Glyphosate-resistant Cultivars Research Articles

Cotton and corn rotation under reduced tillage management: impacts on soil properties, weed control, yield, and net return

A 6-yr rotation study was conducted from 2000 to 2005 at Stoneville, MS to examine the effects of rotating glyphosate-resistant (GR) and non-GR (conventional) cultivars of cotton with corn under reduced tillage conditions on soil properties, weed control, crop yield, and net return. There were four rotation systems (continuous cotton, continuous corn, cotton–corn, and corn–cotton) for each non-GR and GR cultivar arranged in a randomized complete block design with four replications. Field preparation consisted of disking, subsoiling, disking, and bedding in the fall of 1999. After the fall of 2000, the experimental area received no tillage operations except rebedding after harvest each year to maintain reduced tillage conditions. A glyphosate-based program in GR cultivars and a nonglyphosate-based program in non-GR cultivars were used for weed management. Soil organic carbon in the top 5-cm depth progressively increased from the first year to the sixth year, regardless of rotation. In 2005, organic carbon was higher in corn grown continuously and in rotation compared to continuous cotton, partly due to higher plant residues from corn compared to cotton. Control of most grass and broadleaf weeds was sufficient to support cotton and corn production, regardless of rotation and herbicide program. Control of yellow nutsedge was reduced in continuous non-GR cotton; this apparent weed species shift toward yellow nutsedge was mitigated by breaking the cotton monocrop with corn. Plant populations of both GR and non-GR cotton rotated with corn were similar to that of continuous cotton suggesting cotton stand establishment was not affected by corn residues from the previous year. Cotton yield increased every year following rotation with corn by 10–32% in the non-GR cultivar, and by 14–19% in the GR cultivar compared to continuous cotton. Similarly, corn yield increased by 5–13% in non-GR cultivar and by 1–11% in the GR cultivar when rotated with cotton. As a result, net returns were higher from rotation management as compared with monoculture in both crops. This study demonstrated that alternating between cotton and corn is agronomically feasible and a sustainable option for farmers in the lower Mississippi River alluvial flood plain region who are looking for simple cultural practices that provide economic and environmental benefits.

Economics of integrated weed management in herbicide-resistant canola

Integrated weed management (IWM) decision strategies in herbicide-resistant canola-production systems were assessed for net returns and relative risk. Data from two field experiments conducted during 1998 to 2000 at two locations in Alberta, Canada, were evaluated. A herbicide-based experiment included combinations of herbicide system (glufosinate-, glyphosate-, and imazethapyr-resistant canola varieties), herbicide rate (50 and 100% of recommended dose), and time of weed removal (two-, four-, and six-leaf stages of canola). A seed-based experiment included canola variety (hybrid and open-pollinated), seeding rate (100, 150, and 200 seeds m−2), and time of weed removal (two-, four-, and six-leaf stages of canola). For the herbicide-based experiment, strategies with glyphosate were profitable at Lacombe, but both imazethapyr and glyphosate strategies were profitable at Lethbridge. Weed control at the four-leaf stage was at least as profitable as the two-leaf stage at both sites. For the seed-based experiment, the hybrid was more profitable than the open-pollinated cultivar, seed rates of 100 and 150 seeds m−2were more profitable than 200 seeds m−2, and weed control at the two- and four-leaf stages was more profitable than at the six-leaf stage. When risk of returns and statistical significance was considered, several strategies were included in the risk-efficient set for risk-averse and risk-neutral attitudes at each location. However, the glyphosate-resistant cultivar, the 50% herbicide rate, and weed control at four-leaf stage were more frequent in the risk-efficient IWM strategy set. The open-pollinated cultivar, 200 seeds m−2rate, and weed control at the six-leaf stage were less frequent in the set. The risk-efficient sets of IWM strategies were consistent across a range of canola prices.

