia, and wild buckwheat in furrow-irrigated corn. KIH-485 was applied at three rates (166, 209, and 250 g ai/ha) and two timings (PRE and POST) and compared to standard rates of S-metolachlor, acetochlor, and pendimethalin. All PRE treatments were applied alone, whereas POST treatments were combined with 1,261 g ae/ha of glyphosate. All rates of KIH-485 applied PRE controlled velvetleaf and kochia 88% or greater at 4 mo after planting (MAP). Wild buckwheat was controlled 89% or greater with the high rate of KIH-485 applied PRE, which was superior to control achieved with any other PRE herbicide treatment. Velvetleaf, kochia, and wild buckwheat were controlled 91% or greater when any herbicide treatment was combined with glyphosate. Corn treated with KIH-485 applied at 209 g ai/ha PRE produced yield that was similar to that produced by the weed-free control in both years.

Reducing herbicide rates can reduce production costs and the potential for carryover injury in subsequent crops, but may also compromise residual weed control. When used at the highest recommended rates (0.35 kg ai/ha), fomesafen may cause carryover injury on sweet corn. One of the most valuable attributes of fomesafen use in snap and dry beans is control of common ragweed. Field experiments were conducted in 1999, 2000, and 2002 to investigate both the residual ragweed control from fomesafen at 0.09, 0.18, and 0.36 kg ai/ha and the potential for a second application to extend the duration of residual control, while reducing the total amount applied per season. Results were significantly affected by weather conditions. In a very wet year (2000), with the greatest ragweed pressure, a single application of fomesafen at 0.20 kg/ha was predicted to provide at least 90% control of ragweed throughout snap and dry bean seasons. In the driest year (1999), 0.09 kg ai/ha provided greater than 90% control for the same period. Regardless of whether a single or double application was made, the total amount of fomesafen required per season was less than the manufacturer's recommended rate. Compared with making a single application, two low-rate applications did not reduce the total fomesafen rate required per season. To help insure at least 90% control of ragweed throughout the snap and dry bean seasons a grower should apply 0.20 kg ai/ha early in the season. Lower rates may be used, but in wet seasons, a second application may be needed to prevent yield loss or interference of ragweed with mechanical harvesting.