Recent weather has placed Arkansas soybean production in an awkward situation. Cool, wet conditions in April limited planting and decreased seedling vigor tremendously. May arrived with warmer temperatures, but soil moisture has been marginal for planting.
A lot of soybeans were planted in the first two weeks of May. Where conditions were favorable, acceptable stands emerged in less than 14 days. Where soybeans were planted into minimal soil moisture or just plain dry soils, farmers are hoping for rains.
Farmers will soon make decisions about stands on many of those acres. Recommended soybean populations depend upon maturity groups and production systems (irrigated or non-irrigated) used. For indeterminate varieties (Group 3 and Group 4) a final population of 130,000 plants per acre is recommended. For determinate varieties (Group 5, Group 6 and Group 7), final populations of 100,000 plants per irrigated acre and 80,000 plants per non-irrigated acre are recommended.
Estimating plant stands
The population of a plant stand can be determined in many ways. If soybeans are produced on rows (15 inches to 38 inches), I prefer to use “plants per row foot” to estimate final plant populations. To determine if a population is adequate, count the number of surviving plants per row feet and convert the number to plants per acre.
For narrower rows (less than 15 inches), estimating soybean stands per square foot is sometimes more appropriate. This can be done easily with a number of tools. Many use squares constructed from plastic pipe ranging in size from 1 to 10 square feet.
Begin by counting the plants within the square and convert the number to plants per square foot. Multiply the number of plants per square foot by 43,560 (square feet in an acre) to obtain the soybean population.
Another popular tool is a “hula hoop.” With this method you will more than likely use some of the high school geometry you swore you never would. Most “hula hoops” have a 3-foot diameter, with an area of 7.07 square feet.
Count the number of soybean plants inside the hoop and multiply by 6,161 to obtain the number of plants per acre.
Regardless of the method used, be sure to count several sites throughout a field for a better idea of the field average. Also, use commonsense. Many weather-damaged plants alive when you estimate the stand could die in the future.
Having estimated the plant population, should you replant or keep the existing stand? This is going to be a field-specific question. If you have planted Group 4s on non-irrigated land and the population falls between 60,000 to 70,000 plants per acre, seriously consider keeping the stand. However, if you can irrigate and the field has a history of strong yields, replanting since you have time to make an excellent crop.
Chris Tingle is the Extension agronomist for soybeans with the University of Arkansas Cooperative Extension Service.