By Ryan Van Roekel, Research Assistant and Jeff Coulter, Extension Corn Agronomist
With the 2009 growing season winding down, now is the time to prepare for corn harvest and start thinking about next year. Volunteer corn in soybean fields serves as a reminder to properly adjust combines. In addition to being an eyesore, volunteer corn can cause significant economical losses through added weed control costs and lost bushels at harvest. It only takes two corn kernels on the ground per square foot to equal one bushel per acre (Carlson and Clay, 2002), and that's not to mention dropped whole and partial ears that contain hundreds kernels. For specific suggestions on combine adjustment, see the article from Mark Hanna of Iowa State University: http://www.extension.iastate.edu/CropNews/2009/0903hanna.htm
Yield Monitor Calibration:
Another component of most combines is a yield monitor. This valuable tool must not be forgotten in harvest preparations. Yield monitors provide vital data with which to make hybrid and maturity comparisons. The well-below normal rainfall in much of Minnesota this year will likely increase variability in grain yield and harvest moisture within and among fields, making proper calibration of yield monitors even more critical. For specific information on yield monitor calibration, see the following article from Bob Nielsen of Purdue University: http://www.agry.purdue.edu/ext/corn/news/timeless/YldMonCalibr.html
Harvest preparation should also include a walk through the fields to determine stalk strength. This is particularly important in fields that have experienced considerable moisture stress. Pushing plants, pinching lower internodes, and splitting lower stalks will indicate whether stalk strength has been compromised by stalk rot. Stalk strength can easily be evaluated using the push test, where plants are pushed 45 degrees (about 10 inches) from vertical at ear level. Plants that break following the push test are at risk for stalk lodging. Stalk strength can also be evaluated by pinching the lower stalk at the first internode above the brace roots, as hollow and deteriorated stalks will easily collapse when pinched.
With both the push and pinch tests, a minimum of 20 plants should be tested in five representative locations within a field (Malvick and Nicolai, 2005). Fields with 10 to 15% or more of the plants failing the push or pinch test are at risk for severe stalk lodging, and should be put on top of the harvest list to prevent harvesting downed corn later. For a complete set of photos and more discussion, see the following article from Bob Nielsen of Purdue University: http://www.agry.purdue.edu/ext/corn/news/timeless/StalkHealth.html
Yield can be estimated prior to harvest using the following procedure, but accurate yield estimation requires multiple samples from representative locations throughout a field (Nielsen, 2008).
1) Step off the length of row equal to 1/1000th of an acre (17 feet 5 inches in 30-inch rows or 23 feet 9 inches in 22-inch rows) and count the number of harvestable ears in that length of row.
2) Husk every fifth ear (only husk representative ears), and count the number of rows per ear (always an even number) and the approximate number of kernels per row.
3) Calculate the average number of rows per ear and kernels per row from the husked ears.
4) Estimate the number of kernels per ear by multiplying the average number of rows by the average number of kernels per row.
5) Multiply kernels per ear by the number of harvestable ears in 1/1000th of an acre to estimate the number of kernels per acre.
6) Divide the number of kernels per acre by 90 (assuming 90,000 kernels per bushel) to get bushels per acre. To be even more conservative and assume poorer grain fill, divide by a larger number such as 95. If you think grain fill has been exceptional and that kernels are larger than normal, consider dividing by a lower number such as 85.
When pulling back husks, one may find poor tip fill (Figure 1). This can be caused by poor pollination and kernel abortion. For example, the ovules may not have been pollinated due to silk clipping by insects, delayed silk emergence, silk deterioration due to heat or drought stress, or a lack of viable pollen due to the same heat or drought stress. Kernel abortion can be caused by stress resulting from heat, drought, or severe nutrient deficiencies (Nielsen, 2003). Tip fill correlates with plant population (Figure 1), and the optimum plant population will typically have about one inch of poor tip fill (Lauer,2006), as grain yield is a balance between plants per acre and kernels per ear.
Figure 1. Reduced tip fill associated with increased moisture stress at higher plant populations.
Photo by Jeff Coulter.
Crop Maturity and Early Frost:
The USDA crop report on September 14 showed 56% of Minnesota corn to be in the dent stage, compared with 78% last year (USDA-NASS, 2009). Kernel moisture at the onset of denting is around 59%, and it generally takes about an additional 25 days from the start of denting to reach physiological maturity (black layer). A light frost killing only the leaves at the half milkline stage can be expected to reduce final grain yield by 5%, while a hard frost killing the whole plant at this same stage would reduce final grain yield by about 10% (Coulter, 2008). Average long-term dates of critical fall temperatures for various locations in Minnesota are listed in Table 1.
Table 1. Median dates of critical fall temperatures (1948-2005).
Remember that no matter what happens this fall, now is the time to be thinking about next year. When selecting hybrids, spread your risk by planting multiple hybrids of differing maturity. The key is to identify and select those hybrids which are consistently top performers over multiple sites or years within a region. The importance of consistency over multiple environments is critical, since we cannot predict next year's growing conditions.
Hybrid selection should be based on information from numerous sources, including universities, grower associations, seed companies, and on-farm strip trials. Past hybrid trials are available from the University of Minnesota at http://www.extension.umn.edu/Corn/trials.html and the Minnesota Corn Growers Association at past year trial results. Results from this year's trials will be posted soon after harvest. When in the combine this fall, watch that freshly calibrated yield monitor and start thinking about hybrids for next spring. Success next year begins with decisions made this fall.