The Stability of Weed Seedling Population Models and Parameters in Eastern Nebraska Corn (Zea mays) and Soybean (Glycine max) Fields

Intensive field surveys were conducted in eastern Nebraska to determine the frequency distribution model and associated parameters of broadleaf and grass weed seedling populations. The negative binomial distribution consistently fit the data over time (1992 to 1993) and space (fields) for both the inter and intrarow broadleaf and grass weed seed- ling populations. The other distributions tested (Poisson with zeros, Neyman type A, logarithmic with zeros, and Poisson- binomial) did not fit the data as consistently as the negative binomial distribution. Associated with the negative binomial distribution is a k parameter. k is a nonspatial aggregation parameter related to the variance at a given mean value. The k parameter of the negative binomial distribution was con- sistent across weed density for individual weed species in a given field except for foxtail spp. populations. Stability of the k parameter across field sites was assessed using the likeli- hood ratio test. There was no stable or common k value across field sites and years for all weed species populations. The lack of stability in k across field sites is of concern, because this parameter is used extensively in the development of paramet- ric sequential sampling procedures. Because k is not stable across field sites, k must be estimated at the time of sampling. Understanding the variability in k is critical to the develop- ment of parametric sequential sampling strategies and un- derstanding the dynamics of weed species in the field. Nomenclature: Corn, Zea mays L.; soybean, Glycine max L. Merr. Additional index words. Negative binomial distribution, k pa- rameter, likelihood ratio test, sampling strategies.

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