Field studies examined the interaction of soil pH with differing levels of atrazine exposure over 4 yr. Soil pH was 5.2 to 7.1 with atrazine exposures ranging from none (0) to eight applications over a 4-yr period (for each year, one application at planting and one early postemergence). The entire plot area was uniformly managed to reduce potential confounding effects due to cropping history, tillage, and other factors. Soil from all plots previously treated with atrazine displayed rapid atrazine dissipation, with half-lives under laboratory conditions being less than 4 d in plots of pH 5.5 or greater and less than 8 d in the field. Soil pH had a marked effect, with slower atrazine dissipation in those plots that had a pH 5.5 or less. This effect of pH and previous atrazine history was consistent in laboratory and field environments. Implications of these findings include probable reduction in weed control due to more rapid atrazine dissipation and potentially reduced loadings into surface water due to this phenomenon.

Concentrated sulfuric acid (H2SO4) scarification of balloonvine (Cardiospermum halicacabum L.) for 3 h reduced balloonvine seed coat thickness by 25%, and resulted in increased water absorption and maximum germination. Germination of balloonvine occurred at temperatures of 15 to 40 C with optimum germination occurring at 35 C. Intact balloonvine seed was more sensitive to simulated moisture stress than soybean [Glycine max (L.) Merr. ‘Bragg’]. No balloonvine germination occurred at osmotic potentials of −2 bars and less with intact seeds. Balloonvine seed with seed coats removed germinated at osmotic potentials of −8 bars. Maximum emergence of balloonvine occurred at the 1- and 3-cm depths with limited emergence at a depth of 12 cm.

CGA-154281 and temperature effects on metolachlor absorption and metabolism were studied in corn seedlings grown in untreated soil or soil treated with metolachlor, CGA-154281, or both. Seedlings were grown under a cool (21/13 C, 16-h day) or warm (30/21 C, 16-h day) temperature regime and exposed to 14C-metolachlor for 10 min at either 21 or 30 C. Corn grown under the cool temperature regime absorbed slightly more 14C-metolachlor than corn grown under the warm temperature regime. Corn held at 21 C during a 10-min 14C-metolachlor exposure period metabolized metolachlor more slowly than corn held at 30 C. Decreased corn tolerance to metolachlor observed at lower temperatures may be due in part to slower metolachlor metabolism. Corn grown in the presence of metolachlor plus CGA-154281 metabolized 14C-metolachlor to the glutathione conjugate twice as fast as corn grown in the presence of metolachlor alone. The increase in metabolism rate was due to a fivefold increase in glutathione-S-transferase (GST, EC 2.5.1.18) activity and not to an increase in glutathione (GSH) content. Results are consistent with the hypothesis that CGA-154281 protects corn from metolachlor injury by enhancing GST activity, which accelerates metolachlor detoxification via GSH conjugation.

Separate field experiments were conducted for cotton and soybean in 1990 and 1991 to determine the influence of planting date on yield loss due to interspecific interference from entireleaf morningglory and sicklepod and to determine the relative competitiveness of each weed species. Percent soybean yield loss due to weed interference increased as planting date was delayed from early May to early June. Averaged over weed species, yield losses from 1.7 weeds m−1 row were 10, 18, and 20% for soybeans planted in early May, mid-May, and early June, respectively. Yield loss from 6.7 weeds m−1 row were 17, 31, and 35% at the early May, mid-May, and early June planting dates, respectively. Percent seed cotton yield losses averaged over weed species in 1990 were 33 and 28% for the early May and early June planting dates, respectively, at 1.7 weeds m−1 and 50% for both planting dates at weed densities of 6.7 plants m−1. The only experimental factor that significantly affected seed cotton yield in 1991 was weed density. Unlike soybeans, planting date had little effect on weed interference in cotton. Entireleaf morningglory was more competitive than sicklepod in both crops. Results suggest that selection of optimum soy bean planting dates may be a viable means of reducing losses due to weed interference.

Cotton maintained free of annual grass and broadleaf weeds for 6 or more weeks after emergence in 1969 produced maximum yields, whereas in 1970 and 1971, 8 or more weeks of weed-free maintenance were required. Nitrogen applications at planting of 67 and 100 kg/ha did not influence the weed-free requirement of cotton in any year. Cotton yields were not reduced in 1969 when weeds were allowed to compete for 4 or 2 weeks. In 1970 and 1971 weed competition for 7 or fewer weeks did not result in yield reductions. Nitrogen did not affect the competitive relationship between cotton and weeds in 1969 and 1971. In 1970, however, cotton tolerated only 6 weeks of competition at the zero nitrogen rate, whereas it tolerated 7 weeks of competition at the 67 and 100 kg/ha nitrogen rates. Height and stem diameter of cotton were less reliable indicators of the crop-weed relationship than was yield. Neither height nor stem diameter of cotton revealed a consistent difference in the competitive relationship at the three nitrogen rates.

