Some portions of south-central Minnesota received some much needed rainfall on the evening of Sunday, July 8; however, much of southwest and south-central Minnesota missed the rainfall entirely. Rainfall amounts ranged from .1 in. to nearly 1 in. in the areas that received rain, which may not be adequate to relieve the dry conditions that exist in most parts of the region. Most portions of southwest and south-central Minnesota have been extremely hot and dry since mid-June, and soil moisture is getting critical in some areas, especially on lighter, sandier soils. Yield prospects for the 2007 corn and soybean crop are definitely being hurt in some areas by the continued hot, dry weather pattern.

Rainfall at the University of Minnesota (U of M) Research Center at Waseca on Sunday evening was .84 in., and brings the total July rainfall amount to over 1 in., which should be very helpful. The U of M Research Center at Lamberton reported no precipitation on July 8, and has not recorded any measurable rainfall since June 21, when .17 in. was recorded. The last significant rainfall recorded at Lamberton was June 16-18, when 1.45 in. was recorded. As of July 9, the accumulated growing degree units (GDU’s) since May 1 at Waseca were 1,212 GDU’s, compared to a normal of 1,054 GDU’s, which is about 15% ahead of normal. At the Lamberton station, the accumulated GDU’s since May 1 are running about 8-9% ahead of normal. The result is that most corn and soybeans in the region are 10 days to two weeks ahead of normal development.

At the Lamberton Research Center, there was 5.43 in. of stored soil moisture measured in the top 5 ft. of soil on July 1, which is about 12% below normal for early July. Most portions of southern Minnesota had ample supplies of stored soil moisture as we headed into the current hot, dry spell; however, those stored soil moisture supplies are being depleted rapidly, especially in the upper portions of the soil profile. Lamberton recorded 3.69 in. of rainfall in June, which is slightly below normal; however, all of that rainfall occurred in the first three weeks of the month. In May, Lamberton recorded 2.49 in. of rainfall, which was 25% below normal. At Waseca, the total June rainfall was 4.20 in., which is the normal June amount. The total May rainfall at Waseca was 3.36 in., or .60 in. below normal.

Most corn in the region is now in the tasseling and silking stage, which is an important development stage for determining corn yield potential. In recent days, considerable crop stress is being exhibited on lighter soils across the region. On the heavier soils, stored soil moisture has greatly helped in reducing crop stress thus far from the current hot, dry weather pattern. However, with corn now at the critical tasseling and silking stage, and the soybeans rapidly moving into the pod-setting stage, more rainfall will be needed in the coming weeks in order to maintain the good to excellent yield potential of the 2007 corn and soybean crop that exists in many areas of southern Minnesota.

Corn Water Use
Interestingly, it takes 2,400 gal. of water per acre to produce 1 bu. of corn per acre. So, for a 200 bu./acre corn crop, it would take over 480,000 gal. of water per acre, or the equivalent of about 18 in. of rainfall and stored soil moisture. The highest water usage by a corn plant is from the early tassel stage, through silking, to the early ear development (blister) stage, exactly the stages where most corn in southern Minnesota is now at. During this period, corn plants will utilize approximately .30-.35 in. of water per acre per day. Normally, corn will rely on timely rainfall and using available stored soil moisture to get through this critical period.

Typically, corn roots grow about 3 ft. on each side of the corn plant, and extend about 5-7 ft. into the soil. In very moist conditions, the corn plant will draw almost 80% of its moisture needs from the top 1 ft. of soil. During extended dry weather patterns that dry out the top 1-2 ft. of soil, the corn plant will get over 70% of its moisture needs from the available stored soil moisture at the 3-5 ft. level in the soil profile. This rooting pattern and the soil moisture adaptability of the corn plant is what has made the corn plant so flexible in a wide range of weather patterns, and helped result in consistent corn yields in recent years.

Editor’s note: Kent Thiesse is a former University of Minnesota Extension educator and now is Vice President of MinnStar Bank, Lake Crystal, MN. You can contact him at 507-726-2137 or via e-mail at kent.thiesse@minnstarbank.com.