Planting Date, Temperature, Spacing, and Emergence: What Really Matters?

Is there an ideal planting date?

Planting date has a significant impact on corn yield when all other factors are equal. Research across the land grant system shows that there is an "ideal" planting window that exists, with a decline in yield with each additional day as less light and growing degree days are available to the plant. Of course that "ideal" time each year may vary due to the weather in that given year. Under ideal conditions, optimum planting dates range from April 15th to May 1st in the southeastern corner of the state to May 15th to 25th in the northern tier and Laurel Highlands.

Figure 1:  Corn Yield as Affected by Planting Date. A 30-day (approx.) window exists where corn can achieve 95% or more of maximum yield potential.  Once the planting date is about two weeks beyond the ideal date (as shown here as May 15th) the amount of potential yield loss per day begins to increase.

The graph in figure 1 assumes that all other factors throughout the year are equal.  Heat stress during pollination may severely impact yields or rain later in the season may favor hybrids planted outside of the optimum date. Also, it's important to note that because we're considering relative yields, late planted corn in one year may yield more than that same hybrid planted during the "ideal" date in another year (1). An additional point to consider is whether your hybrid will have enough time to mature.  Fortunately, full-season hybrids can "adapt" to shorter growing seasons by requiring fewer growing degree days to reach maturity. Research from Ohio and Indiana show that a hybrid planted 30 days late can reduce the necessary growing degree days to reach black layer by 6.8 degrees per day of late planting. This suggests that a hybrid that takes 2600 growing degree days (GDDs) to mature can do so with 200 fewer GDDs when planted 30 days late. However late-planted full-season hybrids may take longer to dry down than a shorter season hybrid planted on the same date.

Temperature

Soil temperature is still one of the most important factors in determining when corn can be planted. Ideally, soil temperatures, measured at seeding depth taken at 8:00 A.M. should be 50° F or greater. In addition, the weather outlook for the following 5 days should indicate warm conditions. Planting before cool, rainy conditions may result in poor emergence due to chilling injury or by seeds imbibing cold water, with "corkscrewing" of the germinating plant being a sign of such conditions.

What about emergence?

The effect of uneven emergence on corn yields has been well documented, with the vast majority of research showing that when corn plants emerge late relative to neighboring plants, the yields of the late emerged plants are reduced, with neighboring plants not showing a proportionate increase in yield. Older research showed that when 25% of plants emerged 10 days later than the rest of the stand, yields were reduced by 6-7% for the entire stand (2). However, more recent research suggests that yield impacts can happen with late emerging plants occurring much closer to early emergers and with fewer late emergers in the stand. Research in Ontario, Canada, determined that stand yields were reduced by 4% when 1 out of 6 (15%) plants emerge at a 2-leaf stage delay, which occurred between 7 and 16 days after planting (3).  Research in Oklahoma showed that when 30% of plants were delayed 2 and 5 days, yields were reduced by between 3-14% and 10-25%, respectively (4).

A study conducted in 2015 and 2016 by Virginia Cooperative Extension performed a similar analysis across multiple sites in the state looking at yields per acre of plants that showed delayed emergence (5, 6). They found when comparing to plants that emerged on the first day, plants that emerged one day later had yields that were 3% to 26% lower per plant.  Yields of plants that emerged two days after the first emergers were 2% to 44% lower per plant. If these per plant yields represented 15% of a corn stand, it would carry over to a yield loss of up to 6% across the stand if plants emerged 2 days late.

Caption: Pictures from the 2015 Corn Emergence Study in Virginia. Ears in the top picture emerged on the 1st day. Ears on the bottom picture from left to right emerged on days 2, 3, and 4+. Photos by Keith Balderson, Virginia Cooperative Extension.

Does Spacing Matter?

As much as current research suggests that uneven emergence has an impact on yield, the effect of variability or uneven spacing of corn plants on yield is not as clear. Recent research in Indiana showed for each inch that spacing deviates (is ahead of or behind) from 'ideal' location, grain yields decrease 1.7 bu./ac. for the stand (7). However, research performed in Ontario and Wisconsin showed that there was no relationship between within-row plant spacing and yield at typical deviations from ideal spacing (8, 9). Both researchers concluded that spacing that clumps plants together into hill-like patterns, far outside of conventional planter performance eventually affects stand yields.

If I'm Behind, Should I Drive Faster?

With the advent of advanced seed tube designs and metering systems, there is more interest in increasing planting speed to plant more acres per day.  New technology now allows for planting speeds of 10 mph or more, with little change in seed spacing, as shown in this research from Iowa State. But what if your planter doesn't have high-speed seed tubes? The answer likely depends on the age and setup of your plating equipment.

