James J. Kells

Effect of Glufosinate-Resistant Corn (Zea mays) Population and Row Spacing on Light Interception, Corn Yield, and Common Lambsquarters (Chenopodium album) Growth1

Abstract: Management of corn (Zea mays) row spacings and populations has been used for many years to increase corn productivity. In 1998 and 1999, nonirrigated corn was grown in 38-, 56-, and 76-cm row spacings at populations averaging 59,300, 72,900, and 83,900 plants/ha in Michigan. Glufosinate at 0.29 kg/ha was applied to common lambsquarters (Chenopodium album L.) averaging 5 cm in height in each plot. Corn population and row spacing did not influence weed emergence following application of glufosinate. Common lambsquarters biomass and seed production were reduced when grown under canopies of corn planted in populations exceeding 72,900 plants/ha. Common lambsquarters biomass was reduced as corn row spacings were reduced from 76 to 38 cm. Early-season interception of photosynthetic active radiation (PAR) by corn canopies increased as row spacings decreased, but differences were not apparent later in the season. Interception of PAR was similar throughout the season when corn populations exceeded 72,900 plants/ha. Corn yields were not affected by row spacing, but they were increased with corn populations of 72,900 plants/ha or higher. Nomenclature: Glufosinate; common lambsquarters, Chenopodium album L,. #3 CHEAL; corn, Zea mays L. ‘DK 493GR’. Additional index words: Crop density, herbicide-tolerant crops, narrow-row corn. Abbreviation: PAR, photosynthetic active radiation.

Effect of Glyphosate Application Timing and Row Spacing on Corn (Zea mays) and Soybean (Glycine max) Yields1

Corn and soybean were planted in narrow and wide row spacings to determine the effect of glyphosate application timing and row spacing on crop yield. Glyphosate was applied when average weed canopy height reached 5, 10, 15, 23, and 30 cm. Weeds present in these studies included velvetleaf, redroot pigweed, common ragweed, common lambsquarters, jimsonweed, barnyardgrass, fall panicum, giant foxtail, yellow foxtail, green foxtail, and eastern black nightshade. Under highly competitive growing conditions (below normal rainfall and high weed density), corn yield was first reduced when weeds reached 10 and 15 cm in height with corn planted in 38- and 76-cm rows, respectively. Under similar conditions, soybean yield was first reduced when weeds reached 15 and 23 cm with soybean planted in 19- and 38-cm rows, respectively. Yield losses occurred only in the untreated control when soybean was planted in 76-cm rows. When growing conditions were less competitive (adequate rainfall and lower weed density), yield losses occurred only when weeds reached 30 cm or more in corn and only in the untreated control in soybean. Corn and soybean yields were higher when planted in narrow rows in three of 4 yr but were more susceptible to early- season weed interference than corn and soybean in wide rows. Corn yield was affected more by weed interference than was soybean yield. The product of weed height by weed density, as the independent variable, resulted in the best linear fit for both corn and soybean yields. High weed densities increase the risk of yield loss and must be considered when determining the appropriate timing for total postemergence herbicide applications such as glyphosate. Sequential glyphosate applications in corn did not increase yield. Nomenclature: Glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; eastern black nightshade, Solanum ptycanthum Dun. # SOLPT; fall panicum, Panicum dichotomilflorum Michx. # PANDI; giant foxtail, Setaria faberi Herrm. # SETFA; green foxtail, Setaria viridis (L.) Beauv. # SETVI; jimsonweed, Datura stramonium L. # DATST; redroot pigweed, Amaranthus retroflexus L. # AMARE; velvetleaf, Abutilon theophroasti Medik. # ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. # SETLU; corn, Zea mays L. ‘DK 493RR’; soybean, Glycine max (L.) Merr. ‘Pioneer 92B71’. Additional index words: Application timing, competitive load, narrow-row corn, narrow-row soybean, weed competition, weed interference. Abbreviations: DAE, days after crop emergence; GDDAE, growing degree days after emergence; POST, postemergence; SAS, Statistical Analysis Systems.

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.

Effect of Glyphosate Application Timing and Row Spacing on Weed Growth in Corn (Zea mays) and Soybean (Glycine max) 1

Corn and soybean were planted in narrow- and wide-row spacings to study the effects of glyphosate application timing and row spacing on light interception and subsequent weed growth. Corn planted in narrow rows (38 cm) had greater light interception than corn planted in wide rows (76 cm) from 35 to 55 d after crop emergence. Soybean planted in narrow rows (both 19 and 38 cm) had greater light interception throughout the growing season than soybean in 76-cm rows. At maximum canopy closure, narrow-row soybean (both 19 and 38 cm) intercepted more light than narrow-row corn. Biomass of weeds emerging after glyphosate application was greater when soybean was planted in 76-cm than in 19- or 38-cm rows. However, weed biomass was generally similar in both row spacings of corn. Sequential glyphosate applications reduced weed biomass in corn each year compared with a single glyphosate application at the 5-cm weed height. Sequential glyphosate applications that followed initial glyphosate application to 10- or 15-cm-tall weeds did not reduce weed biomass compared with a single application. Nomenclature: Glyphosate; corn, Zea mays L. ‘DK 493RR’; soybean, Glycine max (L.) Merr. ‘Pioneer 92B71’. Additional index words: Light intensity, narrow-row corn, narrow-row soybean, shading, weed control, weed interference. Abbreviations: DAE, days after crop emergence; POST, postemergence.

