Kevin D. Gibson

Farmer Perceptions of Problematic Corn and Soybean Weeds in Indiana1

Corn and soybean farmers across Indiana were surveyed in 2003 to determine the perceived importance of weeds at the state and district levels. Weeds were considered the primary crop pest by 69 to 84% of farmers, depending on district. Diseases or insects were ranked first by no more than 14% of farmers and nematodes were ranked first by no more than 11%. Giant ragweed, Canada thistle, common lambsquarters, common cocklebur, and velvetleaf were considered the most problematic summer annual and perennial weeds statewide. Chickweed, horseweed, dandelion, and henbit were considered the most problematic winter annual weeds statewide. However, no weed species was listed by more than 41% of farmers statewide suggesting that relatively unique weed management problems may exist on many farms. Also, the perceived importance of most weed species varied substantially among Indianas nine districts. For example, velvetleaf was not listed as a problematic weed by any farmers in three districts. Burcucumber was not considered a statewide problem but was listed among the top three weeds by 14 and 16% of farmers in two southern districts. This survey supports the idea that educational programs focused on weed management should be tailored to geographic regions within Indiana. Nomenclature: Burcucumber, Sicyos angulatus L. #3 SIYAN; Canada thistle, Cirsium arvense (L.) Scop. # CIRAR; chickweed, Stellaria media (L.) Vill. # STEME; common cocklebur, Xanthium strumarium L. # XANTH; common lambsquarters, Chenopodium album L. # CHEAL; dandelion, Taraxacum officinale Weber in Wiggers # TAROF; giant ragweed, Ambrosia trifidia L. # AMBTR; henbit, Lamium amplexicaule L. # LAMAM; horseweed, Conyza canadensis (L.) Cronq. # ERICA; velvetleaf, Abutilon theophrasti L. # ABUTH; corn, Zea mays L.; soybean, Glycine max (L.) Merr. Additional index words: pests, weed rankings, winter annual, summer annual, perennial, Sorghum halepense, Sorghum bicolor, # SORVU, # SORHU. Abbreviations: GDD, growing degree days; IASS, Indiana Agricultural Statistics Service.

Glyphosate-Resistant Weeds and Resistance Management Strategies: An Indiana Grower Perspective1

Corn and soybean growers across Indiana were surveyed during winter 2003/2004 to assess their perceptions about the importance of glyphosate-resistant weeds and management tactics to prevent development of resistant populations. The survey showed two intriguing observations. First, 65% of survey respondents expressed moderate or low levels of concern about weeds developing resistance to glyphosate, whereas 36% expressed a high level of concern. Second, when asked an open-ended question regarding the factors that contribute to development of glyphosate-resistant weeds, 58% of the responses included repeated use of the same mode of action. Other factors such as poor application techniques or timing (33%), unique weed characteristics (8%) and changes in tillage practices (1%) were also mentioned. The survey showed that even though a relatively low percentage of respondents were highly concerned about resistance, they still expressed a willingness to use field scouting, tank-mix partners with glyphosate for burn-down and postemergence weed control, and soil-applied residual herbicides as resistance management strategies. This survey also showed that growers who farm 800 ha or more were more concerned about glyphosate resistance and more likely to adopt resistance management strategies than smaller growers. Nomenclature: Glyphosate; corn, Zea mays; soybean, Glycine max (L.) Merr. Additional index words: Survey. Abbreviations: GR, glyphosate-resistant.

Influence of Weed Management Practices and Crop Rotation on Glyphosate-Resistant Horseweed (Conyza canadensis) Population Dynamics and Crop Yield-Years III and IV

