Mark M. Loux

Effect of Planting Date, Residual Herbicide, and Postemergence Application Timing on Weed Control and Grain Yield in Glyphosate-Tolerant Corn (Zea mays)1

Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss. Nomenclature: Glyphosate; giant foxtail, Setaria faberi Herrm. #3 SETFA; corn, Zea mays L. Additional index words: Herbicide-resistant crops, weed interference. Abbreviations: CEC, cation exchange capacity; POST, postemergence; PRE, preemergence; RS, respray; WAP, week after planting.

Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-yr multistate study

Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and Amaranthus spp. or reduced control of Ipomoea spp. With a single application of glyphosate, corn grain yield was most often reduced when the application was delayed until weeds were 23 cm or taller. Averaged across all sites in 1998 and 1999, corn grain yields from a single glyphosate application at the 5-, 10-, 15-, 23-, and 30-cm timings were 93, 94, 93, 91, and 79% of the weed-free control, respectively. There was a significant effect of herbicide treatment on corn grain yield in 23 of the 35 sites when weed reinfestation was prevented with a second glyphosate application. When weed reinfestation was prevented, corn grain yield at the 5-, 10-, and 15-cm application timings was 101, 97, and 93% of the weed-free control, respectively, averaged across all sites. Results of this study suggested that the optimum timing for initial glyphosate application to avoid corn grain yield loss was when weeds were less than 10 cm in height, no more than 23 d after corn planting, and when corn growth was not more advanced than the V4 stage. Nomenclature: Glyphosate; Amaranthus spp. #3 AMASS; Ipomoea spp. # IPOSS; corn, Zea mays L. ‘Roundup Ready®’ # SETFA. Additional index words: Herbicide-resistant crops, weed interference. Abbreviation: POST, postemergence.

Growth and seed production of horseweed (Conyza canadensis) populations resistant to glyphosate, ALS-inhibiting, and multiple (glyphosate+ALS-inhibiting) herbicides.

Abstract Horseweed populations with mixtures of biotypes resistant to glyphosate and acetolactate synthase (ALS)–inhibiting herbicides as well as biotypes with multiple resistance to glyphosate + ALS-inhibiting herbicides have been documented in Indiana and Ohio. These biotypes are particularly problematic because ALS-inhibiting herbicides are commonly tank mixed with glyphosate to improve postemergence horseweed control in soybean. The objective of this research was to characterize the growth and seed production of horseweed populations with resistance to glyphosate or ALS-inhibiting herbicides, and multiple resistance to glyphosate + ALS-inhibiting herbicides. A four-herbicide by four-horseweed population factorial field experiment was conducted in the southeastern region of Indiana in 2007 and repeated in 2008. Four horseweed populations were collected from Indiana or Ohio and confirmed resistant to glyphosate, ALS inhibitors, both, or neither in greenhouse experiments. The four herbicide treatments were untreated, 0.84 kg ae ha−1 glyphosate, 35 g ai ha−1 cloransulam, and 0.84 kg ae ha−1 glyphosate + 35 g ai ha−1 cloransulam. Untreated plants from horseweed populations that were resistant to glyphosate, ALS-inhibiting, or multiple glyphosate + ALS-inhibiting herbicides produced similar amounts of biomass and seed compared to populations that were susceptible to those herbicides or combination of herbicides. Furthermore, aboveground shoot mass and seed production did not differ between treated and untreated plants. Nomenclature: Cloransulam; glyphosate; horseweed, Conyza canadensis L. ERICA; soybean, Glycine max L. Merr.

Response of Horseweed Biotypes to Foliar Applications of Cloransulam-methyl and Glyphosate1

Studies were conducted from 2001 through 2003 to determine the extent of resistance to acetolactate synthase (ALS) inhibitors and glyphosate in Ohio horseweed biotypes. The response of 66 horseweed biotypes to cloransulam-methyl and glyphosate was determined in the greenhouse. Application of 0.07 kg ai cloransulam/ha reduced plant biomass by less than 60% for 38 of the 66 biotypes. Application of 3.4 kg ae glyphosate/ha reduced biomass by at least 80% for the 51 biotypes collected in 2001, but biomass was similar to that of nontreated plants for 11 of the 15 populations collected in 2002. A dose–response study was conducted with selected biotypes, and a nonlinear, logistic dose–response curve was fit to the data to calculate the herbicide dose required to reduce fresh weight 50% (GR50). On the basis of GR50 values, the resistance ratio (R/S) for two ALS-resistant biotypes was 34 and 943 for chlorimuron-ethyl and 32 and 168 for cloransulam, respectively. The R/S ratio for two glyphosate-resistant biotypes was 33 and 39. Results of these studies indicate that, in 2002, ALS-resistant horseweed was widespread throughout Ohio, whereas resistance to glyphosate occurred primarily in several counties in southwestern Ohio. Nomenclature: Chlorimuron-ethyl; cloransulam-methyl; glyphosate; horseweed, Conyza canadensis (L.) Cronq. #3 ERICA. Additional index words: Acetolactate synthase, herbicide resistance. Abbreviations: ALS, acetolactate synthase; GR50, herbicide dose required to reduce fresh weight 50%; R/S, resistance ratio.

