U.S. corn prospects have continued to improve overall in the past two weeks, according to CropProphet, although the second half of July was not as favorable as the first half. Rainfall deficits persisted and intensified in much of Iowa, and the U.S. Drought Monitor now shows moderate drought conditions in more than a third of the state. Preliminary CropProphet data indicate that Iowa’s statewide June-July rainfall was the lowest since the 2012 drought, and 18 of 99 counties experienced either the driest or second driest June-July period in recent decades (1979-present). The CropProphet forecasts showed yield losses of 5-10 bushels per acre in western and southern portions of the state in the past two weeks (see figure below).
In contrast to the deterioration in Iowa, however, widespread improvement has continued in many of the important corn growing districts east of the Mississippi River. Rainfall has been generally above normal in these areas, and satellite indications of crop health have continued to improve; the state-average vegetative health index (NDVI) for corn growing areas is now the highest on record for Indiana and the second highest for Illinois. As a consequence, the significant yield deficits that were evident earlier in the season have been almost entirely eliminated across the eastern Corn Belt. Above-trend yields are now expected in Illinois and Ohio, and Indiana’s state-average corn yield is rapidly approaching the long-term trend (see figure below). According to CropProphet, the only state with a large corn yield deficit remaining is South Dakota, where drought is severe.
The pollinating stage of corn development is now approaching completion in much of the Corn Belt. Corn is particularly susceptible to heat stress during the pollination phase, and market observers expressed concern about the detrimental effects of a heat wave that peaked around July 20 in the central U.S. However, as anticipated in the July 18 CropProphet commentary, the hot spell was followed quickly by cool weather, and the average temperatures of the past two weeks have been only slightly above normal across the Midwest as a whole. Based on CropProphet calculations of the average daily high temperature during each county’s silking period, the net U.S. heat stress has been moderately but not dramatically higher than normal so far this year (see figure below). CropProphet uses the “silking temperature” as a predictor for corn yield, and some negative impacts from heat stress may have contributed to the recent yield declines in southern and western areas.
The next update to the corn commentary will occur on August 15.