WORTHINGTON — 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 between April 25 and May 10. Long-term data also shows that soybean yield is optimized when planting occurs around May 1. Planting earlier than these guidelines rarely leads to greater yields, but does increase risk of stand loss from frost or cool conditions after planting. This can lead to reduced yield or even the need to replant.

Chances are many of us have learned valuable lessons in recent years, perhaps pushing a bit too hard and planting into sub-optimal conditions.

Keep these lessons in mind as we head into a planting season that looks like it may bring better conditions than recent years.

Let’s revisit 2019

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Many of us want to forget the excessively wet conditions in the spring of 2019. In many areas, farmers faced the decision to plant into sub-optimal conditions, plant very late (e.g. corn and soybeans into June) or take prevent plant.

Information on very late planting dates in corn and soybeans in Minnesota is limited, so U of M Extension requested farmer input in a planting date survey following the 2019 season. The goal was to determine how those decisions turned out.

Not surprisingly, planting date was the most important factor influencing corn and soybean yields in 2019. As expected, 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, which also corresponded to an increase in fields planted into wet or very wet conditions — the timeframe when people got nervous and started pushing too hard.

The yield variability within fields was considerable, with some fields ranging from 0 to 265 bushels per acre in corn and 0 to 70 bushels per acre in soybeans. Variable soil conditions and drown-out spots were a significant contributor to this extreme yield variability within a field.

Conditions at planting also key

One key source of variability of corn yield was the condition of the field at planting. The average yield in fields where farmers reported “good” field conditions at planting was 188 bushels per acre. Yield was 8% lower when conditions were “slightly wet” at planting, and it plummeted to 144 bushels per acre 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 yield was adjusted for planting date, planting into “good” conditions resulted in the greatest corn yield. Yields dropped two bushels per acre when conditions were “slightly wet,” and dropped 10 bushels per acre when conditions were described as “very wet.”

Soybean yields, meanwhile, were maximized when planted into “good” conditions. On average, yields were 7% lower in “slightly wet” planting conditions were “slightly wet,” and 18% lower when conditions were “very wet.” Similar to corn, soybeans planted into wetter conditions also tended to be planted later, which confounds these results.

When adjusted for planting date, “good” conditions still resulted in the greatest yields, where “slightly wet” fields yielded 1.5 bushels per acre less and “wet” conditions yielded 2.5 bushels per acre less. Soybean fields planted under wet or slightly wet conditions with poor stands (less than 80% of normal) were the lowest yielding. These fields also tended to have other issues as well, such as symptoms of nutrient deficiency.

Survey results reflect the reality of 2019 for many, which was filled with challenging decisions. Of the 215 fields reported on, only 20% were planted into “good” conditions. As planting was delayed into late May and June, there was a tendency to push field conditions to get the crop in. One lesson learned from 2019 is that it can be OK to push a little, but don’t push too hard. Some 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

There are many factors that influence yield potential that were not addressed in this survey, including genetics, crop maturity, disease tolerance, agronomic traits and pest pressure. As of April 1, the U.S. Drought monitor lists almost all of Minnesota as “abnormally dry,” with moderate to severe drought in some areas.

As we start out with a drier than normal spring, hopefully planting conditions will be much better than they were in 2019. As planting season progresses, don’t forget the lessons learned in recent years.

Thanks to everyone who participated in the survey. Your input was appreciated and will help us provide better guidance if or when we are faced with planting delays again.