How is the “Golden Number” for the U.S. Corn Belt Working out This Year?

In a previous post, I talked before about the “golden number” for crop production in the U.S. Corn Belt. While it is certainly not the only important yield factor, July precipitation is clearly at the top of the list. Historically, average July precipitation in Corn Belt states is very close to 4 inches. When July precipitation is near or above this average, then corn and soybean yields are usually pretty good. Below this average, then yields tend to disappoint. This is why I call 4 inches of precipitation in July the golden number for yields in the Corn Belt.

So, what happened in July 2023? The chart below shows a mix of outcomes. Illinois (IL), Indiana (IN), Missouri (MO), and Ohio (OH) were clearly above the golden line, while Iowa (IA), Minnesota (MN), and Wisconsin (WI) were below. Iowa and Minnesota were well below 4 inches, with Minnesota more than 2 inches below the average. It will be interesting to see if Iowa and Minnesota corn and soybean yields are as negatively impacted as this simple analysis suggests. The USDA August crop report will be released this Friday, and it will give us our first “official” yield estimates. I will be watching closely whether the state yield estimates in the USDA report line up with this simple but important indicator.

What about July precipitation in Great Plains states, where there is now a bunch of corn and soybean acres? The golden number is lower out west due to a drier climate, with the average being roughly 3 inches for July precipitation. Using this standard, the following charts shows that Nebraska (NE) is looking good, with South Dakota (SD) hurting a bit and North Dakota (ND) hurting a lot. July precipitation for North Dakota was only 1.33 inches, less than half of the golden number for that part of the world. Again, it will be interesting to see whether the state yield estimates for the Great Plains in the August USDA crop report line up with this data.

While July precipitation is certainly important in determining Corn Belt and Great Plains corn and soybean yields, we know that other factors are also important, like June precipitation and July temperature. As a specific example, we know that Missouri was very dry in June of this year, but the state received good rains in July. The precipitation in July also may not have hit the heavy corn and soybean production areas within Missouri. So, like your grandmother always said, take this analysis with a grain of salt.

Finally, if you are interested in a thorough and in-depth historical analysis of the relationship between growing season weather variables and corn and soybean yields, I recommend taking a look at a research report I co-authored with Mike Tannura (a graduate student at the time) and Darrel Good entitled, “Weather, Technology, and Corn and Soybean Yields in the U.S. Corn Belt.” It is definitely one of my favorites.