AMES, Iowa -- Severe rainfall over the last week to 10 days has caused many concerns for many farmers. In some areas more than 15 inches of rainfall has caused many low lying areas to go under water. What will happen to those crops?
It’s hard to tell, according to Palle Pedersen, soybean agronomist with Iowa State University Extension. “Most of our research dealing with excessive rainfall has been conducted in the spring when we often are trying to estimate if we need to replant. However, it isn’t very common to be in the situation that we currently are in. For that reason, no data have been published on this kind of scenario.”
The rain first really started in the middle of August, Pedersen said. “At that point most fields were at R5.5 or R6. From approximately R1 to around R5.5 the daily rate of dry weight accumulation by the whole plant is essentially constant. It will gradually decrease during the late seed filling period (shortly after R6) and measurably stop shortly after R6.5. Therefore, the rain came at a good time since it will help to maximize the seed size. In the areas where the plant was under stress from the excessive moisture, there probably will not be a huge impact on yield size because the dry weight accumulation was about finished.”
However, for fields with standing water, the scenario is different, he said. “If your crop is completely under water it will die within 24 hours because of the high temperatures that we currently are dealing with. On the other hand, what if the crop is standing in six to 12 inches of water? That situation is more of a challenge, and we do not have any data to explain what may happen. It is speculated that the crop probably will rot if it is sitting in this water for too long. I would guess that the crop could remain in water for 48 to 72 hours with the current temperatures. If crops are in water for longer periods, stems will rot, the crop will lodge and then the seed also will rot.”
Another issue is insect feeding, Pedersen continued. “Often second generation bean leaf beetles will feed on the pod, so moisture can penetrate the pod wall and the seed will then rot. Finally, we also may have to deal with a lot of foliar diseases that we often don’t have to worry about in Iowa, such as frog eye, cercospora and downy mildew. Phomopsis also will be an issue and will hurt seed quality. For all diseases, however, there is a huge difference among varieties.”
What’s the bottom line? “There isn’t a lot that we can do, and we don’t have a lot of data to accurately document the impact, since late season flooding is not very common,” Pedersen said. “Looking at the fields now, it seems that the worst areas are the low lying areas where we often don’t get too much yield anyway. However, the areas in north central Iowa and south central Iowa could see significant yield loss if the water doesn’t recede quickly.”