Late-Season Dry Weather and Corn

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, 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, 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) 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.

Kernels at 83,000 per bushel are considered to be normal sized, so the yield potential for this plot was probably realized, even though there has been no appreciable rainfall here in Champaign-Urbana for more than 6 weeks. These plots had an average population of about 40,000 plants, and at about 500 kernels per ear (20 million kernels per acre), the yield estimate on September 4 (20,000,000÷83,000) came to about 240 bushels per acre. This is a little dangerous due to the small sample size, but if it’s accurate it means that in 10 days of fill, the crop added about 91 bushels per acre, or about 9 bushels per acre per day. This is in line with previous rates we’ve measured, and it shows a well-functioning crop.

I’ve heard a number of comments coming from drier areas that “test weight” was going to be hurt by dry conditions, with yield lowered as a result. When kernel fill stops before kernels have as much starch as they can hold, kernels tend to be “shrunken” on the end that attaches to the cob. When canopy photosynthesis decreases and the supply of sugars starts to run out by late dough, starch tends not to pack normally in the kernel, and this can lower the density of the starchy part of the kernel – the endosperm. Misshapen kernels that don’t fit together well and kernels that are less dense than normal both contribute to lowering of test weight.

Of course, an early end to starch accumulation means lowered kernel weight. And lowered kernel weight, not lower test weight, directly translates to lower yield. It is certainly the case that test weight and kernel weight are often related, as explained above. But yield is the product of kernel number per acre and weight per kernel, while test weights are often not well-correlated with yield level, unless of course stress lowers both at the same time.

It’s time to remind ourselves that “black layer” – the darkened layer of cells at the tip of the kernel that indicates that the tissue that transfers sugars into the kernel is no longer active – always forms in corn, whether or not the grainfilling process come to its natural end or ends early due to canopy loss. For practical purposes, the disappearance of the milkline at the base of the kernel means that little or no additional weight will be added to the kernel.

While we would have preferred normal rainfall and normal temperatures during August, the periods of cool weather did help stretch the supply of soil water, which helped plants fill grain even under the recent spell of high temperatures. Any green leaf area on plants means that they are capable of producing sugars, and green leaf area that persists even after some weeks of stress conditions indicates that the crop has had access to at least some water. I also think that we are seeing the advantage of cool night temperatures for some periods during grainfill, and that this has translated into slightly better than normal photosynthetic efficiency.

I’ve also heard comments to the effect that low solar radiation might have decreased photosynthesis and yield potential in 2013. It’s certainly true that sunlight amounts have been low in parts of the 2013 season: according to the Illinois State Water Survey (ICN data), solar radiation in July 2013 was 20% less than the average of the previous three years, and in August was 13% below the 2010-2012 average. These two months were warm and dry in all three previous years, with the exception of August 2012, but sunlight amounts in 2013 were considerably less than normal.

Any effect of the lack of sunlight might have had on photosynthesis was moderated by the fact that the 2013 crop was behind in its development and the weather was cool in parts of July and early August. Conditions during and after pollination generally resulted in good kernel numbers, and by the start of rapid grainfilling, when the amount of daily photosynthesis becomes the critical factor for yield, sunlight had returned to more normal levels. This, along with higher temperatures, meant high rates of photosynthesis in fields with enough soil water. As is always the case, more sunlight usually means less rain, and on balance water tends to be more limiting than sunlight, so depending on when it happens and whether it means more rain, low sunlight is not always a negative factor.

Another concern as we head towards harvest is how well stalks will stand in fields where stress at the end of the season means an early end to grainfilling. Keeping stalks healthy depends on having enough sugars in the stalks up to the end of the grainfilling period. In years like this, when stress lowers the production of sugars, stalks can lose sugars to the ear, and stalk health can suffer. One factor that may counter this to some extent in 2013 is that the favorable conditions in July seem to have allowed the plant to produce more than normal amounts of lignin, which is the woody material that strengthens stalks. If there is enough lignin in stalks, they will often stand well even there is not enough sugars to keep their cells alive. One piece of supporting evidence for this is the fact that wind storms have tended to push stalks over (root lodging) this year, but not broken the stalks. Still, it pays to check stalk strength at or before maturity to identify fields for early harvest.

Corn drydown rates should benefit considerably from earlier maturity brought on by high temperatures. It helps that husks seem to be drying well as maturity comes on; this usually means that they will loosen so air can reach the grain to help drying. We can expect drying rates of a half point or more per (warm, sunny) day in September, but this will slow as the weather cools and if rain returns. As we often see when it’s dry in September, grain moistures often drop more quickly than expected.

Finally, I’ve had a few questions about the potential for aflatoxin problems given the recent dry weather and high temperatures. While there are no guarantees, the Aspergillus fungus that produces aflatoxin infects kernels in mid-season, and tends to be favored by hot, dry conditions and plant stress. Conditions in July 2013 were not hot and dry like those in 2012. We hope this means that we won’t see the problem this year.