Field studies were conducted in 1988 and 1989 to examine the interrelations of cultivation and herbicide band width in controlling pitted morningglory and sicklepod in soybean. Alachlor + imazaquin PRE followed by imazaquin + surfactant POST were not applied or were applied on bands 15, 30, 45, 60, 75 cm wide or were broadcast. Plots were cultivated zero, one, two, or three times. Without cultivation, at least a 60-cm wide band was needed to achieve maximum soybean seed yields. A slight linear increase in soybean seed yield in response to increasing band width was observed with one cultivation. With two cultivations, soybean seed yields were similar with and without herbicides, but a 15-cm wide herbicide band was needed to achieve maximum production with three cultivations. Greatest gross returns exclusive of weed management costs were realized with two or three cultivations and a 15-cm wide band.

The influence of environmental factors on germination and emergence of horseweed was examined in growth chamber experiments. Germination was highest (61%) under 24/20 C day/night temperature under light. Horseweed seed germination was observed under both light (13 h photoperiod) and complete darkness (24 h), but germination under continuous darkness was only 0 to 15% compared with 0 to 61% under light. All other experiments were conducted under 24/20 C and 13-h light conditions. Germination was 19 to 36% over a pH range from 4 to 10, with a trend toward higher germination under neutral-to-alkaline conditions. Horseweed germination was > 20% at < 40 mM NaCl concentration and lowest (4%) at 160 mM NaCl. These data suggest that even at high soil salinity conditions, horseweed can germinate. Germination of horseweed decreased from 25% to 2% as osmotic potential increased from 0 (distilled water) to −0.8 MPa, indicating that germination can still occur under moderate water stress conditions. Horseweed seedling emergence was at its maximum on the soil surface, and no seedlings emerged from seeds placed at a depth of 0.5 cm or higher.

Field experiments were conducted on the Eastern Shore of Virginia from 1999 to 2001 to evaluate the effects of tank mixture applications of isopropylamine or trimethylsulfonium salts of glyphosate with two liquid formulations of manganese (Mn lignin or Mn chelate) on spray solution pH and weed control in glyphosate-resistant soybean. Additions of manganese to herbicide solutions resulted in a reduction in the acidifying effects of the herbicides as well as in the control of common lambsquarters, large crabgrass, morningglory spp., and smooth pigweed. Reduced control caused by manganese could be overcome with higher rates of the herbicides on some species, but reduced control of common lambsquarters was seen when manganese was included with any herbicide application rate. For most species, Mn chelate caused a greater reduction in control than did Mn lignin. Although manganese caused significant decreases in weed control, soybean yield was not influenced by glyphosate salt, application rate, or manganese. Reduced weed control caused by the addition of manganese to herbicide solutions may be due to the complexing of the herbicide formulations, which could result in the formation of insoluble salt complexes that are not readily absorbed through the plant cuticle, resulting in decreased glyphosate phytotoxicity.

Survival of horseweed in several glyphosate-tolerant cotton and soybean fields treated with glyphosate at recommended rates preplant and postemergence was observed in Mississippi and Tennessee in 2001 and 2002. Plants originating from seed collected from fields where horseweed escapes occurred in 2002 were grown in the greenhouse to the 5-leaf, 13- to 15-leaf, and 25- to 30-leaf growth stages and treated with the isopropylamine salt of glyphosate at 0, 0.025, 0.05, 0.1, 0.21, 0.42, 0.84, 1.68, 3.36, 6.72, and 13.44 kg ae/ha to determine if resistance to glyphosate existed in any biotype. All biotypes exhibited an 8- to 12-fold level of resistance to glyphosate when compared with a susceptible biotype. One resistant biotype from Mississippi was two- to fourfold more resistant than other resistant biotypes. Growth stage had little effect on level of glyphosate resistance. The glyphosate rate required to reduce biomass of glyphosate-resistant horseweed by 50% (GR50) increased from 0.14 to 2.2 kg/ha as plant size increased from the 5-leaf to 25- to 30-leaf growth stage. The GR50 rate for the susceptible biotype increased from 0.02 to 0.2 kg/ha glyphosate. These results demonstrate that the difficult-to-control biotypes were resistant to glyphosate, that resistant biotypes could survive glyphosate rates of up to 6.72 kg/ha, and that plant size affected both resistant and susceptible biotypes in a similar manner.