Greenhouse, growth chamber, and laboratory studies were conducted to determine anatomical and morphological characteristics and cultural practices limiting the distribution of cogongrass, torpedograss, and johnsongrass in the United States. Cogongrass did not produce axillary buds along most of the rhizome nor regenerate when apical six-node-long rhizome segments were buried deeper than 8 cm. Both torpedograss and johnsongrass produced axillary buds along the entire lengths of their rhizomes. Torpedograss shoot emergence decreased at burial depths between 8 and 16 cm. Shoot emergence from johnsongrass rhizomes was not affected by burial as deep as 16 cm. Rhizomes of all three species were tolerant of desiccation. Cogongrass grew better in soil at pH 4.7 than in soil at pH 6.7, whereas torpedograss and johnsongrass grew equally well in either pH. It is postulated that cogongrass spread is limited by lack of axillary bud formation on most of the rhizome and the inability of rhizomes to send up new shoots if buried deeper than 8 cm. These factors could account for the intolerance of cogongrass to cultivation. Torpedograss appears to spread only vegetatively due to the lack of viable seed production.

In laboratory experiments, the safener CGA-92194 {α-[(1,3-dioxolan-2-yl-methoxy)imino] benzeneacetonitrile} increased the in vitro chemical reactivity of the herbicide metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] for glutathione (GSH) when tested at 1, 50, and 100 μM. Pretreatment of sorghum [Sorghum bicolor (L.) Moench. cv. ‘G623’] seeds with the safener CGA-92194 (1.25 g ai/kg of seeds) enhanced significantly the amount of 14C-metolachlor absorbed by the roots and translocated to the shoots and leaves of grain sorghum seedlings grown hydroponically in a nutrient solution. Thin-layer chromatographic analysis of methanol-soluble extracts from roots or shoots and leaves of sorghum seedlings treated with 14C-metolachlor revealed the detection of 8 to 10 metabolites of this herbicide. One of the major metabolites detected was tentatively identified as the conjugate of metolachlor with GSH. A safener-induced stimulation of metolachlor metabolism via conjugation to GSH was observed in sorghum seedlings grown from seeds pretreated with CGA-92194. Addition of tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane], a potent inhibitor of plant GSH-S-transferase enzymes, to the nutrient solution of hydroponically grown sorghum seedlings reduced the formation of GS-metolachlor in both control and safener-treated seedlings and minimized the stimulatory effect of the safener CGA-92194 on the conjugation of metolachlor to GSH. It is suggested that the safener CGA-92194 may protect grain sorghum against injury from the herbicide metolachlor by stimulating the spontaneous and enzymatic conjugation of this herbicide with GSH.

Autotrophic growth, photosynthesis, and respiration of Chlorella sorokiniana Shihira and Krauss were inhibited by the cupric ion, but photosynthesis was more sensitive than respiration. The percent inhibition was determined by the ratio of cells to cupric ions present. Photosynthesis and respiration were inhibited within 2 and 5 min, respectively, after adding 1.0 mM cupric ions. Chlorella cells which had been incubated for a short time in concentrations of the cupric ion that completely inhibited photosynthesis were not able to grow when cultured in a fresh medium without cupric ions, indicating high concentrations of the ion may have destroyed the photosynthetic apparatus and deprived the cells of their ability for autotrophic growth. Dark preincubation of the cells, as well as high bicarbonate concentrations in the assay medium, decreased inhibition. Treatment with cupric ions reduced the cellular chlorophyll and sulfhydryl content, but anaerobiosis, a condition that increased toxicity, had little effect on the sulfhydryl content. Electron transport in photosystems I and II in intact Chlorella cells was inhibited, but the specific sites of inhibition in the photosynthetic electron transport chain could not be determined using intact cells.

