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Drift of sulfonylurea and phenoxy herbicides from spring cereal fields to nearby spring pea and lentil crops was simulated by spraying pea and lentil with 2,4-D or the 2:1 commercial mixture of thifensulfuron and tribenuron at rates of 0, 0.33, 1, 3.3, or 10% of the use rates (X) for spring cereal crops approximately 3 and 5 wk after planting pea and lentil. 2,4-D had minimal inhibitory effects on both crops at all rates tested. Lentil was slightly more sensitive than pea to 10% X 2,4-D. Thifensulfuron:tribenuron had no effect on either crop at rates less than 3.3% X. Two weeks after application of thifensulfuron:tribenuron, 10% X, and to a lesser degree 3.3% X rates, caused newly emerged leaves to become chlorotic, reducing chlorophyll content 25 to 50%. These treatments also reduced net photosynthesis by 37% and reduced or halted growth of the main stem. Early formation of leaves was reduced, thus tripling light penetration through the canopy. Five to six weeks after application, 10% X thifensulfuron:tribenuron had, in some treatments, more than tripled branching in pea, more than quadrupled branching in lentil, and reduced biomass as much as 42%. Flowering and maturity were delayed. Plants recovered from stunting by thifensulfuron:tribenuron to varying degrees depending on environmental conditions, and final seed yield generally was reduced less than 25%. In controlled greenhouse experiments, rate response to thifensulfuron generally was similar to that observed in field experiments. Pea was stunted less at 30 C than at 10 C, whereas lentil was affected similarly at these temperatures. Overall, visual symptoms from thifensulfuron:tribenuron exposure were more pronounced in pea than in lentil and were detectable at levels substantially lower than those that affected final seed yields.
Greenhouse studies were conducted to determine the influence of glyphosate [N-(phosphonomethyl)glycine] and surfactant concentration on Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] control. Five, 10, and 30% (v/v) solutions of the commercial formulation of glyphosate (356 g ae/L glyphosate and 178 g ai/L MON 0818) applied in 2-μl droplets to Canada thistle leaves at an equal dose per plant did not reduce plant growth, whereas a 2.5% solution reduced growth by 76%. Varying the glyphosate and the polyethoxylated tallow amine surfactant (MON 0818) concentrations independently showed that low glyphosate and MON 0818 concentrations controlled Canada thistle better than high concentrations. High glyphosate concentration (108 μg/μl), high MON 0818 concentration (54 μg/μl), and large droplet size (2 μl) reduced 14C-glyphosate absorption and translocation compared with low glyphosate concentration (9 μg/μl), low MON 0818 concentration (4.5 μg/μl), and small droplet size (0.2 μl). High glyphosate and high MON 0818 concentrations may cause rapid tissue toxicity resulting in reduced translocation and poor perennial weed control.
Canada thistle [Cirsium arvense [L.] Scop. # CIRAR] is a major weed problem in birdsfoot trefoil (Lotus corniculatus L. ‘Norcen’) seed production in northern Minnesota. Several systemic herbicides applied with selective applicators (roller and ropewick) were evaluated for Canada thistle control in birdsfoot trefoil. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), and glyphosate [N-(phosphonomethyl)glycine] controlled Canada thistle better than MCPA [(4-chloro-2-methylphenoxy)acetic acid] and dicamba (3,6-dichloro-2-methoxybenzoic acid). Glyphosate and MCPA did not injure birdsfoot trefoil foliage or bloom, whereas, picloram, clopyralid, and dicamba injured both. Although selective applications of glyphosate controlled Canada thistle for a short term with the least birdsfoot trefoil injury, long-term Canada thistle control in birdsfoot trefoil does not appear feasible with selective herbicide applications.
Bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] was evaluated for Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] control and birdsfoot trefoil (Lotus corniculatus L. ‘Norcen’) tolerance. Bentazon applied in split applications controlled more Canada thistle than a single application of equal rate in both greenhouse and field studies. Bentazon applied four times reduced the level of total nonstructural carbohydrates (TNC) in the roots of both greenhouse- and field-grown Canada thistle and increased birdsfoot trefoil injury more than bentazon applied in a single application of equal rate. All bentazon treatments caused unacceptable injury when birdsfoot trefoil was grown for seed.
