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item Koger Iii, Clifford

Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2005
Publication Date: 5/1/2005
Citation: Koger III, C.H., Poston, D.H., Eubank, T.W. 2005. Factors affecting germination of horseweed (conyza canadensis). Proceedings of Southern Weed Science Society.

Interpretive Summary:

Technical Abstract: Horseweed (Conyza canadensis) is an erect summer or winter annual herbaceous weed species that is native to north North America. It is commonly found in cultivated and abandoned fields, roadsides, pastures, utility right-of-ways, and waste areas of the continental United States. It has also become problematic and difficult to control in glyphosate-resistant (GR) crop production systems of the eastern United States. Achenes of horseweed are easily spread by wind and individual plants are capable of producing 500,000 seed . Horseweed is often susceptible to common tillage practices of conventional-tillage cropping systems; whereas, it often thrives in conservation- or no-tillage systems. In recent years, it has proliferated in minimum tillage GR cropping systems. Emergence of horseweed is typically observed in late summer to early fall, but flushes have been documented throughout the year when sufficient moisture is available. Little is known about the biology of this species or population and emergence dynamics of horseweed under different timings of cultural and chemical weed management practices. Field studies were conducted in 2003 ' 2005 to investigate the effects of soil temperature and timing of tillage and non-selective herbicide application on emergence and populations of GR and glyphosate-susceptible horseweed in Mississippi. Studies were conducted on two fields containing glyphosate-susceptible horseweed populations and two fields containing GR populations. Treatments included disking or glufosinate (0.5 kg ai ha-1 plus 0.25% v/v non-ionic surfactant) in September, November, January, or March; disk in September followed by glufosinate in March; and nontreated check. Glufosinate was applied with a CO2 pressurized backpack sprayer calibrated to deliver 95 liter ha-1 at 190 kPa. Plot size was 4.5 by 9 m and treatments were replicated four times in a randomized complete block design experiment within each field. Horseweed seedling were counted bi-weekly in two 1-m2 quadrats in each nontreated plot beginning September 1. Emerged seedlings were killed with glufosinate after counting. Plant counts were recorded from two 1-m2 quadrats per plot just prior to initiation of each treatment. Total plant counts were recorded from one 1- by 3-m quadrat per plot on August 1 just prior to study termination. One automated soil temperature logger was placed 2.5 cm deep in a nontreated check plot of each study and set to record temperature every 4 h. Horseweed germinated in the late summer, fall, and spring of each year. Extensive germination occurred in September and October of both years, with densities ranging from 70 to 155 plants m-2. Horseweed germinated in both years when soil temperatures were above 10 C, and no differences were observed in germination patterns between GR and susceptible populations. Burial of horseweed seed with tillage reduced densities compared to nontreated plots, but densities remained above 10 plants m-2. Fall tillage followed by spring burndown reduced populations, but flushes emerged in April and May after burndown. Plasticity of horseweed germination was evident, and tillage alone or coupled with non-residual herbicide control did not eliminate germination of horseweed.