Issue No. 3, Article 7/April 23, 2010
Should I Start Planting Soybeans in April?
The favorable weather has expedited corn planting across much of the state, and many growers are moving right on to soybean planting, or contemplating it. Planting early, or earlier than normal, is often touted as a primary way to increase soybean yields. The yield response can be rewarding, but there is no doubt that the earlier you plant, the greater the risk and the greater the required management. Instead of telling producers to "plant early" to increase soybean yields, I always advise "plant timely." What do I mean? In general, timely soybean planting means planting in late April or early May to maximize yields, but the timing has to be coupled with good planting conditions and a favorable weather forecast.
Recent soybean yield response data in Illinois suggests there is not a consistently large difference in yield between soybeans planted in late April and those planted the first week of May for most of the state. Sometime during the second week of May, some incremental daily yield loss starts to occur. That loss increases with time and is accelerated by late May and June. In the northern tier of Illinois, the risk of killing frost in late April is higher than in central and southern Illinois, but the yield loss for late planting will also be higher due to the shorter growing season in terms of available temperature and sunlight. Data for eight site-years at Monmouth and DeKalb are shown in Figure 4; yield loss was 0.1 bushel/day May 1 through May 10, with the highest yield attained by the April 27 planting. Data for five site-years at Brownstown and Dixon Springs are shown in Figure 5; yield loss was 0.1 bushel/day May 10 through May 20, with the highest yield attained by the May 9 planting. This year we are starting soybean planting date studies again at all six Department of Crop Sciences research centers, so we will have more data to report.
Figure 4. Response to planting date over eight site-years in the 1990s at Monmouth and DeKalb. The planting date for the highest yield was April 27; yield losses were 0.10, 0.23, 0.36, and 0.54 bushels per day of delay for May 1-10, May 11-20, May 21-30, and June 1-10, respectively.
Figure 5. Response to planting date over five site-years at Brownstown and Dixon Springs, 2006-08. The planting date for the highest yield was May 9; yield losses were 0.10, 0.26, 0.42, and 0.59 bushels per day of delay for May 10-20, May 20-30, June 1-10, and June 10-20, respectively.
When you are deciding how early to plant, additional considerations are the risk that seed will experience prolonged emergence in cold and wet soils, the risk of a killing frost once seed is emerged, and the increased risk of pathogens and insect pests. To avoid prolonging the time that germinating seeds spend in cold, wet soils, you must keep in mind seedbed preparation, with the goal of enhancing the warming and drainage abilities of your soils. Soils can warm better if you prepare the seedbed with tillage or even strip tillage that removes the residue from the seed furrow, in comparison with no-till situations with heavy residue. Obviously, darker soils will also warm faster than lighter soils, as will better drained soils following rain, which will be cold this time of year. This is also where the forecast comes into play. Look at the 7- and 10-day forecasts and assess the risk of prolonged cold and rainy spells--the forecast isn't always accurate, of course, but it is an indicator.
Once seedlings emerge, killing frost is another concern. The risk of a killing frost in May is pretty low for most of Illinois. It is slightly higher in the first week of May in the most northern region, but still fairly low in most years. However, the growing terminals of soybean plants are above ground as soon as the cotyledons emerge, so they cannot recover from a damaging frost as well as corn plants, whose growing points remain under the soil much longer. With that in mind, the date for which you want to assess the potential for killing frost in soybean is emergence, not necessarily planting. In cool April soils, you can expect emergence to take at least 7 to 10 days, so that buys a little time after planting--you could experience a frost that would not freeze the young growing tissue if it's not yet above the soil surface. Following this emergence guideline means that if the risk of a killing frost in May is low, then the risk of losing a soybean crop planted between April 20 and 23 to a killing frost is also low. But again, the earlier you plant, especially ahead of April 20, the greater the risk. Use the near-term weather forecast for your area as an indicator.
I have included six figures showing daily minimum and daily average temperatures for 1989 through 2009 for Dixon Springs, Champaign, and DeKalb to illustrate the risks related to killing frost and temperature responses over the last 20 years (Figures 6-11). The graphs include regression trend lines for temperature that regress on the number of days past April 1 and go through May 31 (i.e., 51 would be May 21). The average daily temperatures at all three locations were 11 to 12°F higher than the average daily low temperatures. I included a line at 32°F to tally the number of days that temperatures were near or below that point at each location. Keep in mind that there are 20 years of data represented for each day, so when one point is at or below 32, that represents just 5% of the the data. In other words, every day at or below 32°F in a five-day window represents a 1% chance.
Figure 6. Average daily temperatures for DeKalb, 1989-2009. Data generated from the Illinois Climate Network.
Figure 7. Minimum daily temperatures for DeKalb, 1989-2009. Data generated from the Illinois Climate Network.
Figure 8. Average daily temperatures for Champaign, 1989-2009. Data generated from the Illinois Climate Network.
Figure 9. Minimum daily temperatures for Champaign, 1989-2009. Data generated from the Illinois Climate Network.
Figure 10. Average daily temperatures for Dixon Springs, 1989-2009. Data generated from the Illinois Climate Network.
Figure 11. Minimum daily temperatures for Dixon Springs, 1989-2009. Data generated from the Illinois Climate Network.
The last concern regarding early soybean planting is increased potential for pathogens and insect diseases. It would not be best to plant early in fields with a history of seedling diseases or sudden death syndrome, and the chance of early-season insect feeding also increases. Soybean seed treatments may help protect seedlings from some of these pests and are generally a better "insurance" investment under early planting conditions than later. Regardless of seed treatment use, if you plant early, early-season scouting will be essential.--Vince M. Davis