Greenhouse experiments were conducted to evaluate phytotoxicity, absorption, and translocation of thifensulfuron when applied to pea at different rates, droplet sizes, and as dry particles. Thifensulfuron rates that reduced shoot dry weight by 25% were 0.46, 0.59, 0.98, and 1.21 g ai ha-1 for droplet diameters of 110, 155, 300, and 450 μm, respectively. The 14C-thifensulfuron absorption as percent of applied increased twofold, whereas translocation decreased 36% as herbicide concentration increased from 18 to 146 mg L-1 in 980-μm droplets. Small droplets (50 μm) that dried to particles before contact with leaves did not damage pea plants. However, under dew conditions, dry particles damaged peas to a similar degree as liquid droplets. We concluded that small and concentrated droplets of thifensulfuron can damage peas more than large and diluted droplets as a result of increased herbicide absorption.
A better understanding of the influence of various crop and weed management practices on spatiotemporal dynamics of weeds could improve the design of integrated weed management systems. We examined the influence of 18- and 76-cm soybean row spacings on emergence pattern and spatial aggregation of giant foxtail, common lambsquarters, and velvetleaf seedling cohorts. In addition, we characterized the soil seedbank and determined the quantitative and spatial relationship between the seedbank and seedling populations. Viable seeds of about 10 weed species and twice as many species of seedlings were identified in the weed community. Giant foxtail and common lambsquarters were the predominant species in the seedling and seedbank population, respectively, each accounting for 60 to 70% of the total weed species density. Emergence of giant foxtail, common lambsquarters, and velvetleaf depleted 12 to 33%, < 2% and 12 to 49% of the seedbank in the upper 10 cm of the soil profile. Peak time and periodicity of weed emergence was not influenced by soybean row spacing, and peak time of emergence of giant foxtail, common lambsquarters, and velvetleaf occurred 3 to 4, 3 to 6, and 3 to 9 weeks after soybean planting (WAP), respectively. Magnitude of giant foxtail emergence 5, 6, and 9 WAP was 98, 96, and 76% greater in 76- than in 18-cm row soybeans only when the population of 76-cm row soybeans was 57% lower than the 18-cm soybeans in 1997. Giant foxtail and common lambsquarters seeds in the seedbank were aggregated in 1996 and 1997 according to the Taylor power law (TPL) and the negative binomial distribution (NBD). The TPL and the NBD were similar in describing the spatial aggregation of giant foxtail and common lambsquarters but not some velvetleaf seedling cohorts. The spatial aggregation of seedlings varied among cohorts for different weed species and was likely due to species-specific biological characteristics that influence seed dispersal, germination, and seedling emergence. Within a 1.5-ha area, aggregation declined with decreasing density. Within a 24-m2 area, the level of aggregation of all weed species decreased as seedling densities increased. These results indicated that soybean row spacing influenced neither weed emergence pattern nor weed spatial aggregation; thus, several management decisions can be similar in 18- and 76-cm row soybeans.
Farmers need information on herbicide technology and crop performance to assess the profitability of new herbicide-resistant crop technologies. First-generation imazethapyr-resistant corn hybrids evaluated at the University of Wisconsin yielded less than other commercial hybrids. To determine if this resistance trait affected yield or agronomic traits, 10 near-isogenic pairs of imazethapyr-resistant and -susceptible corn hybrids were compared. Whether treated with imazethapyr or not, imazethapyr-resistant hybrids yielded the same when averaged across hybrids, although yield varied among a few individual hybrids within single experiments. Seven of the imazethaypr-resistant hybrids yielded the same, two yield more, and one yielded less than their susceptible near-isogenic counterpart during eight site-years. Grain moisture was not affected, but imazethapyr-resistant hybrids had fewer broken stalks than did susceptible hybrids. The imazethapyr resistance trait does not appear to affect yield potential, but the backcrossing procedure may have caused early resistant hybrids to lag behind in yield compared to other new hybrids.
