It seemed to take forever this spring, but hopefully all of your soybeans are planted – for the first and only time. Ohio’s biggest challenge is replanting; it is costly (new seed, cost of planting, lower yields due to delay in planting). The first step is assessing overall stand health – do you have enough plants to obtain the best yields? Based on a substantial amount of data, for soybeans planted in May, a harvest population of at least 100,000 plants/acre is generally adequate to maximize yield. Data from the Ag Crops Team on-farm trials indicate that a stand of 50,000 plants/acre only reduced yield by 15% compared to a stand of 116,000 plants/acre (when planting in May). Soybeans have the ability to compensate for low populations by increasing the number of branches, nodes, and pods per plant.
After evaluating the stand, the next step is to diagnose what happened, was it disease, insects or slugs, herbicide injury or flooding injury? Each of these will require a different approach when it comes time to replant the field. The more accurate the diagnosis, and the shorter the time frame to replant limits the yield losses that may occur due to any delay in planting. Yield loss resulting from delayed planting ranges from 0.25 to 1.0 bu/acre/day.
- Diseases – there are numerous soil borne pathogens that can attack seeds and seedling and they can be broken into two groups: true fungi and watermolds. The watermolds
require free water in the soil – these are most common when 2 or more inches of rain fall between the day of planting and when seedlings emerge (~ 14 days). More than 30 different species of Pythium as well as Phytophthora sojae have contributed to seedling blights in Ohio. In fields where the disease was caused by watermolds, the seed should be treated with a combination of fungicides that are specific for watermolds. If the seed or seedling has deep brick-red lesions, this is often caused by Rhizoctonia solani. There are several fungicides that have very good efficacy towards this pathogen as well as Fusarium graminearum. Figures 1-4 - Early-season pests of soybean can be above or below ground. Below-ground pests include seed corn maggot, wireworms, and white grubs. These can reduce stand by feeding on seeds and roots, and there are no rescue treatments. Aboveground early season pests of soybean include bean leaf beetle, cutworms, and slugs, which feed on stems or foliage. If the main stem is cut or nibbled to nothing above the cotyledons, the plants will usually regrow. If plants are cut off below the growing point below the cotyledons, they will not recover. So a certain amount of feeding on foliage, and stem above the cotyledon is tolerable. In vegetative soybeans the plant can later compensate for as much as 30 to 40% defoliation. In cases where feeding is more extreme, a number of foliar insecticides are labeled for soybean pests. There are fewer, but some, options for controlling slugs (https://ohioline.osu.edu/factsheet/ENT-20)
- Flooding injury occurs when plants are submerged for more than 48 hours. Places in the field where ponding occurs can cause the CO2 to build-up in the soil and killing many of the organisms, including soybeans. When you are walking through these fields, there is often a strong smell associated to the area where the flooding occurred. Focusing on drainage is warranted for these fields.
- A number of residual soybean herbicides can cause minor stunting of new stands, which is usually readily outgrown and not of much concern. Historically, chlorimuron has been an active ingredient that has caused stunting most frequently, especially under high soil moisture conditions. Stunting can be more extreme from chlorimuron compared with other herbicides, requiring more time to be outgrown, but still should not reduce yield. Products containing flumioxazin (Valor etc.) and sulfentrazone (Authority etc.) have generally caused the most frequent and severe early symptoms in our research plots, and around the state. Part of the reason for the frequency is the extremely widespread use of these products under a broad range of tillage and weather conditions. Injury symptoms can consist of stunting, leaf malformation, and leaf necrosis, but not usually stand reduction. We observe injury from these products most often in our conventional tillage plots when they are applied immediately following planting. Injury has been much less frequent under no-till conditions, where we have most often applied them a week or more ahead of planting. Based on the previous comments, combining either of these actives with chlorimuron can result in higher risk of injury compared with other combinations. We have observed soybeans to eventually outgrow injury of even 25% or greater, but row closure can be delayed compared with uninjured soybeans. Our primary recommendation to minimize injury from these products is to apply a week or more ahead of planting.
Herbicide injury – during the past 2 seasons we have been able to replicate the herbicide injury that can occur when PPO herbicides are applied to soybeans after planting. This is not recommended, but we can all appreciate in Ohio how tough it is to get that herbicide down 2 weeks early. Especially in years when the soil temperatures never got above 40F, winds did not die down until it was 2 weeks after the best calendar date to plant! This study is one of those we do in Extension to remind us why we have these recommendations in place to begin with. The study was planted at two locations, Northwest Branch and Wooster, for the evaluation of herbicides applied 2 to 3 days after planting. The damage from each herbicide is in the next series of Figures. Special Thank you to Pioneer Seeds for supplying the seed for this study so we could take it to yield.