Andrew J. Price

Absorption and translocation of glyphosate in glyphosate-resistant cotton as influenced by application method and growth stage

Abstract The influence of herbicide placement and plant growth stage on the absorption and translocation patterns of 14C-glyphosate in glyphosate-resistant cotton was investigated. Plants at four growth stages were treated with 14C-glyphosate on a 5-cm2 section of the stem, which simulated a postemergence-directed spray (PDS) application, or on the newest mature leaf, which simulated a postemergence (POST) application. Plants were harvested 3 and 7 d after treatment and divided into the treated leaf or treated stem, mature leaves, immature leaves and buds, stems, roots, fruiting branches (including the foliage on the fruiting branch), squares, and bolls. The PDS versus POST application main effect on absorption was significant. Absorption of 14C-glyphosate applied to stem tissue was higher in PDS applications than in POST applications. Plants receiving PDS applications absorbed 35% of applied 14C-glyphosate, whereas those receiving POST applications absorbed 26%, averaged over growth stages at application. Absorption increased from the four-leaf growth stage to the eight-leaf stage in POST applications but reached a plateau at the eight-leaf stage. Plants with PDS applications showed an increase in absorption from the four- to eight- to twelve-leaf stages and reached a plateau at the 12-leaf stage. Translocation of 14C-glyphosate to roots was greater at all growth stages with PDS treatments than with POST treatments. Herbicide placement did not affect translocation of 14C-glyphosate to squares and bolls. Squares and bolls retained 0.2 to 3.7% of applied 14C-glyphosate, depending on growth stage. Separate studies were conducted to investigate the fate of foliar-applied 14C-glyphosate at the four- or eight-leaf growth stages when harvested at 8- or 10-leaf, 12-leaf, midbloom (8 to 10 nodes above white bloom), and cutout (five nodes above white bloom, physiological maturity) stages. Thirty to 37% of applied 14C-glyphosate remained in the plant at cutout in four- and eight-leaf treatment stages, respectively. The concentration of 14C-glyphosate in tissue (Bq g−1 dry weight basis) was greatest in mature leaves and immature leaves and buds in plants treated at the four-leaf stage. Plants treated at the eight-leaf stage and harvested at all growth stages except cutout showed a higher concentration of 14C-glyphosate in squares than in other plant tissue. Accumulation of 14C-glyphosate in squares reached a maximum of 43 Bq g−1 dry weight at harvest at the 12-leaf stage. This concentration corresponds to 5.7 times greater accumulation of 14C-glyphosate in squares than in roots, which may also be metabolic sinks. These data suggest that reproductive tissues such as bolls and squares can accumulate 14C-glyphosate at higher concentrations than other tissues, especially when the herbicide treatment is applied either POST or PDS during reproductive stages (eight-leaf stage and beyond). Nomenclature: Glyphosate; cotton, Gossypium hirsutum L. ‘Delta Pine 5415RR’.

Evaluation of Three Winter Cereals for Weed Control in Conservation-Tillage Nontransgenic Cotton 1

The increased use of conservation tillage in cotton production requires that information be developed on the role of cover crops in weed control. Field experiments were conducted from fall 1994 through fall 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue, conservation-tillage, nontransgenic cotton production system. Black oat, rye, and wheat were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were used: no herbicide, preemergence (PRE) herbicides alone, and PRE plus postemergence (POST) herbicides. The PRE system consisted of pendimethalin at 1.12 kg ai/ha plus fluometuron at 1.7 kg ai/ha. The PRE plus POST system contained an additional application of fluometuron at 1.12 kg/ha plus DSMA at 1.7 kg ai/ha early POST directed (PDS) and lactofen at 0.2 kg ai/ha plus cyanazine at 0.84 kg ai/ha late PDS. No cover crop was effective in controlling weeds without a herbicide. However, when black oat or rye was used with PRE herbicides, weed control was similar to the PRE plus POST system. Rye and black oat provided more effective weed control than wheat in conservation-tillage cotton. The winter fallow, PRE plus POST input system yielded significantly less cotton in 2 of 3 yr compared to systems that included a winter cover crop. Use of black oat or rye cover crops has the potential to increase cotton productivity and reduce herbicide inputs for nontransgenic cotton grown in the Southeast. Nomenclature: Black oat, Avena strigosa Schreb. ‘SoilSaver’; rye, Secale cereale L. ‘Elbon’; wheat, Triticum aestivum L. ‘Pioneer P26 J61’; cotton, Gossypium hirsutum L. ‘Deltapine DP 5690’, ‘Deltapine NuCotn 35B’. Additional index words: Allelopathy, cover crops. Abbreviations: DAP, days after planting; PDS, postemergence-directed spray; POST, postemergence; PRE, preemergence.

