Is cool weather good or bad for grain fill?
Recent early mornings with temperatures hovering in the upper 40's (F) and afternoon highs less than 80F are certainly not synonymous with those we typically experience during these "dog days of summer". While comfortably pleasant for us humans, there are those who question whether such unseasonably cool temperatures are good or bad for the corn crop as it progresses through the important grain filling period.
All things equal, cooler temperatures during grain fill typically result in heavier kernels and higher grain yield than do warmer temperatures, especially stressfully hot temperatures. Even though the rate of grain filling per day is slower when temperatures are cool, the duration of the grain filling period (number of days) is longer. The advantages of a lengthy grain fill period typically outweigh the disadvantages of a slower grain fill rate per day.
Record statewide corn yields in Indiana have generally occurred in years where average temperatures during July and August were cooler than normal. The greatest positive departures from trend yield since 1990 occurred in 1992 (+15.8%), 1994 (+10.4%), 2001 (+9.5%), 2004 (+13.7%), and 2009 (+9.3%). Four of those five growing seasons were characterized by average monthly temperatures for August ranging from 1.7 to 4.7 degrees F below normal.
Graybeards like myself consider 2004 to have been the nearly perfect growing season, in part due to the season-long moderate temperatures. Daily average temperatures across Indiana in July and August of that year were 2.1 and 3.4 degrees F below normal. Across the Midwest, July 2004 ranked as the 9th coldest and August as the 3rd coldest since 1895 (Midwest Climate Watch). In eastcentral Indiana (Farmland), half of the daily low temperatures recorded during July and August of 2004 were below 60F and four days dropped to 46F. Statewide grain yield averaged 13.7% above the trend yield for that year and set a new record (up to that year) of 168 bu/ac.
So, what does this all mean for the current 2013 growing season? By comparison, as of mid-August, there have been 16 days since July 1 with daily low temperatures in the 50's and a couple down into the 40's at the same eastcentral Indiana reporting station. Average daily temperatures across Indiana for the past 30 days have ranged from 1 to 3 degrees (F) below normal.
While it is true that some fields pollinated during the warm, muggy days of mid-July, most of the state's crop pollinated during much cooler temperatures. While scattered reports have been received describing incomplete or scattered pollination problems, the bulk of the state's corn crop has pollinated well. Much of the state's crop has escaped severe stress during the crucial 1 to 2 weeks following pollination and so kernel survival has generally been quite good. Consequently, I believe the table is set for maximizing kernel numbers and ear weights in many fields.