A few weeks ago, I was asked whether the wheat crop would catch up in its growth and development now that planting was delayed compared the last few years. To explore this question we went back into the NASS crop progress reports between 1990 and 2017 and gleaned the date that seeding commenced, reached the halfway mark, and was near completion in Minnesota. We then used those three dates to calculate the heading date using the Fargo NDAWN station.
Figure 1 shows the three regression lines that resulted from this exercise. The blue line represents how much quicker the spring wheat cropped reached heading when planting was delayed from April 1 through the end of the month of April.
The green line represents how much quicker the cropped reached heading when planting was between April 15 and May 15 and the red line represents how much quicker the crop reached heading when planting was delayed from the beginning of May through early June. Each of the three regression lines explained about 80% of the observed variation in the data.
The loss of the number of days to heading, i.e. the faster the pace of development as planting is delayed, is most severe in the first period and the least severe in the last period. This may seem a bit counter-intuitive but think of it this way – in early April the average temperatures are much lower than in early May or June and thus fewer growing degree days are accumulated each day.
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Planting delays will move the crop into a time period where the average differences in daytime high temperatures and nighttime lows are smaller when compared to the previous two week period.
So what does this mean in terms of actual heading date? Using the three regression lines, a crop seeded in April 1 near Fargo is expected to head on June 10, while a crop seeded on May first will head on June 22. The difference of a month in seeding date is reduced to less than two weeks.
How does this correlate to yield? The relationship with yield is less clear than with days to heading. In the same analysis, the yield loss was about ¾ bushel per day of planting delay when using the mid-point planting date data set but the model only explained about 105 of the observed variability.
This suggests that yield potential is reduced as planting is delayed but that weather conditions during grain fill (i.e. nighttime and daytime temperatures) and absence or presence of disease (remember the early nineties are included in this data set) have more to do with the final yield than the fewer number of days to heading.