Higher and Lower Corn Planting Rates Evaluated in On-Farm Trials


By Tom Doerge and Arnie Imholte

Summary | On-Farm Trial Methods | Grower Expectations vs. On-Farm Results | Standability | Key Learnings and Summary | Acknowledgements

Summary

  • North American farmers have steadily increased corn yields by planting improved genetics at higher populations.
  • However, on average, current rates are more than 5,000 seeds/acre below the levels suggested by Pioneer replicated small-plot studies.
  • To determine if this planting rate "gap" is justified, Pioneer conducted over 350 on-farm planting rate trials. Grower-cooperators planted 1) their current rate, 2) 4,000 seeds/acre above that rate, and 3) 4,000 seeds/acre below.
  • When trial locations received adequate moisture, the +4,000 planting rate was the most profitable.
  • Current planting rates below the levels suggested by small-plot research were justified when dry soil conditions were prevalent, especially during grain fill.
  • Decreasing the grower's current planting rate by 4,000 seeds/acre was always unprofitable, regardless of soil moisture status.
  • Increasing planting rates by 4,000 seeds/acre significantly increased stalk lodging only 9% of the time and root lodging 4% of the time.

North American farmers have steadily increased corn yields by planting ever better genetics at progressively higher plant populations. However, surveys have shown that farmers, on average, are planting about 5,000 seeds/acre less than rates suggested by Pioneer Agronomy Sciences small-plot research. To evaluate whether this planting rate gap is justified, Pioneer sales professionals and agronomists conducted 353 on-farm planting rate trials in 2003 and 2004. Specifically, the study was designed with these objectives:

  1. to understand better the risks and benefits of planting above and below the farmers' current planting rates, and
  2. to test whether small-plot plant population research results are applicable to on-farm conditions.

On-Farm Trial Methods


Cooperators were asked to plant and evaluate a Pioneer® brand hybrid of their choice at three differing planting rates. These three side-by-side strips were planted at their "current" planting rate, and at 4,000 seeds/acre above and below that level (Figure 1). They were also asked to predict the harvest stand, grain yield and overall standability in the three strips. At the end of the season, cooperators collected stalk and root lodging data, plant population counts and yield results. In addition, overall soil moisture status for the season was classified as too wet, optimum, somewhat dry, very dry or drought. A total of 101 different hybrids were used.

Figure 1. Aerial view of an on-farm planting rate trial.1) Current planting rate, 2) current plus 4,000 seeds/acre, and 3) current minus 4,000 seeds/acre.

Current Planting Rates

The average current planting rate used in the study was 30,390 seeds/acre. This compares to the North American average of 28,591 from a 2004 industry survey. The +4,000 and -4,000 planting rates averaged 34,540 and 26,260 seeds/acre, respectively.

The Planting Rate "Gap"

Pioneer conducts extensive field research on all new hybrids to determine the plant populations that produce maximum grain yield. For the 101 hybrids used in this project, the range in optimum plant population per acre for maximum grain yield was from 27,700 to 42,000, according to Pioneer Agronomy Sciences.

Growers consistently planted fewer seeds/acre than the optimum plant populations suggested by small-plot research. This under-planting, or planting rate gap, is the number of seeds/acre a grower plants below the suggested optimum plant population established for a particular hybrid.

Figure 2. On-farm planting rate trial locations in 2003 and 2004.

Fully 94% of the cooperators in this study selected planting rates that were lower than the suggested optimum. In fact, 59% of the cooperators chose planting rates that were 4,000 or more seeds/acre below suggested optimum levels. The average planting rate gap for the entire study was 4,952 seeds/acre.

These results point to two important questions, 1) are plant population recommendations developed from small-plot research applicable to on-farm conditions, and 2) is the current planting rate gap justified?

Grower Expectations vs. On-Farm Results


Income Advantage

Expected: On average, cooperators expected to earn $10.75 more per acre in the +4,000 planting rate strip compared to their current planting rate (corrected for yield and seed costs, assuming 2.50/bu corn and $1.50/1,000 kernels). In contrast, growers expected to realize a $16.00/acre income penalty from dropping their current seeding rate by 4,000 per acre.

Actual: The size of the planting rate gap greatly affected the profitability of increasing the current planting rate by 4,000 seeds/acre. However, this response was also influenced by the overall soil moisture status at the trial location during the growing season and especially during the grain fill period (see Tables 1 and 2).

