Does It Make Sense to Switch to Earlier Maturing Wheat Varieties?

Very little if any spring wheat has been seeded up until now.  This will be a very late seeded crop and that begs the question of whether you should switch to earlier maturing varieties.  The simple answer is 'Probably Not'.  I'll try to explain below why.

Most, if not all, spring wheat varieties that are grown in Minnesota are daylength insensitive.  That means that their growth and development are temperature-driven. As heat units are accumulated each day, the crop advances in its development.  This is a linear function from emergence through the end of the grainfill period.  The earliest maturing varieties need about 5% less and the latest maturing varieties need about 5%  more heat units to reach each point in their development.  We will start collecting more heat units per day as the season progresses, after all, average daytime highs and nighttime low temperatures should increase if summer is to be. 

As a result, the crop will need fewer days to reach the same growth stage compared to a crop that was seeded much earlier.  Using the 30-year average daytime high and nighttime low temperatures in Fargo, I calculated the number of days it takes the crop to reach physiological maturity starting on April 15 and every two weeks after that up to June 30 (Figure 1).  The bottom curve in green shows that it takes the spring wheat crop about 78 days to reach physiological maturity when seeded on April 15 and the number of days to reach physiological maturity while a crop seeded on June 15 and June 30 reaches physiological maturity in 54 and 53 days, respectively.  

Because the relationship between the number of accumulated heat units and crop development is linear, the differences between early and late maturing varieties get smaller as seeding is delayed; a 10% difference of 78 days is larger than a 10% difference of 53 days.  Simply put - the maturity differences get compressed and you are not gaining as many days as the reported differences in heading dates suggest.

But will the crop have enough days to dry down after it reaches physiological maturity?  Once the crop has reached physiological maturity differences in maturity do not matter any longer.  The remaining canopy and mature grain need to lose enough moisture to allow for mechanical harvest. Unfortunately, the number of days favorable for dry-down dwindles as the planting dates get later and approach fall (after all, the return of winter is a certainty in Minnesota). To give you an idea, I used the same climate data and calculated the number of days with daytime high temperatures above 68F from the day the crop reached physiological maturity until the end of the month of September.  The choice of 68F was somewhat arbitrary but shows the trend well.  The top curve in red shows the later seeded crop will have fewer days to dry down and the decline in 'drying days' accelerates as seeding is delayed (Figure 1).

Figure 1 - Number of days from seeding to physiological maturity and the number of days (defined as the number of days with daytime high temperatures above 68F) from physiological maturity to the end of the month of September in Fargo for six different planting dates using the climate normal average daytime high and nighttime low temperatures

As you go north in the Red River Valley the number of days to reach physiological maturity takes longer over the whole range of planting dates and the number of 'drying days' decreases faster compared to Fargo (Figure 2). Using the NDAWN station in Humboldt, the difference in reaching physiological maturity was about 4 days at the latest seeding date.  The decline in the number of 'drying days', however, was 11 days or nearly a third when compared to Fargo.  Again, that shouldn't come as a surprise as average daytime highs and nighttime lows are just a few degrees less when compared to Fargo.  But again, switching to an earlier maturing variety does not allow you to make up as much as you would like to think or need. 

Figure 2 - Number of days from seeding to physiological maturity and the number of days (defined as the number of days with daytime high temperatures above 68F) from physiological maturity to the end of the month of September in Humboldt for six different planting dates using the climate normal average daytime high and nighttime low temperatures