Abstract Horseweed is an increasingly common and problematic weed in no-till soybean production in the eastern cornbelt due to the frequent occurrence of biotypes resistant to glyphosate. The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual non-glyphosate herbicides, and preplant application timing on the population dynamics of glyphosate-resistant (GR) horseweed and crop yield. A field study was conducted from 2003 to 2007 in a no-till field located at a site that contained a moderate infestation of GR horseweed (approximately 1 plant m−2). The experiment was a split-plot design with crop rotation (soybean–corn or soybean–soybean) as main plots and management systems as subplots. Management systems were evaluated by quantifying in-field horseweed plant density, seedbank density, and crop yield. Horseweed densities were collected at the time of postemergence applications, 1 mo after postemergence (MAP) applications, and at the time of crop harvest or 4 MAP. Viable seedbank densities were also evaluated from soil samples collected in the fall following seed rain. Soybean–corn crop rotation reduced in-field and seedbank horseweed densities vs. continuous soybean in the third and fourth yr of this experiment. Preplant herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall. Spring-applied, residual herbicide systems were the most effective at reducing season-long in-field horseweed densities and protecting crop yields since the growth habit of horseweed in this region is primarily as a summer annual. Management systems also influenced the GR and glyphosate-susceptible (GS) biotype population structure after 4 yr of management. The most dramatic shift was from the initial GR : GS ratio of 3 : 1 to a ratio of 1 : 6 after 4 yr of residual preplant herbicide use followed by non-glyphosate postemergence herbicides. Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA.

A Field Survey to Determine Distribution and Frequency of Glyphosate-Resistant Horseweed (Conyza Canadensis) in Indiana

In-field surveys, which directly estimate weed population densities, typically utilize either random or nonrandom field selection methods. We used both methods to characterize the distribution and frequency of glyphosate-resistant (GR) horseweed populations and other late-season soybean weed escapes and to develop a database for tracking weed shifts, control failures, and the presence of other herbicide-resistant biotypes over time in Indiana. In-field surveys were conducted in a total of 978 Indiana soybean fields during September and October of 2003, 2004, and 2005. Information from fields with horseweed was obtained from 158 sites (19%) sampled through a systematic random site selection method and 128 fields through a nonrandom site selection method. When present, horseweed seed was collected and germinated in the greenhouse; rosettes 5 to 10 cm wide were sprayed with 1.72 kg ae/ha of glyphosate. Populations with less than 60% control at 28 d after treatment were determined to be glyphosate resistant. A selected subset of glyphosate-resistant populations was confirmed resistant by subsequent glyphosate dose response experiments. All populations in the subset with less than 60% control at the 1.72 kg ae/ha rate of glyphosate demonstrated 4- to 110-fold levels of resistance (R : S ratios). Glyphosate-resistant populations were found in all regions of Indiana; however, the highest frequencies were in the southeastern (SE) region with 38% of fields sampled and only 1, 2, and 2% of fields sampled in the northwestern (NW), northeastern (NE), and southwestern (SW) regions, respectively. Information gathered in this survey can assist in the development of applied research, as well as reactive glyphosate-resistant horseweed management education in the SE region of the state. Moreover, detecting resistance at low frequencies can direct proactive resistance education to farmers and practitioners in the other regions of the state as a means of providing an early warning system to address glyphosate resistance in weeds. Nomenclature: Glyphosate, horseweed, Conyza canadensis (L.) Cronq. ERICA; soybean, Glycine max L. Merr

Detection of Weed Species in Soybean Using Multispectral Digital Images

The objective of this research was to assess the accuracy of remote sensing for detecting weed species in soybean based on two primary criteria: the presence or absence of weeds and the identification of individual weed species. Treatments included weeds (giant foxtail and velvetleaf) grown in monoculture or interseeded with soybean, bare ground, and weed-free soybean. Aerial multispectral digital images were collected at or near soybean canopy closure from two field sites in 2001. Weedy pixels (1.3 m2) were separated from weed-free soybean and bare ground with no more than 11% error, depending on the site. However, the classification of weed species varied between sites. At one site, velvetleaf and giant foxtail were classified with no more than 17% error, when monoculture and interseeded plots were combined. However, classification errors were as high as 39% for velvetleaf and 17% for giant foxtail at the other site. Our results support the idea that remote sensing has potential for weed detection in soybean, particularly when weed management systems do not require differentiation among weed species. Additional research is needed to characterize the effect of weed density or cover and crop–weed phenology on classification accuracies. Nomenclature: Sethoxydim; giant foxtail, Setaria faberi Herrm. #3 SETFA; velvetleaf, Abutilon theophrasti Medicus # ABUTH; soybean, Glycine max (L.) Merr. ‘Asgrow 2733’. Additional index words: Precision farming, site-specific agriculture, weed maps. Abbreviations: ACRE, Agronomy Center for Research and Education; DPAC, Davis-Purdue Agricultural Research Center; FLL, Fisher linear likelihood; MED, minimum Euclidian distance; SAM, spectral angle mapper; SSWM, site-specific weed management.

