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Planting date is just one factor affecting yield potential

By Lizabeth Stahl and Jared Goplen, Extension educators - crops

It is well known that planting date plays a key role in determining yield potential in corn and soybean. 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 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 (only for a little while)

Many of us have been trying 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 to take prevent plant. Information on very late planting dates in corn and soybean in MN is limited, so U of MN 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 yield of corn and soybean 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/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 yield variability 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. Average yield in fields where farmers reported conditions at planting were “good” was 188 bu/ac. 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 (Table 1). 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 yield was 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 maximized when planted into “good” conditions. On average, yields were 7% lower when conditions were “slightly wet,” and 18% lower when conditions were “very wet.” Similar to corn, however, 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 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 also tended to have other issues as well, such as symptoms of nutrient deficiency.

Table 1. Average reported yield for corn and soybean planted into various soil conditions across planting dates (left) and average yield reductions for corn and soybean planted into sub-optimal soil conditions when adjusted for planting date (right). (2019 Survey: Corn, n=136; Soybean n=79).
Yield based on planting conditions Yield reduction based on planting
conditions adjusted for planting date
Good Slightly wet Very wet Good Slightly wet Very wet
Corn 188 bu/ac 173 bu/ac 144 bu/ac 0 bu/ac -2 bu/ac -10 bu/a
Soybean 52 48 43 0 -1.5 -2.5

Survey results reflect the reality of 2019 for many, which was filled with challenging decisions. Out 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. Figure 1 shows the general locations of the 2019 survey fields within Minnesota.
Figure 1. Distribution of survey responses for corn (left) and soybean (right) in 2019 with the corresponding average yield (bu / ac). A total of 136 corn and 79 soybean fields were submitted to the survey. Individual points were randomly placed within the county they were submitted and do not correspond to exact field locations.  

Lessons learned

One of the lessons learned from 2019 is that it can be okay to push a little but don’t push too hard. Some of the 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.

We would like to extend our thanks to everyone who participated in the survey – your valuable input was much appreciated and will help us provide better guidance if/when we are faced with planting delays again in the future!

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