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David Moseley and Andre Reis, LSU AgCenter Scientist
It is an understatement to say soybean farmers have had a challenging year in 2022. In April, farmers had to plant around several rain events. Despite the several April showers, 59% of the soybean fields in LA were planted by May 1 compared to the five-year average of 43% and 23% from last year. The timely planting season gave hope for a good crop since previous research suggests higher yield potential when soybean is planted in April. By June unfortunately, the perspective changed. Many regions experienced heat and drought stress as the pods were developing. An article from the University of Maryland Extension explains final pod size can be decreased as of result of heat and drought stress during development. In late august, when growers were preparing for harvest and still uncertain about the yield penalty due to the heat and drought, excessive rain fell across the state until early-September. Data from the Louisiana Agriclimatic Information System shows the Chase Research Station had a total of 20.38 inches of rain from August 1 to September 11. From August 18 to September 8, in the south, central, and north regions of Louisiana, there were 19, 18, and 18 days of measurable rain with a total of 5.1, 12.5, and 17.8 inches of water, respectively.
The excessive rain volume and duration provided enough moisture to cause damage to mature and soon-to-mature seeds. As the seed swelled due to moisture, the ventral suture of many pods split open allowing additional moisture to enter the pod and for the sprout to grow out of the pod (Figures 1 and 2). By the end of the seed filling phase, the concentration of abscisic acid hormone is reduced which allows the seed to germinate if certain thresholds of temperature and hydration are matched. More information on seed sprouting can be found in this article from the University of Missouri. Pod splitting and subsequently seed sprouting were possibly exacerbated due to a smaller pod size caused by the heat and drought stress. In addition to seeds sprouting in the pods, most plants maturing towards the end of August have had seed damage ranging from minimum to more than 60%.
While traveling around Louisiana to assess the damage, it was noted that fields planted in early to mid-April were reaching maturity during the middle of the excessive rain event and presented greater damage. On one farm, soybean was planted on April 2 and April 29. On the day of evaluation, the soybean planted on April 2 had severe damage to mature seed, however the soybean planted on April 29 still had green seed with no apparent injury (Figure 3). It was clear that the level of damage was affected to differences in planting date or maturity group. No tolerant variety was identified. At the time of writing this article, it is still too early to have a completely accurate estimate of the seed damage. However, a statewide survey indicates approximately 30-50% of the acres could have quality damage from the excessive rain.
Figure 1. As the seed swelled due to moisture, the ventral suture of many pods split open allowing additional moisture to enter the pod and for the sprout to grow out of the pod
Figure 2. Soybean seed sprouting out of the pod
Figure 3. Soybean seed from three fields (pictures A, B, and C) at a farm in Northeast Louisiana. Fields A and B were planted on April 2 with the same variety. Field A was desiccated before the excessive rain but the field B was not desiccated. The third field (picture C) was planted on April 29.
Figure 4. Soybean pods and seed from the core-block in Franklin parish. The demonstration was planted on April 22. The seed damage consisted of sprouts and weathered seed. There were also green seed that showed minimum damage from sprouting or weathering. There was some damage to green seed which may have been a result of stink bug damage. The many days of consecutive rain limited some field work including spraying for insects and disease.
Figure 5. Many fields with mature soybean looked good after the rain. Unfortunately, the seed was damaged inside the pods.
