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Late-season management for early-planted soybeans

LITTLE ROCK, Ark. -- One problem with the early soybean production system is late-season weed interference. Many weeds that emerge too late to cause any yield loss can cause harvest losses and increase moisture and trash content of the harvested crop.

Since a number of producers took advantage of August prices, many may try to accelerate the maturity process with pre-harvest desiccants to meet their contracts. Research is limited in this area, but indications are that applications of desiccants to soybeans prior to physiological maturity may result in uneven maturity, higher seed moisture levels at harvest, and seed quality problems.

The best course is to allow the plant to naturally complete the oil conversion process.

Pre-harvest herbicide applications

Compounds such as Gramoxone Extra and Boa at 0.67 pint per acre plus a surfactant or sodium chlorate at 6 pounds per acre can be applied for desiccation of green weed foliage when 50 percent of the soybean leaves have dropped and the remaining leaves are yellow.

Glyphosate at 1 to 2 pints per acre can also be used for desiccation of green weed foliage, but this application must be delayed until the R8 growth stage (all pods on main stem have reached their mature pod color).

If morningglories are the specific problem, Aim at 1.4 ounces per acre plus a surfactant can be applied at the R8 growth stage.

Insecticide termination

Soybeans should be closely monitored for pod-feeding insects through the R7 growth stage (one normal pod on the main stem that has reached its mature pod color). Soybeans well into seed fill remain susceptible to feeding from insects such as stink bugs. Yield and quality losses can be expected if the pests are not treated.

In addition, pay close attention to the pre-harvest interval (PHI) of insecticides before application.

Irrigation termination

The goal of late-season irrigation is to have adequate soil moisture to insure maximum seed weight. Yield losses have been as high as 10 bushels per acre when irrigation was terminated too soon. Current University of Arkansas recommendations for irrigation termination are when at least 50 percent of pods on a plant are at the R6 growth stage (have seed that are touching within the pod) and adequate soil moisture.

If soil moisture is rapidly depleting, an additional irrigation may be needed to maximize yields. This final irrigation should be implemented as a quick flush across the crop. Soil moisture and crop development should be evaluated again in five to seven days.

Chris Tingle is the Extension agronomist for soybeans with the University of Arkansas Cooperative Extension Service.

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