June in this part of the world can be a tough month for soybeans. Depending on growing conditions and other factors, crop yellowing can occur along with periods of slow or stunted plant growth. But what are we really looking at here? What’s normal and transitory and what should we worry about?
The Period of N Starvation – Often Forgotten
All soybean plants, to a greater or lesser extent, go through a period of nitrogen starvation early in their development. This occurs at the point where the nutrition within the cotyledons, that fueled the plant up until that point, is running out and future growth will depend on the plant’s ability supply itself with nitrogen via nitrogen fixation. What is often forgotten is that nodule formation and the start up of N fixation demands a lot of energy from the plant. Energy that the young plant with its relatively small root system, can be hard pressed to supply depending on growing conditions. General yellowing of plant leaves and slow plant growth during this time is not unusual and will be transitory as N fixation takes off. However, there is something that should be monitored…
Has Nodulation Begun?
By the time your soybean plant gets to the V2 growth stage, nodule formation should be starting unless the field you are in has very high residual soil N levels that likely should have been avoided. Bear in mind that the objective here is only to verify that nodulation is starting. A later evaluation of how well the plants nodulated should best carried out at R1. For this first early season evaluation, follow these steps:
- Assess root nodules on 5-10 plants from 2-3 different areas of the field.
- Use a shovel to gently dig out each root system. At this stage the few nodules that are on the plant are very small and can be easily torn off. If soil cannot be gently removed from the plant root, soak the roots in a pail of water to remove soil. Make sure to inspect the root base or crown area as this is where nodules usually form first, particularly if a seed applied inoculant product was used.
- Verify that nodules are starting to form on the roots. Basically, you are only looking for little bumps starting to form on the roots as root hairs are transformed into nodules through the interaction of the plant and the Rhizobia bacteria. Some of these bumps may be starting to look a bit spherical. Our staff often bring along a magnifying glass to make this inspection easier. You may notice that some of the nodules are displaying a bit of pinkish-red colour, particularly if you squash them. That tell us that N fixation is starting.
An Opportunity to Evaluate Seedling Health…
Since you are digging up plants anyways, take the opportunity to look for any root rot that may be present. In particular look to for root pinching off or near the soil surface. That constriction of plant tissues is often the first sign of root rot setting in. Take note of the colour and firmness of the root tissue. White roots that are firm to gentle squeeze are a good sign. Conversely, brown or black root discoloration and the root feeling mushy to the touch indicate that problems have set in. Lastly, the shape and growth pattern of roots can provide important information. For example, roots growing to the side or even taking on a bit of corkscrew look can be an indication that compaction issues are hampering plant development.
For more information on seedling diseases, Click Here!!
IDC – The Annual June “Yellow Flash”
Symptoms of IDC (iron deficiency chlorosis) include interveinal yellowing (chlorosis) of new growth, where leaf veins remain green. In extreme cases, interveinal browning (necrosis) can occur. Symptoms show up in soybeans as early as V1 (1st trifoliate).
Prior to V1, cotyledons supply stored iron (Fe) to the soybean plant. Once this source of Fe is depleted, plants must acidify their root zone to access it in a plant-available form from the soil. Despite the abundance of Fe in Manitoba soils, factors such as excess calcium carbonates, soil moisture, soluble salts and/or high nitrate levels can impede Fe uptake into the plant and increase the risk of IDC. Wet years can bring soluble salts to the upper soil profile. If dry conditions follow, it is expected that these salts remain in place, increasing the risk of IDC.
Producers should monitor the patterns, persistence and severity of IDC in your fields. While it seems that some IDC shows up on most soybean fields in Manitoba every year, the condition should be transitory, relatively short in duration (a week or two) and plants should fully recover. In fields where severe symptoms persist over multiple weeks, changes in management should be considered.
For detailed information on IDC in Soybeans, Click Here!!
Since You’re Down There – How About a Plant Count?
We’re just never going to stop bringing this up!
In terms of your soybean costs of production, seed is likely your most expensive item . A lot of consideration should go into targeting plant populations that will set you up best for reaching maximum yield potential. Assessing plants stands in June is a fast and easy exercise that will quickly let you know if you are hitting you plant stand targets. And if you are not, plant stand assessments can give you important information to help you figure out why not. Over years, the knowledge gained from annual plant stand assessments will improve your ability to set your equipment for the desired depth and placement, to adjust to changing spring soil moisture conditions, to account for specific seed lot quality concerns and to minimize seed damage from handling.
For more information on assessing plant stands, Click Here!!
Other Factors to Consider – Unavoidable Stress Stacking
Nothing happens in isolation in a soybean field. Plants are experiencing several factors at once and stresses can accumulate to cause plants to be pale and underperforming. In other words, stresses stack up and June is the most likely month for this to happen. Besides the factors already discussed, consider that these three additional items that are likely at play:
Puberty – Soybeans exhibit photoperiodism which means they measure night length to determine when to go reproductive and flower. They initiate flowering when nights become longer which is typically after the summer solstice (June 20 in 2025). The initiation of this significant physiological change will cause stress in a soybean seedlings that are already taxed. The larger the soybean plants are before the summer solstice (more trifoliates) the more quickly they begin to flower. By controlling what we can to minimize other stressors, we promote rapid plant growth and development which helps plants with tolerating the unavoidable.
Herbicide Applications – June is when most herbicide applications happen to emerged soybean crops. Every application of herbicide has some amount of metabolic cost to the crop plants taking up the applied product. On its own, this cost is usually quite minimal for most herbicides but, in a stress stacked environment, it all adds up. We advocate the early and aggressive control of weeds in soybeans. Healthy plants that are growing rapidly can better tolerate this unavoidable stressor.
Environmental Stress – Manitoba weather is variable no doubt but, from the perspective of soybean production, June is likely the worst month for environmental stress on the crop. Temperatures, in particular, can fluctuate from summer heat to near freezing cold in quick order. Physical signs of environmental stress on soybean plants often include discoloration (yellowing, tissue necrosis) and deformation (plant stunting, leaf shape and texture deformities and loss of apical dominance resulting in multiple shoots). The rule remains – by controlling what we can to minimize other stressors, we promote rapid plant growth and development which helps plants with tolerating the unavoidable.
