April 18, 2013

Figure 4. Soybean planting date trend from 1982 to 2012.

For several years you've heard UNL soybean physiologist Jim Specht advise: "Your soybean canopy should be green to the eye by the 4th of July!"

This is for good reason as Specht and his team have found that as much as 0.25 to 0.625 bu/ac can be lost for every day planting is delayed after May 1. The photos in Figure 1 were taken a few days after the longest day of the year which occurs around June 21-22. Note the amount of green (leaves) vs. brown (soil surface) from a late April or early May planting date to a mid-June planting date. Producers grow crops to "harvest the sun" so planting earlier is consistent with that notion. Also, keep in mind that plants must exchange water to get carbon dioxide (via the stomata), so why not use more of the early rainfall for transpiration too?

An analysis of 32 years of data shows more producers are taking notice of this potential yield loss and are planting earlier. Each spring since 1980 the Nebraska Office of the National Agricultural Statistics Service (NASS) has conducted a weekly survey to assess the percentage of soybean planting progress. When that data (Figure 2 solid symbols) are plotted for two example years (1980 and 2012), the progression percentage follows an S-shaped "sigmoid" pattern that rises from zero to 50% before plateauing at 100%. A model can be used to estimate the day when 50% planting progress occurs. For the two example years, the 50% Day of Year (DOY) is 146 and 132, respectively.

Such a model was fit to the data available for each of the 33 years (1980-2012), as shown in Figure 3. As you can readily see here, the S-shaped distribution pattern of planting progress varies among years. Some of this is due to unexpected rainfall events that can delay planting and splay out the curve, and some is due to good weather in spring that allowed for fast planting seasons. The blue curves denote the planting progression patterns reported in the past five years (2008-2012) and the red curves reflect the first five years (1980-1984). This graph shows how the 50% mid-point in the logistic curves varies. Given the positions of the red and blue curves, it is apparent that soybean producers, on average, now plant soybeans earlier in the season than they used to.

Let's take another look at this trend in Figure 4. It plots the day of the year (DOY) when the curves pass the 50% planting point for 33 years (1980-2012). Although there is substantive scatter in the data, the green trend line indicates that, on average, soybean producers are planting their soybeans earlier than they used to. In fact, the trend line linear regression equation indicates that Nebraska producers are advancing their soybean planting date by about 1/2 day per year.

The Bottom Line

Nebraska data indicates that advancing your planting date to be closer to May 1 can potentially increase your final soybean yield by 0.25 to 0.625 bu/ac per day of advance. A 15-day advance translates into increased yield of at least 3.75 bu/ac (15 x 0.25) and in some years as much as 9.375 bu/ac (15 x 0.625). And that is just the beginning. Note that the green trend line ends at a DOY of 136 (May 16). By planting closer to May 1 (as some of your neighbors already do), you could capture another 3.75 to 9.375 bu/ac.

So, to paraphrase the ad, "Let's do it and get that soybean canopy green to the eye by the 4th of July!"

Jim Specht, UNL Professor of Agronomy and Horticulture
Patricio Grassini, UNL Research Assistant Professor, Agronomy and Horticulture
Jenny Rees, UNL Extension Educator