Hybrid selection is a key step for profitable corn production. Since 1948, the average corn yield in Minnesota has increased steadily at a rate of two bushels per acre per year (Figure 1). This has been due to a combination of improved hybrids and agronomic practices, emphasizing the importance of hybrid selection. Although yield gains resulting from improved agronomic practices may be more difficult to obtain now than in the past, yields from 209 to 272 bushels per acre reported by Minnesota growers in the 2008 National Corn Growers Association yield contest (NCGA, 2008 [pdf]) indicate that crop management and growing conditions are major limitations in corn yield, and are likely the reason why yields of this magnitude are not more common.
Figure 1. Average Minnesota corn yields from 1948 to 2008. Source: USDA-National Agricultural Statistics Service.
In the northern Corn Belt, maturity is one of the most important factors to consider when selecting hybrids. Hybrid maturity is rated using the RM and/or GDD rating systems. With the RM system, hybrid maturity is expressed in terms of days. This does not represent the number of days required from emergence to physiological maturity. Instead, it is a relative measure of maturity for a given hybrid when compared to hybrids of known maturity. Although there is no industry standard for measuring RM, hybrid RM across companies is closely related to the number of GDDs required from planting to maturity. In general, a 95-day RM hybrid needs between 2,350 and 2,400 GDDs from planting to maturity, with each one-day increase in RM increasing the GDD requirement for a hybrid by an average of 22 GDDs (Figure 2).
Figure 2. Relationship between growing degree days (GDDs) required for physiological maturity and relative maturity for 480 corn hybrids from four companies. Each point represents the average of multiple hybrids for a given relative maturity.
The GDD system for rating hybrid maturity allows one to compare a hybrid’s GDD requirements with the number of GDDs that typically occur during the growing season for a given location and planting date (Table 1 and Figure 3). Daily GDDs are calculated based on daily high and low temperatures (measured in degrees Fahrenheit) according to the following equation:
GDD = [(High + Low)/2] – 50ºF
where “High” is the daily high (set at 86ºF if the daily high is above 86ºF) and “Low” is the daily low (set at 50ºF if the daily low is less than 50ºF).
Selected hybrids should reach maturity at least ten days before the first average frost (32ºF) (Hall and Reitsma, 2009 [pdf]). This will allow time for grain dry-down prior to harvest and also provide a buffer in case planting is delayed or GDD accumulation is behind normal. In addition, consider planting multiple hybrids of varying maturity, as this reduces the probability that one’s entire crop will experience hot and dry conditions during the critical pollination period.
Table 1 shows the average GDD accumulation until the first average frost (32ºF) for six planting dates at numerous locations across Minnesota. To adjust for ten days before the average frost, subtract those GDDs from the larger number at your planting date. While the number of GDDs available to a crop is reduced with delayed planting, it is important to realize that a hybrid’s GDD requirement is also reduced as planting date is delayed. For example, research from Indiana has shown that a hybrid’s GDD requirement is reduced by 6.8 GDDs for each one-day delay in planting beyond May 1 (Nielsen and Thomison, 2003 [pdf]).
Table 1. Average growing degree day (GDD) accumulation (1971-2000) for various planting dates, along with median dates of critical fall temperatures (1948-2005) across Minnesota.
