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.

Experiments were conducted to determine the efficacy, absorption, and translocation of nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron on barnyardgrass, green foxtail, longspine sandbur, and large crabgrass. In the greenhouse, nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron were applied at 0.0625, 0.125, 0.25, 0.5, 0.75, 1, and 2 times their label rates of 35, 13, and 26 + 13 g ai ha−1, respectively, on 5- to 10-cm plants. Three weeks after treatment (WAT), barnyardgrass was the most susceptible species to all three herbicides, and large crabgrass was the least susceptible. The nicosulfuron, rimsulfuron, or nicosulfuron + rimsulfuron rates causing 50% visible injury (GR50) for barnyardgrass were 10.9, 4.8, and 6 + 3 g ai ha−1, respectively. Similarly, the GR50 for large crabgrass were 25.6, 9.9, and 14.3 + 7.2 g ai ha−1, respectively, 3 WAT. Absorption of nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron was greater in barnyardgrass than in large crabgrass. Absorption of nicosulfuron + rimsulfuron in barnyardgrass and large crabgrass was 74% and 57%, respectively, 7 d after treatment (DAT). In addition, translocation of nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron out of the treated leaf was 14, 12, and 14% higher, respectively, in barnyardgrass than in large crabgrass. The differential response of these weed species to nicosulfuron, rimsulfuron, and nicosulfuron + rimsulfuron might be due to differences in herbicide absorption and translocation.

Field and greenhouse studies were conducted to evaluate mesotrione alone and in combinations with low rates of atrazine and bentazon for control of yellow and purple nutsedge. Mesotrione alone at rates of 105 to 210 g ai/ha controlled yellow nutsedge 43 to 70%. Mixtures of mesotrione with atrazine at 280 g ai/ha did not always improve yellow nutsedge control over that by mesotrione alone, but increasing atrazine to 560 g ai/ha in these mixtures generally provided more consistent control of yellow nutsedge. Mesotrione at 105 g ai/ha mixed with bentazon at 280 or 560 g ai/ha controlled yellow nutsedge 88% or greater which was similar to control from the standard halosulfuron at 36 g ai/ha. Mesotrione, atrazine, and bentazon alone did not control purple nutsedge. Mixtures of mesotrione plus bentazon, however, did improve control of purple nutsedge over either herbicide applied alone, but this control was not considered commercially acceptable.

Populations of eastern black nightshade suspected of being resistant to acetolactate synthase (ALS) inhibitors have been reported since 1999 in different locations in Ontario, Canada. This event has threatened the use of ALS inhibitors for control of this species. The objectives of this study were to evaluate the spectrum of resistance to different ALS-inhibiting herbicides and to examine the effectiveness of alternative modes of action herbicides. Growth room experiments were conducted to determine the response to imazethapyr and atrazine in seven suspected ALS inhibitor– resistant populations. One resistant and one susceptible population were further characterized for their response to ALS inhibitors and chloroacetamides. Seven populations were able to survive imazethapyr at 100 g ai/ha, while there was no resistance to atrazine. Compared to a susceptible (S) population, resistant (R) population SOLPT 1 had 726-, 31-, 6-, and 4-fold resistance to postemergence (POST) applied imazethapyr, imazamox, primisulfuron, and flumetsulam, respectively. Preemergence (PRE) application of imazethapyr, flumetsulam, cloransulam, nicosulfuron, prosulfuron, and rimsulfuron did not provide control of the R population, whereas they totally controlled the S population. The chloroacetamide herbicides metolachlor, dimethenamid, and flufenacet all provided at least 90% control of both R and S populations when applied PRE at the recommended field rates. The ALS inhibitors will not provide adequate control of these resistant populations, but acceptable control could be achieved with chloroacetamides or with atrazine.