Two Italian ryegrass populations from Mississippi, Tribbett and Fratesi, were suspected to be tolerant to glyphosate. A third population from Mississippi, Elizabeth, known to be susceptible to glyphosate, was included for comparison. Plants were treated with the isopropylamine salt of glyphosate at 0, 0.11, 0.21, 0.42, 0.84, 1.68, 3.36, and 6.72 kg ae/ha. GR50 (herbicide dose required to cause a 50% reduction in plant growth) values for the Tribbett, Fratesi, and Elizabeth populations were 0.66, 0.66, and 0.22 kg/ha, respectively, indicating that the Tribbett and Fratesi populations were threefold more tolerant to glyphosate compared with the Elizabeth population. These three populations were also treated with diclofop at 0, 0.13, 0.25, 0.5, 0.75, 1, and 2 kg ai/ha. Diclofop GR50 values for the Tribbett, Fratesi, and Elizabeth populations were 0.25, 0.28, 0.21 kg/ha, respectively, indicating similar tolerance to diclofop in the three populations. Response of all three populations to clethodim rate (0, 0.02, 0.03, 0.05, 0.06, 0.08, 0.09, and 0.13 kg ai/ha) was measured. Clethodim GR50 values for the Tribbett, Fratesi, and Elizabeth populations at the small growth stages were 0.016, 0.023, 0.014 kg/ha, respectively, and at the large growth stage were 0.04, 0.034, 0.02 kg/ha, respectively.

Greenhouse and laboratory studies were conducted to confirm and quantify glyphosate resistance, quantify pyrithiobac resistance, and investigate interaction between flumiclorac and glyphosate mixtures on control of Palmer amaranth from Mississippi. The GR50 (herbicide dose required to cause a 50% reduction in plant growth) values for two glyphosate-resistant biotypes, C1B1 and T4B1, and a glyphosate-susceptible (GS) biotype were 1.52, 1.3, and 0.09 kg ae ha−1 glyphosate, respectively. This indicated that the C1B1 and T4B1 biotypes were 17- and 14-fold resistant to glyphosate, respectively, compared with the GS biotype. The C1B1 and T4B1 biotypes were also resistant to pyrithiobac, an acetolactate synthase (ALS) inhibitor, with GR50 values of 0.06 and 0.07 kg ai ha−1, respectively. This indicated that the C1B1 and T4B1 biotypes were 7- and 8-fold, respectively, more resistant to pyrithiobac compared with the GS biotype, which had a GR50 value of 0.009 kg ha−1. Flumiclorac was antagonistic to glyphosate by reducing glyphosate translocation. The C1B1 and T4B1 absorbed less glyphosate 48 h after treatment (HAT) compared with the GS biotype. The majority of the translocated glyphosate accumulated in the shoot above the treated leaf (that contains the apical meristem) in the GS biotype and in the shoot below the treated leaf in the resistant biotypes, C1B1 and T4B1, by 48 HAT. The C1B1 biotype accumulated negligible shikimate levels, whereas the T4B1 and GS biotypes recorded elevated levels of shikimate. Metabolism of glyphosate to aminomethylphosphonic acid was not detected in either of the resistant biotypes or the susceptible GS biotype. The above results confirm multiple resistance to glyphosate and pyrithiobac in Palmer amaranth biotypes from Mississippi and indicate that resistance to glyphosate is partly due to reduced absorption and translocation of glyphosate.

Greenhouse and field studies were conducted in 1998 and 1999 to compare the growth and development of one imidazolinone-susceptible (S) and four -resistant (R1, R2, R3, and R4) smooth pigweed biotypes under noncompetitive and competitive conditions. Under noncompetitive conditions in the greenhouse, S plants accumulated biomass, grew faster during early seedling development, and accumulated leaf area sooner than plants from R2, R3, and R4 biotypes. At various times during the experiment, S plants grew faster and more efficiently used leaf area to accumulate more biomass than did R2, R3, and R4 plants. In addition, leaves emerged faster on S plants than on R2, R3, and R4 plants. R3 and R4 biotypes had significantly less chlorophyll per gram of plant tissue compared with S. In contrast, most growth parameters measured for S and R1 plants were similar. Biomass production in the field under intra- and interbiotypic competition was similar for S and all R biotypes. Findings from noncompetitive growth studies in the field were inconclusive, and further investigations are warranted. On the basis of these findings, S displayed an advantage in vegetative growth and development over three out of four imidazolinone-resistant biotypes during the early stages of development, but competitive differences were not confirmed in the field.

