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Late-Season Dry Weather and Corn

September 17, 2013
illinois corn crop tour 2013
  
 
 

Despite recent weeks of hot, dry weather, corn in many Illinois fields has retained some green color. That doesn't mean there aren't problem areas.

By Emerson Nafzinger, University of Illinois Extension

The 2013 corn growing season has had its share of ups and downs, with late planting due to early rainfall, more rain in June, and temperatures that were at or below normal most of the season until recent weeks. Pollination conditions were good in most places in Illinois, with adequate soil moisture and generally good temperatures. By late July most fields were in good shape, with good kernel counts and good canopy color and leaf health.

Much of the crop reached the middle part of August in good shape, helped along by continued cool temperatures. But rainfall became infrequent or stopped at some point in July or August, depending on location. At Champaign, Ill., July rainfall was slightly above average, with 5.03 inches at the airport, more than half of which fell on July 21. Many areas received much less than this.

Temperatures continued to be part of the weather story, with growing degree day accumulations well below normal during the last week of July (with barely more than 100 GDD at Champaign, Ill.) and again the third week of August. That changed to above-normal temperatures the last week of August, and since then, weekly GDD accumulations have been above normal, with the total for the first 10 days of September 234 GDD, nearly half the normal total for the month.

Despite the hot, dry weather of recent weeks, corn in many fields has retained some green color, at least in the upper canopy, and it appears that kernels continue to fill. That may not be the case in some of the drier parts of the state, where the crop either ran out of water earlier in August or where canopy color wasn’t very good even before that.

Many have commented on the poor leaf color in some fields, even in areas where there has been some rainfall. Some have reported that adjoining fields with the same hybrid, planted and treated alike, show differences in canopy color and crop condition. Reasons for this are often not clear, but might be related to soil condition at planting, differences in how well nitrogen stayed in the soil and available to roots, or root damage due to flooding or insects. I think that root-related problems are more likely to be issues than loss of N. As is sometimes the case when roots have issues, corn following corn may not perform as well as corn following soybean again this year.

The high temperatures of recent weeks along with drying soils have acted to move the crop toward maturity faster than we had expected. In most areas this is the result of high temperatures rather than of lack of water. At Champaign, GDD accumulations from May 1, May 15, and May 30 through September 10 totaled 2,760, 2,590, and 2,310, respectively. Mid-season hybrids used in central Illinois typically need 2,700 to 2,750 GDD from planting to maturity, so early May plantings should be at or near maturity by now. Those planted in mid-May should mature by September 20 or so, while corn planted in late May or early June is still be several weeks from maturity, unless dry soils bring on early maturity.

As we have mentioned before and as the information above indicates, late planting this year has not decreased the number of GDD required to reach maturity, probably because of the periods of low temperatures during the season. One of the main reasons that late-planted corn often uses fewer GDD than early-planted corn is that late-season stress causes loss of canopy photosynthesis and brings an early end to grainfilling. In such cases the crop almost always loses yield.

The good news this year is that in fields where the crop is taking its normal number of GDDs to reach maturity, yields are likely not to be diminished much by the recent heat and dryness. While there has been a considerable amount of "tip-back" (tip kernel abortion) in fields in drier areas, most reports in areas with some soil moisture have indicated that kernel numbers are good. Of course, it’s never sure until maturity that kernels will end up heavy enough to produce the yields that their numbers would suggest.

To see how kernel weight was progressing, I took some kernel samples early on August 26, at the early dent stage (beginning R5), and oven-dried them. This crop was planted at Champaign on May 15. Adjusted to 15% moisture, these kernels weighed 241 milligrams, which translates to about 105,000 kernels per bushel. By early dent, kernels are expected to have 50 to 60% of their final dry weight. Since these kernels were already more than three-fourths the weight (316 mg) of kernels at 80,000 per bushel, it appeared that the actual filling progress was running ahead of the visual indicators.

I sampled the same plots again late in the day on September 4, or about 10 filling days (about 250 GDD) after the first samples were taken. The milkline – the separation between hard and soft starch – was about halfway down the kernel. According to the Iowa State University publication "Corn Growth and Development" (PMR 1009), kernels at half milkline have accumulated about 90% of their maximum dry weight, are at about 40% moisture, and have about 200 GDD left to go before maturity. These kernels weighed 306 mg, or about 83,000 kernels per bushel, and they were at 31% moisture, which is considered typical for grain at physiological maturity. Thus it’s likely that these kernels were at or very close to their final weight.

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