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May 2018 Growing Degree Days, Precipitation & Drought Monitor

Growing degree days (GDD), also known as heat units, are calculated by taking the average of the high and low temperature in a day, minus the base temperature of 50°F. The optimal number of growing degree days varies depending on the crop and maturity. Tracking growing degree days is not only helpful in monitoring your crop progress throughout the growing season, but it can also be used in determining when certain insects are more likely to occur. It is no surprise to see in the graph below that Des Moines, Red Oak, and Fairfield have accumulated many more growing degree days so far this year than the northern part of the state.

The second graph shows a comparison of GDD for a few years between 2012 and 2018 at various locations. As you can see, the 2018 Iowa average GDD totals were behind the other years early on, but have recently caught back up to pace with other recent years such as 2017, 2016, and 2014. Most locations shown are now at a cumulative GDD similar to the past few years since recent temperatures have been extremely warm to make up for the cool April.

The map below shows the cumulative precipitation beginning in April for several locations across the state. Charles City and Des Moines currently have the highest totals out of these locations, while Red Oak is estimated to have the least cumulative rainfall since April 1, 2018. As you can see, several of these locations went from the middle of May until the end of May without many significant rainfall events, so the scattered showers across the state this week were much needed in many locations. The current drought monitor for Iowa is also shown below – as you can see, much of the bottom third of the state is either abnormally dry or suffering from a moderate drought as of May 29th.

Michelle Mensing, Research Analyst, Decision Innovation Solutions (DIS). DIS is an Iowa-based economic research firm which provides regular farm economic research for Iowa Farm Bureau staff & members.



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