The June 23, 2008 USDA NASS report stated that 11 percent of Iowa's corn crop was replanted or is to be replanted. Eight percent was flooded. Governor Culver declared on June 20 that three million acres of our corn and soybean acres were damaged by the floods.
Many things come to mind when we consider replanting any crop, let alone corn in late June. Replanting corn this late is typically not recommended. Extension publications from previous years often say something like this, "As planting is delayed, the yield potential of corn declines at a faster rate than soybeans. In most cases the relative advantage will move to soybeans after approximately June 10 to June 15."
So, why are producers even thinking about replanting corn? A one word answer -- Economics. Never-before seen corn prices challenge our conventional thinking.
Summary of Previous Research
Guidelines for replanting are posted on our Web site. One variable important in any replant decision is what yield to expect. Estimated yield based on planting date and final population is listed in a table in the Corn Planting Guide. Using this information, we find that 52 percent relative yield potential is possible with a population of 32,000 plants per acre when corn is planted between June 24 and June 28. This table was based on research conducted in the late 1990’s combined with previous data.
Garren Benson, retired ISU Extension corn agronomist, reviewed planting date experiments and summarized the findings in 1990 (Table 1). Regardless of hybrid maturity, yields when planted on July 1 ranged around 30 percent. Earlier maturing hybrids resulted in drier grain at all planting dates, but early maturing hybrids also yielded less than adapted full-season hybrids especially if planted on or before June 1.
Table 1. Influence of planting date and hybrid maturity on corn grain yield and harvest grain moisture in north central Iowa. Maximum yield in this table was 144 bu/acre. Table adapted from Benson, 1990.†
† Benson, Garren O. 1990. Corn replant decisions: A review. J.Prod. Agric. 3:180-184 (1990).
With either late planted corn or replanted corn, an early fall frost can destroy or dramatically reduce seed yields. Table 2 summarizes locations and years which had either a late frost or an early frost in Benson's series of trials.
Table 2. Impact on corn grain yields and harvest grain moisture when fall freeze was 3 weeks later or 2 weeks earlier than normal in north central Iowa. Maximum yield in this table was 148 bu/acre. Table adapted from Benson (1990).
† Benson, Garren O. 1990. Corn replant decisions: A review. J.Prod. Agric. 3:180-184 (1990).
`Adapted' hybrids in these trials were likely between 105 and 110 days relative maturity; `early' hybrids were likely 10 days earlier, and `very-early hybrids' 15 or more days earlier. Year to year variability among hybrids was much greater for later planting dates than for early planting dates. Early frosts profoundly affected corn yields and grain moisture contents. For example, an adapted hybrid planted on July 1 yielded 61 percent of optimum in years with a late frost, but only 7 percent in years with an early frost.
Iowa research on extremely late planting dates is limited to Tables 1 and 2; most efforts center on yield response to earlier planting dates. Much effort though has focused on developing computer simulation models to predict corn yield. I used a corn simulation model, Hybrid-Maize developed by colleagues at the University of Nebraska, here to estimate effects of late planting dates with different combinations of hybrid maturities at three Iowa locations. As the developers of the model caution, ``...the results of model simulations should be considered approximations and not taken as fact.'' The model will help us meet our objectives of estimating yield potential and assessing the risk of frost associated with late planting.
Background information on the variables used in the Hybrid-Maize model relative to planting dates and hybrids used for each location are shown in Table 3.
Table 3. Variables used in Hybrid-maize simulations. Constants in all of the simulations were optimal water during the growing season, a seeding rate of 30,000 plants per acre, and a seedling depth of 1.5 inches.
† These years of actual weather data were used to simulate corn responses at the different locations.
¶ Hybrid growing degree day (GDD) requirements/ approximate crop relative maturities in days.
§ Assumes a 90 day hybrid adjusts for late planting dates by shortening GDD requirements. I only ran this comparisons at the Nashua location.
Data in Table 4 show a full-season (early planting, full-season hybrid) simulation coupled with a June 30 planting with either an early or very early hybrid. Late-replant yields in seventy-five percent of the years will range from 50 to 60 percent. Yields of 60 to 70 percent are possible in the very best years.
Thus, if a field near Ames normally produces a full-season crop yield of 200 bu/acre if planted on June 30 with a 2160 GDD hybrid, we might expect 100 bu/acre in an average year, 140 bu/acre in an extraordinary year, and in the worst year, 28 bu/acre. The chance of frost at maturity for this planting is 39 percent, or about two years out of five.
Table 4. Early planting with full-season hybrids compared to June 30 planting with early or very-early hybrids at three Iowa locations.
These data are also shown graphically in Figures 1-3 along with all of the other simulations at the three locations. The figures show the effect of delayed planting yield potential (as a percent of maximum).
