A Retrospective Analysis of Nebraska Corn and Soybean Planting Progress (1980-2022)

A Retrospective Analysis of Nebraska Corn and Soybean Planting Progress (1980-2022)

Key Points

  • Delays in the Nebraska corn 50% planting progress date that occur within the May 2-12 planting period do not have an appreciable impact on resultant corn yield.
  • The Nebraska soybean 50% planting progress date has been advanced by producers from a 43-year historic mean of May 22 to a projected May 14 for the 2023 season.
  • That eight-day advance in the Nebraska soybean 50% progress date was associated with a historic 4 bu/ac increase in irrigated fields and a 2 bu/ac increase in rainfed fields.
  • Advancement in the Nebraska soybean 50% progress date is expected to continue, given producer adoption of wider higher-speed planters that enable faster corn and soybean planting.

Weekly crop planting progress in United States has been estimated since 1980 by NASS. Full-text reports for the 43 past years and current year 2023 are accessible online. We examined the historical yearly record of corn and soybean planting progress in Nebraska, and in this article, provide some insights about the planting date trends and impact on past and future yields. About two decades ago, UNL research and Nebraska on-farm research, plus later UNL research, showed that a soybean crop, when planted earlier, captured more early-spring solar radiation for use in photosynthesis, thereby greatly optimizing its yield potential. The question of interest is whether Nebraska producers have advanced the planting dates for corn or soybean during the past four decades. This article will share an analytic overview of Nebraska corn and soybean planting progress.

NASS Weekly Nebraska Corn and Soybean Planting Progress (Example Years)

The two stacked Figure 1 panels (top-corn; bottom-soybean) show NASS weekly planting progress estimates (solid symbols) for each of four notable past years (1982 - red, 2012 - black, 2019 - green, and 2020 - blue). Planting progress typically follows a Sigmoid (S-shaped) curve, from inception (0%) to completion (100%), and thus can be modeled using a Logistic Equation (described in the graph text boxes). Progress is initially slow, but gradually accelerates until reaching a 50% progress inflection mid-point (depicted by thick central horizontal line in each graph), after which it gradually decelerates upon its approach to final completion.

The equation was used to compute a Logistic Xm value — demarking the spring date when each colored curve reached its 50% progress point. The colored vertical lines point to the Day of Year (graph bottom axis) or Month-Day (top axis) when that crossover occurred. A Logistic k rate value was also estimated for each curve. It is a numerical indication of planting progress speed. A steeper curve is generated with a faster k rate (e.g., 1982 soybean curve), whereas a flatter curve results from a slower k rate (e.g., 2019 corn and soybean curves). The grey band bracketing each curve is a statistical 95% confidence interval (CI) that reflects the intrinsic degree of random error associated with the estimation of planting progress. Error-free trend estimation is never possible, so successive data points would rarely, if ever, perfectly track a modeled curve trend line.

The four curves highlighted in Figure 1 were chosen to be representative (to date) of the earliest, latest, fastest and slowest of each crop’s 43-year set of curves. For example, the latest 50% date of corn progress was attained in a slow 1982 (May 24) curve, whereas the latest soybean 50% date was attained 19 days later in the fastest ever 1982 (June 12) curve. Alternatively, the earliest 50% date of corn progress was attained in the 2012 (May 2) and 2020 (May 3) curves, whereas the earliest 50% soybean date was attained a respective eight to nine days later in the 2020 (May 10) and 2012 (May 12) curves.

Corn, Soybean, Yield to Planting Date graph
Figure 1. Notable example years of planting progress in corn (top) and soybean (bottom).

The April sections of the stacked corn/soybean graphs show that while Nebraska corn planting reached 50% progress point on May 3 in 2020, Nebraska soybean planting that year attained 25% progress a day earlier — on May 2. That remarkably early phase soybean progress in 2020 (and in 2021 and 2022 — see next figure) indicates that many Nebraska producers (though not yet a majority) are currently planting soybean fields before finishing all their corn fields. In that regard, we note that this past Monday, NASS released its April 30, 2023 planting progress estimates which for Nebraska were 25% for corn and 17% for soybean, likely making 2023, as per the preceding three years, another year for early-to-50% soybean planting progress.

NASS Weekly Nebraska Corn and Soybean Planting Progress (1980-2022)

The Figure 2 stacked panels display all 43 years of the Logistic-modeled curves of NASS weekly planting progress estimates. Differing curve coloration of the first six years (1980-1985) versus the last six years (2017-2022) helps to reveal that the month-day of 50% planting progress was, on average, much later in the “bygone days” versus “nowadays”.

