B. Pollination and Grain Fill Conditions–Summer (June–August)
Drought hit the majority of the key corn growing states with some exempt areas
Corn pollination typically occurs in July. At pollination time, greater 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.
High winds moved through the Midwest at the end of June 2012, possibly affecting the growth of the young corn plants in some fields in the Gulf ECA. Then, the drought rapidly increased in severity from June to July. The percent of cropland experiencing severe or greater drought increased from 20% on June 19 to 51% and 57% by July 17 and August 14, respectively.
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 July 2012 map of the Palmer Z index shows the wide area of the corn-growing region covered by extreme drought.
The record heat in June and July, especially during pollination time, and the drought conditions led to less than average yields in the drought stricken areas. It is important to note, however, that those plants which experienced poor pollination and fewer kernels frequently had remaining kernels grow larger and accumulate more protein per kernel than average.
Conversely, Minnesota, Wisconsin, North Dakota, South Dakota, and some areas in Iowa had timely rains in June, which helped moderate the summer stresses, leading to more starch production and yield in those areas. Cooler weather came in August, but by then, the leaves were dying and ceasing photosynthesis, and therefore grain starch accumulation ended. However, some additional grain protein accumulation was still possible due to remobilization of nitrogen from the leaves and stalk to the grain. In some areas, later-planted or re-planted corn escaped the worst of the heat, with the kernels still growing and developing in August, as would happen in an average year.