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Growers keep close eye on soybean growth stages

At mid-June, Louisiana growers had planted over 725,000 acres of soybeans in 2005 and continued to plant where it was dry enough to get in the field. In many areas, producers were waiting for soils to dry and were dodging afternoon thunderstorms.

Insect populations — especially bean leaf beetles, three — cornered alfalfa hoppers and stink bugs — were building quite rapidly. Some aerial blight had been reported.

The majority of the calls I have gotten are on three things: lack of growth from certain varieties, insects, and explanations on growth stage descriptions. It is good to get the growth stage calls because it means that folks are more in tune with what is going on with their soybean crops than ever before and they want to maximize the effect of fungicide applications.

Before I get into an explanation of growth stages and some great Internet resources, I have to comment about a recent Delta Farm Press article by Alan Blaine, Mississippi's Extension soybean specialist. I cannot agree more with what he said about spraying at R1 in an environment with no Asian soybean rust. No research documentation says there is a benefit from this spray timing. I have been telling agents, consultants and producers who are calling or e-mailing me that they need to spend their money on something more productive, such as another insect application or an additional irrigation — the return will be much greater.

Research has shown that if you are going to spray once for an assortment of diseases, the R3 timing is when you want to make the application. Let's not forget about the other diseases. Left untreated they can be just as devastating as rust, especially Cercospora leaf blight.

Most soybeans in Louisiana were between growth stages R1 and R2 at mid-June. Some beans were near R3 and would begin getting fungicide applications. Some of the earliest beans had already been treated.

I know some producers sprayed a little too early trying to piggyback the second Roundup application with a fungicide. I have been telling producers a fungicide/insecticide tank-mix probably would be more logical if the growth stages and insect populations warrant a tank-mix application.

This brings me to the refresher on soybean reproductive growth stages.

R1 is first flowering or beginning bloom. Regardless of where on the plant a flower is visible, the plant has begun the reproductive phase.

R2 is full-bloom stage. R2 is when there is an open flower at one of the four uppermost nodes on the main stem with a fully developed leaf.

R3 is pod initiation or beginning pod stage. This is the stage when there is a 3/16 inch pod at one of the four uppermost nodes on the main stem with a fully developed leaf.

R4 is full pod stage or pod elongation. This stage is classified when a 3/4 inch pod is visible at one of the four uppermost nodes on the main stem with a fully developed leaf.

R5 is beginning seed stage or seed initiation. This stage is when there is a 1/8 inch seed in a pod at one of the four uppermost nodes on the main stem with a fully developed leaf.

There are two things to remember about these growth stages. First, they are based on evaluations of the four uppermost nodes starting at a fully expanded trifoliate and working down the plant. Second, do not evaluate the branches because they are not used in growth stage evaluations.

How do you determine when a field has reached a respective growth stage? Progressions of growth stages are generally very close in a soybean field. Scout across the field just as you would do for insects and when 50 percent of the field has reached the target growth stage, the field or crop can be classified as such.

Some Internet sites have great descriptions and images of the growth stages. Just a sample of some are:

I am very pleased by the interest in soybean growth stages. It assures me that people are out scouting beans this year more than they have ever been before. What this could correlate to is improved yields from timely fungicide applications.

David Y. Lanclos is the soybean, corn and grain sorghum specialist at LSU AgCenter.

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