Allan S. Hamill

Risks and reliability of using herbicides at below-labeled rates.

Abstract: The objectives of this study were to determine the efficacy and risk of controlling weeds at reduced herbicide rates under various environmental and biotic conditions, through analysis of published data on the use of below-labeled rates of herbicides. A database was established by extracting information from previously published papers on weed control at below-labeled rates of herbicides in crop production systems over large geographical and temporal scales. The database was then analyzed to evaluate the efficacy and risk of using herbicides at various reduced rates under different management systems. Using below-labeled herbicide rates in conjunction with interrow cultivation is an effective way of reducing herbicide input in agricultural systems while maintaining satisfactory weed control. There are greater opportunities for herbicide reduction using preemergence (PRE) than preplant incorporated (PPI) or postemergence (POST) herbicides, in coarse-textured than in fine-textured soils, and in corn than in soybean or wheat. The success of reducing herbicide rates does not depend on whether the herbicides are applied in conventional or conservation tillage systems or whether they are used with or without adjuvants. The above conclusions are based on studies conducted in experimental fields where weed pressures may be subjectively chosen to be high. Greater potential for herbicide reduction may exist at locations or in cropping systems were weed pressure is low. Nomenclature: Corn, Zea mays L.; soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L. Additional index words: Database, herbicides, reduced rates, weed control efficacy. Abbreviations: POST, postemergence; PPI, preplant incorporated; PRE, preemergence.

Critical Period of Weed Control in No-Till Soybean (Glycine max) and Corn (Zea mays)1

The critical period of weed control for crops grown under conventional tillage systems has been well studied, and the results generated by these studies have been proven to be very useful in developing ecologically and economically sound weed management practices. However, these management systems may not be directly applicable under no-till situations because the species composition, total amount, and temporal pattern of seedling emergence change substantially with tillage. The objective of this study was to identify the critical period of weed control for soybean and corn in fields that had been under no-till management for 1 yr. Although estimates of the critical period for a crop vary from year to year and site to site, some interesting comparisons can be made between no-till and conventional tillage. The start of the critical period in no-till corn was stable, usually beginning at the six-leaf stage. The end of the critical period was more variable ranging from the 9- to 13-leaf stage. The critical period for corn under no-till conditions tended to start and end earlier than under conventional tillage practices. In soybean, we were unable to identify a critical period at one of the sites. At the other location (sandy loam soil), the critical period was estimated to begin at the first or second node developmental stage, whereas the end was determined to be at the R1 stage (early flowering). The critical period in soybean was longer than that observed under conventional tillage. Nomenclature: Corn, Zea mays L. ‘Pioneer 3573’; soybean, Glycine max (L.) Merr. ‘Northrup King 2492’. Additional index words: Weed interference, crop yield loss. Abbreviation: DAE, days after emergence.

Impacts of Zone Tillage and Red Clover on Corn Performance and Soil Physical Quality