The two stacked graphs also provide a comparison of the left-to-right “dispersive spread” of each crop’s set of 43 curves. All but five of the corn progress curves had 50% dates concentrated within a May 2-12 (10-day) time span, whereas all but eight of the soybean progress curves had 50% dates occupying a later May 12-25 (13-day) time span. The staggered difference between these two respective “planting windows” is a consequence of most Nebraska producers being motivated to first plant all their corn fields before planting any soybean fields.

However, that “corn-first” sentiment has moderated in recent years. Indeed, a significant fraction of corn-soybean producers now plant soybean either before or concurrently with corn planting. This is inferably evident when one views the leftward shifted positions of the starting and accelerating phases of the 2020, 2021 and 2022 soybean plant curves (see the bottom left quadrant of the soybean graph).

Notably, the most recent soybean progress curves have a less steep shape (i.e., slower k rate). A slower recent progress rate would seem to be counter-intuitive, but it results from an earlier start of soybean planting by those Nebraska producers motivated to plant soybeans before (or with) corn, followed by a later start of soybean planting (and completion) by the many Nebraska producers who have not yet committed to implementing that change. As a result, the front half of the curve is advanced forward by the foregoing producers, whereas the back half of the curve is not. This mathematically leads to a less steep curve (i.e., slower curve k value).

Corn, Soybean, Yield to Planting Date graph
Figure 2. Historic record of 43 years of planting progress in corn (top) and soybean (bottom).

Trends in the 1980-2022 Nebraska Corn and Soybean 50% Planting Progress Date

The Figure 3 stacked panels depict graphs of the spring date of 50% planting progress (circle symbols) associated with each year’s Logistic-modeled curve from 1980-2022. For reference, the maximum, minimum, and mean spring date values are shown as dashed horizontal lines in each crop’s graph.

In the corn graph, it is evident that no advancement of Nebraska corn 50% progress has occurred in the past three decades. Simple linear regression did detect a statistically significant, but very minimal (-0.17 day per year; dashed green line) trend towards earlier corn planting. However, a linear-plateau regression model was statistically more probable (based on the Akaike Information Criterion — AIC test). Thus, May 7, 1989 could serve as a useful baseline prediction for future spring dates of 50% progress in Nebraska planted corn.

In that regard, Nebraska producers could use May 7 as a benchmark spring date guideline relative to aiming for a 50% completion of corn field planting on their farms. Note, however, that since 2000, spring dates for 50% corn planting progress have varied from as early as May 2 (2012) to as late as May 13 (2019). For the present year, we note that a NASS May 7 estimate of 2023 Nebraska corn planting progress will be released this coming Monday.

In contrast, the soybean graph shows that Nebraska producers have markedly advanced the 50% progress date over the past four decades. Simple linear regression indicated an annual hastening of the spring date by 0.38 days per year, leading to a projection of May 14 for the spring date in the year 2023. The less probable (as per the AIC test) linear-plateau model suggested a faster spring date advance (0.46 days per year), but only up to 2011, thereafter leading to a flat projection of May 17 for future soybean spring dates. Notably, if the late 2019 spring date is treated as an extreme outlier data point, and if omitted from regression analyses, the linear-plateau model fit becomes even less probable.

In any event, Nebraska producers could choose to use either May 14 or May 17 as a future benchmark spring date goal for attaining 50% completion of soybean field planting on their farms, though again noting the variance from as early as May 10 (2020) to as late as May 25 (2019). Again, see next Monday’s release of the NASS May 7 estimate of 2023 Nebraska soybean planting progress.

Corn, Soybean, Yield to Planting Date graph
Figure 3. Change over 43 years in the date of 50% planting progress in corn (top) and soybean (bottom).

The 50% Planting Progress Date — Is it Predictive of Corn/Soybean Yield?

The Figure 4 panels display graphs with a vertical yield axis and a horizontal 50% spring date axis (bottom Day of Year, or top Month-Day). Within each graph are shown the data points that pair each year’s NASS-estimated crop yield for Nebraska Irrigated-○ (blue), Rainfed-○ (brown), & Both-□ (i.e., combined IR&RF, represented in green) production systems with the same year’s NASS-estimated 50% planting progress date. Note that the two graphs were constructed to have matching bottom DOY and top Month-Day axis coincidence to more clearly highlight the difference in the spring planting windows that producers have typically allocated to corn (prior to May 15) and to soybean (generally thereafter) over this 43-year historical record.

In both graphs, only the recent year 2011 to 2018 IR data points are year-labeled — readers can easily gaze straight down to find the corresponding same-year (non-labeled) RF and Both data points. Note: NASS ceased providing IR and RF data after 2018, so year labels for 2019 to 2022 are depictable only for the Both data points. The downward sloping IR, RF, Both solid trend lines were derived from linear regression analysis of the respective same colored data points (see the upper right text boxes).