PERFORMANCE OF DIFFERENT ROLLER DESIGNS IN TERMINATING RYE COVER CROP AND REDUCING VIBRATION

Rollers may provide a viable alternative to herbicides for terminating cover crops; however, excessive vibration generated by rollers and transferred to tractors hinders the adoption of this technology in the United States. To avoid excessive vibration, producers must limit their operational speed, which increases time and cost of rolling. The effect of speed on cover crop termination rate and vibration level was tested on several roller designs. Two field experiments were conducted with different roller designs to terminate a cover crop of rye (Secale cereale L.). In the first experiment, three single-section roller designs (long straight bars, curved blunt bars, and a smooth roller with an oscillating crimping bar) were tested at 1.6, 4.8, and 8 km/h operating speeds. In the second experiment, two triple-section commercial width rollers, one with long straight bars and the other, a smooth roller with an oscillating crimping bar, were tested at speeds 3.2 and 6.4 km/h. Data from the first experiment showed that all three roller designs terminated at levels greater than 90% with the highest termination rate produced by the smooth roller with crimping bar (93.4%). Three weeks after rolling, termination rates varied from 88.3% to 94.0% for all designs and speed ranges, all of which were sufficient mortality rates for rye before planting a cash crop without need to use herbicide. Reduced vibration levels measured on the tractor’s frame were generated by the smooth roller with oscillating crimping bar with the highest vibration levels being generated by the roller with the straight bars. In the second experiment, three weeks after rolling significantly higher rye termination rates resulted from the roller with long straight bars (96%) in comparison with the smooth roller (94%). Despite these differences, both rollers effectively terminated rye prior to planting without use of herbicides. The smooth roller with crimping bar transferred significantly lower vibration levels to the tractor’s frame than long straight bar roller at both speeds but vibration levels exceeded acceptable health and comfort levels.

Annual Grass Control in Peanut (Arachis hypogaea) with Clethodim and Imazapic

Field experiments were conducted to evaluate possible interactions of clethodim with imazapic applied as mixtures or sequentially for control of broadleaf signalgrass, fall panicum, goosegrass, and large crabgrass. Imazapic at 70 g ai/ha alone controlled grass weeds inconsistently, whereas clethodim at 140 g ai/ha alone controlled the same weeds at least 99%. Imazapic did not affect broadleaf signalgrass control by clethodim. Reduced control of fall panicum, goosegrass, and large crabgrass was observed when clethodim and imazapic were applied in mixture. Antagonism of clethodim occurred when clethodim was applied 1 d before or up to 3 d after application of imazapic (fall panicum and large crabgrass). Antagonism of goosegrass control was noted when imazapic was applied 3 d before or up to 7 d after application of clethodim. In other experiments, large crabgrass and Texas panicum control by clethodim (70 and 140 g/ha) applied alone or with imazapic (70 g/ ha) or bentazon (1.1 kg ai/ha) plus 2,4-DB (0.28 kg ai/ha) either with or without ammonium sulfate (2.8 kg/ha) was evaluated. Texas panicum control by clethodim was reduced by imazapic regardless of the ammonium sulfate rate. However, large crabgrass control by imazapic was not affected in these experiments. Control of both grasses by clethodim was reduced substantially by bentazon plus 2,4-DB, although in some instances ammonium sulfate improved control when in mixture. Ammonium sulfate improved control by clethodim in some instances irrespective of the broadleaf–sedge herbicide treatments. Nomenclature: Bentazon; clethodim; 2,4-DB; imazapic; broadleaf signalgrass, Brachiaria platyphylla (Griseb) Nash #3 BRAPP; fall panicum, Panicum dichotomiflorum L. # PANDI; goosegrass, Eleusine indica L. Gaertn. # ELEIN; large crabgrass, Digitaria sanguinalis L. Scop. # DIGSA; Texas panicum, Panicum texanum Buckl. # PANTE. Additional index words: Ammonium sulfate, antagonism, herbicide compatibility, herbicide interaction, sequential application.

