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Issue No. 23, Article 5/October 9, 2009

Pondering a Springtime Sea of Yellow

Vibrant colors in Illinois corn and soybean fields might not be the most welcome sight when farmers' attention is focused on harvesting the mature, drab-colored crops. So it may be asking a bit much for readers to recall the bright yellow flowers that have become common across Illinois' springtime landscape. From a distance, folks often presume that the abundant yellow flowers of spring belong to one of the mustard species common in Illinois. Sometimes that assumption is correct, but often the flowers belong instead to a nonmustard species known by several common names, including cressleaf groundsel and butterweed. What do we know about this species? When might be ideal times to implement strategies for its control?

Native to the United States, butterweed can be found from Texas east to Florida, northward along the Atlantic coast to Virginia, and west to Nebraska. Herbarium specimens from the Illinois Natural History Survey indicate that butterweed specimens were collected in the state as early as 1932. The earliest herbarium specimens generally originated from counties in southern Illinois. During the 1980s and '90s, the survey augmented its collection with specimens from more northern counties, including Champaign and Vermilion. As of 1999, butterweed specimens had been collected from 48 counties in southern and central Illinois. Currently butterweed can be found from approximately as far north as Peoria, and from border to border east to west. Typically favoring moist to saturated soils, butterweed thrives in such areas as wastelands, pastures, fence rows, and roadsides. With the increased adoption of no-till and reduced tillage conservation practices, butterweed has become more prevalent in crop production areas.

Prior to Packera glabella, the Latin binomial for butterweed was Senecio glabellus. Similar to other plants in the genus Senecio, butterweed plants contain compounds with poisonous properties. Plants of this genus contain hepatotoxins in the form of pyrrolizidine alkaloids. Butterweed contains five pyrolizidine alkaloids: florosenine, senicionine, integerrimine, otosenine, and senkirkine. Pyrrolizidine alkaloids from the genus Senecio have been studied and shown to cause toxicosis of livestock across various regions of the world.

Butterweed completes its life cycle within one year (an annual growth habit). Field research was conducted at the University of Illinois from fall 2004 through spring 2006 to determine the emergence timing and growth characteristics of butterweed in no-till fields. Emergence tended to predominate in fall and was essentially complete by November, although some occurred in the spring (Table 1). From these results, we generally consider butterweed to be a winter annual species.

Following fall emergence, the formation of rosettes occurs before overwintering. The rosette leaves have petioles that connect the leaves to the stem. Often the underside of the rosette leaves is deep purple. Bolting (stem elongation), flowering, and seed production occur the following spring, often during May. The stem of butterweed is glaborous and hollow. After bolting, petioles are absent from leaves on the upper part of the plant. The leaves are pubescent, generally irregular in shape, and deeply cut to the midrib. The elongated stem often has a purplish tint. A member of the Asteraceae family, butterweed produces two types of composite flowers. The outside portion of the flower contains ray florets, while the center part contains disk florets. The flowers are bright yellow and grouped in clusters on several flowering stalks of the plant. Seeds are easily disseminated by wind due to the white hairs (pappus) on the apex of the achene.

We have coupled what we learned about the emergence characteristics of butterweed with additional field research (four Illinois locations in fall 2004 and three of these locations in spring 2005, then repeated at four locations in fall 2005 and spring 2006). We wanted to evaluate and compare several herbicides and herbicide combinations for butterweed control and to determine if control of butterweed is achievable following a fall or early-spring herbicide application. Results are presented in Table 2.

Nineteen herbicide treatments, including a nontreated control, were included at each location. Many treatments were selected based on common-use practices in Illinois, while other treatments, which are not commonly used (or labeled) in fall or early spring preplant applications, were included to examine their efficacy against butterweed. All treatments were applied at two timings: fall (mid- to late November) and spring (early to mid-April). Average diameter of butterweed rosettes was 2 to 4 inches and 6 to 8 inches when fall and spring treatments, respectively, were applied.

Butterweed control was determined by visually assessing plant biomass reduction in the treated area using a scale of 0 to 100 (0 representing no control and 100 indicating complete control). Additionally, five uniformly sized butterweed plants (2-4 and 6-8 inches in diameter for fall and spring applications, respectively) per plot were marked before herbicide application and subsequently harvested 14 days after treatment to determine herbicide efficacy based on biomass reduction. Visual assessments of herbicide efficacy were made before soybean planting, approximately 24 and 4 weeks after fall and spring applications, respectively.

Several herbicides or herbicide combinations provided better than 90% control of butterweed when applied in the fall (Table 2). Glyphosate alone provided 95% control of butterweed regardless of application rate; combining glyphosate with 2,4-D ester, Pursuit, or Prowl plus 2,4-D ester did not improve control compared with glyphosate alone. Control with 2,4-D ester improved when the application rate was increased from 1 pint to 2 pints but did not exceed 86%.

It's interesting to note that while some fall-applied treatments provided very good control of butterweed, less control was achieved when the identical treatments were applied in the spring. Larger cressleaf groundsel plants likely reduced the efficacy of these spring-applied treatments.

In summary, these results suggest that butterweed can be effectively controlled with several herbicides or herbicide combinations. Overall, many of these herbicides controlled butterweed when applied in the fall, but control was not always as great following an early-spring application, primarily because of larger plant size. Delays in spring burndown herbicide applications often allow butterweed plants to reach flowering and/or the stage before treatments are applied, often resulting in reduced or greatly slowed herbicide efficacy.--Aaron Hager

Aaron Hager

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