Early termination of cover crops, a multi-degree spline plateau function, and spatial-temporal modeling of crop yield distributions

The first essay examined the impact of cover crops and termination timing on wheat forage yield, quality, and profitability in Oklahoma. While cover crops did not significantly affect forage yield or nutritional quality, early termination increased yield by 387.7 kg ha⁻¹ over late termination. Economic analysis found the no-till summer fallow system most profitable, with sensitivity analysis indicating cowpea seed costs would need to drop 40% for this system to break even with no-till fallow. The second essay introduced a multi-degree spline plateau model for optimizing nitrogen fertilizer rates, which is linear in its parameters, simplifying estimation compared to traditional methods. Using data from PRNT, MRTN, and Experiment 502, results indicate that the spline model—with appropriate knot selection—outperforms linear and quadratic plateau models, showing promise for precision agriculture applications. Future research will integrate spatial random effects to enhance predictive accuracy and efficiency. The third essay estimated crop yield distribution using Bayesian spatial-temporal models for Oklahoma hay and Iowa corn, focusing on the influence of historical data spans on model performance. Both data sets had strong spatial patterns with yields increasing from south-to-north for Iowa corn and increasing from west-to-east for Oklahoma hay. The out-of-sample fit for the spatial models did not exceed the models that ignored spatial information. For the spatial-temporal models, time spans at or near the longest available provided the best fit. When trend was modeled separately, time spans of fifteen years provided the best fit. The results show that spatial information did not serve as a substitute for long time-series of data in modeling crop yield distributions.