What’s the optimum agronomic plant population for corn? What’s the optimum economic population? Can they vary by hybrid?

These questions were first posed a couple of years ago when Bob Nielsen was helping producers understand the difference between optimum agronomic seeding rate, which looks at yield, and optimum economic seeding rate, which looks at return. After a two-year study with the same two hybrids, it was clear that optimum agronomic and economic seeding rates aren’t always the same. Likewise, it’s clear that both can vary greatly from hybrid to hybrid.

Here’s a report on that two-year study conducted at the Throckmorton Purdue University Agricultural Center near Romney, Ind. Farm Progress helped sponsor this trial. Seed was provided by Beck’s, Atlanta, Ind.

Both hybrids were tested at five seeding rate targets, varying from 26,000 to 38,000 seeds per acre. The trial was replicated three times.

Karen Mitchell, Tippecanoe County Extension ag educator, and Daniel Bechman, Beck’s product specialist, assisted with stand counts before harvest. Nielsen interpreted the data.

Here are 10 key findings from the study:

1. Trends were the same in 2016 and 2017. Hybrid A yielded more than Hybrid B at every population both years. The gap was slightly larger in 2016. The optimum economic seeding rate was lower for Hybrid A compared to Hybrid B in both years. The optimum economic seeding rate was lower for Hybrid A in 2017 vs. 2016. It was higher for Hybrid B in 2017 vs. 2016.

2. Both hybrids produced the highest yield at the highest seeding rate in 2017. The agronomic optimum seeding rate for Hybrid A in 2017 was 38,000 seeds per acre, with a harvest population of 37,167 plants, producing 225.2 bushels per acre. The agronomic optimum seeding rate for Hybrid B was also 38,000 seeds per acre. Actual population for Hybrid B was 35,667 plants per acre. Yield was 212.4 bushels.

3. Optimum economic populations compared to optimum agronomic population varied by hybrid. The final plant population producing the highest net income for Hybrid A in 2017 was 26,833. Based on the exceptional 98.5% average stand establishment in the trial, economic optimum seeding rate for Hybrid A in 2017 would have been 27,245 seeds per acre. The optimum economic population for Hybrid B was 35,667 plants per acre. Based on this number, the optimum economic seeding rate was 36,215.

4. The gap in economic optimums was narrower in 2016. The economic optimum, the point at which you make the most money, was around 30,000 plants per acre at harvest for Hybrid A in 2016, and just under 34,000 for Hybrid B.

5. Estimate required seeding rate from population. In the 2017 trial, actual population averaged 98.5% of seeding rate. To estimate seeding rate from any final population, divide by 0.985.

6. A seeding rate sweet spot was determined from several trials. Based on 90 Purdue field-scale trials, maximum yield occurs at final populations between 28,000 and 35,000 plants per acre on better soils. Read the research summary.

7. A late planting date didn’t impact yields in 2017. This trial was planted June 8 due to weather delays. Yield averages for each treatment ranged from 200 to 225 bushels per acre.

“This year featured a very good, cool late-summer period for grain fill,” Nielsen says. “Early planting date by itself isn’t a guarantee of higher yields.”

8. Hybrid A performed best in 2017 at the lowest seeding rate. The regression line in the chart above indicates the hybrid might have performed even better economically at lower rates than those in the trial.

9. Hybrid B performed best at the highest seeding rate in 2017. Economic return was still increasing at the highest rate tested.

10. Yield and population response don’t correlate. This is a classic example, Nielsen says. Hybrid A responded to increasing population slowly in 2017. Yet the higher the seeding rate, the more Hybrid B yielded. However, across the entire trial, in two years, Hybrid A yielded more than Hybrid B and produced higher net returns at all populations and seeding rates.