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Research Highlights

Research Highlights
Checkoff research shows a change in optimum planting date

By Kentucky Soybean Board

For the past four years, the University of Kentucky (UK) and the Kentucky Soybean Promotion Board (KSPB) have partnered together to investigate the Optimum Planting Date for Soybeans. Under the research of Dr. Jim Herbek, the project has been conducted at the UK Research and Education Center in Princeton, Kentucky.

Currently the recommended planting dates for soybeans is early May through early June; however, with an increasing trend for soybean producers to begin planting before those dates, this project was created to investigate the benefits and risks of planting earlier then the recommendation. The proj­ect’s objectives were to:

  • determine if the current recommendations were still valid
  • define the optimum planting date range for maximum yield of soybeans
  • determine if early planting achieves maximum yield
  • determine if management practices need to be altered for early planning to succeed and to
  • define the latest planting date that achieves maximum yield

The project consisted of seven planting dates, occurring about every 10 to 15 days, beginning in early to mid-April and ending in early July. A soybean variety from Maturity Group IV was planted at all seven planting dates and a similar seeding rate was also used at all the planting dates. Data was taken on date of emergence, percent emergence, soybean stand achieved, beginning flowering occurrence and full maturity occurrence.

Many factors have influenced the results of this study over the past three years. One of these major factors is that the weather and growing season conditions over the course of the study were not similar. In 2008 and 2010 the growing season was hot and dry, whereas in 2009 the growing season was cool and wet. Despite the weather differences between these three years, the planting date yield results were quite similar in that all three years, the highest yields were obtained at the three earliest planting dates (mid-April, late April, and early May).

Herbek explained that these different weather conditions are beneficial to the project, “Weather plays a major role in soybean planting and the different growing conditions we have experienced the past three years can give us some idea of how weather conditions may impact planting at different times.”

He also noted that although the weather conditions were not the same, the results were still comparable.

“Despite the different growing season conditions, the planting results were still very similar and the highest yields occurred from mid-April to early May.”

 The results from the 2008 thru 2010 studies indicated that the earlier than normal planting dates achieved full yield poten­tial and resulted in the highest yields. The results also showed that the yields of planting after mid-May significantly declined for each successive planting date. The results of the present research were unexpected based on the planting date recom­mendations from previous years. Yield data for the past three years of the present soybean planting date research project indicates that a yield decline occurs prior to early June and also occurs for a mid to late May planting. Yields continued to decline further for later plantings.

 “At least 150,000 – 180,000 seeds/acre would be recommended,” Herbek noted.

Another risk of planting soybeans earlier would be the plant’s susceptibility to Bean Leaf Beetle infestations. In 2008 an 2009, the plants were affected shortly after emergence and were sprayed to prevent damage. An insecticide seed treat­ment was used in 2010 and prevented the early infestations. It is recommended that producers scout and spray their crop if needed or use an insecticide seed treatment in order to prevent Bean Leaf Beetle damage at early planting dates.

Other risks that Herbek mentioned might occur with earlier planting dates is the disease, Soybean Sudden Death Syn­drome (SDS), and a late spring freeze. SDS is caused by a soil born fungus and is favored by poorly drained soils and cool, wet weather early in the growing season. Although SDS was not observed in the three years of research, it is still a risk and farmers need to take precaution by planting SDS toler­ant or resistant soybeans varieties and avoid planting in fields where SDS has occurred previously. Earlier planted soybeans are also at risk to a late spring freeze. In the past three years, Herbek’s research did not encounter a freeze; however, farmers must take this into consideration.

At the time of this report, the 2011 project results are not complete due to later planted soybeans yet to be harvested; however, Herbek did offer insight into the results to come, “Preliminary results indicate a similar trend as the previous three years.”

As a result of the previous three years of research and expectations of this year’s results, Herbek has updated the previous recommendation for the optimum soybean planting window of early May to early June and currently recommends, “Based on the current research, the optimum planting window for full-season soybeans would be mid-April thru mid-May, which is earlier than previously thought.” He also noted that although this research has been conduct­ed in Western Kentucky, it is likely to correspond to a planting date window of approximately one week later in other parts of the state. If the opportunity exists, based on weather and field conditions, the optimum time to plant soybeans should occur as early as possible to achieve their full yield potential.

To find research related to this Research Highlight, please visit the National Soybean Checkoff Research Database.