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Issue No. 20, Article 2/August 7, 2009

It's Been Cold and Wet--For Soybeans' Sake, Let's Not Get Too Hot and Dry

I don't need to remind anyone that we had one of the most delayed soybean planting schedules in Illinois history this year due to high rainfall in April, May, and June. July rainfall was close to normal statewide as well, so very few areas have below-normal soil moisture as we head into August.

In addition to late planting, we just logged 2009 as the coldest July on record for the state, with average daily temperatures (°F) of 70.4, according to the Illinois State Climatologist Office. In second to fifth place in the record book are 71.5°F in 1924, 72.0 in 1967, 72.4 in 1950, and 72.5 in both 1915 and 1947. You can see that it's been a few years since we've experienced this little heat in July. For most Illinois locations, growing degree days are 100 to 200 units behind for May planting dates. You can use the crop degree-day calculator on the Illinois State Water Survey website (www.isws.illinois.edu/warm/cropdata/cropddcalc.asp#) to determine accumulations for your area of the state based on planting date.

USDA NASS Weather and Crops reports have indicated 63% of soybeans were blooming and 18% were setting pods on August 2. This is compared to 71% blooming and 29% setting pods on the same date in 2008, and 89% blooming and 58% setting pods for the same date for the five-year average. It is apparent that crop development continues to be behind, as it has been all year. I'd like to tell you I know exactly what all this means in relation to soybean yield expectations, but I can't, because I don't know. We are certainly in uncharted territory in terms of the time of year our soybean crops will advance through their upper reproductive stages, including seed set and seed fill. I suppose one could easily fearmonger, but final yield has certainly not been determined in our soybean crops yet, so let's examine a couple of thoughts.

  • Many experiments have found that increasing the number of soybean flowers results in increased pods and subsequently increased yield. Also, the earlier in the season canopies are closed and reproduction starts, the longer plants have to "make" yield during pod set and seed fill. So the slightly delayed growth and flowering doesn't seem to be in our favor for making the most from our late-planted soybeans.
  • On the other hand, we also know that soybeans produce roughly 2/3 more flowers than the number of pods they retain. That is, for every three flowers soybeans produce, only one has a chance at becoming a seed-bearing pod. Whether a pod is aborted or retained is regulated primarily by plant stress. So if temperatures remain average to cool coupled with adequate soil moisture, there is a chance that most of our soybeans will experience less stress during these critical reproductive stages. If this equates to an improved percentage of retained flowers and pods (less flower and pod abortion), then we could end up with as many seed-bearing pods as we might have had there been a longer period of reproduction with several intermittent periods of extreme heat and/or drought.
  • The best comparison we can make at this point is to 2008. Statewide flower and pod numbers are lagging behind last year at about the same rate as did the percentage of acres planted this spring. Furthermore, we had close to "average" yields and above-average expected yields when they were regressed on the dates that 50% of the state soybean acres were planted. What saved our bacon last year? It was mild and moist late-season growing conditions.

Maybe I'm overly optimistic, although I'm rarely accused of this, but I still think we can achieve yields that are average for our expectations after accounting for the delayed planting dates. This will require, however, no higher than average August temperatures and continued adequate soil moistures to maintain low-stress environments as soybeans continue to develop flowers and pods and to fill seeds. A hot and dry August and/or early September could be very detrimental to soybean yields.

Unfortunately, weather forecasts are calling for dryer and warmer weather than we've been experiencing so far, so all we can do is hope for the best. The only real advice I can offer is to keep scouting and make sure you are protecting what yield potential you do have from pests when thresholds call for action. One bright spot is soybean aphid numbers, which have remained low to date, but you should continue to scout for soybean aphids all through August. In addition to scouting, you can check the Regional Soybean Aphid Suction Trap Network. Critical yield developing stages in soybean are certainly still ahead of us.--Vince M. Davis

Vince Davis

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