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A Good Crop Takes Shape

Emerson Nafziger
July 23, 2014
Recommended citation format: Nafziger, E.. "A Good Crop Takes Shape." Department of Crop Sciences, University of Illinois at Urbana-Champaign, July 23, 2014. Permalink

The “walls” of dark green corn as the crop begins its push to fill grain and make yield is always an inspiring sight, and the 2014 crop is the best I’ve seen at stage of development. It’s been almost this good in June several times, but not at the end of July. The crop condition rating has been stuck at about 80% good to excellent, and this remarkable uniformity is apparent in travels around northern and western Illinois in recent weeks.

The same is true for the soybean crop, though the condition rating isn’t quite as high, and there are areas where the crop doesn’t look great.

Without many serious problems to look at, this is a good time to consider whether this crop will turn out to be as good as it looks, and what threats might linger as we move into the 6 weeks over which yield will be made:

  • Nitrogen: Despite a lot of rainfall in May and June (into early July in some places), the deep green color of the corn crop in most fields tells us that the crop is well-provided with nitrogen. While water moving through the soil has undoubtedly moved some of the nitrate deeper, the fact that adequate N has clearly been taken up has to be a consequence of having a healthy root system that is active deep enough to take up what N is there. Plots without N in rate trials are very deficient, probably due to both downward movement of N and root systems that are feeble due to poor N nutrition and growth of the crop. But the crop is green all the way to the lower leaves in fields with normal N rates, and so we don’t anticipate that the 2014 crop will require more than normal amounts of N. Recent N Watch samples show that soil nitrate values have dropped considerably over the past month, but that’s as is should be – we apply fertilizer N for the crop, not to accumulate in the soil. Uptake of N by the crop has slowed considerably as the crop moves past pollination, and we see little reason to worry about having enough N in the soil now. Mineralization of organic matter is providing N that will serve as backup, but the crop has taken up 80 to 85% of the N it will take up by the end of the season. Stress from hot, dry weather is about the only threat to the N status of corn plants beyond this point in the season, and with another stretch of cool weather coming now, chances of having this happen in time to hurt yields are diminishing
  • Water: I’ve likened the corn crop this year to last year’s crop about 10 days later than this; the 2014 crop was planted earlier and had normal temperatures through June, plus enough water to make good growth and to reach at least normal height. Excess water has certainly done some damage in some fields, and in cases the roots in low areas that were flooded for a week or more, they (and crop prospects in those areas) are damaged to the point where they won’t recover much. In 2013, there was in some areas little rainfall after August 1 all the way to crop maturity; the soils had to have provided the 8 inches or more of water required to fill grain after the milk stage of kernel development. This year, soil water is in better supply than it was at the beginning of August last year, and the cool conditions over the coming weeks will help stretch the water supply even further. The crop might benefit from some additional rainfall within the next month, but highs of only 80 or so plus lots of sunshine are excellent conditions for corn grainfilling, and with roots that have done a good job up to now, we don’t see much danger of having the crop run out of water.
  • Kernel number: We cannot produce good corn yields without high kernel numbers, even though in very good years kernels often get larger than they would otherwise. A starting point is maybe 15 million kernels per acre, which at 75,000 kernels per bushel (such kernels are a little above average, but not huge) would produce 200 bushels per acre. At 35,000 ears per acre, it takes only about 430 kernels per ear to make 15 million per acre. That’s not a large ear – at 16 rows of kernels, it’s only 27 kernels long. At 14 rows of kernels, it takes 4 more kernels per row to get to the same count. Someone recently mentioned the disappointment of finding ears with only 14 or 16 rows of kernels; it seems that people like to see 18 or 20 rows. The number of rows of kernels is influenced much more by hybrid genetics than by conditions in the field, and fewer rows are typically compensated by having more kernels per row, so have similar numbers of kernels per ear. This year, we may see kernel number approaching or exceeding 20 million per acre, which at 35,000 ears per acre would be 570 kernels per ear. At 14 rows that’s about 40 kernels per row, still easily within range of possibility. Kernel abortion should be less than normal, so kernels we can count by the time silks are brown will be mostly retained into grainfill, with any effect of stress late in the season seen as a reduction in kernel size.
  • Ear number: Stands are mostly good and ear numbers should be as well. There have been some reports of more than one ear per plant, including what Dr. Bob Nielsen calls “multiple ears on the same shank” or MESS (see The second (or third) ear on the same shank usually doesn’t produce kernels. In contrast, having ears at two separate nodes, with both producing grain, is the way that some hybrids today cope with having more than usual amounts of resources (sugar from photosynthesis) available before and at the time of pollination. Many older hybrids respond to this by increasing ear size, up to 800 or more kernels. Many hybrids today form a second ear instead. Making two smaller ears instead of one big one carries a little high “construction” cost, but with a shorter path from the stalk to the tip kernels, two smaller ears may have some advantage in getting the kernels filled if filling conditions are very good. To estimate yield potential of there are a lot of second ears, kernels per plant should be counted, with kernel size perhaps adjusted downward slightly if there are more than 750 or so kernels per plant. Should populations have been higher if there are 700 or 750 kernels per plant? Perhaps, but that many kernels may end up smaller, and not yield much less than more plants with fewer kernels each.
  • Canopy: If you can see down rows of corn, a good canopy will have healthy leaves interlaced to intercept nearly all of the sunlight, and you’ll find little light hitting the soil surface when the sun is high. If the crop gets enough water to maintain this canopy up to close to the end of grainfill, kernels will approach their maximum size (which has been determined by now in most fields, and with good conditions at pollination is likely to be large) and yields will be high. The corn crop needs about 1,300 growing degree days from pollination to maturity (black layer). At normal temperatures from mid-July into September, we get about 24 GDD per day, so getting 1,300 GDDs takes about 55 days. With temperatures on the cool side, at 55 nighttime and 80 daytime, that’s 17.5 GDD per day and it takes some 70 days to get to 1,300 GDD. With pollination nearly completed now, the crop should reach maturity by late September even if it stays this cool, but some warmer temperatures would be welcome, as long as it doesn’t go to the mid-90s and stay there. Cool nights are good, but 60 or 62 instead of 55 degrees would probably help more than hurt.
  • Standability: Having lots of kernels and conditions for potentially high yields often brings concerns about whether high yields might mean stalk quality problems later in the season. It is certainly the case that in the competition for sugars between developing kernels and stalk tissue, stalks usually lose. But when conditions are very good after pollination, as they were in 2013, plants are able to allocate enough sugars to help stalks deposit lignin, which is the woody material that gives stalks strength and helps keeps them standing even after the stalk tissue dies. So while there’s always some cause for concern about the possibility of high winds and downed corn, I see no reason for much alarm because we expect high yields.
  • So how high will yields be? This is the point at which we should humbly admit that we don’t know everything about the corn crop, that every year is different, and that we don’t want to guess. Facts in favor of a new record corn yield in Illinois in 2014 include a crop that’s in great shape now, above-average soil water stored in most areas, and lack of any evidence that a blocking high will set up to bring hot, dry conditions in time to shorten the filling period by much. But such hopes aren’t always realized, and it remains safer not to speculate on final yields.

I’ll continue the story on soybeans later, but will only note here that the dry weather we’ve had in recent weeks has been favorable for the early-planted crop, which had been starting to show signs of making the large leaves and tall plants that we have in the past found to produce mediocre yields. But the late-planted crop seems stunted in some areas, and lack of moisture could be limiting growth enough to affect canopy cover and yield potential. These problems are not beyond fixing in soybean fields, but at this point in the season what may be ideal conditions for the corn crop may be less than ideal for soybeans.

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