Glyphosate affects soybean root exudation and rhizosphere micro-organisms

Glyphosate is a non-selective, broad-spectrum herbicide that kills plants by inhibiting the enzyme 5-enolpyruvylshikimic acid-3-phosphate synthase (EPSPS), which is necessary for synthesis of aromatic amino acids. A secondary mode of action involves infection of roots of glyphosate-susceptible plants by soil-borne micro-organisms due to decreased production of plant protection compounds known as phytoalexins. Varieties of several crops, including glyphosate-resistant (GR) or Roundup Ready soybean, are genetically modified to resist the herbicidal effects of glyphosate and provide farmers with an effective weed-management tool. After glyphosate is applied to GR soybean, glyphosate that is not bound to glyphosate-resistant EPSPS is translocated throughout the plant and accumulates primarily in meristematic tissues. We previously reported that fungal colonization of GR soybean roots increased significantly after application of glyphosate but not after conventional postemergence herbicides. Because glyphosate may be released into soil from GR roots, we characterized the response of rhizosphere fungi and bacteria to root exudates from GR and non-GR (Williams 82; W82) cultivars treated with and without glyphosate at field application rates. Using an immunoassay technique, glyphosate at concentrations >1000 ng plant-1 were detected in exudates of hydroponically grown GR soybean at 16 days post-glyphosate application. Glyphosate also increased carbohydrate and amino acid contents in root exudates in both soybean cultivars. However, GR soybean released higher carbohydrate and amino acid contents in root exudates than W82 soybean without glyphosate treatment. In vitro bioassays showed that glyphosate in the exudates stimulated growth of selected rhizosphere fungi, possibly by providing a selective C and N source combined with the high levels of soluble carbohydrates and amino acids associated with glyphosate treatment of the soybean plants. Increased fungal populations that develop under glyphosate treatment of GR soybean may adversely affect plant growth and biological processes in the soil and rhizosphere.

Glyphosate-Resistant Soybean Management System Effect on Sclerotinia Stem Rot

The impact of the management variables soybean cultivar, row spacing, population density, and shading was evaluated on the incidence of Sclerotinia stem rot (SSR) on glyphosate-resistant soybeans in an irrigated glyphosate-resistant soybean management system. Soybean canopy development, flower number, soil moisture, disease severity, and soybean yield were evaluated on three glyphosate-resistant cultivars, Pioneer ‘92B71’ (upright), Asgrow ‘AG2701’ (bushy), and Asgrow ‘AG2702’ (bushy). Three different row spacing–target population combinations of 76 cm, 430,000 seeds/ha; 19 cm, 430,000 seeds/ha; and 19 cm, 560,000 seeds/ha were evaluated. Cultivars 92B71 and AG2701 had 42 and 15% lower disease severity indexes and 38 and 19% greater yields than AG2702, respectively. The actual average population of 92B71 was 9 and 20% lower than actual average populations of AG2701 and AG2702, respectively. Disease severity indexes were lower and yield was higher when population was reduced from 560,000 seeds/ha to 430,000 seeds/ha in 19-cm rows. When averaged over the entire study, population was positively correlated with disease severity index (r2= 0.33; P < 0.0001) and negatively correlated with yield (r2= −0.13; P = 0.0140). Reduction of soybean population was more important than increasing row spacing to manage SSR in an irrigated system. Average actual spacing between plants within a row was 18 and 4 cm for 19- and 76-cm rows, respectively, at a target population of 430,000 seeds/ha, which may have contributed to greater plant-to-plant transfer of theSclerotinia sclerotiorumpathogen in the 76-cm rows.

Weed density and diversity under glyphosate-resistant crop sequences

The effect of regular and exclusive application of glyphosate in crop sequences on associated weed composition, richness and diversity was studied over five years. Three sequences (wheat–soybean, soybean monoculture and soybean–maize) including soybean and maize glyphosate-resistant cultivars were investigated under two tillage systems (conventional or no-tillage). Regardless of sequence and tillage system, regular glyphosate application appeared to reduce richness and density of the most competitive weeds such as early-emergence annual broad-leaved and grassy annuals and increase that of the less competitive late-emerging annual broad-leaved weeds in no-tillage systems. Some late-emerging annual broad-leaved species, e.g. Bowlesia incana Ruiz et Pav. were present from the beginning of the experiment and others species, e.g. Parietaria debilis G. Foster were not present before initiating the study. This species was the only tolerant to glyphosate at the recommended dose of the herbicide. Species richness in pre-glyphosate application counts showed no response in the wheat–soybean rotation but decreased in the soybean–maize rotation and soybean monoculture in both tillage systems. In post-glyphosate application counts, richness increased for all sequences. Diversity showed an inconsistent response with time. It can be concluded that the effect of glyphosate application was a more important factor than crop sequence to explain weed community changes in summer crops. It may be predicted that continual glyphosate application for longer periods of time might lead to the development or higher increases in abundance of weeds tolerant to the herbicide.

