By George Jared
Erratic weather patterns have impacted row crops in recent years in the Arkansas Delta. One crop in particular has been affected by these weather patterns soybeans.
Farmers typically plant 3 million or more acres of the crop in Arkansas, making it the state’s largest by a wide margin.
Flooding impacts soybeans during all points from planting to harvest. Those impacts include a loss of yield due to a lack of oxygen from high water during flood events early in the season. Sprouting, plant lodging, fungi infestation, and other problems result from too much water in soybeans fields, according to the Institute of Agriculture and Natural Resources.
On the flip side, long droughts are becoming more commonplace as well. Long periods without water can cause soybeans to “pod-shatter” when they are re-whetted.
Finding soybean breeds that are more weather resistant would be beneficial for producers.
Those who use zero-grade fields for rice as their main production system are also interested in flood-tolerant soybean varieties for crop rotation, said Caio Vieira, assistant professor of soybean breeding and a researcher for the Arkansas Agricultural Experiment Station.
Zero-grade fields are leveled to minimize drainage, a water-conservation strategy that has become a key sustainability practice in rice production.
Vieira is the lead author of a study offering more insight into how soybean plants respond to flooding in the critical early reproductive stage when the plant begins to flower. The study, “Impact of flooding at the early reproductive growth stage on soybean yield and seed composition,” was published in the journal Crop Science.
“Flooding research has focused on the early reproductive stage simply because it is when the stress is most pronounced and causes the greatest yield loss,” Vieira said. “Across the Mid-South, soybean is most susceptible to flooding at the R1 stage.”
R1 is the name given to the early flowering stage in soybean growth. There are eight stages of soybean plant reproductive growth, from R1 beginning blooming to R8 full maturity.
Temperature changes in the United States have allowed earlier soybean planting dates so that shifting rain patterns are also resulting in additional stress on the soybean plants. Soybeans are also highly sensitive to flooding during seed germination, the study notes.
“The intensity and frequency of rain has been shifting to earlier in the season,” Vieira said. “We’re pretty much getting the potential for flooding throughout the season. It’s been tougher.”
Soybeans in Arkansas are planted from early April to mid-May, which puts the R1 stage in late June to early July. As growers see the benefits of earlier planting, including improved yields, Vieira said, the early reproductive stage would naturally also shift.
“It can be hit or miss,” he said. “You can get a year where that period is a full-on drought, or you can get a year where that typical R1 period is completely wet with intensive rains. It’s hard, hard to predict.”
Experiment station researchers have also worked to improve drought tolerance in soybeans.
Jeremy Ross, professor and extension soybean agronomist for the University of Arkansas System Division of Agriculture, said he has noticed more farmers planting earlier at the end of February or early March instead of April and May.
“There’s been a big push to plant a month earlier than 10 years ago because we have data showing it leads up to 10 percentage points better production,” Ross said.
Weather pattern shifts have allowed for earlier planting and may assist in avoiding late-season insect problems and possibly outrun some hotter temperatures in July. An earlier harvest has also allowed those early planters to prepare their fields for the following season when the ground is dry in the fall.
Grain yield losses from flooding stress have been well-documented, but Vieira said there has been a gap in literature on the impact of flooding on soybean seed composition.
The study was conducted at the Division of Agriculture’s Rice Research and Extension Center in Stuttgart and compared 31 soybean genotypes over the 2019 and 2020 growing seasons. The plants were exposed to non-flooded and flooded conditions to look at yield and seed composition — specifically its oil and protein content.
A surprising discovery, Vieira said, was that four-day flooding in the early reproductive stage did not significantly alter the soybean seed composition of any of the varieties tested compared to the non-flooded control group.
Visual inspection of flood-damaged soybeans has been useful, Vieira said, but it doesn’t tell the whole story. The study found that some genotypes visually classified as “moderately tolerant” to flooding had higher yields than those classified as “tolerant.”
“Although flood tolerance has been shown to provide yield security under short-term flood exposure, losses are still observed in tolerant genotypes, and little is known regarding the stability of tolerance and the multitude of environmental effects of the trait,” the study noted. “The limitations of subjective visual scoring, combined with spatial variability, emphasize the need for novel and improved methods to accurately classify genotypic responses.”
On average, for each unit increase in flood damage score, the study showed that grain yield decreased by 17.4%. Tolerant genotypes experienced roughly 33% yield losses between flooding and non-flooding treatments, while “moderately” tolerant and “susceptible” genotypes experienced 44% and 51% yield losses, respectively.
Vieira said the study will help his team identify and incorporate flood-tolerant characteristics into future soybean genetics, potentially mitigating flooding-induced yield losses across diverse environmental conditions.
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