Increased crop densities and postplant tillage were evaluated as nonchemical methods to supplement metribuzin for improved broadleaf weed control in dry pea and lentil. The effects of 50, 100, and 150% of recommended 220 kg/ha pea and 67 kg/ha lentil seeding rates and two dates of rotary hoeing and harrowing on pea, lentil, and broadleaf weeds were studied with and without metribuzin for two years. Under favorable growing conditions, crop competition gave 72 and 99% weed control in pea and 33 and 70% weed control in lentil with the 50 and 150% seeding rates. Under less favorable conditions, control was 21 to 39% with the low and high pea and lentil seeding rates. At recommended seeding rates, metribuzin gave greater than 90% control in either crop or year. Postplant tillage 12 to 27 d after planting slightly reduced crop densities in three tillage treatments in one year, but not the second. Postplant tillage did not affect crop yield or improve weed control. In all studies, pea was similar to or more competitive than lentil in suppressing broadleaf weeds. Because neither non-chemical practice significantly improves weed control, changes are not recommended for weed management in pea and lentil.
PALWEED:WHEAT is a bioeconomic decision model for determining profit-maximizing postemergence herbicide treatments for winter wheat in the Washington–Idaho Palouse region. PALWEED:WHEAT performed relatively well economically in 2 yr of on-farm field tests. However, the model was less sensitive than desired in prescribing postemergence broadleaved herbicides in the presence of high densities of broadleaved weed seedlings. Therefore, PALWEED:WHEAT was revised in response to the field testing. This paper compares the revised model's agronomic and economic performance to the original model in computer simulations. The revised model, PALWEED:WHEAT II, differs from the original model in several respects: (1) exponential functions replace linear functions in predicting weed survival, (2) preplant application of a nonselective herbicide is entered as an exogenous binary variable, (3) separate indices of broadleaved and grass competition are substituted for an aggregate weed competition index in the wheat yield function, (4) hyperbolic replaces logistic functional representation of weed damage to wheat yield, and (5) separate models are estimated for winter wheat after spring dry pea and for winter wheat in all examined crop rotation positions. In simulations including a variety of agronomic and economic conditions, PALWEED:WHEAT II recommended postemergence herbicide types and rates that consistently complied with agronomic and economic theory. Furthermore, the revised model, especially when estimated from the relevant wheat after pea data set, was markedly more balanced in recommending both broadleaved and grass herbicides in response to observed densities of both weed groups. The substantial change in herbicide recommendations in response to changes in model functional specifications following field testing confirms the importance of field testing and revision of bioeconomic decision models.
The mechanism of glyphosate tolerance was investigated in nine birdsfoot trefoil selections that exhibited a threefold difference in glyphosate tolerance. Single-stemmed ramets (vegetative clones) established from the nine selections were used to evaluate tolerance to glyphosate, spray retention, and 14C-glyphosate absorption and translocation in growth chamber experiments. The tolerance of greenhouse-grown ramets correlated with the tolerance of field-grown plants, indicating that tolerance was not a function of plant size or affected by environment. The nine selections differed in spray retention and 14C-glyphosate translocation but not in glyphosate absorption. The differences in retention and translocation were not correlated with the level of glyphosate tolerance but could contribute to the tolerance of an individual plant. The specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) (EC 184.108.40.206) ranged from 1.3 to 3.5 nmol min−1 mg−1 protein among the nine selections assayed and was positively correlated with plant tolerance level. These results indicate that the primary mechanism of glyphosate tolerance in birdsfoot trefoil is based on the level of EPSPS activity.