Characterization of Selected Common Lambsquarters (Chenopodium album) Biotypes with Tolerance to Glyphosate

Abstract Biotypes of common lambsquarters with tolerance to glyphosate have been identified in a number of states, but little is known about their fitness characteristics. Field and greenhouse studies were conducted to characterize the response of selected glyphosate-tolerant common lambsquarters biotypes to glyphosate, and also their biological and reproductive characteristics. In a greenhouse dose-response study, GR50 and GR90 values for four tolerant biotypes ranged from 1.48 to 3.22 and 8.73 to 18.7 kg ae ha−1, respectively, compared to 0.57 and 2.39 kg ae ha−1, respectively, for a glyphosate-sensitive biotype. In a field dose-response study, the GR50 and GR90 values were 0.06 and 0.48 kg ae ha−1, respectively, for a tolerant biotype, compared to 0.036 and 0.19 kg ae ha−1, respectively, for the sensitive biotype. The growth rate, time until flowering, and seed production of eight tolerant and two sensitive biotypes was evaluated in a field study. The tolerant biotypes grew taller, amassed more leaf area and dry weight, and advanced through growth stages more rapidly than sensitive biotypes during the early portion of the growing season. The tolerant biotypes were taller than sensitive biotypes at 6 and 10 wk after transplanting, but had lower dry weight at maturity. Tolerant biotypes initiated flower primordia approximately 6 to 8 wk after transplanting, whereas sensitive biotypes required 12 wk. However, no apparent fitness penalties were observed in glyphosate-tolerant biotypes based on seed-production estimates. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L.

Response of ALS-Resistant Common Ragweed (Ambrosia artemisiifolia) and Giant Ragweed (Ambrosia trifida) to ALS-Inhibiting and Alternative Herbicides

Abstract: Three studies were conducted in 1999 and 2000 to determine whether acetolactate synthase (ALS)–resistant common ragweed and giant ragweed biotypes were present in Ohio. Results of field studies indicated that biotypes of both species had cross-resistance to three chemical families of ALS-inhibiting herbicides. Cloransulam-methyl applied postemergence at 9, 18, and 36 g/ha controlled more than 85% of two susceptible populations of common and giant ragweed 28 d after treatment, whereas less than 35% control of resistant populations was achieved at the same rates. Fomesafen, lactofen, and glyphosate applied alone at the recommended rates provided the most effective control of ALS-resistant common and giant ragweed. Mixtures of cloransulam-methyl with either fomesafen or lactofen did not significantly increase ALS-resistant common and giant ragweed control compared with each diphenylether herbicide used alone. Dose–response bioassays conducted in the greenhouse indicated that susceptible common and giant ragweed tended to be more sensitive to cloransulam-methyl and chlorimuron than to imazamox. ALS-resistant common ragweed demonstrated a high level of resistance to all the herbicides tested because GR50 values were not reached with rates 1,000 times higher than the recommended rate. ALS-resistant giant ragweed treated with 13,000 g/ha of chlorimuron and 18,000 g/ha of cloransulam-methyl was not inhibited enough to obtain a GR50 value, thus also demonstrating a high level of resistance. The GR50 for ALS-resistant giant ragweed treated with imazamox was 1,161 g/ha. Results of these studies confirmed the presence of ALS–cross-resistant populations of common and giant ragweed in Ohio and suggest that herbicides with different mechanisms of action will be required to manage these weeds effectively. Nomenclature: Chlorimuron; cloransulam-methyl; fomesafen; glyphosate; imazamox; lactofen; common ragweed, Ambrosia artemisiifolia L. #3 AMBEL; giant ragweed, Ambrosia trifida L. # AMBTR. Additional index words: Acetolactate synthase, herbicide resistance. Abbreviations: ALS, acetolactate synthase; COC, crop oil concentrate; DAT, days after treatment; NIS, nonionic surfactant; POST, postemergence; PPF, photosynthetic photon flux; PRE, preemergence; UAN, urea ammonium nitrate; 1× rate, the recommended label rate; 2× rate, twice the recommended label rate.

Integration of cover crops with postemergence herbicides in no-till corn and soybean

Abstract The integration of cover crops with selected postemergence herbicides was evaluated on the basis of weed control and grain yields in no-till soybean and corn. Soybean was planted into wheat residue, whereas corn was planted into hairy vetch residue. Full, half, and quarter rates and sequential herbicide applications were made. The wheat cover crop did not increase weed suppression but increased soybean grain yields. Half rates of thifensulfuron plus quizalofop-P as single or split applications were as effective as full rates in reducing weed weight in soybean. Soybean grain yields were similar in the half- and full-rate treatments in 1994, but yield was highest in the full-rate treatment in 1995. The hairy vetch cover crop did not increase weed suppression but lowered corn stands and grain yields in 1995 and enhanced corn grain yields in 1996. Full, half, and quarter rates (1996 only) of nicosulfuron plus primisulfuron were equally effective in reducing weed weight. Corn grain yields were similar at all herbicide rates in 1995 but were inversely related to herbicide rate in 1996. Split herbicide applications did not improve weed suppression over single applications of the same herbicide rate in either crop. Results indicate that cover crops can improve crop productivity and reduced rates of environmentally benign herbicides can minimize the herbicide requirements in no-till corn and soybean. Nomenclature: Glyphosate; nicosulfuron; primisulfron; quizalofop-P; thifensulfuron; corn, Zea mays L.; hairy vetch, Vicia villosa Roth; soybean, Glycine max L.; wheat, Triticum aestivum L.

