Herbicide-resistant Amaranthus spp. continue to cause management difficulties in soybean. New soybean technologies under development, including resistance to various combinations of glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, and mesotrione, will make possible the use of additional herbicide sites of action in soybean than is currently available. When this research was conducted, these soybean traits were still regulated and testing herbicide programs with the appropriate soybean genetics in a single experiment was not feasible. Therefore, the effectiveness of various herbicide programs (PRE herbicides followed by POST herbicides) was evaluated in bare-ground experiments on glyphosate-resistant Palmer amaranth and glyphosate-resistant waterhemp (both tall and common) at locations in Arkansas, Illinois, Indiana, Missouri, Nebraska, and Tennessee. Twenty-five herbicide programs were evaluated; 5 of which were PRE herbicides only, 10 were PRE herbicides followed by POST herbicides 3 to 4 wks after (WA) the PRE application (EPOST), and 10 were PRE herbicides followed by POST herbicides 6 to 7 WA the PRE application (LPOST). Programs with EPOST herbicides provided 94% or greater control of Palmer amaranth and waterhemp at 3 to 4 WA the EPOST. Overall, programs with LPOST herbicides resulted in a period of weed emergence in which weeds would typically compete with a crop. Weeds were not completely controlled with the LPOST herbicides because weed sizes were larger (! 15 cm) compared with their sizes at the EPOST application ( 7 cm). Most programs with LPOST herbicides provided 80 to 95% control at 3 to 4 WA applied LPOST. Based on an orthogonal contrast, using a synthetic-auxin herbicide LPOST improves control of Palmer amaranth and waterhemp over programs not containing a syntheticauxin LPOST. These results show herbicides that can be used in soybean and that contain auxinic-or HPPD-resistant traits will provide growers with an opportunity for better control of glyphosateresistant Palmer amaranth and waterhemp over a wide range of geographies and environments. Nomenclature: 2,4-D; dicamba; glufosinate; glyphosate; HPPD, 4-hydroxyphenylpyruvate dioxygenase; isoxaflutole; and mesotrione; Palmer amaranth, Amaranthus palmeri S. Wats; waterhemp (tall and common, respectively), Amaranthus tuberculatus (Moq.) Sauer, and Amaranthus rudis Sauer; soybean, Glycine max (L.) Merr. Key words: Herbicide-resistant crop traits, herbicide-resistant weeds, new technologies, weed control.Amaranthus spp. resistentes a herbicidas continúan causando problemas de manejo en soja. Nuevas tecnologías para soja que están actualmente en desarrollo y que incluyen resistencia a varias combinaciones de glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, y mesotrione, harán posible el uso de sitios de acción que no están actualmente disponibles para uso en soja. Cuando se realizó esta investigación, estas tecnologías estaban todavía bajo regulación y la evaluación de programas de

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance. Therefore, selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the individuals to dicamba was evaluated, and its susceptibility to 2,4-D was characterized. Based on the greenhouse study, following three generations of dicamba selection, the dose required to cause 50% mortality increased from 111 g ae ha−1for parental individuals (P0) to 309 g ae ha−1for the P3. Furthermore, reduced susceptibility of the P3to 2,4-D was also evident. This research presents the first evidence that recurrent use of sublethal dicamba doses can lead to reduced susceptibility of Palmer amaranth to dicamba as well as 2,4-D. Here, we show that selection from sublethal dicamba doses has an important role in rapid evolution of Palmer amaranth with reduced susceptibility to auxin-type herbicides.

ThreeEchinochloa crus-galli(barnyardgrass) populations from rice fields in Arkansas (AR1 and AR2) and Mississippi (MS1), USA, were recently confirmed to be resistant to imazethapyr. Experiments were conducted to characterize cross-resistance to acetolactate synthase- (ALS-) inhibiting herbicides and determine if malathion, a known cytochrome P450 monooxygenase (CYP) inhibitor, would overcome resistance. The AR1 and MS1 populations were cross-resistant to bispyribac-sodium; however, AR2 was sensitive to bispyribac-sodium. The AR1, AR2, and MS1 populations were >94, >94, and 3.3 times, respectively, more resistant to imazamox; >94, 30, and 9.4 times, respectively, more resistant to penoxsulam; and 15, 0.9, and 7.2 times, respectively, more resistant to bispyribac-sodium compared to a susceptible population. Addition of malathion to penoxsulam reduced dry weight of all populations and increased mortality of AR2 and MS1 populations compared to penoxsulam alone. Addition of malathion to imazethapyr and bispyribac-sodium increased the mortality of MS1 population in mixture with imazethapyr and AR1 population in mixture with bispyribac-sodium compared to treatments with imazethapyr and bispyribac-sodium applied alone. Synergism of ALS-inhibiting herbicides with malathion indicates increased herbicide degradation by CYP as partial mechanism of resistance to penoxsulam in all resistant populations and probably to imazethapyr in MS1 and bispyribac-sodium in AR1 populations.

Common cocklebur and sicklepod are troublesome weeds in soybean in the southern United States. A field experiment was conducted from 1991 through 1995 to determine (1) the influence of tillage (no-till and tilled after initial seed deposition) and intraspecific and interspecific interference on seed production potential, emergence pattern, and soil seedbank of common cocklebur and sicklepod, and (2) the dominant species after introduction into a weed-free field. Under intraspecific interference, 1,430 and 1,392 common cocklebur achenes m−2and 1,827 and 5,435 sicklepod seed m−2were deposited to the seedbank after 1 and 2 yr of seed production, respectively. For both species, approximately 11% of the initial seedbank emerged under tilled conditions the first year after deposition. Under no-till conditions, only 0.7% of common cocklebur and 1.6% of sicklepod emerged. The second year after deposition, common cocklebur emergence in no-till decreased to 0.25% of the initial seedbank, while sicklepod increased to 8% of the initial seedbank and remained higher than in tilled plots. Under tilled conditions, common cocklebur became the dominant species, and sicklepod became dominant under no-till conditions. Seedbank depletion was greater for both species under tillage. Three years after initial seed deposition, sicklepod seed was 100% viable but common cocklebur achenes were not viable. Under no-till conditions, common cocklebur was depleted in the seedbank but sicklepod was not. Thus, sicklepod poses a greater long-term weed problem than common cocklebur, especially under no-till conditions.

Field studies were conducted in Arkansas in 1999, 2000, and 2001 to evaluate mesotrione applied preemergence (PRE) and postemergence (POST) for weed control in corn grown in the Mississippi Delta region of the United States. Mesotrione was applied PRE (140, 210, and 280 g/ha) alone and POST (70, 105, and 140 g/ha), alone or in tank mixtures with atrazine (280 g/ha). Standard treatments for comparison were S-metolachlor/atrazine PRE and S-metolachlor plus atrazine PRE followed by atrazine POST. All PRE treatments controlled velvetleaf, pitted morningglory, entireleaf morningglory, prickly sida, and broadleaf signalgrass 95% 2 wk after emergence (WAE). Mesotrione controlled velvetleaf 89% or more 4 and 6 WAE. Control of morningglory species by mesotrione POST averaged 92% 6 WAE. Prickly sida was controlled at least 90% by all treatments 4 WAE. Mesotrione applied alone PRE and POST controlled broadleaf signalgrass 83 to 91% 4 WAE. All treatments controlled broadleaf signalgrass less than 90% 6 WAE, except treatments that contained S-metolachlor, which gave 94% or greater control. Corn yield ranged from 10.5 to 12.4 Mg/ha and did not differ among treatments. Mesotrione PRE and POST provided excellent control of broadleaf weeds, but S-metolachlor was needed for broadleaf signalgrass control.