Based on six years of data from a field experiment near Pullman, WA, a bioeconomic decision model was developed to annually estimate the optimal post-emergence herbicide types and rates to control multiple weed species in winter wheat under various tillage systems and crop rotations. The model name, PALWEED:WHEAT, signifies a Washington-Idaho Palouse region weed management model for winter wheat The model consists of linear preharvest weed density functions, a nonlinear yield response function, and a profit function. Preharvest weed density functions were estimated for four weed groups: summer annual grasses, winter annual grasses, summer annual broadleaves, and winter annual broadleaves. A single aggregated weed competition index was developed from the four density functions for use functions for use in the yield model. A yield model containing a logistic damage function performed better than a model containing a rectangular hyperbolic damage function. Herbicides were grouped into three categories: preplant nonselective, postemergence broadleaf, and postemergence grass. PALWEED:WHEAT was applied to average conditions of the 6-yr experiment to predict herbicide treatments that maximized profit. In comparison to average treatment rates in the 6-yr experiment, the bioeconomic decision model recommended less postemergence herbicide. The weed management recommendations of PALWEED:WHEAT behaved as expected by agronomic and economic theory in response to changes in assumed weed populations, herbicide costs, crop prices, and possible restrictions on herbicide application rates.
Herbicide-resistant weeds are becoming a major problem in the Midwest, and strategies must be adopted to delay further selection. Strategies of rotating and tank-mixing herbicides with different modes of actions should be effective, but adoption may be limited and certain limitations may exist. Therefore, integrating nonchemical practices that indirectly lower selection pressure or restrict the growth of resistant populations is desirable. Appropriate integration of mechanical weeding, crop rotation, increased crop competition, and decision aids may further delay the development of resistance. Understanding the effect of these practices on weed population dynamics is required to more accurately predict their contributions toward resistance management. This knowledge will aid in justifying the adoption of improved management systems.
Knowledge of weed community structure in vegetable crops of the north central region (NCR) is poor. To characterize weed species composition present at harvest (hereafter called residual weeds) in processing sweet corn, 175 fields were surveyed in Illinois, Minnesota, and Wisconsin from 2005 to 2007. Weed density was enumerated by species in thirty 1-m2 quadrats placed randomly along a 300- to 500-m loop through the field, and additional species observed outside quadrats were also recorded. Based on weed community composition, population density, and mean plant size, overall weed interference level was rated. A total of 56 residual weed species were observed and no single species dominated the community of NCR processing sweet corn. Several of the most abundant species, such as common lambsquarters and velvetleaf, have been problems for many years, while other species, like wild-proso millet, have become problematic in only the last 20 yr. Compared to a survey of weeds in sweet corn more than 40 yr ago, greater use of herbicides is associated with reductions in weed density by approximately an order of magnitude; however, 57% of fields appeared to suffer yield loss due to weeds. Sweet corn harvest in the NCR ranges from July into early October. Earlier harvests were characterized by some of the highest weed densities, while late-emerging weeds such as eastern black nightshade occurred in fields harvested after August. Fall panicum, giant foxtail, wild-proso millet, common lambsquarters, and velvetleaf were the most abundant species across the NCR, yet each state had some unique dominant weeds.
Solanum ptycanthum plants putatively resistant to acetolactate synthase (ALS) inhibitors were identified in a Wisconsin Glycine max field in 1999. Three- to four-leaf-stage S. ptycanthum plants in the greenhouse were 150, 120, and 5.9-fold resistant to imazethapyr, imazamox, and primisulfuron, respectively, compared with susceptible plants. In vivo ALS was 170- and less than 20-fold more resistant to imazethapyr and primisulfuron, respectively. These results suggested that the S. ptycanthum accession was highly resistant to imazethapyr and imazamox, and that resistance was associated with insensitive ALS. This is the first confirmed occurrence worldwide of S. ptycanthum resistance to ALS inhibitors.