Postemergence herbicides to control grass weeds in grain sorghum are limited. Acetolactate synthase (ALS) –inhibiting herbicides are very effective at controlling many grass species in many crops; unfortunately, use of ALS-inhibiting herbicides is not an option in conventional grain sorghum because of its susceptibility to these herbicides. With the development of ALS-resistant grain sorghum, several POST ALS-inhibiting herbicides can be used to control weeds in grain sorghum. Field experiments were conducted in 2007 and 2008 to evaluate the efficacy of tank mixtures of nicosulfuron + rimsulfuron applied alone or in combination with bromoxynil, carfentrazone–ethyl, halosulfuron + dicamba, prosulfuron, 2,4-D, or metsulfuron methyl + 2,4-D. In addition, these treatments were applied with and without atrazine. Nicosulfuron + rimsulfuron controlled barnyardgrass, green foxtail, and giant foxtail 99, 86, and 91% 6 wk after treatment (WAT), respectively. A decrease in annual grass control was observed when nicosulfuron + rimsulfuron was tank mixed with some broadleaf herbicides, although the differences were not always significant. In addition, nicosulfuron + rimsulfuron controlled velvetleaf and ivyleaf moringglory 64 and 78% 6 WAT, respectively. Control of velvetleaf was improved when nicosulfuron + rimsulfuron was tank mixed with all broadleaf herbicides included in this study with the exception of atrazine, bromoxynil, and prosulfuron + atrazine. Control of ivyleaf morningglory was improved when nicosulfuron + rimsulfuron was tank mixed with all of the herbicides included in this study with the exception of metsulfuron methyl + 2,4-D. Weed populations and biomass were lower when nicosulfuron + rimsulfuron were applied with various broadleaf herbicides than when it was applied alone. Grain sorghum yield was greater in all herbicide treatments than in the weedy check, with the highest grain yield from nicosulfuron + rimsulfuron + prosulfuron. This research showed that postemergence application of nicosulfuron + rimsulfuron effectively controls grass weeds, including barnyardgrass, green foxtail, and giant foxtail. The research also showed that velvetleaf and ivyleaf morningglory control was more effective when nicosulfuron + rimsulfuron were applied with other broadleaf herbicides.

Resistance to protoporphyrinogen oxidase (protox)-inhibiting herbicides was identified in a population of common waterhemp that had been treated with acifluorfen for several years. The protox-resistant biotype of common waterhemp was approximately 34, 82, 8, and 4 times more resistant than a susceptible common waterhemp biotype to acifluorfen, lactofen, fomesafen, and sulfentrazone, respectively. The resistant biotype also showed a high level of resistance to acetolactate synthase–inhibiting herbicides thifensulfuron and imazethapyr but not to glyphosate or paraquat. An organophosphate insecticide was applied with acifluorfen or lactofen to determine if metabolism could be the mechanism of resistance. No differences were observed between resistant plants treated with an organophosphate plus a protox-inhibiting herbicide and plants treated with a protox-inhibiting herbicide alone.

The lack of POST herbicides to control grasses in grain sorghum prompted researchers to develop acetolactate synthase (ALS)–resistant grain sorghum. Field experiments were conducted to evaluate the differential response of ALS-resistant grain sorghum to POST application of nicosulfuron + rimsulfuron applied at three growth stages. ALS-resistant grain sorghum was treated with 0, 13 + 7, 26 + 13, 39 + 20, 52 + 26, 65 + 33, 78 + 39, and 91 + 46 g ai ha−1 of nicosulfuron + rimsulfuron when plants were at the three- to five-leaf, seven- to nine-leaf, or 11- to 13-leaf stage. In general, as nicosulfuron + rimsulfuron rates increased, visible injury increased at the three- to five-leaf and seven- to nine-leaf stages. Injury was greatest 1 wk after treatment for the three- to five-leaf and seven- to nine-leaf stages across all ratings, and plants then began to recover. No injury was observed at any rating time for the 11- to 13-leaf stage. Plant height and sorghum grain yield were reduced as nicosulfuron + rimsulfuron rates increased when applied at the three- to five-leaf stage. However, nicosulfuron + rimsulfuron applied at the seven- to nine-leaf and 11- to 13-leaf stages did not decrease sorghum yield. This research indicated that nicosulfuron + rimsulfuron application at the three- to five-leaf stage injured ALS-resistant grain sorghum; however, application at the seven- to nine-leaf or 11- to 13-leaf stages did not result in grain yield reduction.