Home | Past Issues

Issue No. 6, Article 9/May 1, 2009

April Showers Bring May . . . Planted Soybeans, We Hope!

A couple of weeks ago I wrote about the importance of choosing the right soybean variety to maximize yield and profits on your farm. I suggested that a few extra hours choosing the right seed was likely worth more than a few hours of planting the wrong seed earlier. Nonetheless, the date you plant is a critical decision for maximizing your profit potential. It is, however, a choice that is also subject to the influence of the weather. Last year, 2008, shaped up to be the second-wettest year in Illinois history. It had a challenging spring many people are still trying to forget. As I write this, April 2009 is nearly behind us, and many people are already making comparisons to 2008. Though I hope this doesn't remain the case for long, April hasn't been favorable for planting crops. Nearly all of Illinois has received rainfall 1 to more than 4 inches above normal, along with daily temperatures 1 to 3 degrees (F) below normal. This has resulted in the lowest reported percentage (4%) of corn acres planted by April 26 in the last 10 years (USDA NASS 2009b). I'm guessing few, and likely no, soybeans have been planted yet. This eliminates the possibility of increasing soybean yields by planting in April, as some people have proposed. But as we progress through May, the effect of planting date on soybean yield will slowly but significantly increase in importance.

As we look ahead to the next week, planting is likely to be challenging for several areas in Illinois due to wet conditions. With few corn acres completed, planting corn is likely to take precedence over planting soybeans. This stands to reason, as the penalty for late-planted corn is still proportionately greater than it is for late-planted soybeans. We are certainly not past a critical date where we are losing major amounts of soybean yield potential. However, I believe we are past a date where there is any reason, other than improper field moisture conditions, that soybean planting should not be done.

So when should I plant soybeans to maximize yield? This question is being asked regularly among soybean farmers and researchers alike. In general, it tends to make sense that earlier planted soybeans will produce higher yields. Egli and Cornelius (2009) recently looked at data from 24 sites in the Midwest and concluded that soybean yields were relatively stable over planting date over the whole month of May. Beyond May 30, yields decreased at rates up to 0.5 bushel per day in high-yield environments. Most of the data they looked at were from the 1980s, so while this provides some perspective on effects of planting date, the question remains whether newer varieties respond the same.

More recent research more centrally located to Illinois would suggest that while some uncertainty remains about how much earlier to start planting, the critical date to have finished for highest yields is certainly earlier than May 30. De Bruin and Pedersen (2008) reported average soybean yields of 62.4, 60.0, 56.7, and 46.7 bushels per acre for soybeans planted in late April, early May, late May, and early June, respectively, over 13 years in Iowa. This equates to a loss of 0.15 bushels per day between late April and early May, 0.28 between early May and late May, and 0.86 between late May and early June. There was enough variability in these results to know that yield loss will vary considerably over years, but the compounded loss of nearly 16 bushels per acre from late April to early June would be $160 an acre at $10 a bushel for soybean. In Iowa, Dr. Pedersen recommends starting planting on April 25 for the southern third of the state and on May 1 for the northern third.

Recent research conducted in Indiana produced similar results. Robinson et al. (2009) found no significant advantage to planting before early May in either 2006 or 2007. The highest and most stable yields were from planting in the first to second week of May, and yields decreased at a rate of 0.5 bushels per day for planting after May 10.

Both of those recent studies, conducted in our neighboring "I" states, produced results similar to those in Illinois. Recent soybean yield response to planting date from research conducted in higher yielding environments at DeKalb Research Center and Monmouth Research Center were very similar to those cited from Indiana, with reduced yields for early April planting dates. The DeKalb and Monmouth results showed yields decreasing by 0.10, 0.23, 0.36, and 0.54 bushels per day of delay for the May 1-10, May 11-20, May 21-30, and June 1-10 periods, respectively (Figure 1). Recent research conducted in lower yielding environments at Brownstown and Dixon Springs agricultural centers showed yields decreasing by 0.10, 0.26, 0.42, and 0.59 bushels per day of delay for the May 10-20, May 20-30, June 1-10, and June 10-20 periods, respectively (Figure 2). Those are very similar losses to the ones reported by both De Bruin and Pedersen (2008) and Robinson et al. (2009).

Figure 1. Response to planting date over 8 site-years in the 1990s at Monmouth and DeKalb. The planting date for the highest yield was April 27, and the yield loss was 0.10, 0.23, 0.36, and 0.54 bushels per day of delay for the May 1-10, May 11-20, May 21-30, and June 1-10 periods, respectively.

Figure 2. Response to planting date over 5 site-years at Brownstown and Dixon Springs, 2006-08. The planting date for the highest yield was May 9, and the yield loss was 0.10, 0.26, 0.42, and 0.59 bushels per day of delay for the May 10-20, May 20-30, June 1-10, and June 10-20 periods, respectively.

So when do we recommend planting soybeans in Illinois? From the time you are reading this, as soon as soil moisture conditions allow it. For those who ask if they can increase yields by planting earlier, the answer is "by large amounts" if you routinely plant in June and "yes" if you routinely plant in late May. The answer is "maybe a little" if you already plant in early May, but increases will hinge more on environmental interactions, such as cool weather conditions after planting and potential disease pressure. The earlier soybeans are planted, the more yield will also be influenced by proper seed selection and management. Choose varieties with good SDS and SCN resistance, and soybean seed treatments will also be of greater value.

Another way to weigh the importance of planting date is to consider the statewide average annual yields in relation to dates of planted soybean acres. Over the last 14 years, there has been a loss of roughly 5 bushels per acre for statewide yield averages as the date that 50% of soybean acres are planted is delayed from the second week of May to the middle of June (Figure 3). There have been a lot of recent articles and discussions about the "plateau" in soybean yields over the last five years. Average state yields were 50, 46.5, 48, 43.5, and 47 bushels per acre for 2004 through 2008, respectively (USDA NASS 2009a). However, only 50% of Illinois soybean acres were planted before mid-May and 80% before the end of May, during that same time frame (Figure 3; USDA NASS 2009b). Furthermore, in 2008 less than 15% of Illinois soybean acres were planted before May 15, and 50% were not planted until the first week of June (Figure 3). While planting date alone is not the reason for stagnated yields, back-of-the-envelope math would suggest 2008 yields should have been much higher if planting was completed much earlier. To improve yields in Illinois, we must plant soybeans in a more timely way (early to mid-May). To accomplish that, we need a change in the weather.

Figure 3. Correlation between date of 50% completed soybean planting in Illinois and statewide yield (bu/acre), 1994-2008.

I will finish this article like I started it: let's hope the next few weeks do not further resemble 2008.--Vince M. Davis, with Emerson Nafziger


De Bruin, J.L., and P. Pedersen. 2008. Soybean seed yield response to planting date and seeding rate in the upper Midwest. Agronomy Journal 100:696-703.

Egli, D.B., and P.L. Cornelius. 2009. A regional analysis of the response of soybean yield to planting date. Agronomy Journal 101: 330-335.

Robinson, A.P., S.P. Conley, J.J. Volenec, and J.B. Santini. 2009. Analysis of high yielding, early-planted soybean in Indiana. Agronomy Journal 101:131-139.

USDA National Agricultural Statistics Service. 2009a. Illinois Crop Progress and Conditions Report. www.nass.usda.gov/Statistics_by_State/Illinois/index.asp.

USDA National Agricultural Statistics Service. 2009b. Illinois Quick Stats: County--Crops. www.nass.usda.gov/Statistics_by_State/Illinois/index.asp#.html.

Vince Davis
Emerson Nafziger

Click here for a print-friendly version of this article

Return to table of contents