David Moseley, Andre Reis, Manoch Kongchum, Rasel Parvej, Boyd Padgett, Thanos Gentimis, Brandi Woolam and Laura Lee

The early soybean production system (ESPS) was proposed in the mid-1990s for the Southern United States. This practice consisted of growing varieties with a maturity group of III and IV and a planting date between mid-April to mid-May instead of the traditional recommendation of planting a maturity group V-VII variety between May and June, which was known as a full-season production system. Research indicated that the early soybean production system improves yield and stability, mainly by avoiding heat and drought stress during seed fill and late season disease and insect pressure.  

In 1998, Donald Boquet reported results from a planting date and maturity group research project conducted at the LSU AgCenter Northeast Research Station near St. Joseph. The research results supported the previous report that there was an equal or greater attainable yield with more stability with an early soybean production system than the traditional production system. Since 1998, soybean planting date and maturity group research across the southern United States, including the northern region of Louisiana, has supported the early soybean production system with an optimal planting date centered around mid-April.

According to U.S. Department of Agriculture survey data, Louisiana soybean producers began to largely adopt the early soybean production system in 1999. Before that, soybean crops were planted earlier than May 1 only twice, in 1988 and 1994, when 1% and 5% of the soybean crop was planted by May 1, respectively. However, in the 1999 season, 19% of the soybean crop was planted by May 2. Since 1999, an average of 35% of the crop has been planted by the beginning of May. Based on an April planting date, the adoption rate of the early soybean production system from 1999 to 2017 saw an annual increase of 1.74% per year and with a maximum of 66% planted by the beginning of May in 2017.

A planting date, maturity group and variety trial was conducted at the AgCenter Dean Lee Research Station in 2021 to determine how the shift to earlier maturing varieties affected the vegetative development rate and timing of growth stages. In addition, the research was designed to revisit the reported yield advantage of an early soybean production system indicated from across the Southern United States.

Temperature and daylength affect soybean growth and development. Warmer temperatures increase the growth rate, and shorter daylengths can decrease the number of days to the beginning of the reproductive stages. The planting date trial at the Dean Lee Research Station was designed to determine if a maturity group III–IV variety planted in April is the optimal practice in the central region of Louisiana. In addition, producers often ask, “How early is too early to plant soybean?” Soybean plots were planted in March to determine if soybean plants would respond favorably to an extremely early planting date.

The overall average yield of all 12 varieties (all three maturity group sections), was the highest for the April 6 planting date (57.4 bushels per acre). The average yield decreased for the March and late April to July planting dates. There was a 9%, 4.7%, 5.1%, 8.1% and 16.3% drop in yield compared to the April 6 planting date for the March 22, April 29, May 31, June 11 and July 1 planting dates, respectively. In 2021, the drop in yield potential for a late May to July planting date seemed to be lower than historical observations. This may have been from less heat and drought stress throughout the growing season. However, the highest yield from an early to mid-April planting date is consistent with previous planting date trials across the Southern United States.

The attainable yield of a soybean crop can be higher when the plant canopy is closed by the beginning pod development growth stage, called R3, to full pod development, or R4. The average percent canopy coverage at R4 across all 12 varieties was 52.5% and 55% at a 38-inch row spacing for the March 22 and April 6 planting dates, respectively. The average canopy coverage at R4 was the highest at 77.7% for the April 29 planting date.

In 2021, the optimal percentage of canopy coverage at R4 and highest yield potential combination was from a planting date between April 6 and April 29. Although this was a one-year trial of 12 varieties split evenly across the maturity group sections III to V, the results indicate the optimal soybean planting date in the central region of Louisiana is between early to late April.

A decrease in yield potential and rate of canopy coverage does not support planting soybean in mid to late-March. In addition to lower yield potential, plants with slower vegetative growth will not shade out weeds as well.

The planting date research at the Dean Lee Research Station indicates the early soybean production system is the optimal planting practice for the central Louisiana region. The data from 2021 supports previous research results that suggest planting soybean earlier than the traditional May and June planting dates will maximize yield potential. However, planting earlier than April may reduce the maximum yield potential.

Ongoing planting date research from the AgCenter H. Rouse Caffey Rice Research Station, the Dean Lee Research Station and the Macon Ridge Research Station will focus on disease pressure and fungicide efficacy and will include analysis of the seed protein and oil content and stability for different environments across the state. In addition, a planting date, maturity group and population rate study will be planted to determine best management practices for production systems that rotate sugarcane and soybean.

David Moseley is the state soybean specialist and an assistant professor at the LSU AgCenter Dean Lee Research and Extension Center; Andre Reis is a soybean and sugarcane agronomist at Dean Lee; Manoch Kongchum is an associate professor at the H. Rouse Caffey Rice Research Station; Rasel Parvej is an assistant professor at the Macon Ridge Research Station; Boyd Padgett is a professor at Dean Lee; Thanos Gentimis is an assistant professor in the Experimental Statistics Department; and Brandi Woolam and Laura Lee are research associates at Dean Lee.

(This article appears in the spring 2022 issue of Louisiana Agriculture.)

Figure 1: Soybean plants of maturity groups 4.3 (1), 4.7 (2), and 5.6 (3). The planting date was March 22, and the picture was taken on July 13. The plants were approaching physiological maturity to beginning senescence, or terminal, growth stages (R6.5–R7: No. 1); late seed development (late R5: No. 2); and early seed development (early R5: No. 3), respectively. It can be advantageous for a soybean plant to complete seed development before the possible heat and drought stress that is common late in the growing season. Photo by David Moseley