As a follow-up to greenhouse studies, acetolactate synthase (ALS) (EC 4.1.3.18) was extracted from one imidazolinone (IMI)-susceptible (S) and three IMI-resistant (R1, R2, and R3) smooth pigweed populations, and activity was assayed in the presence of imazethapyr, chlorimuron, thifensulfuron, and pyrithiobac. ALS inhibitor concentrations, required to reduce enzyme activity by a specified percentage compared with the untreated control (Ip), were determined for each herbicide using regression analysis, and resistance ratios were calculated from these values. An I50 value of >35 μM imazethapyr was calculated for all R populations compared with a value of 3.4 μM for the S population. With chlorimuron, thifensulfuron, and pyrithiobac data sets, pairwise comparisons of regression coefficients were used to determine significant differences between regression lines. Using this technique, it was established that ALS from R3 was more sensitive than ALS from S to inhibition by chlorimuron and thifensulfuron. Also, ALS from R2 and R3 displayed increased sensitivity to pyrithiobac compared with ALS extracted from the S population. We have confirmed enzyme-level resistance to imazethapyr in all R populations and have documented negative cross-resistance in some R populations to ALS inhibitors other than imazethapyr.

A threefold glyphosate tolerance was identified in two Italian ryegrass populations, T1 and T2, from Mississippi. Laboratory experiments were conducted to characterize the mechanism of glyphosate tolerance in these populations. The T1 population absorbed less 14C-glyphosate (43% of applied) compared to the susceptible (S) population (59% of applied) at 48 h after treatment (HAT). The T2 population absorbed 14C-glyphosate at levels (56% of applied at 48 HAT) that were similar to both T1 and S populations, but tended to be more comparable to the S population. The amount of 14C-glyphosate that remained in the treated leaf was significantly higher in both T1 (67% of absorbed) and T2 (65% of absorbed) populations compared to the S population (45% of absorbed) at 48 HAT. The amount of 14C-glyphosate that moved out of treated leaf to shoot and root was lower in both T1 (25% of absorbed in shoot and 9% of absorbed in root) and T2 (25% of absorbed in shoot and 11% of absorbed in root) populations compared to the S population (40% of absorbed in shoot and 16% of absorbed in root) at 48 HAT. There were no differences in epicuticular wax mass among the three populations. Treating a single leaf with glyphosate solution at the field use rate (0.84 kg ae ha−1) as 10 1-µl droplets killed the S plant but not the T1 and T2 plants (33 and 55% shoot fresh-weight reduction, respectively). Shikimic acid accumulated rapidly at higher levels in glyphosate-treated leaf segments of the S population compared to the T1 population up to 100 µM glyphosate. However, above 500 µM glyphosate, the levels of shikimate were similar in both the S and T1 populations. Furthermore, shikimic acid content was three- to sixfold more in whole plants of the S population treated with 0.22 kg ae ha−1 glyphosate compared to the T1 and T2 populations. No degradation of glyphosate to aminomethylphosphonic acid was detected among the tolerant and susceptible populations. These results indicate that tolerance to glyphosate in the T1 population is partly due to reduced absorption and translocation of glyphosate and in the T2 population it is partly due to reduced translocation of glyphosate.