Results obtained at locations with adequate soil moisture were consistent with recommendations developed from Pioneer's small-plot research (Table 1). Namely, at low planting rate gaps (below 3,000 seeds/acre) there was no income advantage to increasing planting rates. However, as planting rate gap increased, the income advantage of adding 4,000 seeds/acre increased to over $11/acre.

Table 1. Effect of planting rate gap on income advantage for planting 4,000 seeds/acre above and below the grower's current planting rate. Trial locations with optimum or wetter soil conditions.

Adequate Soil Moisture Conditions
Planting Rate Gap (seeds/acre) Planting Rate Change
4,000/A* -4,000/A*
Range Ave. No. of Locs. Income Advantage ($/A)
<3,000 below optimum 661 below 40 -$2.19 -$5.18
3,000-6,000 below optimum 4,472 below 74 +$1.37 -$9.99
>6,000 below optimum 8,824 below 47 +$11.71 -$13.35

* compared to the grower's current planting rate.


Under dry soil conditions, there was no income advantage for increasing the current planting rate regardless of the size of the planting rate gap (Table 2). Apparently, growers that are accustomed to experiencing mild to severe drought conditions have already adjusted their current planting rates to a more-or-less optimum level. Clearly, these results show that more attention needs to be given to identifying optimum planting rates for very dry soil conditions.

Under no circumstances was decreasing the grower's current planting rate by 4,000 seeds/acre profitable. In fact, the growers' expectation of a $16/acre income loss for going to the lowest planting rate was very nearly realized.

Table 2. Effect of planting rate gap on income advantage for planting 4,000 seeds/acre above and below the grower's current planting rate. Trial locations with dry or drought conditions.

Dry Soil Moisture Conditions
Planting Rate Gap (seeds/acre) Planting Rate Change
4,000/A* -4,000/A*
Range Ave. No. of Locs. Income Advantage ($/A)
<3,000 below optimum 1624 below 29 -$12.35 -$12.06
3,000-6,000 below optimum 4,250 below 37 -$8.07 -$3.88
> 6,000 below optimum 9,611 below 26 -$8.41 -$10.87

*compared to the grower's current planting rate.

 

Standability


Many growers expect that small changes in planting rates offer an effective way to manage overall standability in corn. In this study, 71% of the cooperators expected that increasing their current planting rate by 4,000/acre would significantly worsen standability. However, this proved to be overly pessimistic. In fact, increasing planting rates by 4,000 seeds/acre significantly increased stalk lodging only 9% of the time and root lodging only 4% of the time (Figures 3, 4).

Stalk Lodging

Figure 3. Average stalk lodging percentages in the three planting rate strips for all locations (left) and for only the locations with at least 10% stalk lodging (right).

Root Lodging

Figure 4. Average root lodging percentages in the three planting rate strips for all locations (left) and for only the locations with at least 10% root lodging (right).

Likewise, growers overestimated the standability benefits of reducing their current planting rate by 4,000 per acre. While 60% of the cooperators predicted better standability in the lowest planting rate strip, improved stalk and root lodging were measured in only 6% and 2% of the locations, respectively.

Key Learnings and Summary

  • The planting rate "gap" (difference between current and suggested plant populations) raises the question of whether growers can boost income by increasing planting rates with only a slight risk of higher lodging.
  • In general, growers' planting rates were below levels needed to maximize income, possibly as a hedge against perceived standability problems or potentially dry conditions.
  • For maximum profitability, growers should choose planting rates within about 2,000 to 3,000 seeds/acre of the Pioneer suggested optimum plant population levels unless dry growing conditions are expected.
  • More attention needs to be given to identifying optimum planting rates for locations with very dry growing conditions.
  • Modest changes in planting rate had only a minimal impact on stalk and root lodging. Selecting a hybrid with better stalk and root scores should reduce the risk of standability problems much more than decreasing planting rates 1,000 to 2,000 seeds/acre.

Acknowledgements


Thanks are expressed to the Plant Population Steering Team of Steve Paszkiewicz, Mike Blaine, Duane Frederking, Don Aanonson and Keith O'Bryan and the 68 Pioneer agronomists and 199 sales professionals who conducted these field trials.


 
 
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