B. Pollination and Grain Fill Conditions – Summer (June – August)
Near normal temperatures in June and cool July with scattered rainfall
June 2013 saw continued flooding in pockets across the region, improving the long-term drought by helping to replenish the groundwater. However, some fields in these flooded areas were destroyed, thus decreasing harvested corn acres.
Corn pollination typically occurs in July, and at pollination time, higher than average temperatures or lack of rain typically reduces the number of kernels. The weather conditions during the grain filling period in July and August are critical to final grain composition. During this time, moderate rainfall and cooler than average temperatures, especially overnight temperatures, promote starch accumulation and increased yields. Moderate rainfall also aids nitrogen uptake for continued photosynthesis and grain protein and starch accumulation.
From July to mid-August 2013, the majority of the Gulf ECA experienced a flash drought (defined as intense, short-term lack of precipitation necessary for proper plant growth), with limited rainfall. However, cooler than average temperatures and plentiful groundwater helped lessen the severity of the flash drought on grain growth and development.
The Palmer Z index is a relative scale, indicating how monthly moisture conditions depart from normal, ranging from short-term agricultural drought to extreme wetness. The map indicates dry conditions with red and dark red, while sufficient to excess moisture is indicated by darker shades of green. The August 2013 map of the Palmer Z index shows the flash drought areas, mostly in the Gulf and eastern Pacific Northwest ECAs, while areas in the western Pacific Northwest and Southern Rail ECAs had plentiful water.
Despite the flash drought conditions, the cool weather in July and the first half of August, especially during grain fill, led to good starch accumulation and excellent yields in many fields. Hot weather came in late August, further stressing many flash drought stricken fields. This hot weather impeded the uptake and assimilation of nitrogen and its transport to the grain, thereby reducing grain protein concentration in the Gulf growing region. Those plants that were able to withstand the heat wave continued to increase their yield through August and September.