Bradley A. Majek

Glyphosate in Double-Crop No-Till Glyphosate-Resistant Soybean: Role of Preplant Applications and Residual Herbicides1

The role of preplant glyphosate applications and residual herbicides in the efficacy of glyphosate for weed management in double-crop no-till glyphosate-resistant soybean (GRS) was investigated in the coastal plains of Mid-Atlantic United States. The experiment had a two- by two- by five-factorial treatment structure laid out in three or four randomized complete blocks at research centers in Delaware and New Jersey. The factors investigated were preplant weed management: preplant or no preplant glyphosate applications; postemergence (POST) herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and GRS growth stage at herbicide application which ranged from cracking, 5 to 8 d after planting, (DAP) to the V6 stage (35 DAP). Preplant glyphosate applications did not influence the efficacy of POST glyphosate applications alone or with the residual herbicides. Glyphosate alone or with clomazone plus imazethapyr provided excellent control of horseweed and fall panicum irrespective of the time of herbicide application from GRS at cracking to the V6 stage. With other weed species, residual herbicide influence varied with year, weed species, and GRS growth stage at herbicide application. Generally, glyphosate alone was most effective when applied at the V2 to V6 stages (16 to 35 DAP). A tank-mix of glyphosate with clomazone plus imazethapyr extended this window to include applications at GRS cracking and the V1 stage. Herbicide treatments were safe on GRS at all stages of application up to the V6 stage (35 DAP). Nomenclature: Clomazone, glyphosate, imazethapyr, horseweed, Conyza (= Erigeron) canadensis L. #3 ERICA; fall panicum, Panicum dichotomiflorum Michx. # PANDI. Additional index words: Preplant glyphosate applications, critical weed period, integrated weed management, Amaranthus hybridus, Ambrosia artemisiifolia, Chenopodium album, Ipomea hederacea, Xanthium strumarium, AMACH, AMBEL, CHEAL, IPOHE, SETFA, XANST. Abbreviations: DAP, days after planting; GRS, glyphosate-resistant soybean; POST, postemergence; RAREC, Rutgers Agricultural Research and Extension Center; UD-REC, University of Delaware Research and Education Center.

Optimum Glyphosate Timing with or without Residual Herbicides in Glyphosate-Resistant Soybean (Glycine max) under Full-Season Conventional Tillage1

Abstract: Field studies were conducted under full-season conventional tillage in Delaware and New Jersey to determine the critical time to apply glyphosate with or without residual herbicides for optimum weed control in glyphosate-resistant soybean (GRS). The residual herbicides tank-mixed with glyphosate (0.84 kg/ha) were clomazone (0.55 kg/ha) and imazethapyr (0.063 kg/ha). Herbicide application was made at cracking, unifoliate, and one- to six-trifoliate stages of GRS. Weeds varied in growth stages from preemergence (PRE) at cracking to an average height of 30 cm at the six-trifoliate stage of GRS. Herbicide activity varied by year and weed species. Herbicidal action was better under high (>125 mm/mo) than low (<100 mm/mo) rainfall regime. Glyphosate application without residual herbicides was less effective at cracking and unifoliate than at one- to three-trifoliate leaf stages. Mixing residual herbicides with glyphosate at cracking and unifoliate stages enhanced weed control but made no difference when application was delayed until one- to three-trifoliate stages. For optimum weed control in GRS, the window of application for glyphosate alone was between the one- and three-trifoliate leaf stages, approximately 18 to 28 days after planting (DAP). If glyphosate was tank-mixed with residual herbicides, the window of application extended from cracking until the four-trifoliate stage; and weed interference until the four-trifoliate stage (approximately 32 DAP) did not depress GRS yield. Nomenclature: Clomazone, 2-[(2-chlorophenyl)methyl]-4,4-dimethyl-3-isoxazolidinone; glyphosate, N-(phosphonomethyl)glycine; imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid. Additional index words: Critical weed removal period, glyphosate ± residuals, Amaranthus hybridus, Ambrosia artemisiifolia, Chenopodium album, Ipomoea hederacea, Panicum dichotomiflorum, AMACH, AMBEL, CHEAL, IPOHE, PANDI. Abbreviations: DAP, days after planting; GRS, glyphosate-resistant soybean; POST, postemergence; PRE, preemergence; RAREC, Rutgers Agricultural Research and Extension Center; UD-REC, University of Delaware Research and Education Center.

