William G. Johnson

Greenhouse and field experiments were conducted to evaluate the efficacy of nicosulfuron, primisulfuron, glyphosate, glufosinate, imazethapyr plus imazapyr, and quizalofop on johnsongrass biomass reduction, rhizome total nonstructural carbohydrate (TNC) content, and subsequent regrowth from rhizomes. In the greenhouse, johnsongrass plants originating from rhizome segments were controlled 88 to 97% with quizalofop, glyphosate, imazethapyr plus imazapyr, nicosulfuron, and primisulfuron and 56% with glufosinate 3 wk after treatment (WAT). Johnsongrass treated with quizalofop, glyphosate, and nicosulfuron did not regrow 6 WAT, whereas plants treated with primisulfuron, imazethapyr plus imazapyr, and glufosinate regrew from the rhizome of the treated plant. Rhizome TNC levels 3 WAT were not reduced by glufosinate or nicosulfuron, but they were reduced 64% by quizalofop, 32% by primisulfuron, 61% by glyphosate, and 29% by imazethapyr plus imazapyr. When rhizome TNC was reduced by 60% or more compared with nontreated plants, johnsongrass did not regrow from the treated rhizomes. In field experiments, nicosulfuron and glyphosate controlled johnsongrass 94 and 99%, respectively, whereas imazethapyr plus imazapyr (79%) and glufosinate (85%) provided less control 6 WAT. Nomenclature: Glufosinate; glyphosate; imazapyr; imazethapyr; nicosulfuron; primisulfuron; quizalofop; johnsongrass, Sorghum halepense (L.) Pers. #³ SORHA. Additional index words: Biomass reduction, carbohydrate analysis, corn yield, fresh weight. Abbreviations: DAT, days after treatment; POST, postemergence; TNC, total nonstructural carbohydrate; WAT, weeks after treatment.

Grass weed interference and nitrogen accumulation in no-tillage corn - Volume 50 Issue 6 - Kaleb B. Hellwig, William G. Johnson, Peter C. Scharf

