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

Identification of associations between site properties and weed species abundance 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 research was to use a multivariate statistical technique, canonical correlation analysis, to identify the associations. Two continuous Zea mays production fields under center-pivot irrigation in the central Platte River Valley of Nebraska were grid-sampled between 1994 and 1997 for nine site properties and six to seven weed species. Weed species were identified and counted just 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 survived preemergence herbicide banded within the crop row. Canonical correlation analysis identified one to five significant correlations between linear combinations 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 pair of linear combinations described associations between weed species and soil fertility. In all cases, it was hypothesized that management practices strongly interacted with site properties to create the observed associations with weed populations. Other hypothesized mechanisms for weed patchiness include 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 fertility that affects the growth, reproduction, and competitive ability of both the crop and the weed.Nomenclature: Abutilon theophrasti Medicus ABUTH, velvetleaf; Amaranthus spp., pigweed; Apocynum cannabinum L. APCCA, hemp dogbane; Helianthus annuus L. HELAN, common sunflower; Physalis subglabrata Mack. et Bush. PHYSU, smooth groundcherry; Setaria spp., foxtail; Solanum ptycanthum Dun. ex DC. SOLPT, eastern black nightshade; Strophostyles leiosperma (T. et G.) Piper, smoothseed wildbean; Zea mays L., corn.