The crop progress report from NASS showed that on July 1, 40 percent of the Illinois corn crop was silking, the crop rating was 85 percent good + excellent (G + E), and more than 85 percent of Illinois was reported as having adequate or surplus soil moisture. While this combination is very supportive of prospects for high yields, questions remain about whether the crop is actually as good as it looks, and about how dry weather in the coming weeks might affect crop prospects.
Do high crop ratings point to high yields? In a series of articles on Farmdoc, Scott Irwin and Todd Hubbs have pointed out that U.S. crop ratings become better predictors of final yield as the season progresses, with the highest correlation reached by about the third week of July. They also showed that when high early ratings drop, most of this drop tends to occur by the time the crop pollinates, which is normally in mid-July.
The largest drop in the Illinois corn ratings we have seen in recent decades was in 2012, when 79% G+E on May 20 dropped to 26% on July 1, and to 5% by the end of July. That was the year of the worst drought since 1988, and by July 1 of that year, only 10% of Illinois soils were judged as having adequate moisture.
In only one of the past five years did Illinois corn condition ratings drop between late May and mid-July; that was in 2015, when very wet June weather resulted in ratings going from the upper 70s in late May to the mid-50s by mid-July. In 2014 and 2016, ratings rose from the low 70s in late May to the lower 80s by mid-July, while in 2013 and 2017, ratings rose from the upper 50s to the mid-60s over the same period. With the help of the very low rating and low yield (105 bushels per acre) in 2012, correlation between mid-July rating and yield was fairly good over the past 6 years: yields were in the mid-170s in 2013 and 2015, and close to 200 the other three years. The most “out of line” year was 2017, when cool weather in August canceled out the mediocre crop ratings to give Illinois the highest average yield on record.
In the five years before 2012, early ratings were high only in 2007, and they stayed high that year. Ratings bounced around some in the other four years, and in 2011 went from the 60s in early June to 40% by August. But in none of the last 11 years did high ratings in late June or early July turn into low ratings by August, followed by low yields. So we have no precedent for expecting the crop rating to come crashing down this year, and for yields to be low. When we have had low yields, crop ratings at the time of pollination have usually predicted that this is going to happen. Nothing about the crop in 2018 suggests that this year will be different.
After a cool April, the weather turned warm in early May and has stayed warm ever since. As a result, growing degree day accumulation rates have been very high: GDD totals through the end of June were some 300 above average in Illinois, totaling about 1,200 since May 1 in northern Illinois, 1,400 in central Illinois, and 1,500 in southern Illinois. With about 1,400 GDD needed to reach pollination in most hybrids, the 40 percent figure for silking reported for July 1 was probably low, and by July 8 we should see silks on 75% or more of Illinois fields.
GDDs used to track corn development have 86 degrees as the cutoff, meaning that with the same night temperature, a daytime high temperature of 95 degrees produces the same GDD as a high of 86 degrees. This means that above-normal GDD accumulations during the summer result more from above-normal night temperatures than from high daytime temperatures. With night temperatures running in the 70s as July gets underway, we have been adding about 200 GDD per week. This will moderate some with slightly cooler temperatures forecast for July 6-8, but unless the temperature pattern turns consistently cooler soon, we are on pace to reach the 2,700 or so GDD required for mid-season hybrids to reach maturity by the third week of August in central Illinois: 1,400 GDD by July 1 plus 180 per week (a little higher than the average for July) means only about 7 more weeks to crop maturity. A couple of stretches of below-normal temperatures like we saw in 2017 would delay maturity, but would very likely add some yield. This weekend will be such a stretch, but a very modest one.
High temperatures have meant rapid crop development, and while the crop currently has good leaf color and is in good condition in most fields, will continued high temperatures cause problems and lower yields? Good plant height and good crop color now indicate that high temperatures have not been a problem so far. Sunshine amounts were above normal in most of Illinois in June, which helped boost growth. And periods of dry weather have resulted in good root growth, except where water has stood in wetter parts of the state.
This is a good time to remember that plants aren’t people – corn plants have no problem with high temperatures in the 90s, and they are more or less unaffected by what the “heat index” might be. Low temperatures in the 70s mean high dewpoints, which can mean wetter leaves in the morning and higher nighttime respiration, both of which are negatives. Good nitrogen uptake as gauged by leaf color (except where water has stood) and lots of sunshine have been positives, though, and on balance the crop condition and our eyes tell us that the 2018 corn crop is in great shape over most of Illinois.
The worry persists that rains will stop and that heat will build, causing rapid deterioration of the crop and stopping grainfill before maturity. We can’t rule this out, especially in the region with a radius of about 50 miles centered on Quincy, where June rainfall was less than normal. Elsewhere, the water in the soil now will, in fields with medium or heavier-textured soils, provide good protection against crop water shortages in the coming weeks. The crop coefficient, which is the percentage of evaporation that goes through the crop and out the leaves as water vapor, is at its maximum of about 80% at pollination. It will remain high for a few weeks before starting to decline.
Daily evaporation amounts on hot, sunny days are in the range of 0.25 to 0.3 inches, so crop water use (transpiration) on such days with the crop at full canopy like it is now is about 0.2 to 0.25 inches. A quarter acre-inch of water is about 6,750 gallons, so in one day each plant in a field with 35,000 plants per acre takes up and transpires about 3 pints of water. While we sometimes say that plants “lose” this amount of water, having the stomata wide open to allow water vapor to exit also means allowing maximum amounts of carbon dioxide into the leaf, where the carbon is used to make dry matter through the process of photosynthesis. Dry matter production can exceed 500 pounds per acre on a good day, so losing water means making yield.
Our deeper prairie soils hold as much as 8 to 9 inches of plant-available water within reach of healthy roots systems. That amount would supply the crop for a month of high-demand days. So unless there’s no rain at all over the next month, a lot of the Illinois corn crop already has enough water in the soil to set kernels in the coming weeks and to begin the filling process that will end with maturity.
While we know from experience that unexpected problems can come up that threaten to decrease yields, it’s hard to imagine having Illinois yields end up being below normal after the first half of the season we’ve had in 2018. Leaf diseases haven’t blown up in most fields, and as the crop moves past pollination and into grainfilling, the threat of disease outbreaks decreases. Insects such as corn rootworm adults and Japanese beetles will emerge too late to affect pollination. Weather threats like hail have occurred and will occur again, but the size of affected areas is normally small, and hail means rain, whose benefits usually exceed losses from hail over a wide area. With a crop that should be mature by early September, frost is a non-issue. Finally, the fact that it has rained in places in the last few days means that the pattern has not yet turned as dry as some feared that it might.
As is always the case, we will know the potential for yield once we can count developing kernels, by the third week of July in the early-pollinating fields. We won’t reiterate the process here, but it’s a simple one: ear number per acre x kernel number per ear gives kernels per acre. If that number is 16 to 20 million, good yields will likely follow.