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Can late rains still make beans?

By any measure, this crop season has been an extreme challenge. Given the long, hot and dry spell that plagued us in July, I’m guessing that by the time you read this in mid-August we may still be challenged with above-average temperatures and below-average rainfall.

While corn has a relatively short “grain fill” window that was less than ideal this summer, soybean fields have a little more time to flower, make pods and fill those pods; all of which can add back some yield, even if the early part of the growing season was rough. Just how rough the early season was can hang with us and impact the bean fields negatively, though, so we aren’t trying to paint too rosy of a picture.

Dry soils early in the season led to monumental struggles to get good stands established, and many of the attempts to fill in or replant stands weren’t successful simply because we didn’t get enough rain in time for the new seeding to contribute to the stands. The good news is that soybeans can often overcome poor stands; we typically say a stand is a “keeper” if there are around 70,000 healthy, relatively well-spaced plants per acre in early June or later. While growers ended up with acceptable stands in most areas, there are parts of fields all across Iowa that thin stands will negatively impact yields this fall.

Hot and dry conditions during vegetative and early reproductive stages of the beans added to the problems. Our own Dr. Andy Lenssen describes the impact of the drought on beans in June and early July: “Soybean plant shoot growth decreased or stopped, but luckily roots can continue to grow.

“This evolutionary response allows the plant to search for additional soil water while having an overall low water use rate. Assuming adequate rainfall occurs again, soybean have the ability to reinitiate shoot growth, and shoot growth rate may be greater than that observed prior to the onset of drought stress. This is called compensatory growth.”

Andy, who is our ISU Extension soybean agronomist at Ames, points out that this short-term, moderate drought stress during vegetative growth stages generally does not impact soybean yield. Conversely, longer-term severe drought stress can cause irreversible plant cell death causing low growth yield. While conditions varied across the state, in many areas our beans were under moderate stress in the vegetative growth stages, so our focus can shift more toward the reproductive stages that started in mid-June.

Soybean yield is most sensitive to water deficits during reproduction. Soil water deficits during reproductive growth phase results in increased flower abortion, reduced pod number, reduced seed per pod and small seed.

Nitrogen fixation is a key biochemical pathway for soybean yield, and nitrogen fixation can be severely limited or completely halted by even moderate drought stress. Once nitrogen fixation has been stopped, substantial precipitation and soil water accumulation is required to reinitiate the process.

Compensatory reproductive growth rarely will occur in soybean under moderate drought stress at reproductive growth phases. In layman’s terms, if a soybean plant has adequate water around the R3.5 growth stage then N fixation will continue and yields could be decent. If N fixation shuts down early, then yield will likely be poor.

Sensitive during pod fill

We are likely in the R4 to R5.5 wheelhouse now, when our beans are particularly sensitive to moisture stress, due to the large demand for water and nutrients. Estimates are the crop uses 0.2 to 0.3 inches of water a day now, if it has access to it.

While water is a primary driver, temperature also impacts beans. Soybeans grow well in a similar temperature environment to corn (ideal temperature for seed development is around 86 degrees F), although cooler nights can be more of a problem for beans in that there can be less movement of starches out of the leaves (this can inhibit photosynthesis the next day).

Soybean seed set and temperatures go hand in hand, and is most consistent when nights around the low 70s are followed by days in the mid to upper 80s. Nighttime temperatures in the low 60s or less and daytime temperatures in the mid 90s or above can adversely affect seed set.

With all that said, late-season rain and more favorable temperatures could help add some size to immature beans in areas with relatively healthy soybean plants still hanging on. The last few growing seasons we have seen late rains fill pods at the top of the plants of many varieties and add a surprising amount of yield.

Here’s hoping we can end our crop season with more favorable weather than we have had most of the summer.

McGrath is an ISU Extension field agronomist, co-leader of the crops team and author of this Corn-Soybean Insight column. Contact him at cmcgrath@
iastate.edu.

Scout for mites

Scout soybean fields for the two-spotted spider mite, which can be a problem during droughts. The tiny pest gathers at field edges; then disperses with wind, infesting fields. Check edge rows first for mites. You may need to spray an insecticide. Work with your ISU Extension field agronomist and local crop consultant for help with treatment decisions.


This article published in the August, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.