Agronomic research and extension personnel generally recognize the benefits of integrated pest management (IPM) but IPM practices have not been rapidly adopted by farmers. In order for applied research and extension programs to be as influential as possible, strategies and tactics must be evaluated in the context of the real-world constraints experienced by farmers. We investigated the linkage between farmers' pest management behaviors, attitudes, and constraints by analyzing an extensive corn pest management survey distributed throughout Wisconsin in 2002. Our objectives were to (1) create a benchmark against which future changes in pest management practices could be detected and (2) explore potential associations between practices and farm characteristics, e.g., farm size or commodity produced. A total of 213 farmers responded with descriptions of their operations; weed, insect, and disease pest management practices; crop consultant usage; interactions with their local agrichemical dealer; and attitudes regarding pest management decision-making. We compared the relative responses of cash-grain and dairy farmers as well as managers of large and small farms. Larger farm size and percentage of operation in cash-grain production were associated with an increased frequency of rotating crops, rotating herbicide families, and use of a broadcast herbicide application. Managers of large farms and/or cash-grain crops also more frequently indicated considering the level of pest control, price, carryover potential, weed resistance management, environmental safety, and risk to the applicator than did dairy or small-sized operations. Cash-grain farmers had significantly higher scores on a calculated IPM index than did dairy farmers (P < 0.0001). We also found a significant positive relationship between farm size and IPM score (P < 0.0001). Our results provide a benchmark for future comparisons of IPM adoption rates in Wisconsin and highlight the association between IPM research/extension and farmers' management behavior.
Researchers interested in describing or understanding agroecological systems have many reasons to consider on-farm research. Yet, despite the inherent realism and pedagogical value of on-farm studies, recruiting cooperators can be difficult and this difficulty can result in so-called “convenience samples” containing a potentially large and unknown bias. There is often no formal justification for claiming that on-farm research results can be extrapolated to farms beyond those participating in the study. In some sufficiently well-understood research areas, models may be able to correct for potential bias; however, no theoretical argument is as persuasive as a direct comparison between a randomized and a convenience sample. In a 30-cooperator on-farm study investigating weed community dynamics across the state of Wisconsin, we distributed a written survey probing farmer weed management behaviors and attitudes. The survey contained 59 questions that overlapped a large, randomized survey of farmer corn pest management behavior. We compared 187 respondents from the larger survey with the 18 respondents from our on-farm study. For dichotomous response questions, we found no difference in response rate for 80% of the questions (α = 0.2, β > 0.5). Differences between the two groups were logically connected to the selection criteria used to recruit cooperators in the on-farm study. Similarly, comparisons of nondichotomous response questions did not differ for 80% of the questions (α = 0.05, β > 0.9). Exploratory multivariate analyses failed to reveal differences that might have been hidden from the marginal analyses. We argue that our findings support the notion that the convenience samples often associated with on-farm research may be representative of the more general class of farms, despite lack of bias protection provided by truly randomized designs.
Widespread use of crop yield loss models based on weed density has been limited on account of spatial and temporal variability. Furthermore, research characterizing crop yield loss associated with two or more weed species is lacking for many cropping systems. Therefore, research was conducted to characterize giant foxtail and common lambsquarters leaf area, height, and shoot volume in soybean, to quantify the relative competitive ability of giant foxtail and common lambsquarters in a mixed–weed species environment, and to assess weed density, weed relative leaf area, and weed relative volume as predictors of soybean yield loss. Based on weed density, coefficient estimates of percent soybean yield loss as giant foxtail or common lambsquarters densities approached zero differed between years. In contrast, coefficient estimates of maximum soybean yield loss were similar between years. Based on weed relative leaf area, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters leaf area differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail leaf area did not change over time within a growing season or between years. Based on weed relative volume, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters volume differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail volume did not change over time within a growing season or between years. Based on weed density, weed relative leaf area, or weed relative volume, giant foxtail was more competitive than common lambsquarters in terms of soybean yield loss. Temporal variability of weed density, weed relative leaf area, and weed relative volume indicates that additional parameters may be required to accurately predict weed–crop interactions in a multiple–weed species community.
