soybean pods
TIMING IS EVERYTHING: Late July and early August rains can help soybeans retain pods, while rains later in August help plants fill the pods.

Synching soybeans with rain

Soybean Source: Timely late-season rain can rejuvenate a drought-stressed soybean crop.

It might be time to update the old saying “rain makes grain,” especially if talking with farmers from parts of Iowa that fought excess moisture for most of the season so far. For those of us in the southern parts of Iowa that the Drought Monitor has colored in with various stages of drought, “rain makes grain” may be more relevant if it comes in time. Rainfall too late in the season, for any of us, can make harvest and other fall work a nightmare.

“Rain in the right amounts and intensity at specific points in the growing season makes grain” might be more accurate —but probably won’t catch on. I’ll let somebody else take a shot at updating the old adage while we move on to talk timing. Can rains later in the season “make grain”? As usual, it depends on a lot of factors, but in general late-season rains benefit soybeans more than corn.

How those late rains help beans
While corn has a relatively short “grain fill” window, soybeans have a little more time to flower, make pods and fill those pods. This longer time frame can potentially help bean yields even if the earlier part of the growing season wasn’t the greatest. I don’t want to sell too much blue sky though. As any producer can attest to, beans can take a lot of abuse, but eventually adverse weather will take a toll.

While the reasons may vary, this year there are more than a few places across Iowa that have had a rather rough growing season that has already taken a pretty good bite out of yield potential. Timely, late-season rains might still help bean fields like those, but to a lesser degree. 

Just the ‘FACTS’
This is probably a good time to promote the “Forecast and Assessment of Cropping sysTemS” (FACTS) tool from Iowa State University Extension. Find it at crops.extension.iastate.edu/facts. While the scope of the project is much broader, we’ll focus on the real-time measurements and forecasts for weather, soil water and nitrogen, crop water and nitrogen, yield predictions, crop staging, and heat-frost stresses that the FACTS team provides during the growing season. To cover our major soil landform regions and climatic conditions, they monitor 30 crop-management replicated treatments at nine locations across Iowa.

FACTS may sound like an overwhelming amount of information, but if I can get the hang of its website, it’s a good bet that most folks will get along just fine. A good place to start is with the “Forecast tool” on the website. Pick whatever crop and location you want to look at first, and hit the “Yield and staging,” “Weather,” “Water and nitrogen” and “Summary” tabs. You’ll get the hang of it quickly and might find yourself looking at what FACTS says about corn and soybeans at other locations across Iowa.

To circle back around to what we were talking about earlier, FACTS illustrates some of the concepts for us. The Summary tab shows that yield potential varies more than I typically expect between regions, with some regions taking a fairly big hit that could tie directly back to stresses earlier in the season. With a little more digging into the FACTS data, we can try to guess if the rain-makes-grain adage holds water. In many regions, perhaps even some fields in regions that had excess rainfall earlier in the year, timely late-season rain could increase bean yields as things stand in mid-August.

Tough time to run short of water
While too little (or too much) water at planting time is hard on beans, once they get established, they are rather resilient through the vegetative stages. When they hit the reproductive stages, beans get a little more demanding when it comes to water. Soil water deficits during reproductive stages can result in increased flower abortion, reduced pod number, fewer seeds per pod and smaller seed. Nitrogen fixation can be severely limited or even stopped by moderate drought stress, not a good thing for yield either.

To break it down a little more, early on during the reproductive stages, beans can handle short-term water stress fairly well. While they may lose some flowers or pods, yield potential is in pretty good shape if the moisture stress is short-lived. If rains don’t come and we see extended moisture stress —which is the case in much of southern Iowa — well, most of us know the potential implications. But depending on how much stress the beans were under earlier (and a lot of other factors), late-season rains can make a huge difference.

Soybean research done under irrigation tells us that perhaps the most important times for soybeans to have adequate water are during pod development (R3-R4) and seed fill (R5-R6). Rainfall could help a lot of beans in Iowa in that R5-R6 ballpark, probably through August, with some fields possibly benefiting into early September. As we approach beginning maturity at R7 — when one normal pod on the main stem has reached a mature pod color like tan or brown — we can shift gears and hope for relatively dry weather for harvest, as rainfall will be more apt to create headaches than increase yields.

Temperature’s role
While water is a primary driver, temperature impacts our beans as well. Soybeans grow well in a similar temperature environment to corn (ideal temperature for seed development, much like corn, 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.

So, to wrap things up, take a look at the ISU FACTS website if you have time. While rain is the last thing some areas need, in others some late-season rain could make a big difference. Favorable temps could help improve yields for about everyone. Good luck the rest of the season and be safe.

McGrath is the on-farm research and Extension coordinator for the Iowa Soybean Research Center at ISU. Contact him at [email protected].

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