A sentiment I heard a few times this past week is that the wheat crop is heading really early and some question why. Growth and development of HRSW in this region is - in absence of stressors like excess water or low fertility – mostly driven by temperature. Using daily minimum and maximum temperatures allowed Armand Bauer and A.L. Black at the USDA-ARS Northern Great Plains Research Lab in Mandan to develop a robust growing degree models that predict the growth stage of spring wheat. The NDAWN weather observation network uses this growing degree model to predict the growth stage of spring wheat (https://ndawn.ndsu.nodak.edu/wheat-gdd-multiple-planting-dates.html). Their model is for an average maturity variety and can be to be corrected for differences in maturity by a simple coefficient for relatively maturity.
Comparing how heat units have accumulated this season allows us to compare whether the crop is developing faster than other years. Figures 1 through 3 summarize how the growing degree days have accumulated in Grand Forks since April 15 in 2010, 2015, and 2016. Last year growing degrees accumulated slower than either 2016 or 2010 (Figure 1). Upon closer examination of the daily minimum and maximum temperatures it becomes clear that, although 2015 has lower maximum temperatures in the second half of May and the first half of June than either 2010 or 2016 (Figure 2), that the difference in even more pronounced when we look at the daily minimum temperature (Figure 3).
So what does this mean in terms of yield potential? In general, a slower accumulation of growing degree days means higher yields. When the same number of heat units are accumulated, having them accumulated with a larger difference between the minimum and maximum daily temperatures is more advantageous as long as the maximum temperatures doesn’t exceed 85F too long during the daytime. In short, the yield potential at this point in time of the development is slightly lower than what it was in 2015 but similar to 2010. However, yield potential is not the same as actual yield and, especially, the nighttime temperatures during the grain fill period will be very influential.
Figure 1 – Accumulated growing degree days for HRSW seeded on April 15 near Grand Forks in 2010, 2015, and 2016.
Figure 2 – Accumulated deviation from the 30-year climate normal daily maximum temperatures in Grand Forks after April 15 in 2010, 2015, and 2016.
Figure 3 – Accumulated deviation from the 30-year climate normal daily maximum temperatures in Grand Forks after April 15 in 2010, 2015, and 2016.