Why Early Season Weed Control Is Important in Maize

Abstract Control of early-emerging weeds is essential to protect the yield potential of maize. An understanding of the physiological changes that occur as a result of weed interference is required to address variability in yield loss across sites and years. Field trials were conducted at the University of Guelph (UG), the Ohio State University (OSU), and Colorado State University (CSU) during 2009 and 2010. There were six treatments (season-long weedy and weed-free, and weed control at the 1st-, 3rd-, 5th-, and 10th-leaf-tip stages of maize development) and 20 individual plants per plot were harvested at maturity. We hypothesized that, as weed control was delayed, weed interference in the early stages of maize development would increase plant-to-plant variability in plant dry-matter accumulation, which would result in a reduction of grain yield at maturity. The onset of the critical period for weed control (CPWC) occurred on average between the third and fifth leaf tip stages of development (i.e., V1 to V3, respectively). Rate of yield loss following the onset of the CPWC ranged from 0.05 MG ha−1 d−1 at UG 2009 to 0.22 MG ha−1 d−1 at CSU 2010 (i.e., 0.5 and 1.6% d−1, respectively). On average, reductions in kernel number per plant accounted for approximately 65% of the decline in grain yield as weed control was delayed. Biomass partitioning to the grain was stable through early weed removal treatments, increased and peaked at the 10th-leaf-tip time of control, and decreased in the season-long weedy treatment. Plant-to-plant variability in dry matter at maturity and incidence of bareness increased as weed control was delayed. As weed control was delayed, the contribution of plant-to-plant variability at maturity to the overall yield loss was small, relative to the decline of mean plant dry matter. Nomenclature: Atrazine; glyphosate; mesotrione; S-metolachlor; maize, Zea mays L.

Certified Crop Advisors’ Perceptions of Giant Ragweed (Ambrosia trifida) Distribution, Herbicide Resistance, and Management in the Corn Belt

Abstract Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern and mechanisms of spread in crop fields are lacking. To address this gap, we conducted a Web-based survey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada. Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice. Responses were mapped and correlation analyses were conducted among the responses to determine factors associated with giant ragweed populations. Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported giant ragweed populations with herbicide resistance to acetolactate synthase inhibitors, glyphosate, or both herbicides. Results suggest that giant ragweed is increasing in crop fields outward from the east-central U.S. Corn Belt in most directions. Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in noncrop edge habitats, early and prolonged emergence, and presence of the seed-burying common earthworm in crop fields. Managing giant ragweed in noncrop areas could reduce giant ragweed migration from noncrop habitats into crop fields and slow its spread. Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations, disrupt emergence patterns, and select against herbicide-resistant giant ragweed genotypes. Incorporation of a cereal grain into the crop rotation may help suppress early giant ragweed emergence and provide chemical or mechanical control options for late-emerging giant ragweed. Nomenclature: Glyphosate; giant ragweed; Ambrosia trifida L. AMBTR; common earthworm; Lumbricus terrestris L.; corn; Zea mays L.; soybean, Glycine max (L.) Merr.

Interactions of Glyphosate with Residual Herbicides in No-Till Soybean (Glycine max) Production1

Abstract: Glyphosate is often mixed with residual herbicides to control emerged weeds in no-till crop production systems. Field studies were conducted in Ohio from 1992 to 1994 to evaluate the weed control provided by residual herbicides and reduced rates of glyphosate in full-season, no-till soybean. Herbicide treatments were applied at two timings to examine the effect of weed size. At 4 wk after treatment, greater than 85% Pennsylvania smartweed control was obtained with metribuzin plus chlorimuron and linuron plus chlorimuron applied with 280 g ai/ha glyphosate and with imazethapyr and imazaquin applied with 560 g/ha glyphosate. All residual herbicides provided at least 85% common lambsquarters control when applied with 560 g/ha glyphosate. In 1992, the early application of residual herbicides provided this level of common lambsquarters control without glyphosate due to the small weed size at the time of application. Residual herbicides applied with 280 g/ha glyphosate controlled giant foxtail 85% or greater. Higher glyphosate rates were needed to control barnyardgrass. The performance of reduced glyphosate rates was dependent on weed species and weed size. Nomenclature: Chlorimuron; glyphosate; imazaquin; imazethapyr; linuron; metribuzin; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; common lambsquarters, Chenopodium album L. # CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; Pennsylvania smartweed, Polygonum pensylvanicum L. # POLPY; soybean, Glycine max (L.) Merr. Additional index words: Tank mixtures, CHEAL, ECHCG, POLPY, SETFA. Abbreviations: WAT, weeks after treatment.