Data from USDA-ERS show a decreased seeding rate for corn following corn compared to soybean in Iowa (see Figure 1). This difference may simply be due to the locations that generally raise multi-year corn versus other areas of the state. In other words, most acres are in corn-soybean production across Iowa, and therefore, more seeding rates are taken into account when figuring plant populations for a corn-soybean system than corn following corn. The data bring a valid question; should different seeding rates be used for corn following corn than corn following soybean?
Figure 1. Seeding rate for corn following corn and soybean in Iowa during 1996-2005. (United States Department of Agriculture-Economic Research Service)
Corn does require more moisture (generally a few inches) per growing season than soybeans. Therefore, multi-year corn is at a slight disadvantage because of the reduced available moisture. If moisture is taken into consideration in seeding rate decisions, producers may want to reduce their seeding rates if they believe moisture to be limited in the upcoming growing season. This would be an attempt to tailor the seeding rate to what the environment can "handle." The U.S. Drought Monitor as of February 8, 2007 (Figure 2), reveals most of Iowa has normal moisture; the exception is southeast Iowa. Therefore, a moisture deficit may not be a major factor to consider when determining seeding rates in 2007 for most areas, although available moisture in the growing season should always be considered.
Previous Iowa State University data (1997-2000) showed that, in general, a harvest population of 32,000 plants per acre gave the optimum yield across the state. Hybrids continue to be bred for good performance under increased populations though. Based on 2006 data, highest yields were associated with a final population that was approximately 2,000 plants per acre higher (approx. 34,000). Therefore, in typical growing environments, we believe the optimum plant population has increased. The next question is whether seeding rates should be changed to reflect the cropping system they are planted into.
Figure 2. U.S. Drought Monitor, February 8 , 2007.
Research conducted by the University of Wisconsin during 1995-1997 investigated final plant populations relative to the system they were placed into. The yield response to plant population did not differ whether corn followed corn or soybean. The highest yields were associated with a final population of approximately 30,000 plants per acre (this was the highest population included in the study). Based on this research, we can conclude that yields follow the same response trend to population regardless of what system they are placed into.
Research by Iowa State this past year agrees with University of Wisconsin data, in that there was no interaction between population and rotation sequence (i.e., population caused the same yield response trend). Yet, this does not mean that there are not overall population differences between the systems. In other words, are the number of seeds planted and the number of final plants at harvest similar between the two systems? Research from Iowa (2006) shows an overall difference in final population relative to the seeding rate (see Figure 3). Note that at Nashua (northeast Iowa), the final population for corn following soybean was greater than corn following corn by approximately 2,000 plants, although the linear responses are nearly parallel to one another. In this one location, the final population was less for corn planted into corn residue than soybean residue. We expect heavy residue to have been partially responsible for the reduced stand in corn following corn; see Allelopathy: A cause for yield penalties in corn following corn? to understand the relationship among residue, seed germination, and plant growth.
Figure 3. The relationship between seeding rate and harvest population. Ideally, we would like to have every seed planted be present at harvest; this would signify a 1:1 ratio. Most hybrids have a 95 percent germination rate, which means that each seeding rate should parallel the 1:1 line, except that it would be slightly below it. As the seeding rate increases though, the final populations start to drag down from the 1:1 line.
ISU and the University of Wisconsin data show that yields will respond similarly to population regardless of the system in which they are placed. Yet, slightly greater harvest populations could be realized in corn following soybean than in corn following corn. To have the same harvest population between both systems may require a higher seeding rate into corn residue, especially if the residue is heavy. Increasing the seeding rate slightly may not result in increased yields, but rather that your stands between the two systems may be more closely related.
Portions of this text, written by Lori Abendroth and Roger Elmore, originally appeared in the Integrated Crop Management extension newsletter on pages 6-9 of the IC-498(1) - February 5, 2007 issue.