A Negative Binomial Regression Model of the Observed Population Density of after Annual Corn Rotation in Nebraska.

In Nebraska, rotation of soybean () fields to corn () is a major practice to manage the soybean cyst nematode (SCN; ). However, factors associated with SCN population density decline during corn rotation are not clearly defined. This study addresses that question using a modeling approach. Seventy-nine fields were sampled in 2009, 2010, and 2011 to determine SCN population densities (eggs/100 cm of soil) before and after rotation. After rigorous field screening and model testing and validation, the regression model was developed, where Log P is the natural log of SCN eggs at the end of the rotation year, P is the population density before rotation, and pH is the soil pH. Model goodness-of-fit was assessed through residual analysis, information criteria, and other remedial measures. Model overdispersion was 1.04. Validation in a 50 and a 75% random sample from the original data set showed little change in model regression coefficients, standard errors, and associated significance, confirming model fit and performance. The model indicates that for one-unit increase in soil pH, SCN P is expected to increase by 53.7% at constant P, and correspondingly, a 10% change in P will result in about 8.3% change in P at constant soil pH. The model suggested that SCN population levels before corn rotation and soil pH are major determinants of observed SCN population density reduction after annual corn rotation in Nebraska. This model has potential for use in SCN risk analysis and in predicting SCN population decline after corn rotation in the state.