Competitiveness of Rice Cultivars as a Tool for Crop-Based Weed Management

Rice (Oryza sativa L.) is one of the most important crops in the world both in terms of acreage planted and human consumption (Evans, 1998). Millions of people derive a substantial portion of their caloric intake from rice and rice is grown on every continent except Antarctica (IRRI, 1993). Rice is unique among crops in the variety of environments in which it can be grown which range from drought-prone sites to tropical rainforest and from sea level to the slopes of the Himalayas (IRRI, 1993). Cropping systems range from highly mechanized, high yielding, irrigated systems in which fields are continually flooded to lower-yielding rainfed upland rice where prolonged periods of moisture stress may occur (IRRI, 1993). Weed control is essential across rice cropping systems and the economic costs of weed management practices and yield losses can be substantial (Moody, 1996). Weed management practices are also diverse and range from hand-weeding to the near-exclusive reliance on herbicides for control. Despite the diversity of environments and weed management practices, these cropping systems all rely to some extent on vigorous crop growth to interfere with weed growth. Enhanced crop competitive ability has been proposed for reducing the costs of weed control and as an environmentally-compatible tool for rice farmers (Moody, 1996). The rationale for adopting more competitive cultivars varies with cropping systems. In the Ivory Coast, approximately half of the rice yield decline was attributed to increased weed pressure (Becker and Johnson, 1995). Johnson et al. (1998) suggested that more competitive upland rice cultivars might reduce the need for hand-weeding in West Africa where only a small proportion of farmers rely on herbicides for weed control. In contrast, rice production in California is highly dependent on herbicide use and work on competitive cultivars (Gibson et al., 2001 a, b) was driven by concerns related to reducing pesticide loads in the environment and to the evolution of cross-resistant watergrass (Echinochloa spp.) populations (Fischer et al., 2000).

Influence of Weed Management Practices and Crop Rotation on Glyphosate-Resistant Horseweed Population Dynamics and Crop Yield

Abstract Horseweed is an increasingly problematic weed in soybean because of the frequent occurrence of glyphosate-resistant (GR) biotypes. The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual nonglyphosate herbicides, and preplant herbicide application timing on the population dynamics of GR horseweed and crop yield. A field study was conducted at a site with a moderate infestation of GR horseweed (approximately 1 plant m−2) with crop rotation (soybean–corn or soybean–soybean) as main plots and management systems as subplots. Management systems were evaluated by quantifying horseweed plant density, seedbank density, and crop yield. Crop rotation did not influence in-field horseweed or seedbank densities at any data census timing. Preplant herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall. Spring-applied, residual herbicide systems were the most effective at reducing season long horseweed densities and protecting crop yield because horseweed in this region behaves primarily as a summer annual weed. Horseweed seedbank densities declined rapidly in the soil by an average of 76% for all systems over the first 10 mo before new seed rain. Despite rapid decline in total seedbank density, seed for GR biotypes remained in the seedbank for at least 2 yr. Therefore, to reduce the presence of GR horseweed biotypes in a local no-till weed flora, integrated weed management (IWM) systems should be developed to reduce total horseweed populations based on the knowledge that seed for GR biotypes are as persistent in the seed bank as glyphosate-sensitive (GS) biotypes. Nomenclature: Glyphosate; horseweed, Conyza canadensis L. ERICA; corn, Zea mays L; soybean, Glycine max (L.) Merr; winter wheat, Triticum aestivum L.