Several populations of smooth pigweed with resistance to imidazolinone (IMI) herbicides have been identified in recent years. One IMI-resistant population (R2) was 10-fold more sensitive to cloransulam-methyl in the greenhouse when compared with a susceptible (S) population. Laboratory studies were conducted to determine if differences in the absorption, translocation, and metabolism of cloransulam-methyl existed between S and R2 populations and to determine if these differences could account for the whole-plant responses observed in the greenhouse. Enzyme assays were also conducted to determine if differences in acetolactate synthase (ALS) sensitivity to cloransulam-methyl existed between S and R2 populations. Absorption of foliar-applied cloransulam-methyl was rapid and similar in both populations of smooth pigweed. Translocation of the absorbed radioactivity out of the treated leaf was symplastic and generally similar in both populations. Translocated radioactivity was detected primarily in shoots above and below the treated leaf with little movement to the roots. 14C-cloransulam-methyl metabolism was also similar in S and R2 populations. Differential tolerance to cloransulam-methyl in S and R2 populations in the greenhouse cannot be explained by differences in the absorption, translocation, and metabolism of this herbicide. However, ALS from R2 was 25-fold more sensitive to inhibition by cloransulam-methyl than ALS from S.

Field, laboratory, and greenhouse experiments were conducted to determine the seed production potential and effect of environmental factors on germination, emergence, and survival of texasweed. Texasweed produced an average of 893 seed per plant, and 90% were viable. Seed exhibited dormancy, and prechilling did not release dormancy. Percent germination ranged from 56% for seed subjected to no prechilling to 1% for seed prechilled at 5 C for 140 d. Seed remained viable during extended prechilling conditions, with 80% of seed viable after 140 d of prechilling. Texasweed seed germinated over a range of 20 to 40 C, with optimum germination (54%) occurring with a fluctuating 40/30 C temperature regime. Seed germinated with fluctuating 12-h light/dark and constant dark conditions. Texasweed seed germinated over a broad range of pH, osmotic potential, and salt concentrations. Seed germination was 31 to 62% over a pH range from 4 to 10. Germination of texasweed ranged from 9 to 56% as osmotic potential decreased from − 0.8 MPa to 0 (distilled water). Germination was greater than 52% at less than 40 mM NaCl concentrations and lowest (27%) at 160 mM NaCl. Texasweed seedlings emerged from soil depths as deep as 7.5 cm (7% emergence), but emergence was > 67% for seed placed on the soil surface or at a 1-cm depth. Texasweed seed did not germinate under saturated or flooded conditions, but seed survived flooding and germinated (23 to 25%) after flood removal. Texasweed seedlings 2.5 to 15 cm tall were not affected by emersion in 10-cm-deep flood for up to 14 d. These results suggest that texasweed seed is capable of germinating and surviving in a variety of climatic and edaphic conditions, and that flooding is not a viable management option for emerged plants of texasweed.

The effect of formulation and adjuvants on absorption and translocation of 14C-clethodim was determined at 1, 4, 12, 24, 48, and 72 h after treatment (HAT) in bermudagrass under greenhouse conditions. Absorption of 14C-clethodim with the 0.12 kg L−1 (15 to 85%) formulation was higher than with the 0.24 kg L−1 (5 to 40%) formulation, regardless of presence or absence of adjuvant. There was considerable variation in the effect of adjuvant on 14C-clethodim absorption. When either ammonium sulfate (AMS) or AMS plus crop oil concentrate (COC) was added to the 0.12 kg L−1 formulation, 14C-clethodim absorption increased significantly at all harvest times except at 12 HAT compared with 0.12 kg L−1 formulation alone, whereas, 14C-clethodim absorption after addition of COC to the 0.12 kg L−1 formulation was similar to the 0.12 kg L−1 formulation alone up to 24 HAT. Conversely, COC enhanced 14C-absorption at all harvest times when added to 0.24 kg L−1 formulation. Most of 14C-clethodim (79 to 100% of absorbed) remained in the treated leaf, independent of formulation or adjuvant. Formulation did not have an impact on distribution of absorbed 14C-clethodim; however, presence of an adjuvant increased movement of 14C-clethodim out of treated leaf. Of the absorbed 14C-label, most remained in the treated leaf. 14C-clethodim that translocated out of the treated leaf remained in the shoot, and negligible amount of 14C-clethodim translocated to roots. These results demonstrated improved absorption of clethodim with formulations containing half the active ingredient (0.12 kg L−1) and inclusion of both AMS and COC.

