Uneven emergence and plant heights are caused by several factors, including variation in soil temperature, seeding depth, residue distribution, soil crusting, and soil moisture. Photo 1 illustrates the effects that variable soil temperatures had on plant height and vigor in 2006. How much can plant height vary before it causes a real yield loss? And based on this, should producers replant if their fields look like this?
Photo 1: Uneven corn in northeast Iowa.
In general, non-uniform stands result in lower yields because the smaller, late-emerging plants cannot capture enough sunlight. Unfortunately, the yield loss from the "late" plants is not made up by the "normal" plants.
We compiled research from scientists across the Midwest and world to determine how later emerging plants performed within a field of normal emerging corn. These studies are typically conducted by delaying planting of a certain percentage of plants within the field to simulate variable emergence. Within this article, we will refer to "normal" and "late" plants. These refer to seed that was planted on a typical or normal planting date versus corn planted into the existing stand at a later time. Several research reports were also combined in graphical form (Figure 1). The figure illustrates the percent yield loss due to uneven emergence. Go to Tools to calculate yield loss from uneven heights for a worksheet on how to use this figure and estimate yield losses.
Figure 1. Compilation of research results showing percent yield loss when a certain percentage of the stand (x axis) is late compared to the rest of the field. Arrow colors represent which research report they are from. Populations are embedded within parentheses of the references (26 to 32K where K is 1,000 plants per acre).
Below is a description of findings from individual research trials. Ifi you would like to read the summarization of all of these reports, go to the end of this article to Summary.
Research from Europe
European researchers found that corn planted either when the normal seed had germinated or when it had already emerged resulted in reduced yields (Pommel et al., 2005). The later emerging plants had smaller leaf widths, less leaf area, less total biomass, and yielded less than the normal plants. Leaf area is a measure of how much leaf is available to capture solar radiation and was measured throughout the season. It varied somewhat early in the crop development between treatments. By the time that approximately 850 growing degree days (GDD) (refer to How and why we measure degree days for how to calculate GDDs) had accumulated, the normal plantings always had greater leaf area values than the later emerging plants. Corn will be at approximately V10 once 850 GDDs are accumulated. Therefore, if plants are small once they reach 850 GDDs, then they will always be behind.
Research from Canada
Canadian scientists found that when one of six (17%) plants was delayed in emergence by two leaves, overall yield was reduced 4 percent; when delayed by four leaves, 8 percent yield losses were observed (Liu et al., 2004). They targeted a plant population of 27,100 plants per acre. Plants neighboring late-emerging plants only partially offset yield losses of the dominated plants. Plants located near gaps in the row were able to compensate for the gaps.
Research from Argentina
Argentian researchers determined that late-emerging plants are not compensated for in corn as they are in soybean (Andrade and Abbate, 2005). Attaining uniform corn stands is more important because the plant has lower vegetative plasticity; i.e., corn is less able to compensate than soybean. Corn hybrids do differ though among one another in their ability to compensate, and, therefore, yield is determined somewhat by hybrid characteristics.
Research from Wisconsin and Illinois
Illinois and Wisconsin researchers investigated the response of corn when 25, 50, or 75 percent of the plants were planted either 10 or 21 days after the original planting date (Nafziger et al., 1991). Final stands were 26,100 plants/acre. Overall, grain yields were reduced 6 to 7 percent by a delayed planting of 10 days regardless of the percentage of plants delayed. However, when planting was delayed 21 days, yields were reduced 10 percent when 25 percent of the plants were delayed, 20 percent when 50 percent were delayed, and 23 percent when 75 percent of the plants were delayed.
Research from Minnesota
Minnesota scientists compared corn planted normally to that where half of the seeds were planted either 7 or 14 days later (Ford and Hicks, 1992). Normal plants had larger stalks, more tillers, longer ears, more ears, fewer barren plants, and more grain per plant than late plants. Yields were reduced more with a 14-day delay than with a 7-day delay (Figure 1). At lower populations (approximately 16,000 plants per acre) the difference between normal and late plantings was small. The yield reduction from late plants increased in relation to the normal stand though as population and yield level increased (Figure 2).
Figure 2. Corn yield averaged over years at Lamberton, MN, as influenced by planting date and plant population. Early = all seeds planted in early May; Early + 7 = half of seed planted in early May, half 7 days later; Early + 14 = half of seed planted in early May, half 14 days later (adapted from Ford and Hicks, 1992).
Wisconsin and Illinois researchers concluded that even though late-emerging plants reduce yield, replanting would not increase yield potential unless more than half of the plants were delayed by three weeks or more. We agree with these findings and highly doubt that replanting will result in increased yield for most fields that have variable emergence and plant heights. Although yield potential is compromised in these fields, the best decision is still to leave the crop as it is.
To estimate what you may expect for yield loss in your fields, please refer to Figure 1. Estimate the percent of your stand that appears smaller than the normal plants; this is your x-axis value. Next, try to estimate how many vegetative stages the plants are apart, on average. Take this value and multiply it by three to derive an approximation of how many days separate the normal and late plants. Use these values in Figure 1 to derive a fair approximation of percent loss for your field.
It is possible that in some fields the corn is actually at the same growth stage but still appears uneven. The uneven appearance comes solely from a variation in plant height, i.e. some plants have reduced internode elongation and leaf area compared to neighboring plants at the same growth stage. We are not aware of research data that specifically simulates this type of growth; the data discussed above is for corn that is different in terms of growth stages. A yield reduction should be expected, but the severity of it cannot be known for sure.
Andrade, F.H. and P.E. Abbate. 2005. Agronomy Journal. 97:1263-1269.
Ford, J.H. and D.R. Hicks. 1992. Journal of Production Agriculture. 5:185-188.
Liu, W. et al. 2004. Crop Science. 44:847-854.
Nafziger, E.D. et al. 1991. Crop Science. 31:811-815.
Pommel, B. et al. 2002. European Journal of Agronomy. 16:263-277.
Portions of this text, written by Roger Elmore and Lori Abendroth, originally appeared in the Integrated Crop Management extension newsletter on pages 169-171 of the IC-496(15) - June 12, 2006 issue.