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What Makes the “Golden Number” for U.S. Corn Belt Crop Production Golden?

In a recent post, I talked about the “golden number” for crop production in the U.S. Corn Belt in 2023. I called July precipitation the golden number because of its importance as a driver of (dryland) corn yield in the U.S. A state-level review of July precipitation revealed a mixed record across the Corn Belt this year. It occurred to me that some people may not understand exactly why July precipitation is so critical to final yields. Since the annual Pro Farmer Crop Tour is going on this week, I figured it would be a good time to dig into this question.

The corn plant is truly a miraculous producer of calories per acre in the form of its ear and kernels. This is why one can trace explosions in the population of indigenous people in North America during the last three thousand years to the expansion of corn production. In order to achieve this miracle, the corn plant needs to develop as large of a leaf surface area as possible in the vegetative growth phase (for photosynthesis), then hit a switch and direct as much of the plant’s resources as possible toward reproduction. It is no surprise then that the water needs of the corn plant are at their greatest during the reproduction process, as illustrated by this great chart from a University of Nebraska publication.

Source: Kranz et al (2008)

There is one more critical piece of information that one needs to understand. When the corn plant shoots a tassel and starts reproduction, the process cannot be turned off, which leaves the corn plant uniquely vulnerable to drought stress during this stage of its life. There is literally no going back for the corn plant. The classic study estimating the yield risk to drought hitting the corn plant at different stages is Classen and Shaw (1970). Their main results are summarized in the table below. Notice that water deficits that cause corn plants to wilt always lead to yield loss in this study. However, the yield losses are fairly modest prior to tasseling and reproduction. Losses skyrocket during reproduction, particularly the critical pollination period, where yield losses can be as large as 40-50% during severe droughts. While this study is fairly old, I see agronomists still regularly citing it, so I think the estimates are relevant even though corn hybrids are more drought-tolerant than when the study was conducted.

The point here is to not argue about the latest estimate of corn yield reduction from drought during the reproduction phase, but, rather, to show that water availability during reproduction is a critical driver of yield. So, we can finally get to the punch line. Precipitation is crucial to the corn plant during reproduction, which, typically occurs in July in the U.S. Corn Belt. And that is why the golden number for U.S. corn production is July precipitation.

In a future post, I plan to rank the importance of different weather variables on U.S. corn and soybean yields with the aid of a crop weather models like the ones in this publication, “Weather, Technology, and Corn and Soybean Yields in the U.S. Corn Belt.” Should be fun.


  1. Matthew Hickey

    Calling the July precipitation the Golden Number was creative and accurate. Krantz’s drawing made it even more clear. In the early ’70’s, we had several strong storms with ample rainfall between July 1 and 15 in North Central Illinois. That would explain yields between right up to and including ’74. Keep up the good information

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