A recirculating sprayer was developed for applying herbicides in row crops to weeds taller than the crop. Herbicide solutions are applied through solid stream nozzles above and at right angles to the rows, so that a trap collects herbicide spray not deposited on weeds. Trapped solution is returned to the original spray tank by gravity flow. Good to excellent control of pigweed (Amaranthus spp.), cocklebur (Xanthium spp.), and sesbania (Sesbania spp.) was obtained with (2,4-dichlorophenoxy)acetic acid (2,4-D) while monosodium methanearsonate (MSMA) effectively controlled johnsongrass (Sorghum halepense (L.) Pers.). The better treatments increased the harvested yields of soybeans (Glycine max (L.) Merr., var. Lee) 6 to 8 bu/A when weed infestations were severe at time of treatment. Use of the sprayer permits treatment with herbicides normally considered too toxic to crops, and reduces costs through recovery of most of the herbicide solution that is not intercepted by weeds.

Thirty compounds that are precursors, analogs, derivatives, or fragments of strigol were prepared and tested for their activity as witchweed [Striga asiatica (L.) Kuntze] seed-germination stimulators. Five compounds showed significant stimulatory activity. Among these were 5-ethoxy-3-methyl-2(5H)-furanone, 3-hydroxy-2,6,6-trimethylcyclohex-1-ene-1-carboxaldehyde, a two-ring analog of strigol, and two isomers of a three-ring analog. Several of the other 25 compounds showed slight and/or non-reproducible activity when dimethylsulfoxide was used as the carrier at 0.1% (v/v) in aqueous solutions. Compounds with such diverse structures as trimethylcyclohexenes, furanones, and bicycloheptenones were among those that possessed some stimulatory activity.

Glyphosate resistance was found in a rigid ryegrass population in northern California. A sample of the resistant plants were collected and grown under greenhouse conditions. The objective of this study was to evaluate glyphosate resistance in the progeny of the collected plants by recurrent selection, obtain the homozygous resistant and sensitive lines to establish dose-response curves, and to determine the inheritance of glyphosate resistance in rigid ryegrass. Diverse levels of resistance were observed in the first generation with survival of 89, 59, 45, and 9% from glyphosate at 1x, 2x, 4x, and 8x respectively, where x = 1.12 kg ha−1 isopropylamine salt of glyphosate. Clones of plants that died from 1x were allowed to produce seed and were further subjected to recurrent selection to generate the most sensitive plants (S lines), which died from 0.125x glyphosate. The most resistant plants (R lines) were generated from the survivors receiving 8x glyphosate. The ratio between I50 rates for the glyphosate resistant and the glyphosate sensitive plants was > 100-fold. The R and S lines were crossed reciprocally and F1 progeny of both (R × S) and (S × R) showed intermediate resistance. These survived up to 2x glyphosate. The F2 progeny were generated by intercrossing of F1 plants. The ratio of sensitive, intermediate, and resistant plants in the F2 population before the treatment of glyphosate at 0.125x followed by 8x was 1 : 16, 14 : 16, and 1 : 16 respectively, which corresponded to the Mendelian segregation ratio of two genes. The results indicated that the inheritance of glyphosate resistance in rigid ryegrass from California appeared to be nuclear, incompletely dominant, multigenic, and pollen-transmitted with no indication of maternal inheritance.

Previous studies have suggested that the reduction in the root/shoot ratio that accompanies the shade avoidance response may reduce the tolerance of individuals to subsequent nutrient or moisture limitations. In this work, we examined the impact of the shade avoidance response on maize seedling growth and development and the response of these plants to a subsequent abiotic stress. Seedlings were grown in a field fertigation system under two light quality environments, ambient and a low red to far-red ratio, which were designed to simulate weed-free and weedy conditions, respectively. This system also enabled the controlled restriction of water and nutrients, which reduced the relative growth rate of the crop and created a secondary stress. Results of this study indicate that, while the shade avoidance response did reduce the root/shoot ratio in maize, this effect did not reduce plant tolerance to subsequent abiotic stress. Rather, the apparent additivity or synergism of shade avoidance and the secondary stressor on yield loss depended on whether the net effect of these two stressors was sufficiently large to shift the population toward the point where reproductive allometry was broken.