benefits of conservation tillage with the yield benefits of conventional moldboard plow tillage (e.g., Pierce et Despite extensive research, reduced corn (Zea mays L.) performance is still encountered using conservation tillage on fine-textured al., 1992) for cool humid climatic zones. Here, a narrow soils in cool humid temperate climates. These problems are intensified zone 10 to 20 cm wide by 10 to 30 cm deep is conventionwhen corn is planted into residue from a previous crop such as winter ally tilled in the crop row while the rest of the soil surface wheat (Triticum aestivum L.). The objective of this 4-yr study was is left in an untouched no-till state. This supposedly to determine the influence of fall zone tillage (ZT), no tillage (NT), encourages the more favorable soil temperature, moisand conventional moldboard plow tillage (CT) (fall plowing) on corn ture, aeration, density, and strength conditions associperformance and soil physical quality under a winter wheat–corn– ated with conventional tillage in the narrow seedbed soybean (Glycine max L. Merr.) rotation with and without red clover (Trifolium pratense L.) (RC) underseeded in the wheat phase of the zones, while retaining the increased erosion resistance, rotation. A randomized complete block design (3 2 factorial, 4 organic matter protection and reduced energy inputs of replicates) was established on three adjacent fields in the fall of 1996 no tillage between the zones. Although there is much on a Brookston clay loam soil (fine loamy, mixed, mesic, Typic Argiainterest in the zone-till system, it has not yet been tested quoll) at Woodslee, ON Canada, and measurements were collected extensively in cool humid temperate climates, nor on during 1997 to 2000. Over both wet and dry growing seasons from the agriculturally important clay and clay loam soils of 1998-2000, zone tillage following underseeded RC produced average southern Ontario. In nearby Michigan, ZT on sandy corn grain yields (7.23 Mg ha 1) that were within 1% of those obtained using conventional tillage (7.33 Mg ha 1), and 36% higher than those loam soils did indeed improve potato (Solanum tuberoobtained using no tillage and RC (5.33 Mg ha 1). Zone tillage also sum L.) yields and soil physical conditions relative to improved soil quality as evidenced by generally lower soil strength conventional tillage in most years of a 4-yr study (Pierce than no tillage, and near-surface soil physical quality parameters that and Burpee, 1995); however, corn yields were not inwere equivalent to, or more favorable than, those of the other treatcreased by zone tillage in a similar 3-yr study, despite ments. It was concluded that corn production using zone tillage and substantially reduced soil strength (penetration resisRC underseeding is a viable option in Brookston clay loam soil, as it retains much of the soil quality benefit of conventional tillage but tance) within the 0to 30-cm depth range (Pierce et still achieves most of the yield benefit of conventional moldboard al., 1992). plow tillage. Red clover underseeded in cereals can produce large quantities of plant biomass and it fixes N in the nodules, which can in turn provide the equivalent of 90 to 125 C systems, such as no-till, have kg N ha 1 to the following crop (Bruulsema and Christie, been demonstrated to have several advantages 1987). In addition, RC can be effective in cool-temperover conventional moldboard plow systems, including ate climates for increasing microbial biomass, improving reduced soil erosion and surface runoff, slower loss of the structure of fine-textured soils (Drury et al., 1991), soil organic matter, and lower production costs. Howand for accelerating the decomposition of surface crop ever, there are many reports of reduced corn emergence residues (Drury et al., 1999). It was consequently hyand yields under no-till relative to conventional till on pothesized that including RC underseeding in a crop fine-textured soils in humid and cool temperate clirotation might further improve the potential yield and mates. This appears to be primarily a result of spring soil quality benefits of zone tillage on fine-textured soils. soil conditions that are cooler (Graven and Carter, 1991; The objective of this study was to determine, for a Fortin and Pierce, 1990; Fortin and Pierce, 1991) and clay loam soil in southern Ontario, if zone tillage and wetter (Fortin, 1993) relative to conventional tillage, RC underseeding could achieve corn yields comparable plus other factors such as increased soil bulk density with conventional moldboard plow tillage, but still reand strength (e.g., Hill, 1990; Pierce et al., 1992), detain most of the soil quality, environmental, and reduced creased soil air-filled porosity and saturated hydraulic energy inputs of no tillage. To accomplish this, convenconductivity (e.g., Pierce et al., 1992), and desiccation of seeds or seedlings through reopening of the planting tional moldboard plow tillage, no tillage, and zone-tillage slot produced by the no-till planter (Drury et al., 1999). systems were applied to a winter wheat–corn–soybean Zone tillage has been proposed as a possible alternarotation, with and without RC underseeded in the wintive tillage system that may combine the soil quality ter wheat. Evaluations were made on the basis of corn emergence, corn yield, and near-surface soil physical C.F. Drury, C.S. Tan, W.D. Reynolds, T.W. Welacky, S.E. Weaver, quality. and A.S. Hamill, Greenhouse & Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow, ON, Canada N0R 1G0. T.J. Vyn, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907Abbreviations: CHU, corn heat unit; CT, conventional moldboard 1150. Received 22 Jan. 2002. *Corresponding author (druryc@agr. plow tillage; FC, field capacity; NT, no tillage; PR, penetration resisgc.ca). tance; PWP, permanent wilting point; RC, red clover; WAS, wet aggregate stability; ZT, zone tillage. Published in Soil Sci. Soc. Am. J. 67:867–877 (2003).

Control of Volunteer Glyphosate-Resistant Corn (Zea mays) in Glyphosate-Resistant Soybean (Glycine max)1

Volunteer corn in soybean can reduce yields, interfere with harvest, and cause unacceptable levels of contamination by its presence in the harvested soybean. In Ontario, soybean frequently follow corn in rotation. The use of glyphosate-resistant corn and soybean varieties has increased dramatically in Ontario. Field studies were conducted at two locations in southwestern Ontario to determine whether quizalofop-p-ethyl, clethodim, and fenoxaprop-p-ethyl can be tank mixed with glyphosate to provide effective control of volunteer glyphosate-resistant corn in glyphosate-resistant soybean. Soybean plots were overseeded with glyphosate-resistant corn and treatments consisting of glyphosate applied alone and tank mixed with full and reduced rates of each graminicide with and without a recommended surfactant. Tank mixing the graminicides and adjuvants with glyphosate did not affect glyphosate weed control or crop tolerance. Use of a recommended adjuvant significantly improved the effectiveness of the graminicides, particularly when reduced rates were applied. Quizalofop-p-ethyl was the most effective graminicide for controlling glyphosate-resistant volunteer corn, followed by clethodim and fenoxaprop-p-ethyl. Nomenclature: Soybean, Glycine max (L.) Merr. ‘Pioneer 9294 RR’; volunteer corn, Zea mays L. Additional index words: Graminicides, efficacy. Abbreviation: DAT, days after treatment.