Substitutes for ammonium sulfate as additives with glyphosate and glufosinate

Glyphosate and glufosinate are now options for postemergence weed control in herbicide-resistant corn and soybean. Velvetleaf is one of the more difficult to control annual weeds with these herbicides at commonly used rates. Ammonium sulfate (AMS) is generally used with these herbicides to overcome hard water antagonism and to increase herbicide activity. Greenhouse and field trials were conducted with commercial adjuvants that might substitute for AMS. The adjuvants were evaluated in deionized water, tap water, and deionized water containing 500 mg/L CaCO3. In the absence of AMS, hard water reduced velvetleaf control with both herbicides. Regardless of water source, AMS increased velvetleaf control with both glyphosate and glufosinate. Several adjuvants increased velvetleaf control with either herbicide; however, none were superior to 2% w/v AMS. Other adjuvants decreased velvetleaf control with either herbicide. Nomenclature: Ammonium sulfate; glufosinate; glyphosate; velvetleaf, Abutilon theophrasti Medic #3 ABUTH; corn, Zea mays L. # ZEAMA; soybean, Glycine max (L.) Merr. # GLYMA. Additional index words: Ammonium sulfate, glufosinate, glyphosate.

Residual Herbicides used in Combination with Glyphosate and Glufosinate in Corn (Zea mays)1

Weed management strategies are needed to control weeds in corn that emerge after postemergence (POST) application of glyphosate or glufosinate. Field trials were conducted from 1996 to 1999 to determine if residual herbicides could be used with glyphosate or glufosinate to provide season-long weed control in glyphosate-resistant or glufosinate-resistant corn. Preemergence (PRE) applications of several residual herbicides followed by POST applications of glyphosate or glufosinate were compared with POST tank mixtures of glyphosate or glufosinate with residual herbicides. All residual herbicides used in combination with glyphosate, when compared with glyphosate alone, increased control of redroot pigweed and common lambsquarters by an average of 20% and resulted in a 4 to 19% increase in control of giant foxtail. Each residual herbicide tank mixture with glufosinate, when compared with glufosinate alone, increased control of common ragweed and giant foxtail. In most instances, weed control was similar for total POST and for PRE followed by POST systems. Velvetleaf control was reduced by 12% when glyphosate was tank mixed with a half rate of atrazine compared with a full rate of atrazine. Common lambsquarters control was reduced by 13% when glyphosate was tank mixed with a half vs. a full rate of acetochlor. Glufosinate tank mixed with half rates of acetochlor + atrazine, flumetsulam, or pendimethalin reduced control of velvetleaf compared with the full rate of these products. Corn yields were not affected by residual herbicide application timings or rates of residual herbicides. Nomenclature: Acetochlor; atrazine; flumetsulam; glufosinate; glyphosate; pendimethalin; common lambsquarters, Chenopodium album L. #3 CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; giant foxtail, Setaria faberi L. # SETFA; redroot pigweed, Amaranthus retroflexus L. # AMARE; velvetleaf, Abutilon theophrasti Medik. # ABUTH; corn, Zea mays L. ‘DK 493GR’, ‘DK 493RR’. Additional index words: Glufosinate-resistant corn, glyphosate-resistant corn, herbicide-tolerant crops, metolachlor, soil-applied herbicides. Abbreviations: POST, postemergence; PRE, preemergence.

Effect of Weed Removal Timing and Row Spacing on Soil Moisture in Corn (Zea mays)

Glyphosate-resistant corn was grown in 38- and 76-cm row spacings at two locations in 2001 to examine the effect of weed competition and row spacing on soil moisture. Volumetric soil moisture was measured to a depth of 0.9 m in 18-cm increments. Glyphosate was applied when average weed canopy heights reached 5, 10, 15, 23, and 30 cm. Season-long weed interference reduced soil moisture compared with the weed free controls. At Clarksville, MI, where common lambsquarters was the dominant weed species, weed interference reduced soil moisture in the 0- to 18-cm soil depth from late June through early August and at the 54- to 72- and 72- to 90-cm depths from mid-July through the end of the season. At East Lansing, MI, where giant foxtail was the dominant weed species, weed interference reduced soil moisture at the 18- to 36-, 36- to 54-, and 54- to 72-cm soil depths from mid-June to the end of the season. Season-long weed competition reduced yields more than 90% at each location. Weeds that emerged after the 5-cm glyphosate timing reduced soil moisture and grain yield at both locations. Delaying glyphosate applications until weeds reached 23 cm or more in height reduced corn yield at both locations and soil moisture at East Lansing. Grain yields in the 10- and 15-cm glyphosate-timing treatments were equal to the weed-free corn, even though soil moisture was less during pollination and grain fill. Row spacing did not affect grain yield but did affect soil moisture. Soil moisture was greater in the 76-cm row spacing, suggesting that corn in the 38-cm row spacing may have been able to access soil moisture more effectively. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #3 CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; corn, Zea mays L. Additional index words: Narrow-row corn, glyphosate-resistant, weed interference, weed competition, time domain reflectometry, soil water. Abbreviations: DAP, days after planting; FC, field capacity; PRE, preemergence; PWP, permanent wilting point.