Field and greenhouse studies were conducted in 2000 and 2001 in Mississippi to determine the most effective POST herbicide programs for control of Texasweed in glyphosate-resistant soybean. In the field, Texasweed plants recovered from most POST herbicide treatments, and plant death rarely occurred. A sequential application of 1,100 g ae/ha glyphosate followed by 840 g/ha glyphosate was the only treatment that controlled at least 90% of Texasweed 4 wk after treatment. Texasweed control ranged from 80 to 87% for lower rates of glyphosate applied once or twice and 390 g ai/ha fomesafen. The addition of fomesafen or other herbicides to glyphosate did not improve control compared with glyphosate alone. In the greenhouse, glyphosate at 560 g/ha controlled 93 and 90% of one- and two-leaf plants, respectively, but at least 1,400 g/ha was required to obtain 90% control of four-leaf plants. Fomesafen at 260 g/ha controlled 93 to 99% of one- to four-leaf Texasweed plants in the greenhouse.

Greenhouse and field experiments were conducted to investigate the effect of glyphosate rate and degree of glyphosate spray coverage on pitted morningglory control. Pitted morningglory in the two-, four-, and six-leaf growth stages were treated with the isopropylamine salt of glyphosate at 0.28, 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha. Two- and four-leaf plants were controlled 98% with 1.68 kg/ha glyphosate, whereas six-leaf plants were controlled 68%. Control of two-, four-, and six-leaf plants with the commonly used field rate of 1.12 kg/ha was 68, 60, and 50%, respectively. In a separate greenhouse study, four-leaf pitted morningglory plants with 0, 33, 66, or 100% of their total leaf area exposed to herbicide spray were treated with 0.84, 1.68, or 3.36 kg/ha glyphosate. Increasing glyphosate rate from 0.84 to 3.36 kg/ha increased control from 36 to 88%. In contrast, increasing percent leaf exposure to glyphosate from 0 to 100% increased control from 57 to 75%. Increasing glyphosate rate from 0.84 to 1.68 kg/ha always improved control. However, increasing glyphosate rate from 1.68 to 3.36 kg/ha was beneficial only when no leaves were exposed to the spray solution. In the field, glyphosate spray coverage decreased from 85 to 40% as plant density increased from 1 to 32 plants/m2. However, control decreased only 11% (90 to 79%) between the highest and lowest levels of glyphosate spray coverage. These results demonstrated that inadequate control of pitted morningglory with glyphosate was more related to tolerance than glyphosate spray coverage. Glyphosate rates higher than 1.68 kg/ha may be beneficial when spray coverage is severely limited or when plants are beyond the four-leaf growth stage.

Field and greenhouse studies were conducted to evaluate the responses of several imidazolinone (IMI)-resistant Amaranthus hybridus populations to various acetolactate synthase (EC 4.1.3.18)-inhibiting herbicides. In 1996 field studies, IMI resistance was confirmed in one A. hybridus population (R4) that was not cross-resistant to the sulfonylurea herbicides CGA-277476, chlorimuron, or thifensulfuron. Amaranthus hybridus control with triazolopyrimidine herbicides was ≤ 35%, but control with cloransulam-methyl or flumetsulam plus cloransulam-methyl was higher than with IMI herbicides. Follow-up greenhouse studies were conducted in 1997 and 1998 to investigate the response of one IMI-susceptible (S) A. hybridus population collected near Painter, VA, and four IMI-resistant A. hybridus populations (R1, R2, R3, R4) collected from fields in Somerset County, MD, to postemergence imazethapyr, chlorimuron, thifensulfuron, pyrithiobac, and cloransulam-methyl applications. Resistance to imazethapyr was confirmed in all R populations, and no practical level of cross-resistance to chlorimuron, thifensulfuron, pyrithiobac, or cloransulam-methyl was detected. Based on resistance ratios, R populations were slightly more tolerant to chlorimuron and slightly more sensitive to pyrithiobac, thifensulfuron, and cloransulam-methyl than the S population.

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba or metribuzin at 0.42 kg ai/ha. Glufosinate applied alone at 0.47 kg ai/ha resulted in at least 88% control of horseweed and maximized soybean yield. Results indicate that effective management of glyphosate-resistant horseweed can be obtained in glyphosate-resistant soybean in glyphosate-, paraquat-, and glufosinate-based preplant weed control programs.