Evaluation of weed control provided by three winter cereals in conservation-tillage soybean

Information is needed on the role of cover crops as a weed control alternative due to the high adoption of conservation tillage in soybean [Glycine max (L.) Merr.] production. Field experiments were conducted from fall 1994 through fall 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue conservation-tillage, soybean production system. Black oat (Avena strigosa Schreb.), rye (Secale cereale L.), and wheat (Triticum aestivum L.) were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were utilized: no herbicide, a mixture of two pre-emergence (PRE) herbicides, or PRE plus post-emergence (POST) herbicides. The PRE system contained pendimethalin plus metribuzin. The PRE plus POST system contained pendimethalin plus a prepackage of metribuzin and chlorimuron ethyl applied PRE, followed by an additional chlorimuron ethyl POST application. No cover crop was effective in controlling weeds without a herbicide. However, when black oat or rye was utilized with only PRE herbicides, weed control was similar to the PRE plus POST input system. Thus, herbicide reductions may be attained by utilizing cover crops that provide weed suppression. Rye and black oat provided more effective weed control in the PRE only herbicide input system than wheat in conservation-tillage soybean. The winter fallow, PRE plus POST herbicide input system yielded significantly less soybean one out of three years when compared to systems that included a winter cover crop.

Enhanced Atrazine Degradation: Evidence for Reduced Residual Weed Control and a Method for Identifying Adapted Soils and Predicting Herbicide Persistence

Abstract Soilborne bacteria with novel metabolic abilities have been linked with enhanced atrazine degradation and complaints of reduced residual weed control in soils with an s-triazine use history. However, no field study has verified that enhanced degradation reduces atrazines residual weed control. The objectives of this study were to (1) compare atrazine persistence and prickly sida density in s-triazine-adapted and nonadapted field sites at two planting dates; (2) utilize original and published data to construct a diagnostic test for identifying s-triazine-adapted soils; and (3) develop and validate an s-triazine persistence model based on data generated from the diagnostic test, i.e., mineralization of ring-labeled 14C-s-triazine. Atrazine half-life values in s-triazine-adapted soil were at least 1.4-fold lower than nonadapted soil and 5-fold lower than historic estimates (60 d). At both planting dates atrazine reduced prickly sida density in the nonadapted soils (P ≤ 0.0091). Conversely, in the s-triazine-adapted soil, prickly sida density was not different between no atrazine PRE and atrazine PRE at the March 15 planting date (P  =  0.1397). A lack of significance in this contrast signifies that enhanced degradation can reduce atrazines residual control of sensitive weed species. Analyses of published data indicate that cumulative mineralization in excess of 50% of C0 after 30 d of incubation is diagnostic for enhanced s-triazine degradation. An s-triazine persistence model was developed and validated; model predictions for atrazine persistence under field conditions were within the 95% confidence intervals of observed values. Results indicate that enhanced atrazine degradation can decrease the herbicides persistence and residual activity; however, coupling the diagnostic test with the persistence model could enable weed scientists to identify s-triazine-adapted soils, predict herbicide persistence under field conditions, and implement alternative weed control strategies in affected areas if warranted. Nomenclature: Atrazine; prickly sida, Sida spinosa L.

Weed Control and Cotton Response to Combinations of Glyphosate and Trifloxysulfuron1

Greenhouse and field studies were conducted to evaluate potential interactions between glyphosate and trifloxysulfuron on barnyardgrass, browntop millet, hemp sesbania, seedling johnsongrass, pitted morningglory, prickly sida, sicklepod, and velvetleaf control as well as cotton injury and yield. In the greenhouse, glyphosate at 840 g ae/ha controlled all weed species 62 to 99%, which was better than trifloxysulfuron at 2.5 or 5 g ai/ha. Control of four-leaf pitted morningglory and hemp sesbania was 80 to 88% when glyphosate and trifloxysulfuron were mixed compared with 62 to 66% control with glyphosate alone. Mixing trifloxysulfuron with glyphosate did not affect control of other species compared with glyphosate alone. In the field, glyphosate controlled barnyardgrass, prickly sida, sicklepod, seedling johnsongrass, and velvetleaf 68 to 100%. Trifloxysulfuron controlled hemp sesbania, seedling johnsongrass, and sicklepod 65 to 88%. All other species were controlled 36 to 72% with glyphosate and 10 to 60% with trifloxysulfuron. Combinations of glyphosate (840 g/ha) and trifloxysulfuron (5 g/ha) were applied postemergence over-the-top and postemergence-directed to three-, six-, and nine-leaf glyphosate-resistant cotton in the field. Cotton injury at 2 wk after treatment (WAT) was less than 13% for all herbicide treatments and less than 5% by 3 WAT. Herbicides did not affect the percent of open bolls or nodes per plant. Seed cotton yield ranged from 1,430 to 1,660 kg/ha, and only the sequential over-the-top applications of trifloxysulfuron reduced cotton yield compared with the weed-free, nontreated cotton. Nomenclature: Glyphosate; trifloxysulfuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; browntop millet, Brachiaria ramosa (L.) Stapf # PANRA; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A. W. Hill # SEBEX; johnsongrass, Sorghum halepense L. Pers. # SORHA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin & Barnaby # CASOB; velvetleaf, Abutilon theophrasti Medik. # ABUTH; cotton, Gossypium hirsutum L. Additional index words: CGA-362622, crop injury, glyphosate-resistant cotton, herbicide interactions, pesticide interactions, tank mixtures. Abbreviations: ALS, acetolactate synthase; EPOST, early postemergence; fb, followed by; GRC, glyphosate-resistant cotton; LPOST, late postemergence; MPOST, midpostemergence; PD, postemergence-directed; POST, postemergence; POT, postemergence over-the-top; WAT, weeks after treatment.