In the corn graph, the IR trend line indicates that just a one-day delay in the 50% progress date could be expected to lessen the resultant IR corn yield by a statistically significant 3 bu/ac, whereas the RF trend line displayed a (barely) significant 1.8 bu/ac per day of delay in the 50% date. However, when the five extremely late 50% corn plant date years (i.e., 1982-1984 and 1994-1995) enclosed in the dotted box in lower right corn graph were omitted from the regression analyses (as conjectured outliers), all three re-computed (dashed) trend lines (IR, RF, Both) were judged to be NOT statistically significant (P>0.05).

Corn, Soybean, Yield to Planting Date graph
Figure 4. Association of the 43 years of date of 50% planting progress with corn (top) and soybean (bottom) yield.

What does this mean? Delays in the Nebraska corn 50% progress date that occur within the May 2-12 planting period do not seem to have an appreciable impact on resultant corn yield. Our Nebraska corn finding here is not unprecedented, given that it was noted in a prior CropWatch article. The minimal association of yield enhancement with a prior than mid-May corn planting has also been documented in Iowa, Indiana and the U.S. Though there does not seem to be much of a yield penalty associated with planting corn as early as April 15 (see Figure 3 in this Illinois report), a late April date of 50% corn planting progress in Nebraska has, thus far, never been attained. 

In the soybean graph, the downward sloping IR, RF, Both solid trend lines had larger R-square values (i.e., more explanatory regression power) compared to respective corn R-square values. The IR trend line indicates that just a one-day delay in the 50% progress date can be expected to lessen the resultant IR soybean yield by a statistically significant 0.83 bu/ac, with the RF trend line also indicating significant decline of 0.54 bu/ac per day of delay in the 50% date. When the three extremely late 50% soybean date years (1982 and 1994-1995) enclosed in dotted box in the soybean graph were omitted from the regression analyses (as hypothesized outliers), the re-computed (dashed) trend lines (IR, RF, Both) remained statistically significant. Moreover, the re-computed IR and Both trend lines exhibited slightly steeper yield declines of a respective 0.99 and 0.92 bu/ac per day of delay. Though the re-computed RF trend line was just barely significant (P=0.049), it still displayed about 0.5 bu/ac lost yield per day of delay (note: its dashed line is hidden under the solid line).

One other notable feature in this soybean graph is that Nebraska producers have advanced the 50% soybean progress date from a 43-year mean of May 22 (black vertical line) by eight days to a projected date of May 14 (green vertical line) for the year 2023. That eight-day shift to earlier soybean planting in Nebraska would have generated 0.99 x 8 = ~ 8 bu/ac greater yield for irrigated soybean producers and 0.54 x 8 = ~ 4 bu/ac greater yield for rainfed soybean producers. One might anticipate a doubling of those foregoing yield numbers, if in the future, the Nebraska soybean 50% progress date were to be advanced to May 7 (current projected corn 50% date).

Are the foregoing soybean planting progress findings in agreement with recent published university research findings? Yes. In a recently completed NC-USA region project, the UNL and Wisconsin research leaders found that, of all of the examined soybean management practices, earlier planting was documented as the most consistent means of significantly increasing soybean yield potential in most of the Midwest. Earlier soybean planting has also been promoted by the private seed trade sector. Still, to unlock even more of the higher yield potential afforded by early planting in all kinds of fields, soybean producers must also pay attention to proper plant mineral nutrition (notably, P) and ensure adequate control of nematodes, diseases, insects and weeds.

The primary caveat relative to any routine expectation that earlier planting will lead to higher soybean yields is this: Unexpected August drought in rainfed soybean production systems can significantly diminish (or even obviate) that high yield expectation, as shown in Figure 5 of this report. Simply put, early planting optimization of yield potential in rainfed fields can be deprecated by an inadequate soil water balance during soybean pod-setting. This may be why Nebraska rainfed producers have opted to use slightly later varietal MGs to opportunistically try to catch late-August drought-ending rainfall events that would not be of benefit to earlier MGs that have already matured.

To conclude the foregoing retrospective analysis, one could ask what the future might portend relative to advances in planting progress in these two major Nebraska crops? Producers electing to update from an old to a modern planter that has more planter units and is capable of higher-speed planting will likely enable more corn and soybean ground to be planted per day (i.e., steeper crop planting progress curves). Ultimately, advancing the soybean planting window to be closer to the corn planting window may result from emerging robotic planter technology.

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