By Emerson Nafziger
With the exception of a few cool and wet periods in May and some areas of southeastern Illinois that stayed wet and were planted late, the 2016 growing season has been very good so far. The Illinois corn crop was planted a little earlier than normal, stands are excellent, and the crop has had outstanding leaf color throughout the spring and into mid-summer. On July 24, 82% of the crop was rated good or excellent. That matches the late July rating in 2014, the best-ever Illinois corn crop at 200 bushel per acre.
Virtually all of the corn crop has completed pollination, and the half that pollinated by July 10 is now into the “roasting ear” stage (R3), with some already in the dough (R4) stage. With good soil moisture in most fields, kernel numbers are not likely to decrease due to kernel abortion, so the number of kernels in a field now is likely to be close to the final number.
As is always the case, yields are going to depend on two things: kernel counts and canopy. Counting kernels may not be great fun on a warm day, but it’s a straightforward exercise. Simply get a good count of the number of ears per acre (most people do this in 1/1000th of an acre, which in 30-inch rows is 17 ft. 5 in. of row) and then count the number of rows of kernels and the number of kernels in an average row to give kernel number per ear for several ears. Making these counts a few times across a field, and going to those spots randomly will give more realistic estimates. If the goal is to brag, choose the best spots to count. But decency requires that you move into at least the fourth row in from the outside of the field; outside rows have “indeterminable” row spacing so can’t be used to calculate yield.
Once you have the number of ears in 1/1000th of an acre and the kernel number per ear averaged over at least three ears, simply multiply those two numbers to give the number of kernels per 1/1000th of an acre. Typical numbers in a good crop might be 34 ears x 500 kernels per ear, or 17,000 kernels (17 million kernels per acre.) The tricky part of the yield estimate is trying to guess how large the kernels will get. The default we usually use at this point in the season is 80,000 kernels per bushel. We drop the 000s since kernel count is for 1/1000th of an acre, and divide kernel number by kernels per bushel. In our example, that would be 17,000 divided by 80 = 212.5 bushels per acre.
To fill kernels to their maximum will require a full crop canopy that stays green and active up until close to maturity. With warm temperatures continuing, the early-planted crop is on track to mature (reach kernel black layer) by late August or early September. So we need the canopy to maintain its color and activity (the two are very nearly the same thing) for 4 to 5 weeks more.
Leaves that turn from dark green to yellow have lost most of their photosynthetic capacity, so any lightening of canopy color is a concern. You can see canopy color loss by using a drone or otherwise getting above the crop, but a better way is to walk into the field around mid-day to see if the amount of light passing through the leaves – that is, how well-lighted the ground seems to be – has increased from what it was a week or two earlier. Reading a newspaper placed on the ground should require some squinting if the canopy is good; if it’s easy to read, too much light is getting through to the ground. Dark at ground level means the crop is intercepting 97 or 98 percent of the sunlight. We expect yield to drop by as much as 2 percent for each additional percent of light that gets through the canopy.
A number of things can cause loss of canopy light interception. One that many people worry about is having the crop “run out of nitrogen” during grainfill. The good news is that a corn crop growing on average or above-average Illinois soils, and that had enough N to stay green up to pollination, is virtually guaranteed not to run out of N during grainfill. That includes nearly all corn fields in Illinois in 2016.
Canopy color has been outstanding in most fields this year due to good mineralization, adequate (to more than adequate) N fertilizer used, and little potential for N loss in the spring. Crop N needs drop quickly once the crop is past pollination, and by now the crop in most fields is taking up no more than a pound of N per acre per day. Mineralization rates typically exceed that amount at this time of the season, so the crop needs little or no additional N from fertilizer from here on out. Much of the N applied late in fields with dark green plants was unnecessary, and we can expect some of it to exit the field through tile lines.
The main cause of loss of canopy during grainfill is having the soil get dry enough to restrict water availability to the plants. Illinois is generally well-supplied with soil water now, but with uneven distribution there are likely some areas where the water supply may be dwindling to the point that canopy function is starting to slow. Plants during grainfill may not show the distinct leaf rolling that we see with water stress before pollination. Instead, leaves exposed to the sun at the top of the plant may start to lose their green color, and this loss may continue until it rains or until leaves start to dry up.
As plants start to lose photosynthetic capacity as soils dry, they often show characteristic “firing” as leaves, starting with those closest to the ground and moving up, begin to transport their N to the upper leaves and into the ear. This is a defensive mechanism that results in some seed production as photosynthetic activity declines or stops. Because N loss from leaves means development of N deficiency symptoms, there’s a tendency to believe that having more N in the soil would have prevented firing. That’s simply not true: leaf firing results from having too little water available, and has nothing to do with the amount of N in the soil. In fact, high fertilizer N rates can increase plant and canopy size, and larger plants use water more quickly, which can trigger earlier and more severe firing.
Foliar diseases can also take a toll on the crop canopy, decreasing both yield and stalk integrity, with greater potential for stalk rot to occur later in the season. Foliar diseases seem to be fairly low this year, but scout to make sure, and consider fungicide use if diseases begin to move up the plant in the next week or so. The amount of benefit from using fungicides decreases as grainfill proceeds, and once the kernels are into the dough stage the cost may exceed the benefit.
Anthracnose leaf blight is a fungal disease that can appear during grainfill on susceptible hybrids, and when it appears in August, control by foliar fungicides may not be very good. Goss’s wilt, which can also destroy leaf area, is a bacterial disease against which fungicide has no activity. If crop canopy is physically removed by hail, no repair is possible. Insects very rarely cause extensive defoliation in a corn crop during grainfill, but scouting will help you know if there’s a problem developing.
If we continue to get enough rain and the canopy stays healthy, will the crop be as good as it looks? Yes. I’ve heard reports of kernels counts of above 20 million per acre, which would be 250 bushels at 80,000 kernels per bushel. If conditions deteriorate and the canopy declines before maturity, kernels may end up lighter and yields lower. But if conditions through August remain favorable and the canopy stays intact, kernels can get larger than this and yields higher.
What could go wrong with this year’s crop? Wind and hail always come to mind, but the frequency of these events decreases later in the season, and the fact that these have not been widespread up to now is a plus. A sudden onset of high temperatures without rainfall would limit yields in a lot of fields, but better soils in many areas have enough water today to keep the plants going for several more weeks, as daily water use rates are starting to decline. So this would be less devastating than we might expect. Late-developing foliar diseases could end grainfill early, but their larger effect, like that of late-season drought, would be to result in depletion of stalk reserves as ears take in sugars faster than the plant is producing them. This may not greatly diminish grain yields, but depleted stalks are more susceptible to stalk rots, and affected fields could lodge earlier.
With the finish line moving into sight, it’s clear that the crop has an unusually good chance to yield above trendline this year. But as in most things, there’s no guarantee.