The objectives of this study were to model the influence of herbicides, wilt disease, and mechanical treatments on velvetleaf population dynamics, annualized net return (ANR), and economic optimum threshold (EOT) in a 20-yr rotation involving alternate years of corn and soybean. Mechanical treatments were interrow cultivation in corn and rotary hoeing in soybean. Herbicides at a quarter (¼×) rate or lower did not reduce velvetleaf seed banks without mechanical treatments in the absence of wilt. Herbicides at full (1×) and half (½×) rates decreased velvetleaf seed banks 95% within 6 and 20 yr, respectively, when there was no wilt. Herbicides at ½× rates with mechanical treatments reduced the seed bank 95% in only 10 yr, but mechanical treatments did not increase the rate of seed bank decline with 1× rates. Wilt infection had to occur annually to reduce velvetleaf seed banks as effectively as herbicides at 1× rates alone. ANR was maximized with herbicides at reduced rates, even though they were not as effective at reducing seed banks as were 1× rates. The herbicide rate required to maximize ANR increased as the initial velvetleaf seed bank density increased. Mechanical treatments and wilt decreased the herbicide rate required to maximize ANR. In fact, wilt infection increased the ANR of herbicides at reduced rates. The EOT was 0.55 and 0.4 seedlings m−2 when velvetleaf was managed with herbicides at 1× and ½× rates, respectively. Mechanical treatment had no effect on EOT, but wilt increased the EOT. Herbicides at reduced rates should only be used to manage velvetleaf in fields with a low seed bank density when integrated with mechanical treatments or when the field has a history of wilt. Herbicides should be used at 1× rates when fields have a large velvetleaf seed bank and when integrated management practices are not used.
Understanding weed–crop interactions is critical in predicting crop yield loss, but it is also important to understand how these interactions affect weed productivity. Therefore, research was conducted to characterize the weed relative leaf area and weed relative volume of several giant foxtail cohorts in soybean, and to assess weed density and cohort emergence time, weed relative leaf area, and weed relative volume as predictors of giant foxtail shoot biomass and fecundity. Giant foxtail cohorts emerged at VE (emergence), VC (cotyledon), V1 (first node), and V3 (third node) soybean growth stages and were thinned to densities of 0, 4, 16, 36, and 64 plants m−2. Based on weed density and cohort emergence time, the maximum shoot biomass per square meter or the maximum fecundity per square meter differed between years. In contrast, shoot biomass or fecundity per plant, as weed density approached zero, and the rate at which shoot biomass or fecundity decreased exponentially, as time increased, were similar between years. Based on the weed relative leaf area, the cohort effect on giant foxtail shoot biomass differed between years, whereas the cohort effect on giant foxtail fecundity was similar between years. Maximum giant foxtail shoot biomass per square meter or fecundity per square meter differed between years when estimated from weed relative leaf area. Based on the weed relative volume, the cohort effect on giant foxtail shoot biomass per square meter or fecundity per square meter was similar between years, as was the maximum giant foxtail shoot biomass per square meter or fecundity per square meter. The temporal stability of weed relative volume, used to describe giant foxtail shoot biomass or fecundity, may aid in improving bioeconomic weed management models.
The relationship between species richness and sample area has been characterized in many natural communities but has rarely been examined in crop–weed communities. We determined the species–area relationship in short-term (≤4 yr) and long-term (>15 yr) moldboard-plowed (MP), chisel-plowed (CP), and no-tillage (NT) fields cropped to corn and in short-term MP, CP, and NT fields cropped to soybean. A total of 10 corn fields and 10 soybean fields were sampled for species richness in 14 nested sample areas that ranged from 0.0625 to 512 m2. The influence of sample area on frequency of species occurrence was also determined. Species richness was greater in long-term NT fields than in tilled or short-term NT fields. The species–area relationship in tilled and short-term NT fields was best described by an exponential function. In contrast, a power function was the best fit for the species–area relationship in long-term NT fields. The functional minimum area required to represent 75% of the total weed species in tilled and short-term NT fields was 32 m2. A functional minimum area could not be determined in long-term NT fields because species richness continued to increase over the range of sample areas. Regression functions predicted that sample areas of 1 m2 would contain less than 50% of the observed maximum species richness in these fields. Sample areas of 36 m2 in tilled and short-term NT fields and 185 m2 in long-term NT fields were predicted to measure 75% of observed maximum species richness in these fields. Pigweed species and common lambsquarters occurred at high frequencies and were detected in most sample areas. This information could be used to better define sample area requirements and improve sampling procedures for species richness of weed communities.