How and why we measure degree days

Humans and other warm-blooded creatures generate their own internal heat and have regulatory systems to hold body temperature fairly constant. These systems provide insulation from fluctuations of temperature in the environment and allow growth and development based on the passage of time in minutes, hours, weeks, months and years.

In contrast, most of the creatures we manage in agriculture don't have an internal heat regulatory system and environmental temperatures drive their development. For plants, disease organisms, insects and other creatures, development is keyed to the temperature around them. So if we understand the key temperatures needed for a given species, we often can monitor and predict development based on measuring how much heat each species accumulates, relative to its functioning temperature range.

Readers of the Integrated Crop Management Newsletter should be used to the weekly crop development degree day posting (typically on the back page, starting today for the 2004 issues), and the insect-specific degree day models, including those for black cutworm, stalk borer, and bean leaf beetle.

Scientists have estimated a temperature that approximates the coldest temperature where effective development occurs for many species. That is the lowest cardinal temperature. For some species, there is also a high cardinal temperature, which is a point where growth and development are at their peaks. Listed below are some of the cardinal temperatures frequently used in crop management:

Regardless of the base (lower cardinal temperature), the process to calculate degree days is similar. To model crop or pest growth, we estimate the accumulation of heat on a daily basis. We look at each day as a provider of heat that leads to development. Let's go through this process step by step for a given day and degree-day base, as follows:

  • collect the daily high and low temperatures for a site, or average high and low for a region for that date
  • average the high and low temperatures to estimate the average heating for that day
  • because temperatures below the base temperature contribute nothing to development, whenever the actual low temperature is lower than the base you should artificially reset the low to the base temperature, to estimate the heat received more closely. Adjust the high temperature the same way, if actual highs are greater than the maximum cardinal temperatures.
  • subtract the averaged temperature from the base temperature and you have degree days for that date
  • calculate the heat-unit for subsequent days and add them to estimate the accumulated degree days.

Here is an example. Lets calculate how many base 50°F degree days accumulated on two days in May, for a farm somewhere in Iowa.

Day Low High
May 4 38 69
May 5 55 75

May 4

First, adjust the low temperature to the base (50°F) because no development occurs below 50. That means we average the high (69) and the adjusted low (50), which comes out to 59.5. Subtract the base, 50, and that means there are 9.5 degree days for May 4.

May 5

Repeat the process for May 5. However, because the low is above 50, the low temperature needs no adjustment. As with May 4, we average the high (75°F) and the adjusted low (55), which comes out to 65. Subtract the base, 50, and that means there are 15 degree days for May 5. The two-day accumulation is 9.5 plus 15, or 24.5 base-50 DD.

Although there are some assumptions involved in making development models based on degree-days, the information allows pest management efforts to be well timed and effective.

Minimum and maximum cardinal temperatures in crop and pest management and use of information.

Crop or pest minimum maximum information use
Corn development: 50°F 86°F crop development
Soybean development 50°F 86-90°F crop development
Black cutworm 50°F 300 DD from egg to cutting
Stalk borer 41°F predicting migration
Bean leaf beetle 46°F 2nd generation emergence
Seedcorn maggot 39°F seed treatment on replant
Alfalfa weevil 48°F larval presence in fields
Western bean cutworm 50°F adult emergence/ egglaying

This article originally appeared on pages 46-47 of the IC-492 (8) -- May 17, 2004 issue.

Updated 05/16/2004 - 1:00pm