Deep and Shallow Fall Tillage for Irrigated Soybean Grown with Different Weed Management Systems in the Midsouthern USA

Management inputs that maximize economic return from early soybean [Glycine max (L.) Merr.] production system (ESPS) plantings in the midsouthern USA have not been evaluated fully. The objective was to determine the effect of different weed management systems on yield and net return from irrigated ESPS plantings of soybean following deep (DT; 40–45 cm deep) and shallow (ST; <10 cm deep) fall tillage. Adjacent experiments receiving either DT or ST were conducted in 1999 and 2000 on Sharkey clay (very‐fine, smectitic, thermic Chromic Epiaquert) near Stoneville, MS (33°26′ N lat). Weed management systems were (i) glyphosate‐resistant (GR) cultivar with pre‐emergent (PRE) nonglyphosate [N‐(phosphonomethyl)glycine] herbicides followed by one postemergent (POST) application of glyphosate timed to control grasses, (ii) GR cultivar with two POST applications of glyphosate timed to control both grasses and broadleaves, (iii) GR cultivar with PRE nonglyphosate herbicides followed by two POST applications of glyphosate timed to control both grasses and broadleaves, (iv) non‐GR cultivar with PRE herbicides followed by one POST application of a grass herbicide, (v) non‐GR cultivar with POST application of herbicides timed to control both grasses and broadleaves, and (vi) non‐GR cultivar with PRE herbicides followed by POST applications of herbicides timed to control both grasses and broadleaves. Fall DT was more expensive than ST but resulted in taller soybean plants and less weed cover at harvest. Average yields and net returns from DT and ST were 4286 and 4085 kg ha−1 and $364 and $362 ha−1, respectively. Thus, the investment in equipment for fall DT for irrigated ESPS plantings is not justified. Postemergent‐only weed management was the cheapest for both GR and non‐GR cultivars. Weed management that used POST‐only glyphosate resulted in the greatest yield and profit from GR cultivars. With non‐GR cultivars, yields were not affected by weed management, but net returns were lower when the most intensive weed management was practiced.

Glyphosate and shade effects on glyphosate-resistant soybean defense response to Sclerotinia sclerotiorum

Application of glyphosate in combination with planting soybeans in narrow rows is an effective practice for management of weeds in glyphosate-resistant soybean. Farmers in Michigan reported higher levels of Sclerotinia stem rot (caused by Sclerotinia sclerotiorum) in fields of glyphosate-resistant soybean. Studies were conducted to determine if glyphosate or shading reduced the defense response of glyphosate-resistant soybean to S. sclerotiorum. Glyphosate caused shikimate accumulation in glyphosate-susceptible cultivar GL2415 but not in glyphosate-resistant cultivar GL2600RR. Ethylacetate extracts containing the plant defense compound glyceollin inhibited S. sclerotiorum hyphae in a rate-dependent manner. Glyphosate had no effect on either baseline or induced levels of glyceollin in glyphosate-resistant soybean, indicating that glyphosate did not impair plant defense responses to S. sclerotiorum. Shade levels of 60 and 90% in the greenhouse did not inhibit the induction of glyceollin synthesis. Glyphosate herbicide and shading did not affect the glyphosate-resistant soybean defense response to S. sclerotiorum.

Influence of Early‐Season Nitrogen and Weed Management on Irrigated and Nonirrigated Glyphosate‐Resistant and Susceptible Soybean

Field studies were conducted on Sharkey clay soil (very‐fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objectives were to determine the effect of application of 0 and 35 kg N ha−1 applied early in the growing season to glyphosate‐resistant (GR) and non‐GR soybean [Glycine max (L.) Merr.] cultivars using two weed management systems in irrigated and nonirrigated environments. Weed management systems were (i) pre‐emergent followed by postemergent weed management using nonglyphosate herbicides applied to both GR and non‐GR cultivars (PRE + POST) and (ii) postemergent weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non‐GR cultivars (POST). Applied N had no effect on weed management in or yield from soybean and lowered average net returns by $28 to $50 ha−1. Average seed yields from the highest‐yielding GR cultivar in 1999 and 2000 were 135 and 270 kg ha−1 more than 1999 and 2000 yields from a non‐GR cultivar in the nonirrigated environment (all net returns were negative and yields <1500 kg ha−1). In the irrigated environment, use of a non‐GR cultivar compared with a GR cultivar resulted in a significant 200 and 250 kg ha−1 greater yield and greater profits in 2 of 3 yr. Use of PRE + POST compared with POST‐only was not necessary for achieving greatest yield or net return with either non‐GR or GR cultivars. Use of postemergent glyphosate always resulted in the cheapest weed control ($43 to $81 ha−1), even with the greater cost for seed of GR cultivars included. There was no measured effect of glyphosate compared with nonglyphosate herbicides on GR cultivar yield.

