Figure 1. Record of 30 years of precipitation climatology for York, Clay Center, and Mead. The solid line represents average monthly precipitation; the gray areas represent variability in precipitation over the 30-year period. (Source: www.agclimate4u.org)  (All figures link to larger versions.)

An article in last week's CropWatch, discussed the potential benefits of early planting soybean from a yield perspective (http://go.unl.edu/pmmr). This article discusses the benefits of early soybean planting from a climatology perspective.

Is there potential for capturing more precipitation and reducing water shortfall during the growing season by planting soybean early? To answer this question, we analyzed cumulative precipitation and evapotranspiration (ET) for early and late soybean planting dates for three locations: York, Clay Center, and Mead.

Precipitation Climatology

To start with the historical perspective, precipitation data from 1981 to 2010 from www.agclimate4u.org was charted (Figure 1). The shaded area shows annual variability in precipitation patterns; the solid black line represents an average for the 30-year period. Generally, May is at or near the peak of cumulative precipitation followed by June. If planted early, soybean plants could capture more moisture from late April and early May precipitation. If soybean is planted late, precipitation during the growing season might not be enough to fulfill crop water demand. 

Early Soybean Planting Advantage

Cumulative precipitation and evapotranspiration (ET) were compared for selected early (April 24) and late (May 27) planting dates to determine whether early planting could capture more in-season precipitation and reduce shortfall of water during the growing season.

First, let's define terminologies. Evapotranspiration is a measure of water consumed by crops through evaporation and transpiration. Evaporation occurs from the soil and wet leaf surfaces. Transpiration is a process involving water and gas exchange through the leaves. ET varies from day to day depending on crop development stage and weather variables such as temperature, radiation, humidity, and wind. A major crop and water management objective is to reduce evaporation and enhance transpiration. This is positively correlated with yield. Evaporative demands are reduced by leaving more residue on the soil surface and achieving earlier canopy closure, such as by early planting.

Theoretically, soybean attains its full yield potential when ET is satisfied throughout the growing season and all other growth factors are non-limiting. The most cost effective way to do this is through precipitation and stored soil moisture rather than supplemental irrigation.

To further examine this, we compared soybean ET and cumulative precipitation during the growing season for early and late planting dates using data from the last six years (2008-2013) for three locations (York, Clay Center, Mead) from High Plains Regional Climate Center's AWDN stations (www.hprcc.unl.edu). For each location we used mid-season soybean varieties that mature at 2360 growing degree days (GDD). Similarly, seasonal ET for each year was determined from selected planting date to maturity date. Given the weather variability from one year to the next and one site to the next, maturity dates and seasonal ET also vary accordingly. 

The figures show growing season ET for the three locations each year. As you can see, the ET for early planting soybean is slightly higher compared to late planting. However, when you compare cumulative precipitation, you notice that the early planting takes better advantage of accumulated precipitation. In the early-planting scenario, soybean potentially could capture 2 and 2.78 inches of additional precipitation during the growing season. When compared to late-planted soybean, the average shortfall of water (ET minus precipitation) was reduced by 0.94 inches to 1.41 inches across three locations. Table 1 summarizes these results for three locations.

Please note that this is just a scenario-based analysis to compare ET and precipitation in early- and late-planted soybeans. Important factors not considered in this model are the water storage capacity of the soil and soil moisture at planting time. If soil moisture was 100% recharged at the start of the growing season, the excess moisture between field capacity and saturation may be the only moisture lost to evaporation between early and late planting. In this case any true moisture differences or shortages during the growing season would be negated. However, in conditions such as were seen in 2012 and 2013, when soils were not fully recharged at the start of the growing season, this moisture difference could play an important role.

As noted in the previous article, keep in mind year to year variations and consider current weather and soil conditions when making planting decisions.

Tapan Pathak, Extension Educator for Climate Variability
Roger Elmore, Extension Cropping Systems Agronomist
Ken Hubbard, Professor and Climatologist at the High Plains Regional Climate Center

4/18/14