The Corn Crop Needs Rain: It's Time to be Concerned

Joe Lauer, Corn Agronomist

The National Weather Service has classified much of southern and southeastern Wisconsin as "abnormally dry" and "moderate drought" areas. Spotty rain showers occurred and farmers have rated the corn condition from excellent to poor in these areas. No rain is forecasted for the next week. In general, the corn condition improves as one travels north and west within the state. Early planted cornfields seem better off than later planted fields.

Figures 1 and 2 depict weather summaries for the UW Agricultural Research Stations at Arlington and Marshfield. Arlington precipitation from April 1 to July 5 was 4.90 inches in 2005. For the same period during 1988, it was 5.77 inches. It is the driest year on record for this period since records were kept at the farm beginning in 1962. Marshfield precipitation during this period has been 7.41 inches in 2005, and during 1988 was 6.19 inches.

The good news is we are right on the mark for GDU accumulation at both locations meaning it has not been too hot (like 1988). Corn planted June 1 is ahead for development and GDU accumulation. All other planting dates are right on schedule. Silking will begin for the earliest planted fields around July 11, with most of the crop silking around July 17-20 (the normal silking date).

Research indicates that the most sensitive period that affects corn yield is the pollination period. The research methods used to determine these sensitive periods are to quickly withdraw and apply water at different growth stages. That is not our situation this year. Soil water has steadily depleted over a long period. Soil moisture is still present because the crop is still growing. Leaf rolling might occur during the day, but plants are usually turgid the next morning.

The important question is: How much yield potential is lost? We do not have a good answer for this. In general, I think we are in good shape, but we need rain. There might some impact on yield with stress at this time, but the plant responds by increasing root growth at the expense of aboveground leaf area growth. Roots are growing into the soil profile where they typically may not grow every year. Typically, corn roots will grow 3 feet to the side of the plant and 5 to 7 feet into the soil profile in the absence of compaction. Plants experiencing drought will likely be shorter than normal. The "factory" will be smaller and even if field conditions improve, yield will likely be reduced.

The pollination period is the most sensitive time for the corn plant because silk and kernel ovule development are the most water sensitive tissues in the plant. Tassels typically emerge even under severe water stress. Under stress, tassels will emerge and shed pollen earlier than normal, but silk emergence is delayed affecting pollen germination and ovule fertilization. The result is a decrease in kernel number. So, even if field conditions become favorable during August and beyond, grain yield is reduced since no kernels were fertilized and developed during the pollination phase of the corn life cycle.

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