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Spatial stability of weed populations
by Bob Hartzler and Brent Pringnitz
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Feb. 1997 -- Anyone involved with weed management is aware of the patchy nature of weed populations. While weed populations vary widely within the field, most weed control tactics are applied uniformly across the field which may not be the most efficient use of resources. Our ability to exploit the uneven distribution of weeds is hindered due to our poor understanding of the factors that drive the patchiness and the stability of patches within a field. A recent article in Weed Science provides some excellent information concerning the spatial stability of patches ( Gerhards, R., D. Y. Wyse-Pester, D. Mortensen, and G. A. Johnson. 1997. Characterizing spatial stability of weed populations using interpolated maps. Weed Sci. 45:108-119). This article will provide a brief synopsis of some of the information found in this article.

Methods
Seedling populations of four weed species were monitored in two fields in eastern Nebraska over a four year period (I will only report results from one field in this article). The field was planted to soybeans in 1992 and corn in 1993 through 1995. A 10 acre area in the center of the field was sampled on a 23 ft grid. Weed populations in a 4 ft2 quadrat were determined at each grid point approximately three weeks after planting between the rows. Preemergence herbicides were applied in a band and weeds were counted prior to postemergence herbicides, thus the data represent weed populations not exposed to control tactics. The research was conducted on cooperating farmer's fields and all field work was completed by the farmers.

A linear triangulation interpolation method was used to estimate weed seedling density at unsampled positions and to plot continuous weed density maps (Gerhards et al. 1996. J. Agron. Crop Sci. 176:259-266.). A patch was defined as an area with a contiguous weed infestation within a distance of 45 ft (2 cells).

Results
Weed populations of the four species were highly aggragated, but there were differences in stability of patches among species.

Sunflower Average density of sunflower in the field ranged from 0.7 plants/m2 in 1994 and 1995 to 2.4 plants/m2 in 1993. Area of the field infested with sunflower ranged beween 26 and 41% during the four years. Three major patches were observed in all years, with the patches elongated in the direction of tillage operations (E-W) (Figure 1). Two additional, small patches were observed in 1993 and 1994 along the northern edge of the border. The area of the largest patch ranged from 2.7 acres to 3.4 acres during the course of the study.

Velvetleaf Mean velvetleaf populations varied from 2.1 to 22.1 plants/m2 in 1995 and 1993, respectively. Approximately 60% of the field was infested with velvetleaf in all years (Figure 2). Two major patches were found in the field, the largest ranging from 4.1 to 5.5 acres in size.

Foxtail The foxtail population in the field was a mixture of green and yellow foxtail. The average seedling density increased from 0.1 plants/m2 in 1992 to 63.4 plants/m2 in 1995 (Figure 3). In contrast, the foxtail population in the second field decreased from 21.9 in 1992 to 0.8 plants/m2 in 1995 (data not presented here). More than 95% of the sampled area was free of foxtail in 1992, compared to less than 1% weed-free in 1995. The authors suggested the increase in foxtail population may have been due to seed movement into the field from adjacent areas, survival of late emerging seedlings, and poor herbicide performance. The data show the speed with which weed infestations can increase.

Hemp dogbane Average shoot density of hemp dogbane ranged from 0.2 to 0.4 plants/m2 during the course of the study (Figure 4). Almost 63% of the field was free of hemp dogbane in all four years. The area of the major hemp dogbane patch ranged from 1.9 to 2.7 acres. Hemp dogbane patches were more stable than those of the other species, probably due to its spread by vegetative rootstocks. The orientation of the patches was in the direction of tillage.

Summary
Development of this type of information is critical for fulfilling the 'promise' of site specific agriculture. Although significant portions of the field were free of individual weed species, when the distribution maps of the species were overlaid almost 0% of the field was weed-free. The factors that drive the patchiness of weed population are poorly understood. Better sampling methods will need to be developed in order to make development of weed distribution maps affordable to agricultural producers. Increased resources are being invested in evaluating the potential to map weed populations using remote sensing technology, and perhaps this will solve the cost problem.

Prepared by Bob Hartzler, extension weed management specialist, and Brent Pringnitz, extension program specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.