Issue No. 17, Article 5/July 20, 2007
Corn During Grainfill
Rainfall this week has eased concerns about lack of water reducing corn kernel number, and this year should be one with relatively high kernel counts per acre. That means that yields will depend mostly on how well kernels fill before the crop reaches maturity.
Here are things to watch as the crop goes through the process of producing sugars in the leaves and moving the sugars through the stalk to the ear and into kernels, where conversion to protein, oil, and (mostly) starch takes place:
- In general, silks are numerous and lengthy this year, indicating that in many fields silking started a little before pollen started to shed. That usually improves the success of pollination. You can check kernel number by about 10 days after pollination. Remove husks and gently pull on the silks or shake the ear. Silks that remain attached to the "kernel" indicate a kernel that did not get pollinated. Count the number of kernel rows and the average number of kernels in a row, and multiply to get kernel number per ear. Multiply this by the ear count to get kernels per acre. Kernel numbers between 14 and 18 million per acre are normal. The goal is to have enough kernels so that yield is limited by the extent to which kernels can fill, rather than by kernels' reaching their maximum size, as can happen when kernel counts are low.
- While there can be kernel abortion--failure of kernels to fill after they were pollinated--adequate moisture this year should minimize this in most fields. Watch, though, for insect feeding that can remove some kernels after they have started to fill.
- Once kernel numbers are established as adequate, yield will depend almost entirely on how heavy kernels get. This is almost completely determined by the amount of sugars the leaves can produce and send to the developing kernels. Every trip to the field from now to the end of the season should include an effort to see how the health and color of leaves and canopy (number, size, and arrangement of leaves) are holding up. Assess individual leaves for color, indicating nutrient status, and for the presence of diseases that can limit effective leaf area. Insect injury to leaves during grainfill is also possible but less common. Grasshoppers can eat leaves, usually in rows on field edges.
- The completeness of the canopy can be assessed by noting how much sunlight reaches the soil when the sun is high, during midday. If it helps, lay a sheet of white paper on the ground and estimate what percentage of the sheet has sunlight. If it's not windy, you can even outline the sunlit areas with a pencil to better estimate their area. In a complete canopy, sunlight should strike no more than 3% of the ground area under the crop. Areas shaded by the canopy should also be shaded well; light green leaves or only single leaves will not intercept as much of the sunlight as is ideal, and as a result the shaded area beneath the canopy will look light rather than dark. Bright days work better for this than hazy days, because haze tends to disperse light.
- Because yields are usually limited by how well leaves function over a period of about 40 days--from the start of rapid grainfill about 12 to 14 days after silking to the slowing of grainfill several days before maturity. That means an average of about 5 bushels is added per day for a 200-bushel crop. The maximum is about 10 bushels per day, but this ranges down to almost no fill if the crop is under stress or if there is little sunlight during a day.
- The amount of sunlight that falls in a day is a critical component of "yield making" in corn. The Illinois Climate Network of the Illinois State Water Survey maintains a record of daily solar radiation. In July 2006 at Champaign, the average for the month was about 80% of the maximum attainable if every day were clear. This ranged from about 70% to about 83% over the years 1998 through 2006. The lowest year, however, was 2004, the year of record-high Illinois corn yields. This indicates that lack of water (associated with high sunlight) is more often limiting to yield than lack of sunlight. Still, on a daily basis, how much grainfill takes place is tied to the amount of sunlight that day. The minimum amount of sunlight on a cloudy day is generally about one-fifth of the maximum, and we would expect a long period of low sunlight to affect yield, all else being equal.
- Stalk quality, and to some extent leaf quality, is also closely tied to photosynthetic activity. In a sense, stalks get "leftover" sugars for maintenance, since the ear draws so strongly on the sugar supply. So if the canopy remains healthy throughout grainfill, stalk quality usually remains good, and stalks are able to build up some of the woody material (lignin) that will help them stay standing after maturity. Well-nourished stalks also maintain cellular integrity better, and so are less likely to be invaded by disease organisms.
- Roots generally start to decline in growth and function after pollination, and their maintenance during grainfill also depends on how well the canopy functions. A good supply of sugars in the lower stalk usually results in good brace roots, and well-maintained roots are less susceptible to root (and stalk) diseases that invade through roots. It can be helpful to try pulling plants in the second half of grainfill to see how well the root system is maintained. While roots only have to be "good enough" to see the plant through until maturity, compromised roots can mean more root lodging and less-than-ideal uptake of water and nutrients as the crop nears maturity.
While most signs remain positive for the Illinois corn crop in 2007, a return to unfavorable (hot, dry) weather, or disease (even foliar fungicides do not protect directly against diseases that invade through the roots) could result in rapid deterioration of the canopy and a rapid end to grainfill. The earliest-pollinated crop is now starting into the milk (R3) stage, and kernels will fill rapidly from now on. The race is on to see whether the plants can remain healthy long enough to maximize the considerable yield potential in fields now.--Emerson Nafziger