Influence of planting date and environment on Oklahoma wheat grain yield trend from 1963 to 1995

The five-year moving average (SYRMA) wheat (Triticum aestivum L.) grain yield per harvested hectare has declined by more than 18% over the past decade in Oklahoma. By contrast, world wheat yields have increased steadily over the same period. The Oklahoma wheat yield trend during the past decade is inconsistent with expectations. The objective of the research was to determine why the SYRMA wheat grain yield per harvested hectare did not increase in Oklahoma during the past decade. Five types of potential explanatory variables were investigated: structural change (including government programs), fertilizer use, proportion grazed, planting date and environment. Regression analysis was used to determine that the consequences of improvements in technology, research and education programs, as measured by a linear trend variable, were positive. However, during the past decade, these advancements were overwhelmed by changes in planting date and environmental factors.