The top end of each bar in the figures represents the best yield in the years modeled (21 years for Ames and 19 for the other two locations). The bottom end of each bar represents the yield that could occur in the worst year. The lower horizontal line of the box represents the 25 percentile limit, and the upper horizontal line represents the 75 percentile limit. The circle represents the average year in the simulation. The figures display the range of and consistency of reaching certain yield potentials.
Figures 1 - 3. Hybrid-Maize estimates of yield potentials for the various planting date/hybrid combinations at three Iowa locations showing the effect of delayed planting yield potential (as a percent of maximum). Data were simulated using Hybrid-Maize, University of Nebraska. The relative planting dates for all locations were the same: E = 25 April; M = 20 June; L = 30 June.
Probabilities of frost at crop maturity as well as silking and maturity dates are shown in Table 5 for the complete set of computer simulations.
Table 5. Silk & maturity dates and frost probabilities at maturity for all the combinations of planting date and hybrid for three regions of Iowa. Dates listed are based on Hybrid-maize simulations of the median year in the weather data set provided for each location as noted in Table 3. The `median year' is apt to differ in each of the six to seven simulations developed for each location. This is because the year to year ranking in the simulation model is based entirely on estimated yield. Probabilities of frost of less than 29 degrees F are from ISU's Iowa Environmental Mesonet.
† The first letter of these pairs reflects the planting date: E=25 April; M=20 June; L=30 June. The second letter of the pair refers to the hybrid planted: F=full-season; E= early-season; VE = very-early season. Specific hybrid characteristics varied with location as shown in Table 3.
To help explain the simulation findings, here is an example from Figure 2: In the best year a full-season corn replanted in central Iowa (around Ames) on June 20 could yield about 70 percent of normal while in the worst possible year it might yield less than 10 percent of normal. Interestingly though, yields are less than 50 percent of normal only 25 percent of the time.
On the other hand, if an early hybrid is planted on June 20, not only will yields in the best year be higher, 77 percent, but also the estimated yield in the worst years will be higher, 29 percent, than if a full-season hybrid was planted this late.
The figure also shows a reduced range of yields associated with planting an early-season hybrid on June 20 relative to an adapted hybrid planted at the same time. Planting an early-season hybrid also reduced the probability of frost by 68 percent (Table 5). The decision on June 20 is apparent: plant an early-season hybrid. The choice for a June 30 planting date at the same location is not as apparent. Although yields in the best of years are less with the early hybrid, the risk of achieving yields less than 47 percent of normal are much less and the frost risk is also reduced by 47 percent.
Table 6 summarizes previous research with Hybrid-Maize simulations. Yield potential from the Corn Planting Guide is an average for all years, statewide. Benson's data (from north central Iowa) and the Hybrid-Maize simulations allow fine-tuned comparisons given specific types of years. Averages of the latter two, 33 percent and 43 percent, are less than that provided by the Corn Planting Guide.
Simulated results compare well with those of Benson especially when early or very early hybrids are used. In a year with poor conditions or with an early frost, early-season hybrids planted in late June may yield as little as 22 percent of normal potential. But, with a late-frost or excellent conditions, yields may reach 60 to 67 percent of potential. Average performance for these situation ranges between 40 and 50 percent.
Table 6. Summary of findings from previous research and from Hybrid-Maize simulations.
† Freeze dates and data for Benson 1990 data and are from Table 2/ Worst and Best years of simulations are from Hybrid-Maize (Figures 1-3).
∂ 'Adapted' hybrid/`Very Early' Hybrid (see Table 5 footnote for full description).
β from Table 1.
‡ from Table 2.
Concluding Remarks and Cautions
l The decision to replant in late June is not an easy one. The simulations provided here are approximations of what might happen given multiple scenarios based on the last two decades of weather data. Yet, they seem consistent with previous research.
In late-June replant situations, plant hybrids that are 10 and 15 days earlier than hybrids typically used in an area. Shortening RM by 10 to 15 days equates to a reduction of approximately 400 to 500 growing degree days. When growing short-season hybrids, remember they were not developed for Iowa. Because of this they may not be able to tolerate environmental stress, insect and disease pressure quite as well. Fortunately, hybrids in these maturity ranges are available for planting.
Nevertheless, whether corn is planted for the first time or replanted between now and June 30, plant early-season hybrids. These will have the greatest chance of maximizing yield while lowering risk of very poor yields. Be aware that the fall frost risk associated with planting these hybrids late will range between 29 and 39 percent in the south and central regions and up to 66 percent in the north.
Given the high prices of not only corn but also soybeans and other commodities, take care in evaluating all options: replanting with corn, soybeans, grain sorghum, or various spring and summer annual forage crops, or leaving the land fallow. In any case, consider carefully your crop insurance options. This is a time for logical, thoughtful decisions based on sound advice and careful calculations.
[Text originally appeared in the Integrated Crop Management extension newsletter (http://crops.extension.iastate.edu/cropnews) on June 25, 2008.]