The WSSA Group 15 (HRAC Group K3) herbicide flufenacet is a key compound in weed resistance management, primarily used for PRE control of grass weeds in winter cereal–based crop rotations in Europe. Although resistance to compounds of its mechanism of action (inhibition of the synthesis of very-long-chain fatty acids) generally evolves slowly, reduced flufenacet efficacy due to enhanced glutathione transferase (GST) activity has been described in several blackgrass (Alopecurus myosuroides Huds.) populations. The present study aimed to better understand of the mechanism of flufenacet detoxification in A. myosuroides. Therefore, we characterized four A. myosuroides populations with different levels of flufenacet sensitivity. Flufenacet degradation was significantly slowed down in a sensitive population and a population with reduced flufenacet sensitivity by the use of the GST inhibitors tridiphane and ethacrynic acid at sublethal rates. Finally, an RNA sequencing (RNA-seq) study with the four A. myosuroides populations was conducted. In total, six differentially expressed GSTs and nine transcription factors as well as a keto-acyl-CoA reductase involved in the biosynthesis of very-long-chain fatty acids were identified as candidate genes among a set of 319 significantly more highly expressed gene-associated contigs. Among a set of 218 contigs with significantly lower expression levels, receptor kinase activity was the most frequent annotation. In summary, the likely GST-mediated reduction in sensitivity evolves in A. myosuroides at a slow rate and can partially be reversed by an interaction between flufenacet and the GST inhibitors tridiphane and ethacrynic acid. This provides further evidence for enhanced GST activity as a key mechanism in flufenacet resistance in A. myosuroides and supports the hypothesis that the six differentially expressed GSTs detected in the present RNA-seq study are potentially involved in flufenacet resistance.

Root moisture extraction profiles varied from 20 square feet for Kochia scoparia (L.) Schrad, (kochia to 44 square feet for Xanthium pennsylvanicum Wallr. (cocklebur). Weeds within the extremes with increasing root moisture extraction profiles were Eragrostis pectinacea (Michx) Ness (tufted lovegrass), Solanum rostratum Dunal. (buffalobur), Digitaria sanguinalis (L.) Scop. (large crabgrass), Tribulus terrestris L. (puncturevine), Salsola kali L., var. tenuifolia Tausch (Russian thistle), Amaranthus palmeri S. Wats. (Palmer amaranth) and sorghum.

Glyphosate applied POST can provide a high level of efficacy on many weed species in soybean, but delayed application beyond optimal weed growth stages might fail to fully protect yield potential. Further, we do not have a good understanding of the extent to which delayed glyphosate application and its associated yield loss is occurring on-farm. Our goal was to characterize on-farm weed communities in glyphosate-resistant soybean just prior to glyphosate application and estimate potential yield loss associated with early-season soybean-weed competition. In field surveys conducted across 64 site-yr in southern Wisconsin in 2008 and 2009, common lambsquarters, velvetleaf, dandelion, Polygonum spp., and Amaranthus spp. were the five most abundant broadleaf weed species across site-years, present in 92, 69, 64, 42, and 50% of all fields, respectively, at average densities of 14, 5, 5, 14, and 10 plants m−2, respectively. Average height of these species was 21 cm or less at or near the time of glyphosate application. Grass and sedge species occurred in 95% of fields at an average density of 41 plants m−2 and height of 21 cm. The mean and median values of total weed density across site-years were 101 and 41 plants m−2, with heights of 19 and 17 cm, respectively. Recommended height for treatment is 15 cm. Glyphosate application occurred on average at V3 to V4 soybean growth stage, which is later than V2 soybean typically targeted to protect yield. Average yield loss predicted by WeedSOFT® was 5% with a mean economic loss of $47 ha−1. Predicted yield loss was greater than 5% on one-fourth of the site-years, all of which were treated at V4 soybean or later. The maximum predicted yield loss was 27%. These results suggest that glyphosate was applied at weed height and soybean growth stages that were greater than optimal to protect yield in many fields across southern Wisconsin. A soil-residual herbicide applied PRE, or a more timely POST application of glyphosate would alleviate the majority of these losses.

Research was conducted during 1988 and 1989 in Georgia and Florida to determine the densitydependent effects and critical periods of wild poinsettia interference with peanut. Peanut yield loss ranged from 0 to approximately 50% over the wild poinsettia density range from 0 to 32 plants 9 m−1 peanut row. Predicted peanut yield losses across 2 yr in Georgia were 0, 4, 8, 12, 15, 26, 40, and 54% for season-long wild poinsettia interference at densities of 0, 1, 2, 3, 4, 8, 16, and 32 plants 9 m−1 of row, respectively. Across 2 yr in Florida, predicted peanut yield losses were 0, 9, 14, 22, 30, 37, and 41% for wild poinsettia densities of 0, 1, 2, 4, 8, 16, and 32 plants 9 m−1 row, respectively. Peanuts had to be maintained poinsettia free for 10 wk after peanut emergence to prevent yield loss. Wild poinsettia that interfered with peanut for more than 2 wk after peanut emergence reduced peanut yield.

