Identifying associations among site properties and weed species abundance.II. Hypothesis generation

Identification of associations between site properties and weed species abu ndance led to the generation of hypotheses as to why weed populations occur where they do, or do not, in agricultural fields. The objective of this re search was to use a multivariate statistical technique, canonical correlati on analysis, to identify the associations. Two continuous Zed mays producti on fields under center-pivot irrigation in the central Platte River Valley of Nebraska were grid-sampled between 1994 and 1997 for nine site propertie s and six to seven weed species. Weed species were identified and counted j ust prior to postemergence weed control in two adjacent quadrats (1 by 0.38 m) at each grid sampling point. These quadrats represented untreated weed populations emerging between crop rows and treated populations that survive d preemergence herbicide banded within the crop row. Canonical correlation analysis identified one to five significant correlations between linear com binations of site properties and weed species abundance depending on field site, years, and between- vs. on-crop row weed populations. The first pair of linear combinations consistently described an association that separated weed species across-a gradient of topography and soil type. The second pai r of linear combinations described associations between weed species and so il fertility In all cases, it was hypothesized that management practices st rongly interacted with site properties to create the observed associations with weed populations. Other hypothesized mechanisms For weed patchiness in clude patchiness in available soil moisture that would influence weed seed germination, emergence, and seedling growth. Additional variation in plant- available preemergence herbicide concentration across the field site would vary weed control efficacy. Another mechanism would be variation in soil fe rtility that affects the growth, reproduction, and competitive ability of b oth the crop and the weed.