Conventional wisdom has been to limit plating speeds to maintain optimum seed spacing and to reduce doubles and skips, which appears to be sound guidance for finger meter and older air systems (10). However, with the advent of improved (vacuum) meter and aftermarket equipment, such as seed firmers, planting speed at times can be increased with little effect on yield (11, 12). This can circle back to the effects between population and uniform seed spacing and emergence, as shown in an example from Iowa State where seed spacing variability increased and one from Ohio State where the population was reduced with increased planting speed.  However, in both cases, there was no difference in yields when planting speeds were increased to 6 mph or greater.

Putting it All Together

Some of the above points, at times, contradict each other when making management decisions. Planting into poor conditions in order to not lose yield based on planting date may result in uneven stands which may also hurt yields.  Focusing on correcting within-row spacing while ignoring the planter row unit may cause similar results as well. 

When it comes to deciding to plant, you should ideally look for warm soils that are not overly moist and plant approximately 10 to 14 days before the average date of the last killing frost. When considering the tradeoff of soil conditions vs. calendar date, there may not always be a clear answer. When you're within a few weeks of the ideal planting date, the penalty of uneven emergence may outweigh the penalty of planting a few days later into better conditions. When you move beyond two weeks outside of the ideal planting date, yield loss per day approaches 1%. However, as you move further south or later in the growing season and growing degree days accumulate faster, the effect of uneven emergence may become more pronounced as plants are putting on more growth per day relative to cooler growing conditions. Therefore, waiting for planting conditions to improve rather than focusing solely on planting date may again be the better option.

What is undeniable is that consistent planting conditions matter. It should be the goal of all corn growers to plant into consistent soil temperature and moisture. For no-tillers, paying particular attention to even residue distribution out of the combine is important, as well as managing residue at the planter by utilizing row cleaners to promote even warm-up within the row as well as clearing trash away from the gauge wheels to ensure that they are not riding on residue and affecting planting depth. For everyone, planting into conditions that are not too wet is advice that needs to be followed.

In addition, some of the research mentioned above can provide a good starting point when considering what upgrades provide the quickest payback on your planter. Some examples of modifications to the row unit that ultimately result in even emergence are row cleaners, seed firmers, closing wheels and air or hydraulic systems that actively adjust row unit downforce. These modifications may improve uniformity of establishment and provide positive returns through better yields.  Conversely, improvements to metering systems that provide more accurate within-row spacing may take longer to pay you back.

If you're considering increasing planting speeds above 5 mph, a combination of the above may be necessary to not sacrifice yield. Vacuum meters will likely outperform finger meters as speed increases. Row-unit modifications such as seed firmers and active downforce systems will likely help to keep the row-unit on the ground and the seed in the seed slot. As speeds exceed 7 mph, a full-fledged conversion to high-speed planting equipment is likely necessary if you don't want to lose yield. For those making smaller, more marginal improvements, watching planter monitors and experimenting with faster speeds on a small scale is a good approach before fully committing.

Overall, it is important to know that conditions matter and setting up for planting into consistent soil conditions will likely reward you. So when you're ready for the planter to roll, make sure you're doing everything right and hopefully those decisions will result in good yields later in the year.

Sources Cited

1. Nielsen, RL. 2016. The Planting Date Conundrum for Corn. Corny News Network, Purdue Extension. [On-line]. [URL accessed March 2017].
2. Nafziger, E.D., P.R. Carter, and E.E. Graham. 1991. Response of Corn To Uneven Emergence. Crop Sci. 31:811-815.
3. Liu, Weidong, et al. 2004. Response of Corn Grain Yield to Spatial and Temporal Variability in Emergence. Crop Sci. 44:847-854.
4. Lawless, et al. 2012. Effect of Delayed Emergence on Corn Grain Yields. Journal of Plant Nutrition. 35:480-496.
5. Virginia Cooperative Extension. 2016. 2016 Virginia On-Farm Corn Test Plots (ANR-235NP). [On-line]. [URL accessed March 2017].
6. Balderson, Keith, and Wade Thomason. 2016. Corn Emergence Evaluation (CSES-157NP). [On-line]. [URL accessed March 2017].
7. Nielsen, R. L.  2006. Stand Establishment Variability in Corn. Proceedings of the 2006 Indiana CCA Conference, Indianapolis, IN. [On-line]. [URL accessed March 2017].
8. Liu, W., et al.  2004. Within-Row Plant Spacing Variability Does Not Affect Corn Yield. Agronomy Journal. 96:275-280.
9. Lauer, J. G. and M. Rankin. 2004. Corn Response to Within Row Plant Spacing Variation. Agronomy Journal. 96:1464-1468.
10. Nielsen, R. L.  1993  Planting Speed Effects on Stand Establishment and Grain Yield of Corn (AGRY 94-02). [On-line] [URL accessed April 2020]. 
11. Liu, W., et al. 2004. Impact of Planter Type, Planting Speed, and Tillage on Stand Uniformity and Yield of Corn. Agronomy Journal. 96:1668-1672.
12. Staggenborg, S.A, Taylor, R. K., and Maddux, L. D. Effect of Planter Speed and Seed Firmers on Corn Stand Establishment. Applied Engineering in Agriculture.  20(5):573-580.