Keeping Your Soybean Crop Happy

Thursday, July 25, 2024

It’s The Key to Higher Yields

By Paul Herman

Imagine a world where you could see into the future. What would it be like if you could predict the weather? If you could predict which insects, diseases or other issues would affect your crop and what this would mean for final yields.

I always tell growers that if I could do that, I would be a millionaire who travels the world.

The trick is to figure out what these pest issues will be annually for different crops, on different fields, for different yearly growing environments.

In this article I will explore soybean crop physiology and reference some fungicide trial work we conducted during the 2023 growing season.

You may have heard me say that the key to getting higher soybean yields is to set a large pod load, fill a large pod load, and harvest a large pod load. Having as big a factory as possible right from the get-go is crucial to higher yields. This involves planting as early as possible for your given maturity area and having all the foundational management practices in place, like soil fertility, drainage, and variety selection.

From here it is critical to manage the large pod load of flowers that will eventually make it to soybean seed. This is extremely important in the reproductive stages of soybeans from R1 through to R5, which usually starts around the summer solstice for 40 to 45 days.

A quick refresher on reproductive stages:

• R1 has one open flower at any node on the main stem.
• R2 has an open flower at one of the two uppermost nodes on the main stem and has a fully developed leaf.
• R3 has a pod 3/16 of an inch long (4.8 mm) at one of the four uppermost nodes on the main stem with a fully developed leaf.
• R4 is like R3 but the pod is 3/4 of an inch long.
• in R5 the seed is forming, and is 1/8 of an inch long (3.2 mm) in the pod at one of the four uppermost nodes on the main stem with a fully developed leaf.

Taking a closer look at how a soybean crop grows and when soybean yield is determined allows us to understand when we need to make sure the soybean crop is the happiest. Most of a soybean’s life cycle occurs during the vegetative stages. Seventy per cent of the soybean lifecycle occurs when growing plant stems and nodes, followed by about 40 percent of a plant growing branches from nodes. From here, smaller portions of a plant are broken down into seed size development, followed by pollinated flowers and then seeds per pod.

Although most of the plant’s life is in vegetative stages, it is the reproductive stages that are most crucial to soybean yield.

All nodes produce flowers; however, yield on a soybean plant is more located around the middle and upper canopy. This is driven by absorption of light entering the canopy. Less than 25 per cent of light makes it down to the bottom quarter of a soybean plant. Hence the differences in where yield is found on a plant. Soybeans abort higher percentages of flowers in the lower canopy due to differences in light absorption and sugar movement within the plant. Studies have shown that 20 to 80 per cent of flowers produced will be aborted at the R2 stage.

With newer plant breeding, soybeans have changed over time. Plant leaves are smaller and changed in architecture, allowing more sunlight to pass through the middle-lower portions of the plant, reaching lower nodes. Again, the middle to upper portion of a plant are the higher-yielding parts. Hence the reason to make sure this part of the plant is kept healthy from insects, diseases, and other stresses.

Here in Eastern Ontario, keeping soybeans happy means applying fungicides in a timely manner to reduce white mould infection.

In our 2023 white mould fungicide trials, we focused on one application of Viatude fungicide at the R 1.5 to 2.5 stage. Yield results showed a 3.2-bushel advantage to the fungicide applied versus no fungicide gained by yield protection.

The 2023 season was a white mould magnet year. In a few trials, we had two applications applied that showed a 9.9-bushel advantage compared to the one-pass system. There is no surprise here, as flowering occurred during a longer period. This long reproductive stage was compromised by continual wet canopies and soil moisture that aided white mould development.

Interestingly enough, on two plots that were planted across the road from each other, they had varying results. One trial planted earlier, on May 18, with five varieties ranging in maturity from 0.4 to 1.2, saw a 0.5-bushel advantage. The later planted plot, planted May 23 with the same varieties, had a 4.2-bushel advantage. You may ask yourself why this would be. The difference is timing of when flowering occurred, and when most of the crop was experiencing spores from white mould.

In another trial, conducted by P.T. Sullivan Agro, they looked at a range of soybean maturities from 0.3 to 2.8 RM with a total of eight varieties planted. The trial was planted over three planting dates: April 16, May 11, and June 2, 2023. The highest yielding date was April 16 at 62 bushels, followed by June 2 at 42 bushels. The May 11 planting date yielded 40 bushels. White mould plot scores were taken for each planting date. April 16 was 2.25, May 11 was 4.5, and June 2 was 2.2. The May 11 planting date had the highest mould infections (1 being minimal and 5 being heavy pressure). Again, this shows timing effect of critical flowering periods and how they relate to white mould infection.

Going forward, research initiatives are looking at utilizing computer modelling programs that will allow us to get a better handle on application timing to ensure we have better white mould control.

I realize a lot of areas in Ontario do not have white mould pressure like we have in Eastern Ontario, so what would the strategy be in these areas to increase soybean plant health?

One fungicide application in these environments will suffice and aid yield improvements. Timing in the R2.5 to 3 stage is advisable for a one-pass system. Get a good handle on what diseases you have present and what fungicides are most active on those diseases for better returns.

Plant health is the key reason for applying a one-fungicide application in the absence of white mould.

To summarize, yield can be divided into three parts: Total number of pods, number of beans per pod, and the size (weight) of the beans (seed size). What can you do throughout the 2024 growing season to make a positive change to affect one of these three parts?

Wishing you a bountiful soybean cropping season! BF