Interpreting a Variable-Rate Seeding Trial
A 160-acre no-till corn field has uniform soil type but a gentle slope and a history of variable phosphorus fertility. The farmer wants to decide between 28,000 and 34,000 seeds per acre. He plants twelve 24-row strips alternating the two rates, but because the planter turns at the west end, all high-rate strips are placed on the lower-slope half where soil test P averages 38 ppm, compared with 22 ppm on the upper half. The yield monitor shows the high rate out-yielded the low rate by 4.2 bu/ac. There is one year of data, no replicated blocks, and the yield map shows a strong east-west trend. The farmer concludes the higher rate is better and plans to use it on all 160 acres. A dealer offers a variable-rate prescription based on the yield map.
1. Explain why the farmer's conclusion is not supported. Identify at least two confounding variables and the key design flaw.
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The comparison confounds seeding rate with field position. The high-rate strips were all placed on the lower-slope half, where soil test P was 38 ppm rather than 22 ppm. Slope affects water availability, erosion, and temperature, while P fertility affects early growth and yield. Any of these could explain the 4.2 bu/ac difference without the seeding rate mattering. The design also lacks randomization and replication: strips are not independent experimental units, and one field-year cannot separate treatment from year and location effects. The alternating layout is systematic, not random, so the planter-turn pattern and the east-west yield trend are built into the treatment assignment. Yield is therefore correlated with seed rate but not evidence of causation. A valid conclusion would require the two rates to be compared within similar fertility and slope conditions, with several independent replications and a preplanned analysis.
2. Describe a practical one-season on-farm trial that could test the seeding rate more credibly. Include randomization, replication, blocking, and measurements.
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Use a randomized complete block design with four blocks oriented across the fertility and slope gradient. Within each block, randomly assign several strips or plot pairs to 28,000 and 34,000 seeds/ac, keeping hybrid, planting date, planter, fertilizer, and residue management identical. Four blocks give four independent comparisons per rate, which is enough to estimate whether the difference is consistent. Avoid placing all of one rate on one side of the field. Calibrate the yield monitor and account for flow delay and header width; measure stand counts after emergence, harvest moisture, test weight, and yield from a measured area in each plot. Record soil P and elevation for each plot so those variables can be used as covariates or blocking factors. Analyze the paired differences within blocks rather than pooling all strips. If the trial cannot be randomized, treat the result as a demonstration, not a recommendation for the whole field.
3. How should the farmer use the existing yield map and the dealer's variable-rate prescription without overinterpreting them?
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The yield map is useful for describing spatial patterns, not for proving which seeding rate caused them. The farmer should first check monitor calibration, flow delay, and header width, then compare stable low- and high-yield zones with soil tests, elevation, and drainage. Those zones can define blocks for the next trial and guide targeted sampling. The prescription should not be applied as a blanket change based on one confounded year. A better use is to test the prescription on randomized strips against a uniform rate, then compare yield, seed cost, and stand establishment. If the dealer's prescription changes seeding rate within the same field, the farmer should verify that it is not simply following the same P and slope pattern that confounded the original comparison. Multiple years and independent strips are needed before adopting a whole-field variable-rate program.