July 15, updated July 20, 2010

July 22, 2010 Update

It appears that, based on a May 1 emergence date, most locations east of the Panhandle are running ahead of last year in GDD accumulation. Compared to last year, daily  temperatures have been an average 10 degrees warmer. If normal to warmer temperatures continue, early corn in southeast and south central Nebraska could mature from very late August to early September. Currently, northern Nebraska is accumulating approximately 180 GDD a week and southern Nebraska is collecting 210 GDD.

Western Nebraska and some areas of northern Nebraska are behind normal in GDD accumulations. While projected temperatures for the next two weeks are normal to above normal, little reduction in GDD deficits would be expected since maximum temperatures in the GDD calculation are capped at 86°F (see below). Deficits can be reduced on the low temperature side if they remain above normal.  Expect a 1 unit GDD reduction for each 2°F that the low temperature is above normal.  Maximum temperature normals will begin to drop below 86°F by mid-August in northern Nebraska and August 20-25 in southern Nebraska.  If Nebraska has above normal temperatures in September, there could be a significant reduction in the deficit.

Use the following tables to compare GDD accumulations at specific sites for this time last year with accumulations on the same date this year:

• GDD Accumulation — May 1, 2009 Emergence
• GDD Accumulation — May 15, 2009 Emergence
• GDD Accumulation — May 1, 2010 Emergence
• GDD Accumulation — May 15, 2010 Emergence

Al Dutcher
Extension State Climatologist

July 15, 2010

As summer begins to really heat up, growing degree day (GDD) accumulations will increase, likely reaching the maximum accumulation each day.  GDD accumulations across the state have been near the long-term average, with many sites reporting numbers from 1% to 5% below average to 1% to 3% above average

According to records from the High Plains Climate Center comparing 2009 and 2010 GDD accumulations (Table 1), in every case more GDDs had accumulated by July 8, 2010 than by this date last year.  For examplle, in 2010 in Holdrege 1056 GDDs had accumulated by July 8, while in 2009, the number was 979. The long-term average is 1009, placing 2010 slightly ahead of the norm by 2% and 2009 slightly behind it by 1%.

Table 1 shows similar data for many other reporting sites. The same corn maturity – 2569 GDDs – was used for all sites and wouldn’t typically be planted at higher elevation levels with a shorter season, for example in north central and western Nebraska where GDD accumulations were furthest below the norm.

Early emerging crops may have gotten off to a slower start when much of the state experienced cooler temperatures the first two weeks of May.  For example, at York corn emerging May 1 was estimated to have only 100 more GDD than corn emerging two weeks later on May 15. At Alliance, corn emerging May 7 had only 29 more GDD than corn emerging 10 days later on May 17.  

If normal temperatures hold true for the rest of the season, the corn crop would be expected to mature before the average fall frost date, said Nebraska State Climatologist Al Dutcher.  (Also see GDDs Catching up from Early Season Fluctuations.)

CropWatch GDD and Et Data

CropWatch offers several resources for estimating evapotranspiration (Et) and GDD accumulation.  At the top of every CropWatch page, in the grey bar under the red bar under Weather, you'll find links to 

• Growing degree days and estimated crop water use for various crops and emergence dates for 19 sites
• How to calculate crop water use by plant growth stage for corn, soybean, wheat, alfalfa, sorghum, sugarbeets, and potato

Lisa Jasa
CropWatch Editor
Al Dutcher
Extension State Climatologist