HADSS™, Pocket HERB™, and WebHADSS™: Decision Aids for Field Crops1

Row crop weed management decisions can be complex due to the number of available herbicide treatment options, the multispecies nature of weed infestations within fields, and the effect of soil characteristics and soil-moisture conditions on herbicide efficacy. To assist weed managers in evaluating alternative strategies and tactics, three computer programs have been developed for corn, cotton, peanut, and soybean. The programs, called HADSS™ (Herbicide Application Decision Support System), Pocket HERB™, and WebHADSS™, utilize field-specific information to estimate yield loss that may occur if no control methods are used, to eliminate herbicide treatments that are inappropriate for the specified conditions, and to calculate expected yield loss after treatment and expected net return for each available herbicide treatment. Each program has a unique interactive interface that provides recommendations to three distinct kinds of usage: desktop usage (HADSS), internet usage (WebHADSS), and on-site usage (Pocket HERB). Using WeedEd™, an editing program, cooperators in several southern U.S. states have created different versions of HADSS, WebHADSS, and Pocket HERB that are tailored to conditions and weed management systems in their locations. Nomenclature: Corn, Zea mays L.; cotton, Gossypium hirsutum L.; peanut, Arachis hypogea L; soybean, Glycine max L. Additional index words: Bioeconomic models, computer decision aids, decision support systems, weed management. Abbreviations: HADSS, Herbicide Application Decision Support System; PDS, postemergence-directed; POST, postemergence; PPI, preplant-incorporated; PRE, preemergence.

Weed Control in Peanut Grown in a High-Residue Conservation-Tillage System

Information is needed on the role of cover crops as a weed control alternative due to the increase in adoption of conservation-tillage in peanut production. Field experiments were conducted from autumn 1994 through autumn 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue conservationtillage peanut production system. Black oat (Avena strigosa Schreb.), rye (Secale cereale L.), and wheat (Triticum aestivum L.) were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were utilized: no herbicide, preemergence (PRE) herbicides followed by (fb) postemergence (POST) herbicides, and PRE fb sequential POST herbicides. The PRE fb POST herbicide input system consisted of pendimethalin at 1.12 kg ai/ha fb an additional early POST application of paraquat at 0.14 kg ai/ha plus bentazon at 0.56 kg ai/ha. The PRE fb sequential POST herbicide input system contained the aforementioned herbicides fb 2,4-DB at 0.22 kg ai/ha plus chlorimuron at 0.14 kg ai/ha applied late POST. No cover crop was effective in controlling weeds without a herbicide program. However, when black oat or rye was utilized with PRE fb POST herbicides, weed control was similar to the high input system in two out of three years. Yield increased in 14 of 27 comparisons following conservation-tilled peanut using the Brazilian cover crop management system, compared to a winter fallow system. Yields never decreased following a winter cover crop compared to winter fallow. The winter fallow, high herbicide input system yielded between 7 and 26% less peanut compared to the highest yielding system that included a winter cover crop. The Brazilian system using black oat or rye cover crop has potential to increase peanut productivity and reduce herbicide inputs for peanuts grown in the Southeast.

Reducing Vibration while Maintaining Efficacy of Rollers to Terminate Cover Crops

Rollers may provide a valuable alternative to chemicals for terminating a cover crop. Several producers are now using versions that they have made or have purchased. Most of these producers, however, complain about excessive vibration that is caused by the roller passing over the cover crop. To avoid this excessive vibration, they must limit their operational speed. Experiments were performed to determine if two alternative roller blade designs would decrease vibration while maintaining the ability to kill a cover crop. Results showed that a spiral blade system or a short-staggered straight blade system significantly reduced vibration as compared to the standard long-straight blade system typically found on rollers. These two alternative blade systems were also found to kill the cover crop as effectively as the long-straight blade system.