ALS inhibitor resistance in populations of Powell amaranth and redroot pigweed

Abstract In 1997, farmers in Ontario, Canada, reported failure of some ALS-inhibiting herbicides to provide adequate control of pigweed species. Growth room experiments were conducted to confirm resistance to ALS inhibitors in populations of Powell amaranth and redroot pigweed. Twenty-two out of 35 collected seed samples were able to grow in the presence of soil-applied imazethapyr or flumetsulam. Dose–response curves were generated for 11 and 9 populations of Powell amaranth and redroot pigweed, respectively, using foliar-applied imazethapyr and thifensulfuron. Resistance to ALS inhibitors was confirmed in nine and five populations of Powell amaranth and redroot pigweed, respectively. Within each species, comparison of the herbicide rate required to reduce plant dry weight 50% (GR50) between the resistant populations and a susceptible population was conducted to obtain resistance factors. For imazethapyr, resistance factors ranged from 4.2 to 3,438 and from 33 to 168 for Powell amaranth and redroot pigweed, respectively. High-level cross-resistance to thifensulfuron was found in two populations of each species, with resistance factors ranging from 270 to 2,416. In both species, populations could be grouped according to their cross-resistance patterns: some populations were resistant to imazethapyr only, whereas others expressed resistance to both imazethapyr and thifensulfuron. The observed patterns of cross-resistance were not correlated with known herbicide exposure history of the fields where these populations originated. Nomenclature: Flumetsulam; imazethapyr; thifensulfuron; Powell amaranth = green pigweed, Amaranthus powellii S. Wats. AMAPO; redroot pigweed, Amaranthus retroflexus L. AMARE.

Environment and Soil Conditions Influence Pre- and Postemergence Herbicide Efficacy in Soybean

Abstract Deciding on the most efficacious PRE and POST herbicide options and their ideal application timing can be challenging for soybean producers. Climatic events during the 14 d before and after herbicide application can further complicate decisions because of their influence on herbicide effectiveness. Nine field trials were conducted at three locations in southwestern Ontario from 2003 to 2006, to determine the most effective PRE and POST soybean herbicides for control of common lambsquarters, common ragweed, green foxtail, and redroot pigweed. When precipitation was low at least 7 d before and after herbicide application weed control was reduced in treatments that included imazethapyr (PRE or POST) or flumetsulam/S-metolachlor (a premix formulation) (PRE). Cumulative precipitation during the 12 d after PRE application that exceeded the monthly average by at least 60% reduced common lambsquarters control when metribuzin was applied and green foxtail control when imazethapyr was applied. Delaying application of imazethapyr + bentazon to a later soybean growth stage decreased control of common lambsquarters and green foxtail; however, environmental conditions appeared to influence these results. Precipitation on the day of application decreased control of common ragweed and redroot pigweed more with quizalofop-p-ethyl + thifensulfuron-methyl + bentazon compared with imazethapyr + bentazon. Soybean yield varied among POST herbicide treatments because of reduced weed control. This research confirms that environmental conditions pre- and postapplication, as well as application timing, influence herbicide efficacy and should be considered by growers when selecting an herbicide program. Nomenclature: Bentazon; cloransulam-methyl; flumetsulam; glyphosate; imazethapyr; linuron; metribuzin; quizalofop-p-ethyl; S-metolachlor; thifensulfuron-methyl; redroot pigweed, Amaranthus retroflexusL.; common ragweed, Ambrosia artemisiifolia L.; common lambsquarters, Chenopodium album L.; green foxtail, Setaria viridis (L.) Beauv.; soybean, Glycine max L.

Benefits and Risks of Economic vs. Efficacious Approaches to Weed Management in Corn and Soybean

A 3-yr study was conducted on nine farms across southern Ontario to evaluate the risks and benefits of different approaches to weed management in corn and soybean. Weed control decisions were based on field scouting and recommendations from the Ontario version of HADSS™, the herbicide application decision support system. Treatments were selected to maximize profit (economic threshold approach) or to maximize yield (highest treatment efficacy). Reduced rates of the high efficacy treatment for each field also were included. Weed density before and after treatment, crop yields, weed seed return, and the effect of weed control decisions on weed density 1 yr after treatment were assessed. Crop yield varied among years and farms but was not affected by weed control treatment. Weed control at 28 d after treatment (DAT) was often lower and weed density, biomass, and seed production 70 DAT were often higher with the profit maximization approach compared with the yield maximization approach. However, weed density 1 yr later, after each cooperator had applied a general weed control program, did not vary significantly among the previous years weed control treatments. Reduced rates of the high efficacy treatments did not lead to increased weed problems the next year, despite lower weed control and increased weed seed production in some years. During the 3 yr of the study, weed control costs with the profit maximization approach were approximately Can