Influence of Early-Season Nitrogen and Weed Management on Irrigated and Nonirrigated Glyphosate-Resistant and Susceptible Soybean

Field studies were conducted on Sharkey clay soil (very-fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objectives were to determine the effect of application of 0 and 35 kg N ha−1 applied early in the growing season to glyphosate-resistant (GR) and non-GR soybean [Glycine max (L.) Merr.] cultivars using two weed management systems in irrigated and nonirrigated environments. Weed management systems were (i) pre-emergent followed by postemergent weed management using nonglyphosate herbicides applied to both GR and non-GR cultivars (PRE + POST) and (ii) postemergent weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non-GR cultivars (POST). Applied N had no effect on weed management in or yield from soybean and lowered average net returns by $28 to $50 ha−1. Average seed yields from the highest-yielding GR cultivar in 1999 and 2000 were 135 and 270 kg ha−1 more than 1999 and 2000 yields from a non-GR cultivar in the nonirrigated environment (all net returns were negative and yields <1500 kg ha−1). In the irrigated environment, use of a non-GR cultivar compared with a GR cultivar resulted in a significant 200 and 250 kg ha−1 greater yield and greater profits in 2 of 3 yr. Use of PRE + POST compared with POST-only was not necessary for achieving greatest yield or net return with either non-GR or GR cultivars. Use of postemergent glyphosate always resulted in the cheapest weed control ($43 to $81 ha−1), even with the greater cost for seed of GR cultivars included. There was no measured effect of glyphosate compared with nonglyphosate herbicides on GR cultivar yield.

Weed Management, Fiber Quality, and Net Returns in No-Tillage Transgenic and Nontransgenic Cotton (Gossypium hirsutum)1

Studies on weed management systems with bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton cultivars in a no-tillage environment were evaluated in North Carolina and Tennessee in 1997 and 1998. All weed management systems in nontransgenic and bromoxynil-resistant cotton controlled sicklepod 72 to 78%, whereas sicklepod was controlled at least 94% in glyphosate-resistant cotton. Entireleaf and pitted morningglory, smooth pigweed, and common cocklebur were controlled at least 92% with all management systems. Fiber quality was also measured in Tennessee. Micronaire and fiber strength ranged from 4.35 to 4.55 units and 28.6 to 30.1 g/tex, respectively, in bromoxynil-resistant and glyphosate-resistant cultivars that received postemergence over-the-top (POT) herbicides. Harvest trash content ranged from 0.80 to 1.15% in systems where POT and late postemergence–directed (LAYBY) herbicides were used and was 1.65% where multiple glyphosate applications were used with no LAYBY herbicides. Cotton lint yield and net returns were generally highest in systems that included bromoxynil or pyrithiobac POT followed by cyanazine plus MSMA LAYBY or multiple glyphosate treatments where no LAYBY herbicides were used.Nomenclature: Bromoxynil; cyanazine; glyphosate; MSMA; pyrithiobac; common cocklebur, Xanthium strumarium L. #3 XANST; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; pitted morningglory, Ipomoea lacunosa L. # IPOLA; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; cotton, Gossypium hirsutum L. ‘Stoneville 474’, ‘Stoneville BXN 47’, ‘Deltapine 5415RR’, ‘Paymaster 1220RR’.Additional index words: Color grade, extraneous material, fiber length, fiber strength, fiber uniformity, harvest trash, high-volume instrumentation, gin turn out, leaf grade, micronaire, staple length.Abbreviations: ASN, as needed; LAYBY, late postemergence–directed; fb; followed by; PDS, postemergence-directed spray; PRE, preemergence; POT, postemergence over-the-top.

