Corn field

It is well known that planting date plays a key role in determining yield potential in corn and soybeans. Long-term University of Minnesota trials demonstrate, for example, that corn yield is usually optimized when corn is planted from April 25 to May 10. Long-term data also show that soybean yield is optimized when planting occurs around May 1. Planting earlier than these guidelines rarely lead to greater yields but do increase the risk of stand loss from frost or cool conditions after planting. This can lead to reduced yield or even the need to replant. Soil conditions at planting are also a big factor to consider.

Revisiting 2019

Many of us have been trying to forget the excessively wet conditions of 2019. Many farmers faced the decision to plant into sub-optimal conditions, plant very late (corn and soybeans into June) or to take prevent plant. Information on very late planting dates in corn and soybean in Minnesota is limited, so University of Minnesota Extension requested farmer input in a planting date survey following the 2019 season. The goal was to determine how those decisions turned out.

Planting date was most important

Not surprisingly, planting date was the most important factor influencing the yield of corn and soybean in 2019. As expected, the yield on average decreased as planting was delayed, but there was a lot of variability in the data. Variability increased as planting was delayed into mid-May and beyond, corresponding to an increase in fields planted into wet or very wet conditions—the timeframe when many had to push conditions to get the crop in.

Yield variability within fields was considerable, with some fields ranging from 0 to 265 bushels/ac in corn and 0 to 70 bu/ac in soybean. Variable soil conditions and drown-out spots were a significant contributor to this extreme range in yield within a field.

Conditions at planting also key

One of the key sources of variability when looking at corn yield on a particular date was the condition of the field at planting. The average yield in fields where farmers reported conditions at planting were “good” was 188 bu/ac. The yield was 8% lower when conditions were “slightly wet” at planting, and plummeted to 144 bu/ac when fields were planted under “very wet” conditions. Nearly all of the fields planted in “very wet” conditions were also planted after May 14 when many started pushing field conditions given the continued wet forecast in 2019. When adjusted for planting date, planting into “good” conditions resulted in the greatest corn yield. Yields dropped 2 bu/ac when conditions were “slightly wet,” and dropped 10 bu/ac when conditions were described as “very wet.”

On average, soybean yields were greatest when fields were planted under “good” conditions. Average yields were 7% lower where planting conditions were “slightly wet,” and 18% lower when “very wet.” Similar to corn, soybeans planted into wetter conditions also tended to be planted later. When adjusted for planting date, “good” conditions still resulted in the greatest yields, where “slightly wet” conditions yielded 1.5 bu/ac less and “wet” conditions yielded 2.5 bu/ac less. Soybean fields planted under wet or slightly wet conditions that had poor stands (less than 80% of normal) were the lowest yielding. These fields tended to have other issues as well, such as symptoms of nutrient deficiency.

Out of the 215 fields reported on, only 20% were planted in “good” conditions. One of the lessons learned from 2019 is that it can be OK to push field conditions a little but don’t push it too much. Survey comments were telling, such as “Remind me to never plant corn in June again!” and “I should have taken prevent plant.”

Outlook for 2021

Many factors that influence yield potential were not addressed in this survey, including genetics, crop maturity, disease tolerance, agronomic traits, and pest pressure. Field conditions this spring are looking to be much drier than in 2019, but as the season progresses we don’t want to forget lessons learned from the past.