Degree Day Information for Iowa

Departures from normal

The map shows the seasonal departure from long-term averages in base 50 °F degree day accumulations. As we have mentioned before, crop maturity once reproductive phases are reached becomes more dependent on day length and total stresses. Stresses include drought, hail, crop disease, possibly nutrient deficiencies, and other factors. Note that these data reflect deviations from normal within that crop reporting district. The southern 3 districts in Iowa normally accumulate about 300 base 50 °F DD more than the northern 3 districts.

From this chart, 2004 is so far the coldest growing season in the 2000s, while crop year 2002 was the warmest, just nominally over the long-term average accumulations. And 2002 also netted the highest average yields, but remember that late season stresses (that were generally light that year) play a major part in determining yield. The lowest yields of the period were in 2003 for soybean and 2000 for corn. Soybean aphid and late-season drought played special havoc with soybean production last year; corn yields remained quite good.

So what does this all mean? Obviously, weather does play a role in crop development and production, but crop production involves more than just measuring heat. Integrated crop and pest management calls for understanding all the input costs and effects and then fitting them into your plans.

More about degree days:

How and why we measure degree days (5/17/2004)

Degree days and crop management (4/6/1998)