Cultivar and Herbicide Selection Affects Soybean Development and the Incidence of Sclerotinia Stem Rot

Glyphosate‐resistant (GR) soybean [Glycine max (L.) Merr.] is produced on a majority of U.S. soybean hectares. Concerns have been expressed regarding interactions of herbicides such as glyphosate and Sclerotinia stem rot [Sclerotinia sclerotiorum (Lib.) de Bary]. Research evaluated the effect of herbicides on growth, phytoalexin production, incidence of Sclerotinia stem rot, and grain yield of GR and isogenic nonresistant cultivars in 1998 and 1999. Lactofen at 70 g a.i. ha−1 delayed reproductive development, reduced leaf area index (LAI), reduced Sclerotinia stem rot lesion diameter 2 to 26 d after treatment (DAT), increased phytoalexin production 2 to 26 DAT, reduced disease severity index (DSI), and increased yield of ‘Great Lakes 2415’ (GL2415) by 510 kg ha−1 compared with untreated soybean. Glyphosate at 840 g a.e. ha−1 increased disease severity index in ‘Great Lakes 2600’ (GL2600) and ‘Pioneer 93B01’ (P93B01) GR compared with untreated soybean while yield of ‘Novartis S20‐B9’ (S20‐B9) GR treated with glyphosate was 650 kg ha−1 greater than untreated soybean. Thifensulfuron at 4.5 g a.i. ha−1 delayed reproductive development, reduced LAI, and did not affect phytoalexin production of ‘Novartis S 19–90’ (S 19–90) or S20‐B9 GR but increased grain yield of ‘Novartis S 12–49’ (S 12–49) by 450 kg ha−1 compared with untreated soybean. S 12–49 and GL2415 treated with thifensulfuron had lower disease severity index and increased yield compared with GR isolines. Reduction of disease severity index following lactofen may be attributed to increased phytoalexin production, reduced LAI, and delayed reproductive development; however, increased yield was observed for only one of eight cultivars. Producers can reduce Sclerotinia stem rot with postemergence herbicides, but soybean yield did not necessarily increase.

Pest Management Implications of Glyphosate-Resistant Wheat (Triticum aestivum)in the Western United States1

Glyphosate-resistant crop species have increased in number over the past decade as growers eagerly adopt this simple and effective weed management technology. Glyphosate-resistant wheat cultivars are being developed and may soon be available to growers. The objective of this paper is to discuss the pest management implications of glyphosate-resistant wheat in the western United States, a region stretching from the Great Plains to the Pacific Ocean that produces more than 80% of the nation's wheat crop. The benefits of glyphosate-resistant wheat include: (1) improved weed control, particularly of difficult-to-control weeds, such as winter annual grasses belonging to the Aegilops, Avena, Bromus, Lolium, Poa, Secale, and Setaria genera; (2) an ability to control weeds resistant to currently available wheat herbicides; (3) an extended application window for control of late-emerging weeds; and (4) improved crop safety. Although these benefits are not to be minimized, they need to be considered in the light of ...

Yield Ranks of Glyphosate-Resistant Cotton Cultivars are Unaffected by Herbicide Systems

Official Cultivar Trials (OCTs) evaluate transgenic, glyphosate [N-(phosphonomethyl)-glycine]-resistant and nonresistant cotton (Gossypium hirsutum L.) cultivars with a nonglyphosate herbicide regime. Thus, yields of glyphosate-resistant cotton cultivars in OCTs may not reflect cultivar variation in glyphosate resistance or the intended production system because all cultivars are produced under a common herbicide regime. Our objective was to assess in field trials main effects and interaction among glyphosate-resistant cotton cultivars and herbicide systems with and without glyphosate for lint yield. Fourteen glyphosate-resistant cultivars were tested in 1998 while 10 were tested in 1999. Herbicide systems were (i) Standard, preplant soil-applied and postdirect-applied herbicides potentially used in OCTs but no glyphosate; (ii) Residual + Glyphosate, preplant soil-applied and postdirect-applied herbicides including a four-leaf-stage topical glyphosate application; and (iii) Glyphosate Only, a four-leaf-stage topical glyphosate application followed by an eight-leaf directed spray. We found significant (P < 0.05 and P < 0.10, earlier- and later-maturity trials, respectively) herbicide system main effects for lint yield but nonsignificant cultivar × herbicide system interactions. Averaged over years and cultivars, the Glyphosate Only herbicide system produced significantly (P < 0.05) greater yields (597 and 839 kg ha−1 for earlier- and later-maturity trials, respectively) than the Standard herbicide system (419 and 644 kg ha−1 for earlier- and later-maturity trials, respectively). The lack of cultivar × herbicide system interactions for yield suggests OCTs determine relative yield potential among glyphosate-resistant cultivars. The lower yield in the Standard system suggests OCTs can impose a yield penalty on glyphosate-resistant cultivars; thus, comparisons with non-glyphosate-resistant cultivars may not be valid.