Weed Control and Yield Response to Foramsulfuron in Corn

45/ha less than with the yield maximization approach. Nomenclature: Corn, Zea mays L.; soybean, Glycine max (L.) Merr. Additional index words: Economic threshold, net benefit, reduced rate, weed seed rain. Abbreviations: DAT, days after treatment; HADSS, herbicide application decision support system; POST, postemergence.

Response of common lambsquarters (Chenopodium album) to glyphosate application timing and rate in glyphosate-resistant corn

Foramsulfuron has recently been registered for weed control in corn in Ontario, but there is very little information on the rate of foramsulfuron required to obtain at least 90% weed control. Our objective was to determine the foramsulfuron rates giving at least 90% weed control while maintaining crop yield loss due to weed interference and injury at less than 5%. Ten field trials were conducted at five Ontario locations (Exeter, Harrow, Ridgetown, Woodslee, and Woodstock) in 2001 and 2002 to evaluate the effectiveness of foramsulfuron at rates ranging from 8.75 to 140 g ai/ha. To obtain a reduction in biomass of 90% (I90) at 78 d after treatment (DAT), foramsulfuron must be applied to common lambsquarters at 68 g/ha and to common ragweed at 86 g/ha, respectively. For green foxtail a foramsulfuron rate of 25 g/ha was required to achieve 90% control. The application of foramsulfuron caused injury to corn at 7 DAT at Ridgetown and Woodstock only, but did not exceed a rating of 10%; by 14 and 28 DAT no corn injury was recorded at any location. Corn yield of at least 95% of a weed-free check was obtained at Woodstock when foramsulfuron was applied at 70 g/ha. At Exeter and Woodslee yield was 90% of the weed-free check at a foramsulfuron rate of 35 g/ha. Finally, at Harrow and Ridgetown, corn yield was lowered at all foramsulfuron rates because of broadleaved weed interference. Tank-mixing foramsulfuron with dicamba plus prosulfuron improved common lambsquarters and common ragweed control and final corn yield was improved by more than 20% when compared with an application of foramsulfuron alone. Thus, these results show that weed control with foramsulfuron is species specific and that tank mixtures with a broadleaf herbicide may be required for broad-spectrum weed control and to protect the full yield potential of corn. Nomenclature: Foramsulfuron; common ragweed, Ambrosia artemisiifolia L. AMBEL; common lambsquarters, Chenopodium album L. CHEAL; green foxtail, Setaria viridis (L.) Beauv. SETVI; corn, Zea mays L.

Contributions of Weed Science to Weed Control and Management1

Field studies were conducted over 3 yr at two locations to evaluate the effect of glyphosate rate and time of application on common lambsquarters control, density, dry weight, seed production, and the number of seedlings emerging from soil cores taken the year after herbicide application in glyphosate-resistant corn. Glyphosate was applied at 0, 112, 225, 450, 675, or 900 g ai/ha when common lambsquarters were at the two-, four-, or six-leaf stage of growth. Nicosulfuron was applied to all experimental areas to control annual grasses. Visual estimates of percent control increased, whereas density, dry weight, seed production, and seedlings emerging the year after treatment decreased as the rate of glyphosate was increased from 0 to 450 g/ha. Increasing the glyphosate rate above 450 g/ha had little effect on these parameters. Corn yield declined only at glyphosate rates below 450 g/ha. Time of application had no effect on common lambsquarters control and corn yield because little emergence occurred after the first glyphosate application. There was no interaction between glyphosate rate and time of application for any of the parameters evaluated. In these studies, the application of glyphosate at half the manufacturers registered rate provided control of common lambsquarters equivalent to the full-registered rate with no measured increase in weed seed production and no increase in weed seedlings emerging from soil cores the year after herbicide application. The results suggest that in some cases the use of reduced herbicide rates can provide excellent weed control and maintain crop yields, while reducing the cost of production and the environmental impact of herbicides. The use of extremely low rates (112 or 225 g/ha), however, resulted in reduced corn yields, increased common lambsquarters seed production and seedlings emerging the year after application, and possibly increased weed management costs in subsequent years. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #3 CHEAL; corn, Zea mays L. ‘Pride G4286’, ‘DeKalb 493RR’. Additional index words: Reduced rates, weed control, weed seed production. Abbreviation: DAA, days after application.