Varying plant populations within fields is an old idea that has been given new life by the availability of GPS technology. Intuitively, it makes sense to reduce plant populations on soils that have lower yield potential. From the investment cost perspective, variable rate planting is relatively inexpensive, especially for a producer who already has invested in GPS. A variable rate planter controller sells for about $3500-$4000.

But intuitive appeal and low investment cost do not necessarily generate profit. The objective of this analysis was to examine the profitability of variable rate planting for corn, using available information on crop response to plant populations, estimates of crop responses by yield potential from Pioneer Hi-Bred agronomists, and a spreadsheet model. The results indicate that variable rate seeding by yield potential zones is profitable mainly for farmers with some low-yield-potential land (<100 bu./a).

Manual variable rate planting systems have been around for about 20 years, but they failed to catch on because they depended on the alertness of the operator. Farmers who used manual systems often say that they worked when the operator was fresh, but that as fatigue crept in, they would often forget to switch populations. All the gains from lower populations for the low-yield-potential areas could be lost if a few rounds were made at the lower population in higher yield areas. Pioneer agronomists concluded that variable rate seeding may be profit-able on farms with some areas with yield potential below 100 bu./a. Data from a three-year study in Kentucky shows a modest return to variable rate planting on fields with some areas below 100 bu./a yield averages. Some farmers have reported success in variable rate planting by soil type, instead of yield potential.

Methods – The approach was to develop spreadsheet budget examples that estimate returns to various seeding rate strategies and then to vary certain prices to determine the sensitivity of results to assumptions. The Pioneer Hi-Bred population response functions were used to estimate corn yields because they are the best available for Cornbelt conditions. The focus was on corn because no soybean population response functions are available. The analysis was only of varying response by yield-potential zone. Plant population responses by soil type are not available.

One of the key questions raised by agronomic studies is how the changes will be determined if planting rates are varied. Accurately map-ping yield potential is, in itself, a major problem. It was assumed that the yield-potential zones are relatively small, irregularly shaped, and interspersed, so that management by field or other unit would be difficult. It was assumed that yield-potential zones are accurately mapped. For simplicity, this study also assumed that the proportions of the corn area with high, medium, and low yield potential were known and that variable rate equipment could accurately change populations given a map of the zones. The zones were defined as:

➤ High: over 180 bu./a expected yields

➤ Medium: between 120 and 140 bu./a expected yields

➤Low: under 100 bu./a expected yields

Results for three strategies for determining plant population are reported here:

➤ Uniform seeding to achieve a population of 28,000 plants/a at harvest – The control to which other strategies are compared.

➤ VRT, agronomic rule – Variable rate seeding based on Pioneer agronomic recommendations.

➤ VRT, economic rule – Variable rate seeding based on the eco-nomic criteria that the marginal value of the additional product be equal to the marginal cost of the additional input in every management zone.

The plant populations for the variable rate strategy using agronomic recommendations were: low potential, 18,000; medium potential, 28,000; high potential, 30,000. For the variable rate strategy with the economic decision rule, the plant populations were: low potential, 20,000; medium potential, 26,000; and high potential, 30,000. Price assumptions included: corn at har-vest, $3/bu.; corn seed, $67/bag; dryer fuel, $0.50/gal.; variable rate controller and monitor, $3500; interest rate, 10%. Other assumptions included: 1000 acres of corn planted; to allow for germination and other problems the planted populations should be 10% higher than desired population at harvest; and the useful life of the planter controller was 5 years.

In the examples, only the seeding rate was determined site specifically. All other inputs were held constant. Larger gains are to be expected in an integrated system which manages several inputs site specifically.

To keep the example simple, the baseline farm was assumed to have only two yield-potential zones: 50%high-yield-potential land and 50%low-yield-potential land. Scenarios were also developed with a 50/50 mix of high-and medium-yield-potential land and a 50/50 mix of low-and medium-yield-potential soil. Other scenarios included varying seed price from $50 to $110 per 80000 kernel bag and changing the variable rate equipment cost from $2000 to $8000 per farm. The baseline scenario assumed that the producer’s only investment was the variable rate planter controller and monitor. The sensitivity testing considered the case of producers who must also purchase GPS units, computers, or other equipment to implement variable rate planting.

Results and Discussion – Consistent with the agronomic studies, variable rate planting shows eco-nomic advantage on farms with some land with under 100 bu./a yield potential (Table 1). The benefits vary with the proportion of low potential soil, but are of similar magnitude for mixes of high-and low-or medium-and low-potential land. Benefits occur for a very modest proportion of low-yield land. Both variable rate strategies show small positive returns at 10% of land being low yielding.

The benefits are highest when a small part of the farm has high-potential soil. In the baseline exam-ple, when 90% of the farm is low yield potential (10% high yield potential), the gain from variable rate is around $4/a for both variable rate strategies. With the variable rate strategies, the source of eco-nomic benefits depends on the pro-portion of low yield land. When proportion of low yield land is small, yield gains provide most of the benefits and seed savings are small. When the proportion of low yield land is large, the largest source of benefits is seed savings.

If the farm has a mix of medium-and high-productivity land, the cost of variable rate planting is greater than the yield or seed savings benefits (Table 1). Higher seed cost increases value of site-specific variable rate management but does not change the general management advice. Farmers with a mix of high-and low-productivity land may benefit from variable rate seeding at any seed cost in the range from $50 to $110 per bag. Farmers with a mix of medium-and high-yield-potential soil are better off with uniform rate planting.

Similarly, a higher investment cost for variable rate equipment reduces the benefit, but the farmer with a mix of low-and high-yield-potential soil still shows positive returns, even with an $8000 investment on 1000 acres of corn. The farmer with a mix of medium-and high-potential soils does not show positive returns from variable rate even at a $2000 investment cost.

Conclusions & Implications –The general conclusion is that variable rate seeding by yield potential zones has profit potential mainly for farmers with some low-yield-potential land (<100 bu./a). The examples indicate that profitability does not depend on the productivity of the other land in the mix, as long as it is substantially higher than that of the low-productivity soil. Variable rate seeding can be profit-able for mixes of high-and low-or of medium-and low-yield-potential soils. Farmers with a mix of medium, and high-potential land are better off with uniform rate seeding. Sensitivity tests indicate that variable rate seeding is potentially profitable when the proportion of low-yield land is as low as 10%. Results are not particularly sensitive to seed cost or variable rate investment cost. Overall results suggest that variable rate seeding may be most valuable in areas on the fringe of the Cornbelt where some of the low yield potential soils are farmed.

Table 1. Net Gains from Variable Rate Seeding