Response of ‘Wakefield’ Winter Wheat (Triticum aestivum) to Dicamba1

Abstract: Dicamba is a herbicide used for the control of broadleaf weeds in wheat. Dicamba, applied within the recommended growth stage interval, reduced the grain yield of Wakefield winter wheat by 95% in a herbicide sensitivity study at Michigan State University. Growers have also reported yield loss when using dicamba on Wakefield. Field and greenhouse experiments were conducted to characterize the response of Wakefield winter wheat to dicamba and to compare this response to that of ‘Harus’ winter wheat, a cultivar that is not considered sensitive to dicamba. This research was conducted to characterize the sensitivity of Wakefield to dicamba and to develop visual methods for detecting sensitivity of wheat cultivars to dicamba. Field experiments confirmed that dicamba affects the number of spikelets and the seed weight of Harus and Wakefield similarly. However, dicamba, applied within the recommended application interval, caused small, shriveled (underdeveloped) seeds to occur in Wakefield in the field and greenhouse. These seeds weighed very little, did not contribute to grain yield, and could not be harvested mechanically. Dicamba reduced the number of fully developed seeds of Wakefield by as much as 62% in the field and 92% in the greenhouse when applied within the recommended application interval. The number of fully developed seeds of Harus was reduced in the field only when dicamba was applied later than the recommended application interval. Decreases in grain yield due to dicamba were caused primarily by decreases in the number of fully developed seeds. Greenhouse experiments indicated that pollen abnormalities were only a minor cause of the development of underdeveloped seeds. Nomenclature: Dicamba; winter wheat, Triticum aestivum L. ‘Harus’, ‘Wakefield’. Additional index words: Herbicide sensitivity, growth regulators. Abbreviations: F2, Feekes stage 2; F3, Feekes stage 3; F5, Feekes stage 5; F9, Feekes stage 9; F10, Feekes stage 10.

Triazine-resistant common lambsquarters (chenopodium album) control in corn with preemergence herbicides

Triazine-resistant common lambsquarters (TR-CHEAL) is a widespread weed problem in the northcentral United States. Field studies were conducted from 1995 to 1997 to determine the efficacy and consistency of metolachlor, pendimethalin, and acetochlor applied preemergence (PRE) for control of TR-CHEAL in corn. Pendimethalin provided greater (98%) and more consistent control of TR-CHEAL than metolachlor (66%) or acetochlor (86%). Studies were conducted from 1998 to 2000 to examine the potential of isoxaflutole, flumetsulam, and rimsulfuron plus thifensulfuron for control of TR-CHEAL in corn. In 1999 and 2000, isoxaflutole (35 g ai/ha), flumetsulam (35 g ai/ ha), and rimsulfuron plus thifensulfuron (26 g ai/ha) provided 98% or greater control of TR-CHEAL. In 1998 when rainfall was limited after application, isoxaflutole (70 g ai/ha) and flumetsulam (70 g ai/ha) provided 65 and 55% control, respectively, and rimsulfuron plus thifensulfuron (26 g ai/ha) provided 55% control. Results indicate that control of TR-CHEAL with currently labeled PRE herbicides is possible. Nomenclature: Acetochlor; flumetsulam; isoxaflutole; metolachlor; pendimethalin; rimsulfuron; thifensulfuron; triazine; common lambsquarters, Chenopodium album L. #3 CHEAL; corn, Zea mays L. Additional index words: Herbicide resistance. Abbreviations: OM, organic matter; POST, postemergence; PRE, preemergence; TR-CHEAL, triazine-resistant common lambsquarters.

Common lambsquarters (Chenopodium album) interference with corn across the northcentral United States

Abstract Variation in crop–weed interference relationships has been shown for a number of crop–weed mixtures and may have an important influence on weed management decision-making. Field experiments were conducted at seven locations over 2 yr to evaluate variation in common lambsquarters interference in field corn and whether a single set of model parameters could be used to estimate corn grain yield loss throughout the northcentral United States. Two coefficients (I and A) of a rectangular hyperbola were estimated for each data set using nonlinear regression analysis. The I coefficient represents corn yield loss as weed density approaches zero, and A represents maximum percent yield loss. Estimates of both coefficients varied between years at Wisconsin, and I varied between years at Michigan. When locations with similar sample variances were combined, estimates of both I and A varied. Common lambsquarters interference caused the greatest corn yield reduction in Michigan (100%) and had the least effect in Minnesota, Nebraska, and Indiana (0% yield loss). Variation in I and A parameters resulted in variation in estimates of a single-year economic threshold (0.32 to 4.17 plants m−1 of row). Results of this study fail to support the use of a common yield loss–weed density function for all locations. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; corn, Zea mays L. ‘DK404SR’, ‘DK493SR’, ‘DK592SR’, ‘Asgrow RX602SR’.