Late-season dry weather raises concerns
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 ½ 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.