The Effect of Weed Density and Application Timing on Weed Control and Corn Grain Yield1

A 2-yr experiment repeated at five locations across the northeastern United States evaluated the effect of weed density and time of glyphosate application on weed control and corn grain yield using a single postemergence (POST) application. Three weed densities, designed to reduce corn yields by 10, 25, and 50%, were established across the locations, using forage sorghum as a surrogate weed. At each weed density, a single application of glyphosate at 1.12 kg ai/ha was applied to glyphosate-resistant corn at the V2, V4, V6, and V8 growth stages. At low and medium weed densities, the V4 through V8 applications provided nearly complete weed control and yields equivalent to the weed-free treatment. Weed biomass and the potential for weed seed production from subsequent weed emergence made the V2 timing less effective. At high weed densities, the V4 followed by the V6 timing provided the most effective weed control, while maintaining corn yield. Weed competition from subsequent weed emergence in the V2 application and the duration of weed competition in the V8 timing reduced yield on average by 12 and 15%, respectively. This research shows that single POST applications can be successful but weed density and herbicide timing are key elements. Nomenclature: Glyphosate; corn, Zea mays L.; forage sorghum, Sorghum bicolor (L.) Moench. Additional index words: Glyphosate-resistant corn, herbicide application timing, postemergence weed control, weed competition, weed density. Abbreviations: POST, postemergence; WAP, weeks after planting.

Orchard Floor Management Influence on Summer Annual Weeds and Young Peach Tree Performance1

This study compared the effect of weed control and orchard floor management (OFMA) options including organic mulch on summer annual weed interference in a newly established peach orchard. Weed interference where no preemergence (PRE) herbicides were applied, including vole damage, caused 29% peach tree mortality, reduced tree trunk cross-sectional area (TCSA) 62% by the fourth year of orchard establishment, and reduced fruit yield and fruit number in 1999 by 73 and 75%, respectively, but had no effect on fruit size. Compared with a no-till or conventionally tilled orchard floor, the population of grassy weeds within the tree row was greater in killed perennial ryegrass sod (PRS) plus hard fescue residue mulch treatments but was less in killed PRS plus tall fescue residue mulch treatments. Among the no-PRE treatments, the tree row broadleaf weed populations were suppressed in killed PRS with or without the addition of fescue residue mulch to the tree row when compared with the no-till or conventionally tilled orchard floor treatments. PRE herbicide treatments strongly affected peach fruit yield and TCSA but not average fruit size. There was no effect among the killed PRS, with or without hard or tall fescue residue mulch treatments, on peach fruit yield, TCSA, or average fruit size when compared with the no-till or conventionally tilled orchard floor treatment options. All treatments with herbicide had higher yields in 1999 than those without herbicides. Nomenclature: Hard fescue, Festuca longifolia var. Reliant; perennial ryegrass, Lolium perenne L.; tall fescue, Festuca arundinaceae Schreb. var. Kentucky-31; vole, Microtus sp.; peach, Prunus persica (L.) Batsch var. Candor. Additional index words: Drive row, orchard floor management, summer annual weeds, tree row. Abbreviations: OFMA, orchard floor management; PRE, preemergence; PRS, perennial ryegrass sod; SAW, summer annual weed; TCSA, trunk cross-sectional area.

Giant sensitiveplant interference in cassava

Abstract Giant sensitiveplant interference at different population densities in cassava established at 10,000 plants ha−1 was investigated on a Ferric Luvisol in a humid tropical environment. Interference for 12 mo was compared at 0, 10,000, 20,000, 30,000, and 40,000 plants ha−1 and at natural populations (averaging 630,000 plants ha−1) in four randomized complete blocks. Results showed that the order of cassava growth parameter response to giant sensitiveplant interference for 12 mo was leaf number > height > stem girth > leaf size = petiole length. The natural population density of giant sensitiveplant reduced growth faster and more than populations of 10,000 to 40,000 plants ha−1 in cassava. All giant sensitiveplant populations from 10,000 plants ha−1 and higher reduced storage root yield in cassava 12 mo after planting. Yield reduction increased as giant sensitiveplant population increased and the highest reduction of 85% occurred in the natural population of giant sensitiveplant. Nomenclature: Cassava, ‘TMS 30572’, Manihot esculenta Crantz; giant sensitiveplant, Mimosa invisa Mart. MIMIN.

Manual control of thorny mimosa (Mimosa invisa) in cassava (Manihot esculenta)

The optimum weeding regime for thorny mimosa control in cassava established at 10,000 plants/ha was studied at Ibadan, Nigeria (7°22½′N, 3°50½′ E), a humid tropical environment. The study compared six weeding regimes, each comprising manual removal of thorny mimosa three times at different intervals within 13 wk after planting (WAP). Cassava vegetative growth recovered from thorny mimosa interference when the first weeding occurred within 5 WAP, but interference for more than 5 WAP reduced storage root yield. Allowing thorny mimosa infestation after 11 WAP had no effect on cassava growth or root yield. Manual removal of thorny mimosa at 4, 7, and 11 WAP consistently gave the highest cassava root yield. Nomenclature: Thorny mimosa (or giant sensitive plant), Mimosa invisa Mart.; cassava, Manihot esculenta Crantz ‘TMS 30572’. Additional index words: Canopy closure, cassava vegetative growth, storage root yield, weed interference. Abbreviations: IAR & T, Institute of Agricultural Research and Training; MAP, months after planting; WAP, weeks after planting.