By Jeff Coulter, Extension Corn Agronomist
The unusually warm and dry spring has facilitated some of the earliest corn planting dates of all time in Minnesota, with an estimated 63 percent of the state's corn crop planted and 1% emerged by April 25 (USDA, 2010). This early start to the growing season should allow the corn crop to pollinate and fill grain earlier than normal, hopefully under more favorable soil moisture levels. From 1966 to 2009 at the University of Minnesota Southwest Research and Outreach Center in Lamberton, plant-available soil water to a depth of five feet averaged 5.22 inches on July 15, compared to 4.32 inches on August 15 (SWROC, 2010). Early corn planting dates should also allow kernels to fill when day length is longer, reduce the risk of a fall freeze before crop maturity, and allow increased time for grain dry-down prior to harvest.
Figure 1. Early-planted corn off to a vigorous start.
Early-Season Frost Damage:
Early corn emergence dates increase the risk of spring frost damage, especially in low-lying areas in the northern part of the state. Dates of the last spring frost for numerous locations in Minnesota have been compiled by the Minnesota Climatology Working Group, and are available at: http://climate.umn.edu/text/historical/frost.txt.
Frost damage can occur when air temperatures are in the mid-30s on calm nights, as the lack of wind allows the transfer of heat from air near the ground to the air above, resulting in colder temperatures near the soil surface. In general, frost damage tends to be worse in low areas where cold dense air settles, near field edges where vegetation reduces the potential for heat transfer from the soil to the air above, and in fields where high levels of surface residue coverage limit heat transfer from soil. In addition, fields that were recently row-cultivated prior to cold temperatures are more susceptible to frost injury, as tillage dries the surface soil, thereby reducing the amount of heat and moisture that can be transferred between the soil and air. Thus, row cultivation should be delayed if the weather forecast indicates a potential for frost damage.
Symptoms of frost-damage to corn are initially dark and water-soaked leaves, which later dry and turn brown. Since the growing point of the corn plant remains below the soil until the fifth to sixth leaf-collar stage (Nielsen, 2008), frost prior to this stage typically does not kill the plant unless temperatures are low enough to freeze the upper part of the soil where the growing point is located. Frost-damaged corn plants generally show new leaf growth a few days after the frost if their growing point was not damaged, so assessment of damaged fields should be delayed until 3 to 5 days after the frost (Nielsen, 1998).
To determine whether frost-damaged corn will survive, dig up plants and split stems to examine the growing point and the tissue directly above the growing point. Healthy growing points will be firm and white to yellow in color. If the growing point or plant tissue within 0.5 inches above the growing point is damaged, it will be watery and orange to brown in color, and the plant will not likely recover (Carter and Wiersma, 2006). In general, crop recovery tends to be greatest when frost occurs before the third leaf-collar stage or when only a limited amount of leaf area is damaged after the third leaf-collar stage, since recovery is influenced in part by the amount of energy reserves in the seed and leaf area for growth.
Yield Reductions Due to Frost:
Yield loss due to early-season frost damage is influenced by the reduction in plant population and the severity of plant damage. In Minnesota, growers can expect yield losses of 5, 12, and 24% when the final plant population is reduced to 28,000, 22,000, and 16,000 plants per acre, respectively (Coulter, 2009). In addition to reductions in plant population, growers should consider the intensity of frost damage on survived plants. Recent research from southern Wisconsin found that yield was reduced by 8% when all corn plants were cut off at the soil surface at the second leaf-collar stage, but that yield reductions were minimal when only half of the plants were cut off (Lauer, 2007). In addition to yield losses, frost damaged plants may reach maturity a few days later than normal.
Before replanting, growers should consider the yield potential of the existing crop, replanting costs, and the yield potential of a replanted crop. Replant costs including time, fuel, seed costs, and penalties associated with hybrid selection when the best genetics are no longer available. Yield potential of a replanted crop will also be influenced by planting date. Corn planting date studies from 1988 through 2003 conducted at the University of Minnesota Southwest Research and Outreach Center in Lamberton show that on average grain yield was reduced by 5 and 18% when corn planting date was delayed until May 15 and May 30, respectively (Coulter, 2010). If replanting, growers should also consider the length of the growing season that remains and select hybrids of appropriate maturity. The number of growing degree days available for corn production for various planting dates and locations in Minnesota, along with the relationship between growing degree days required for crop maturity and hybrid relative maturity is available at: http://www.extension.umn.edu/distribution/cropsystems/M1276.html
Carter, P., and D. Wiersma. 2006. Early season frost damage to corn. Available at http://www.semomfa.com/images/E0123901/PioneerArticle.pdf (verified 1 May 2010). Pioneer Hi-Bred International, Inc., Johnston, IA.
Coulter, J. 2010. Plan now for successful corn planting. Available at http://blog.lib.umn.edu/efans/cropnews/2010/03/plan-now-for-successful-corn-p.html (verified 1 May 2010). Univ. of Minnesota, St. Paul.
Coulter, J. 2009. Optimum plant population for corn in Minnesota. Available at http://www.extension.umn.edu/distribution/cropsystems/M1244.html (verified 1 May 2010). Univ. of Minnesota, St. Paul.
Lauer, J. 2007. Frost impact on corn at early growth stages. Available at http://corn.agronomy.wisc.edu/WCM/W192.aspx (verified 1 May 2010). Univ. of Wisconsin, Madison.
Nielsen, R.L. 2008. Growing point location in corn at different growth stages. Available at http://www.agry.purdue.edu/ext/corn/news/timeless/GrowingPointsGallery.html (verified 1 May 2010). Purdue Univ., West Lafayette, IN.
Nielsen, R.L. 1998. Assessing frost damage to young corn. Available at http://www.agry.purdue.edu/ext/corn/news/articles.98/p&c9818.html (verified 1 May 2010). Purdue Univ., West Lafayette, IN.
Southwest Research and Outreach Center (SWROC). 2010. Available soil water vs. historic average (1966-2009). Available at http://swroc.cfans.umn.edu/Weather/Charts/Soil/2010/10_soil_water.htm (verified 1 May 2010). Univ. of Minnesota SWROC, Lamberton.
USDA. 2010. Minnesota ag news - crop weather. 26 Apr. 2010. Available at http://www.nass.usda.gov/Statistics_by_State/Minnesota/Publications/Crop_Progress_&_Condition/cw042610.pdf (verified 1 May 2010). USDA-National Agricultural Statistics Service, St. Paul, MN.