|Planting date||Adjustment for 10 days of drying before 32°F||First fall frost|
|Location||April 20||April 30||May 10||May 20||May 30||June 9||32°F||28°F|
|GDD accumulation to first 32°F fall temperaturea||GDDa||Median dateb|
|Lamberton||2,596||2,540||2,458||2,348||2,210||2,046||109||Sep. 28||Oct. 7|
|Marshall||2,703||2,647||2,565||2,456||2,320||2,158||97||Oct. 4||Oct. 14|
|Pipestone||2,460||2,408||2,322||2,230||2,104||1,954||115||Sep. 24||Oct. 3|
|Redwood Falls||2,797||2,734||2,643||2,525||2,378||2,206||109||Oct. 2||Oct. 9|
|Worthington||2,440||2,394||2,322||2,224||2,100||1,951||96||Sep. 30c||Oct. 7c|
|Faribault||2,484||2,434||2,361||2,263||2,138||1,987||103||Sep. 29||Oct. 12|
|Mankato||2,624||2,568||2,487||2,379||2,246||2,088||100||Oct. 2d||Oct. 13d|
|Waseca||2,547||2,494||2,415||2,308||2,175||2,018||105||Sep. 30||Oct. 6|
|Winnebago||2,695||2,637||2,554||2,444||2,308||2,146||95||Oct. 6||Oct. 17|
|Preston||2,342||2,294||2,225||2,133||2,016||1,873||119||Sep. 23||Oct. 3|
|Red Wing||2,560||2,503||2,423||2,318||2,188||2,034||118||Sep. 26e||Oct. 4e|
|Rochester||2,378||2,329||2,258||2,163||2,045||1,904||94||Oct. 1||Oct. 12|
|Winona||2,690||2,633||2,553||2,447||2,315||2,158||91||Oct. 7||Oct. 20|
|Alexandria||2,316||2,271||2,202||2,109||1,995||1,860||83||Oct. 1||Oct. 12|
|Canby||2,713||2,656||2,573||2,465||2,329||2,169||105||Oct. 1||Oct. 10|
|Fergus Falls||2,328||2,282||2,211||2,117||1,999||1,861||92||Sep. 28||Oct. 8|
|Montevideo||2,559||2,506||2,409||2,326||2,196||2,042||102||Sep. 30||Oct. 7|
|Morris||2,474||2,422||2,345||2,241||2,114||1,964||96||Sep. 29||Oct. 6|
|Wheaton||2,531||2,481||2,407||2,308||2,184||2,034||91||Oct. 1||Oct. 10|
|Collegeville||2,660||2,601||2,516||2,405||2,271||2,116||94||Oct. 5||Oct. 18|
|Hutchinson||2,589||2,533||2,451||2,342||2,209||2,051||99||Oct. 1||Oct. 13|
|Melrose||2,415||2,368||2,296||2,197||2,074||1,926||106||Sep. 25||Oct. 5|
|St. Cloud||2,236||2,189||2,118||2,025||1,909||1,775||99||Sep. 24||Oct. 5|
|Staples||2,011||1,969||1,905||1,820||1,715||1,594||95||Sep. 22f||Oct. 2f|
|Willmar||2,525||2,472||2,395||2,291||2,162||2,009||89||Oct. 3||Oct. 15|
|Aitkin||1,904||1,869||1,812||1,735||1,639||1,525||82||Sep. 24||Sep. 30|
|Forest Lake||2,491||2,439||2,363||2,260||2,135||1,987||86||Oct. 5||Oct. 17|
|Hinckley||1,980||1,944||1,886||1,807||1,708||1,591||90||Sep. 22||Sep. 28|
|Rosemount||2,505||2,452||2,377||2,279||2,156||2,007||92||Oct. 4g||Oct. 14g|
|Crookston||2,245||2,201||2,131||2,037||1,919||1,781||98||Sep. 23||Oct. 2|
|Itasca||1,805||1,777||1,728||1,657||1,566||1,456||81||Sep. 20||Sep. 26|
|Moorhead||2,365||2,316||2,242||2,142||2,020||1,876||103||Sep. 24h||Oct. 3h|
|Warroad||1,935||1,906||1,855||1,782||1,686||1,568||77||Sep. 23||Sep. 30|
Figure 3. Average growing degree day accumulation from May 1 to September 30 (1971-2000). Courtesy of State Climatology Office - DNR Waters.
Within a given region, mid- to full-season hybrids generally have higher yield potential than early-maturing hybrids (Figures 4 through 9), as they utilize more of the GDDs available during the season. However, it has been rare in Minnesota for the very full-season hybrids to out-yield the mid-season hybrids. There is also more variability in yield among hybrids within a given RM rating than there is between maturity groups, further emphasizing the importance of hybrid selection. Another important consideration is that grain moisture at harvest increases steadily with increasing relative maturity (Figure 10). On average, grain moisture at harvest increases by 0.25 to 0.44% with each one-day increase in relative maturity (Figure 11). Selecting hybrids of appropriate maturity is important for a balance between yield potential and grain moisture at harvest.
Figure 4. Relationship between relative maturity and corn yield among hybrids in the southwest Minnesota corn hybrid trial at Lamberton in 2008.
Figure 5. Relationship between relative maturity and corn yield among hybrids in the southwest Minnesota corn hybrid trial at Lamberton in 2007.
Figure 6. Relationship between relative maturity and corn yield among hybrids in the southwest Minnesota corn hybrid trial at Lamberton in 2006.
Figure 7. Relationship between relative maturity and corn yield among hybrids in the southwest Minnesota corn hybrid trial at Crookston in 2008.
Figure 8. Relationship between relative maturity and corn yield among hybrids in the southwest Minnesota corn hybrid trial at Crookston in 2007.