Preplant incorporation of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] at 3.6 kg ai/ha, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 2.2 kg ai/ha, and tank mixtures of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} with either alachlor at 0.14 + 2.2 kg ai/ha or metolachlor at 0.14 + 1.1 kg ai/ha controlled >90% red rice (Oryza sativa L. # ORYSA) in soybeans [Glycine max (L.) Merr.]. Also, FMC-57020 [2-(2-chlorophenyl)methyl-4,4-dimethyl-3-isoxazolidinone] at 1.7 kg ai/ha applied preplant incorporated controlled > 80% of the red rice. DPX-Y6202 {2-[4-[(6-chloro-2-quinoxalinyl)oxy]-phenoxy]-propionic acid, ethyl ester} was the best and most consistent postemergence herbicide treatment for red rice control when applied once at 0.28 kg ai/ha or sequentially at 0.14 kg ai/ha each time. Haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} applied at 0.16 kg ai/ha followed by another application at 0.14 kg ai/ha gave excellent red rice control in 2 of 3 years. Soybeans were uninjured by the herbicide treatments and yield was higher from treated than untreated soybeans.

Glyphosate [N-(phosphonomethyl)glycine] was applied at various stages of maturity to corn (Zea mays L. ‘Pioneer brand 3147’ and ‘Dekalb XL 394’), soybeans [Glycine max (L.) Merr. ‘Forrest’ and ‘Essex’], and johnsongrass [Sorghum halepense (L.) Pers.]. Glyphosate applied over-the-top of corn before the grain moisture level decreased to 30% (black layer will have been formed) caused various seed and subsequent progeny abnormalities. Depending on grain moisture level at the time of glyphosate application, seed weight was sometimes reduced and progeny seedling emergence, vigor, and weight were reduced. Also, abnormal seedlings, albino or straited, occurred. Glyphosate applied 2½ weeks or more before soybean maturity reduced seed weight, caused seed discoloration, and drastically reduced progeny seedling emergence, vigor, and weight. Glyphosate applied in September or early October controlled semimature johnsongrass. Later applications were less effective because of advanced senescence.

The effects of photoperiod and temperature on the translocation of triclopyr {[1(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, picloram (4-amino-3,5,6-trichloropicolinic acid) and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] were studied on tanoak [Lithocarpus densiflorus (Hook. & Arn.) Rehd.], snowbush ceanothus (Ceanothus velutinus Dougl.), bigleaf maple (Acer macrophyllum Pursh), bean (Phaseolus vulgaris L. var. ‘Red Kidney’) and barley (Hordeum vulgare L. var. CM-67). Isolation of 14C and analysis for the radioactive herbicides revealed little metabolism of the herbicides. Regardless of herbicide or plant species herbicide movement was greatest under warm temperature and long day conditions. Among the herbicides tested, 14C associated with triclopyr was the most mobile in each species. Each herbicide moved readily in the symplast but root applications of each herbicide revealed limited apoplastic mobility.

Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed in agronomic crop production fields in the United States. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. palmeri using soil moisture sensors under greenhouse conditions. Two A. palmeri biotypes collected from Nebraska were grown in loam soil maintained at 100%, 75%, 50%, 25%, and 12.5% soil field capacity (FC) corresponding to no, light, moderate, high, and severe water stress levels, respectively. Water was regularly added to pots based on soil moisture levels detected by Watermark or Decagon 5TM sensors to maintain the desired water stress level. Amaranthus palmeri plants maintained at ≤25% FC did not survive more than 35 d after transplanting. Amaranthus palmeri at 100%, 75%, and 50% FC produced similar numbers of leaves (588 to 670 plant−1) based on model estimates; however, plants at 100% FC achieved a maximum height of 178 cm compared with 124 and 88 cm at 75% and 50% FC, respectively. The growth index (1.1×105 to 1.4×105 cm3 plant−1) and total leaf area (571 to 693 cm2 plant−1) were also similar at 100%, 75%, and 50% FC. Amaranthus palmeri produced similar root biomass (2.3 to 3 g plant−1) at 100%, 75%, and 50% FC compared with 0.6 to 0.7 g plant−1 at 25% and 12.5% FC, respectively. Seed production was greatest (42,000 seeds plant−1) at 100% FC compared with 75% and 50% FC (14,000 to 19,000 seeds plant−1); however, the cumulative seed germination was similar (38% to 46%) when mother plants were exposed to ≥50% FC. The results of this study show that A. palmeri can survive ≥50% FC continuous water stress conditions and can produce a significant number of seeds with no effect of on seed germination.