Farmer Perceptions of Weed Problems in Corn and Soybean Rotation Systems1

Corn and soybean growers across Indiana were surveyed in 2003 to determine their perceptions of the importance of weed problems in various crop rotations. Growers were asked to list the three most problematic weeds in the following rotation systems: soybean and corn planted in alternate years (SC) and corn (CC) or soybean (SS) planted to the same field for 2 or more years. Although some summer annuals and perennials (common lambsquarters, Canada thistle, and common cocklebur) and winter annuals (chickweed and henbit) were considered problematic by at least 10% of growers in all three systems, there were differences among systems in the relative importance of weed species. Giant ragweed was considered problematic by at least 30% of SC and CC growers but by less than 10% of SS growers. Horseweed was listed as a problematic summer annual by 13% of SS growers but by only 3% of CC growers. Purple deadnettle was listed by 15% of CC growers but by less than 6% of SC and SS growers. Perennial dicots were more problematic in SS than in CC. Annual and perennial grasses were more problematic in CC than in SC or SS. Despite these differences, the results of this survey suggest that the cumulative effect of weed management practices in corn and soybean rotation systems in Indiana has been the promotion of larger seeded, broadleaf, summer annual species. Nomenclature: Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; chickweed, Stellaria media (L.) Vill. # STEME; common cocklebur, Xanthium strumarium L. # XANTH; common lambsquarters, Chenopodium album L. # CHEAL; giant ragweed, Ambrosia trifidia L. # AMBTR; henbit, Lamium amplexicaule L. # LAMAM; horseweed, Conyza canadensis (L.) Cronq. # ERICA; purple deadnettle, Lamium purpureum L.; corn, Zea mays L.; soybean, Glycine max (L.) Merr. Additional index words: pests, weed rankings, winter annual, summer annual, perennial. Abbreviations: ALS, acetolactate synthase; CC, continuous corn rotation; CTIC, Conservation Technology Information Center; fb, followed by; GR, glyphosate-resistant; IASS, Indiana Agricultural Statistics Service; SC, soybean/corn rotation; SS, continuous soybean rotation.

Field Presence of Glyphosate-Resistant Horseweed (Conyza canadensis), Common Lambsquarters (Chenopodium album), and Giant Ragweed (Ambrosia trifida) Biotypes with Elevated Tolerance to Glyphosate

Late-season field surveys conducted in Indiana from 2003 to 2005 showed that common lambsquarters and giant ragweed plants were present in 11 and 22%, respectively, of randomly sampled soybean fields that also contained glyphosate-resistant horseweed. In the fall of 2005 and 2006, seed from 13 common lambsquarters and 22 giant ragweed populations were collected from previously surveyed fields that had confirmed glyphosate-sensitive or -resistant horseweed. The objective of this study was to determine whether the presence of glyphosate-resistant horseweed was correlated with the presence of common lambsquarters and giant ragweed biotypes with elevated tolerance to glyphosate. Through a series of greenhouse screens, 57% of common lambsquarters and 31% of giant ragweed populations collected from fields that had glyphosate-resistant horseweed expressed elevated levels of glyphosate tolerance. However, elevated tolerance to glyphosate was expressed by 33% of giant ragweed and 100% of common lambsquarters populations collected in fields that had glyphosate-sensitive horseweed. Therefore, under the parameters of this experiment and through different types of analyses, we concluded there was not a strong correlation between the late-season presence of glyphosate-resistant horseweed and common lambsquarters and giant ragweed populations with elevated glyphosate tolerance in the same field. A number of the weed populations expressed significant stunting from exposure to glyphosate, but were able to resume growth. Thus, researchers should evaluate plant regrowth in addition to biomass suppression when making assessments of glyphosate resistance in weed populations through greenhouse and field screening. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. CHEAL; giant ragweed, Ambrosia trifida L. AMBTR; horseweed, Conyza canadensis (L.) Cronq. ERICA; soybean, Glycine max (L.) Merr

Late-Season Weed Escapes in Indiana Soybean Fields

Crop advisors and extension personnel have observed a recent increase in the number of complaints regarding weed control with glyphosate (Fig. 1). Since glyphosate-resistant soybeans are grown in 89% of soybean acres in Indiana, we were interested in determining what weeds are escaping glyphosate treatments by recording identity, occurrence, and distribution of weed species in soybean fields just prior to harvest.