Figure 9. Relationship between relative maturity and corn yield among hybrids in the southwest Minnesota corn hybrid trial at Crookston in 2006.
Figure 10. Relationship between relative maturity and grain moisture at harvest among hybrids in the southern Minnesota regional hybrid trials in 2008. Data are averaged over three locations.
Figure 11. Average increase in grain moisture at harvest with each one day increase in relative maturity for the southern, central, and northern Minnesota regional hybrid trials from 1998 to 2008. Data from each region are averaged over two or three locations.
Once an acceptable maturity limit has been defined, select those hybrids that are consistently top performers over multiple sites or years within a region. Yield stability over multiple trials with differing growing conditions is critical, since we cannot predict next year’s growing conditions. To reduce risk, hybrid selection should be based on information from multiple sources, including universities, grower associations, seed companies, and on-farm strip trials. Results from unbiased and replicated trials are of particular importance.
Past hybrid trial results are available from the University of Minnesota and from the Minnesota Corn Growers Association. Results are posted soon after harvest each year. Printed versions of the University of Minnesota trials are available from regional Extension offices and as a special report published each December in Agri-News. Links to corn hybrid trial results from neighboring universities are listed below:
Hybrids are turning over at a faster rate than ever before. Try new and unproven hybrids on limited acreage to see how they do in your fields. As more information becomes available, these hybrids can begin to play a larger role in your corn production system.
After a set of consistently high-yielding hybrids has been identified within your maturity range, further selection within this group can be based on specific agronomic traits. Choosing a hybrid with a strong balance of agronomic traits is important for managing risk, but the relative importance of individual traits differs with production practices and growing conditions. When selecting hybrids, important agronomic considerations include standability, tolerance to diseases and drought, the need for transgenic resistance to insects and herbicides within a given production system, and good emergence under cool conditions.
Standability is a common agronomic trait that is critical for ensuring that the grain produced is harvestable. Since corn has a narrow optimum plant population, unharvestable ears due to stalk and root lodging will have a large impact on yield. In a hybrid trial conducted in northwest Iowa in 2005 where severe lodging was present, each 1% increase in lodging reduced yield by an average of 0.5 bushels per acre (Elmore et al., 2006). Standability is particularly important for corn that will be planted at a higher population, or for corn that is likely to be under drought stress. This is because stalk diameter decreases with increasing plant population, and because drought stress favors stalk rot.
The standability of a hybrid in a given field can easily be evaluated using the late-season 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 [pdf]). 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.
Transgenic traits for insect resistance can be useful for protecting stalks and roots, but it is important to note that these traits alone will not influence yield in the absence of pressure from the target pests.
Emergence should also be considered when selecting hybrids. Early planting is important for high corn yields, but the soil in late April is often cool and wet, causing many stresses on young seedlings. This makes strong emergence a key agronomic trait, particularly for fields with heavy soil and abundant surface residue that will be planted early.
Elmore, R., L. Abendroth, and J. Rouse. 2006. Choosing corn hybrids [Online]. Available at http://www.agronext.iastate.edu/corn/production/management/hybrid/choosing.html (verified 21 Sep. 2009). Univ. of Minnesota, St. Paul.
Hall, R.G., and K.D. Reitsma. 2009. Corn hybrid selection. p. 9-12. In D.E. Clay, K.D. Reitsma, and S.A. Clay (ed.) Best management practices for corn production in South Dakota. EC929 [Online]. Available at http://agbiopubs.sdstate.edu/articles/EC929.02.pdf (verified 21 Sep. 2009). South Dakota State Univ., Brookings.
Malvick, D., and D. Nicolai. 2005. Corn stalk rots in Minnesota this year [Online]. Available at http://www.extension.umn.edu/cropEnews/2005/pdfs/05MNCN48.pdf (verified 21 Sep. 2009). Univ. of Minnesota, St. Paul.
National Corn Growers Association [NCGA]. 2008. State winners [Online]. Available at http://www.ncga.com/files/pdf/2008CYCStateWinners.pdf (verified 21 Sep. 2009). Chesterfield, MO.
Nielsen, R.L., and P. Thomison. 2003. Delayed planting and hybrid maturity decisions [Online]. Available at http://www.agry.purdue.edu/ext/pubs/AY-312-W.pdf (verified 21 Sep. 2009). Purdue Univ., West Lafayette, IN.
Thanks to the Minnesota Corn Growers Association and the Minnesota Corn Research and Promotion Council for their generous support of University of Minnesota corn production research and extension.
Published 25 September 2009, University of Minnesota, St. Paul.
In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.