If one could prescribe the perfect growing season for winter wheat in Pennsylvania, it would go like this. The fall should allow for timely planting, the winter should prevent winter kill to preserve an even plant stand, and spring should be sunny, cool, with no late frost, timely precipitation, and not much fog.
It is very difficult to package all these conditions in one growing season, but in 2019-20 many of these factors aligned, save for a late frost in certain parts of the state that significantly damaged barley stands and moderately damaged some wheat fields in susceptible locations. The results have been record-breaking wheat yields, often 20 bu/ac above the average condition with yields over 150 bu/ac in some cases.
Why is that? The summary is that we got a long, sunny and cool spring that allowed for spectacular spike and grain growth.
Cool, sunny spring weather serves to increase wheat yields by slowing down the maturation of the plant, giving it more days to absorb solar radiation and convert that radiation into spike and grain growth. In plant physiology lingo, this is called the photothermal quotient, or the amount of solar radiation absorbed per growing degree day. Cool, sunny weather in the spring leads to a high photothermal quotient because fewer growing degree days accumulate for each day of high radiation absorption by the plant.
We can also express this as the amount of cumulative solar radiation that occurs during the 1100 degree days (Cd in Celsius degrees) of spring growth after greenup (Figure 1), the growth period that brackets wheat jointing, spike growth, flowering, and early grain filling. When spring temperatures are relatively cool, it takes more days to complete this developmental phase, allowing more solar radiation to accumulate.
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This year’s cumulative solar radiation during wheat growth in spring is the highest it has been since 1997. This enables the survival of tillers which end up bearing fertile spikes. The second and perhaps most important effect is that wheat spikes become large with more grains per spikelet. Unlike barley, which also responds to good growing conditions, wheat can have up to 5 or 8 grains per spikelet (or many spikelets with 4 grains instead of 2 or 3 grains) so it can respond tremendously to good weather conditions. And to top it off, cool and sunny springs lead to effective grain set and longer grain filling.
And all this with the benefit of getting lesser pressure from diseases. The benefit of cool daytime conditions come with the risk of frost, and a late hard frost did damage barley and some wheat fields in susceptible locations, such as lowlands or west facing positions in the landscape. It is not often the case in Pennsylvania that we get a sunny, cool, and relatively dry spring, but 2020 seems to be a year for the record books in wheat production.
Relating weather to wheat yield is usually complicated by the upward trend in yield with time due to technology, and due to the noise brought up by diseases and pests, problems with harvest and other issues. One way to clean up the noise is to use a method called the “first difference”, which looks at the relationship between the difference in yield between two consecutive years and the difference in a given variable of interest.
In that way, and if technology does not change suddenly between years (and it rarely does), one could remove the technology effect. In Figure 2 we show the relationship between the winter wheat yield and the cumulative solar radiation during the 1100 Cd of winter wheat development in spring (as shown in Figure 1) for Lancaster County, but as a first difference, using National Agricultural Statistics Service winter wheat yield data.
Despite the dispersion, there is a striking linear relationship. One could summarize the figure like this: on average for the county, there are about 20 bu/ac at play that can be given or taken by the radiation load received during the spring growth period. In elite fields, that delta can be much larger, perhaps up to 30 or 40 bu/ac judged by the data dispersion. Just looking at this graph, one would predict that for 2020, the area around Lancaster County may get a winter wheat yield boost of up to 20 bu/ac above the average of 2014 to 2016 (we do not have the NASS yield records for 2017 to 2019).
It is possible that certain fields will set the record winter wheat yield for the state, and indeed yield contests have already documented a 153 bu/ac harvest in Lebanon County, 148 bu/ac in Berks County, and two in the mid-130 bu/ac range in Lancaster County. Across the state, many other farmers are experiencing 15-20 bu/ac yield increases over previous years, consistent with projections from this simple climate model.
How likely are we to continue getting years like this in Pennsylvania? On weather matters it would be negligent to make predictions. We can however speculate on weather conditions that can undo a season like this one. The first one would be conditions that delay planting. A wet fall that complicates soybean or corn harvest may delay wheat planting. The second one would be a winter that does not favor winter survival.
Winters should be cold enough to slow wheat growth while allowing vernalization. That requires a protective snow cover or the lack of a cold snap following warm conditions (even more critical for barley). Conditions this year were good for wheat in this regard. The third one is that a relatively warm mild February-March is not followed by a cold snap due to air masses marching south from Canada or east from the Plains.
This year came close, with some cool mornings and spots with frosts. Just 1 or 2°C of lower minimum temperature (or 5°F lower temperatures) could have undone the season. These swings in temperature are likely one of the features to fear the most, and unfortunately, they are likely to become more frequent. And finally, we could get diseases that are difficult to control like Fusarium. The foggy mornings typical of Pennsylvania springs can be lethal in this regard.
We hope farmers and the state can celebrate an exceptional, perhaps record season for winter wheat. And we also hope that this article helps you to understand and appreciate the climatic factors that aligned to make it such an exceptional growing season. This is one of those seasons for winter wheat that we hope for, but do not expect to happen very often. Cheers to those fields with more than 150 bu/ac of wheat, America’s amber waves of grain.