Field trials were conducted at two sites in both 1997 and 1998 to evaluate soybean response and weed control with glufosinate alone or combined with quizalofop, lactofen, imazethapyr, flumiclorac, or bentazon plus acifluorfen in narrow-row, glufosinate-resistant soybean. Soybean injury ranged from 0 to 21% at 2 wk after treatment (WAT) and from 0 to 5% by 4 WAT. Glufosinate alone at 0.29 and 0.4 kg ai/ha controlled velvetleaf, common waterhemp, common ragweed, morningglory species, and giant foxtail greater than 85% in all studies. Mixtures containing glufosinate and other herbicides controlled these species greater than 81% but did not improve control over glufosinate alone. Estimates of weed biomass closely reflected visual control evaluations. However, giant foxtail biomass was higher for mixtures of glufosinate plus lactofen, flumiclorac, or bentazon and acifluorfen, indicating possible antagonism of glufosinate activity. At both locations, soybean yields were similar among most treatments, but that of the glufosinate plus lactofen treatment was lower when compared with other treatments. Additional trials evaluated soybean response and weed control with a preemergence herbicide followed by glufosinate postemergence (POST), glufosinate applied once or twice POST, and mixtures of glufosinate plus imazethapyr or flumiclorac POST in wide-row soybean. Glufosinate applied twice controlled common waterhemp, morningglory species, prickly sida, common cocklebur, and giant foxtail up to 39% greater than did glufosinate applied once. The addition of imazethapyr, but not flumiclorac, to glufosinate improved weed control when compared with glufosinate alone. Nomenclature: Acifluorfen; bentazon; flumiclorac; glufosinate; imazethapyr; lactofen; quizalofop; common cocklebur, Xanthium strumarium L. # XANST; common ragweed, Ambrosia artemisiifolia L. # AMBEL; common waterhemp, Amaranthus rudis Sauer # AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; morningglory species, Ipomoea spp. # IPOSS; prickly sida, Sida spinosa L. # SIDSP; velvetleaf, Abutilon theophrasti Medik. # ABUTH; soybean, Glycine max (L.) Merr. ‘Asgrow 2704 LL’. Additional index words: Herbicide mixtures, herbicide-resistant crop, soybean injury. Abbreviations: COC, crop oil concentrate; EPOST, early postemergence; LPOST, late postemergence; MPOST, midpostemergence; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Field experiments were conducted near DeKalb and Urbana, IL, and Columbia, MO, in 1997 and 1998 to evaluate weed management systems in glyphosate- and glufosinate-resistant soybean planted in 18-cm rows. Overall weed control was improved to a greater extent when the rate of glufosinate was increased from 300 to 400 g ai/ha than when the rate of glyphosate increased from 630 to 840 g ae/ha. Sequential applications of glufosinate improved control over single applications, whereas sequential treatments of glyphosate generally provided no advantages over single applications. When averaged across all weed species in these trials, the systems that provided 95% or higher average control were sequential applications of glufosinate, sequential applications of glyphosate, and clomazone followed by (fb) glyphosate. Single applications of glufosinate provided somewhat variable control of giant foxtail, common lambsquarters, ragweed, and common cocklebur similar to that observed with pendimethalin fb imazethapyr. The addition of fomesafen to glufosinate did not improve control of any of the weeds in this study with the exception of velvetleaf at DeKalb. The addition of clomazone to glufosinate treatments resulted in slightly better giant foxtail and velvetleaf control. Single applications of glyphosate provided somewhat variable control of giant ragweed at DeKalb in 1997 and ivyleaf morningglory and common cocklebur control at Columbia. The addition of fomesafen to glyphosate provided an increase in ivyleaf morningglory and common cocklebur control at Columbia but did not improve control of any other species. The addition of clomazone to glyphosate-based programs resulted in slightly higher velvetleaf, common cocklebur, and ivyleaf morningglory control. In the glyphosate-based herbicide programs there were no substantial differences in relative yield, with all programs protecting over 95% of soybean yield. Glufosinate-based programs were effective in protecting 85 to 92% of soybean yield. Nomenclature: Clomazone; fomesafen; glyphosate; glufosinate; imazethapyr; pendimethalin; common cocklebur, Xanthium strumarium L. # XANST; common lambsquarters, Chenopodium album L. # CHEAL; giant foxtail; Setaria faberi Herrm. # SETFA; giant ragweed, Ambrosia trifida L. # AMBTR; ivyleaf morningglory, Ipomoea hederacea L. Jacq. # IPOHE; velvetleaf, Abutilon theophrasti Medic. # ABUTH; soybean, Glycine max (L.) Merr. Abbreviations: EPOST, early postemergence; fb, followed by; LPOST, late postemergence, POST, postemergence; PRE, preemergence; UAN, 28% urea ammonium nitrate.

