Less stress on soybeans means more yield

If soybean fields are dry, hope and pray for that “million dollar” rain. At today’s prices, it could be worth several million. However, if fields are already wet, pray for dry weather. Conditions that are too dry or too wet stress soybeans in the reproductive phase. Stress at this critical juncture is the last thing you want.

Less stress on soybeans means more yield

If soybean fields are dry, hope and pray for that “million dollar” rain. At today’s prices, it could be worth several million. However, if fields are already wet, pray for dry weather. Conditions that are too dry or too wet stress soybeans in the reproductive phase. Stress at this critical juncture is the last thing you want.

Why August is such a critical month for soybean yields becomes evident if you study growth stages, explains Shaun Casteel, Purdue University Extension soybean specialist. Soybeans move out of R3, or beginning pod stage, and head into R4, or full pod. Technically, soybeans are at R4 when there’s at least one pod three-fourths of an inch long on one of the four uppermost nodes on the main stem with a fully developed leaf.

Key Points

The stakes of avoiding stress get higher as soybeans develop pods and beans.

The time it takes for soybeans to move through stages R4 and R5 varies.

Weather or insect stress causes the most damage at the R5 stage.


Stress, such as dry weather or insect feeding, becomes more damaging at this stage. For example, losing 5% of total leaf area per plant means a 2% yield loss. A 35% leaf loss could cost 8% of expected yield. If your goal was 60 bushels and insects riddle 35% of total leaf area during R4, that’s 4.8 bushels per acre. At $12 per bushel, that’s $60 per acre, more than enough to pay for scouting and insecticide.

Development progresses

Soybeans are typically in the R4 stage for about 10 days, although it can vary from five to 15 days. Next comes stage R5, or the beginning seed phase. Officially, agronomists say it’s when seed in the pod at one of the four uppermost nodes on the main stem with a fully developed leaf is about one-eighth-inch long.

The greatest threat for yield loss due to leaf defoliation occurs during seed development. After all, soybeans need leaves, the photosynthetic factory, to harvest sunlight and grow beans. A 35% loss in total leaf area trims yield an average of 11%. If your original yield goal was 60 bushels per acre and soybeans are $12 per bushel, you would sacrifice nearly $80 per acre.

Soybeans stay in beginning seed stage about 10 days, on average. However, there’s a wide range, depending upon conditions and varieties, from as few as four to as many as 26 days.

Test your knowledge

When should you apply fungicides?

If it seems strange talking about growth stages such as R2 through R5, get used to it. Many fungicide labels utilize them. You’ll need to understand their lingo to know when to make the most effective application.

When you should apply fungicides depends upon whom you talk to. Most sources nail it down to somewhere between R2, or full bloom, and R4, or full pod. Check pesticide labels and discuss it with your crop consultant and fungicide supplier.

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The ‘money’ phase:
Rubber meets the road in stage R5. Actual seeds form within pods.

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Full speed ahead:
Soybeans put on as many pods as possible during R4.

This article published in the August, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.

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