Impact of Planting Date and Growing Degree Day Accumulations in Winter Wheat

Cereal development can be determined by following GDD accumulations.  GDDs can be calculated by taking the maximum and minimum daily temperature adding them together and then dividing them by 2.  You then subtract the base temperature for winter wheat which is 0.  The number you get is the number of GDDs accumulated for that particular 24 hour period.  If the daily mean temperature is equal to or less than the base temperature (base temperature for cereals is 0), the degree-day value is zero.  In order for cereals, or in this instance winter wheat, to germinate they require approximately 80 GDDs and another 50 GDDs to emerge.  So in total winter wheat requires at least 130 GDDs in the fall just to emerge!  While that may not seem like a lot, as we push dates further and further the less GDDs we accumulate before the crop goes dormant and the less yield potential we have.

 

Ontario research has shown that for every day we delay seeding beyond the optimum seeding date for a region we lose 1.1 bu/ac/day.  You’re probably thinking that isn’t a big deal but what if you have to delay your seeding by 7 days?  Suddenly that yield loss jumps to 7.7 bu/ac and if you delay even further to 14 days beyond your optimum seeding date that yield impact is even greater at 15.4 bu/ac.

 

To give you a sense of how this works, I tracked the GDDs for a number of locations across Ontario with the help of historical data from Environment Canada (Table 1).  If we look at the GDD accumulations for winter wheat planted September 15th (blue bars) until October 19th, with the exception of Kapuskasing, all locations received more than 300 GDDs to allow the crop to germinate, emerge and put out some tillers in some cases before this winter.  The more fall tillering that occurs not only maximizes the yield potential of the crop, but it also increases the chances of winter survival.  As you continue across the bars and look at the accumulation since September 29th (green bars), you can see there has been a decline in GDD accumulation and there are still some locations that have not received enough GDDs to allow the crop to emerge.  If you look even further at fields planted since October 6th (purple bars), even fewer locations have received enough GDDs to emerge.

While we will continue to accumulate GDDs, you can see that the trend is showing a decline in accumulation and therefore a reduction in the growth of the crop this fall.  Additionally, as wheat is exposed to temperatures below 5°C for a period of time, vernalization is induced.  As soybean harvest continues for some, it is important to keep this in mind as you make your decisions on winter wheat seeding.  Keep an eye on the forecast and try some GDD calculations yourself.  Then ask the questions, “will I have enough GDDs to get the crop to a point where I am confident it will have a chance to survive the winter?  What are the conditions at seeding going to be like?  Do I have my populations high enough to compensate for a reduction in tillering?  Should I consider adjusting my crop plans and consider seeding a spring cereal instead?”

 

I also challenge you to think about your crop plans for soybeans next spring to allow you to better incorporate wheat into your rotation.  Target shorter season soybean varieties in those fields that are intended for winter wheat next fall.  Some will argue that selecting a shorter season soybean variety doesn’t result in a huge impact on winter wheat seeding date.  However, Ontario research has shown that by selecting a variety that is 0.5 Maturity Group (MG) less than the target for your area will result in the crop being harvested an average of 5 days earlier.  If you select a variety that is 1.0 MG less than the target for your area will advance the maturity by up to 9 days compared to an adapted variety.

 

The other argument that is often made is that you will have to sacrifice too much yield to justify going to a shorter season variety.  However, the Ontario Soybean Performance trials show that by selecting a 0.5 maturity group less than your adapted area results in an average yield loss of 2.0 bu/ac based on normal seeding dates.  There are of course exceptions to the rule.  While regions to the south often see less of a yield impact, regions further north or east may see an even greater yield loss.  Additionally, if you are already growing a longer season variety and planting early the yield sacrifice may be even greater when switching to a variety that is 0.5-1.0 MG below the target for your area.  However, if we use the average loss of 2 bu/ac and the current price of soybeans at $11.50/bu that means a sacrifice of $23.00/ac.  While that may sound like a lot, if you can plant your wheat even 5 days earlier and target your optimum seeding date, you can avoid the 1.1 bu/ac loss per day.  That would result in an extra 5.5 bu/ac of winter wheat at $6.92/bu which would mean an extra $38.06/ac!  If you choose a soybean variety that is 1.0 MG less than the target for your area you may gain up to an extra 9 days which would equate up to 9.9 bu/ac more in your wheat crop at $6.92/bu would mean an extra $68.50/ac!  The earlier seeding window not only has the potential to put more dollars in your pocket, but it also means you are able to stick with a robust rotation.  A rotation that has been proven to improve soil health, provide an opportunity to incorporate cover crops, allows for manure management and gives you a chance to spread the workload and your risk!