Field experiments were conducted in 1997 and 1998 near Columbia and Novelty, MO, and at Urbana, IL, to evaluate corn injury, weed control, corn yield, and estimated economic returns with weed management programs in glufosinate-resistant corn. Herbicide programs included acetochlor preemergence (PRE) followed by glufosinate alone or with atrazine postemergence (POST) and total POST programs consisting of single and sequential applications of glufosinate alone or tank mixed with acetochlor, atrazine, or acetochlor plus atrazine. Metolachlor PRE followed by dicamba plus atrazine early POST (EPOST) and metolachlor plus atrazine PRE were included for comparison. In the total POST treatments, mid-POST applications controlled shattercane and common cocklebur better than EPOST applications. However, yield reductions as high as 23% occurred because of early-season weed interference, although weeds were controlled later in the season. Applying atrazine with glufosinate generally increased control of giant foxtail, common cocklebur, morningglory species, and common waterhemp compared to glufosinate alone, but did not increase control of common lambsquarters, velvetleaf, or Pennsylvania smartweed. Corn yield was positively correlated with weed control (r = 0.88) and more strongly dependent on grass (r = 0.82) than broadleaf (r = 0.70) weed control. Net incomes were positively correlated to corn yield (r = 0.73). Four of the top six net income-producing treatments included two herbicide applications. Three of the treatments were PRE followed by POST programs, and the fourth was a sequential POST treatment of glufosinate. Nomenclature: Acetochlor; atrazine; dicamba; glufosinate; metolachlor; common cocklebur, Xanthium strumarium L. # XANST; common lambsquarters, Chenopodium album L. # CHEAL; common waterhemp, Amaranthus rudis Sauer # AMATA; giant foxtail, Setaria faberi (L.) Herrm. # SETFA; morningglory, Ipomoea spp.; Pennsylvania smartweed, Polygonum pensylvanicum L. # POLPY; shattercane, Sorghum bicolor (L.) Moench. # SORVU; velvetleaf, Abutilon theophrasti (L.) Medikus # ABUTH; corn, Zea mays L. # ZEAMX ‘Pioneer 34A55’. Additional index words: Corn yield, economic returns, glufosinate-resistant crop, AMATA, ABUTH, CHEAL, POLPY, SETFA, SORVU, XANST, ZEAMX. Abbreviations: ALS, acetolactate synthase; CV, coefficient of variation; EPOST, early postemergence; MPOST, mid-postemergence; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Field studies were conducted to determine the effect of early-season and early- plus late-season acetolactate synthase–resistant Helianthus annuus interference on Glycine max and H. annuus growth and yield at two sites in Missouri. Helianthus annuus densities of 3 plants m−2 were established shortly after G. max emergence in all plots except the weed-free check. To study early-season interference, H. annuus were removed with postemergence glyphosate (0.84 kg ae ha−1) 2, 4, 6, and 8 wk after planting (WAP) and kept weed-free for the rest of the growing season. Glycine max yields were not different with 2, 4, 6, or 8 wk of early-season interference at either location. To study early- plus late-season interference, H. annuus densities were established at 3 plants m−2. They were then removed 2, 4, 6, or 8 WAP with glyphosate and subsequently reestablished at the same density within 2 wk after removal by newly emerging and transplanted H. annuus. These H. annuus were allowed to remain in the field for the remainder of the growing season. This provided a weed-free period of approximately 2 wk during the growing season beginning 2, 4, 6, or 8 WAP. Season-long interference and no-interference treatments were also included. Glycine max yields were reduced 47 to 72% with season-long interference. Helianthus annuus vegetative dry matter was approximately 56% lower at Columbia than at Miami. Glycine max yields tended to increase as the weed-free period was delayed into the growing season. Early-season weed-free periods (2 to 4 and 4 to 6 WAP) allowed H. annuus to become re-established before G. max formed a canopy and resulted in larger amounts of H. annuus biomass and seed production as well as G. max yield losses of 15 to 80%. Re-establishment of H. annuus in 6 to 8 WAP and 8 to 10 WAP weed-free treatments generally resulted in the plants surviving for only a few weeks after establishment and not producing seed or reducing G. max yield.

Field experiments were conducted in 1997 and 1998 near Columbia and Novelty, MO, and Urbana, IL, to evaluate crop injury, weed control, corn yield, and net economic returns provided by weed control programs in glyphosate-resistant corn. The herbicide programs evaluated included acetochlor preemergence (PRE) followed by (fb) glyphosate with or without atrazine postemergence (POST) and total POST programs consisting of single and sequential applications of glyphosate alone and tank-mixed with actochlor, atrazine, or both. Metolachlor PRE fb dicamba plus atrazine POST and metolachlor plus atrazine PRE were included for comparison. In the total POST treatments, mid-post (MPOST) applications provided better control than early-post (EPOST) applications on weeds that germinated throughout the growing season such as shattercane and common cocklebur, but also resulted in yield reductions of up to 23% caused by early-season weed competition. The addition of atrazine to glyphosate POST generally increased ...