Method for Estimating Corn Harvest

— Written By and last updated by JoAnne Gryder

tennessee-corn-harvestCorn yield estimations prior to harvest are helpful in planning harvest operations, including drying, storage and marketing of grain. They may also be useful in making preliminary hybrid selections and buying decisions for next year’s crop, as seed-corn discounts are often greater with early purchases. Yield estimate accuracy increases with crop maturity, but early estimates provide more time for developing strategies to deal with the expected volume of grain, and also satisfy a grower’s most basic curiosity – how is my corn crop going to do?

Ultimate corn yield depends on number of ears per acre, number of kernels per ear, and average weight per kernel. Though the first two components may be evident three weeks after silking, the weight per kernel is unknown until five or six weeks later. This is because kernel depth and density (test weight) continue to increase until black layer. Consequently, early estimates have to account for kernel weight by using a broad average. This kernel weight component contributes most to yield estimate variability, especially if growing conditions deviate from normal after the estimate, or a hybrid’s intrinsic kernel weight is much higher or lower than average.

Yield estimation procedures for reproductive growth stages R3 through R5 (milk through dent) involve a random sampling procedure to approximate ears per acre and kernels per ear. An estimate of kernel weight (kernels per bushel) is then applied to determine bushels/acre. The broad average applied in original yield estimation procedures was 90,000 kernels per bushel of corn at 15.5% moisture, or approximately 1600 kernels per pound. Upgraded procedures have allowed users to input a different kernel weight that may be more accurate for the hybrid or growing environment.

Yield estimation methods vary in their sampling procedures, but the goal of sampling is the same; that is, to reasonably represent the field without making the process excessively arduous or time-consuming. This can be accomplished in a number of ways, but will ultimately depend on field uniformity. A uniform field requires relatively few samples; a field that varies due to topography, soil type, compaction, uneven emergence, water damage, drought stress, nitrogen deficiency or other factors requires more samples. In extreme cases, a variable field can be split into two or more fields if the borders of non-uniform areas are clear and the size of each part can be easily determined.

  1. Sampling Intensity:
    • In uniform fields, conduct 5 to 8 individual samples, or about one sample for every 10 to 15 acres.
    • In non-uniform fields, conduct 8 to 12 individual samples, or about one sample for every 6 to 10 acres.
  2. Choose each sample location at random.
  3. For each sample, measure 1/1000 acre. Use a tape or pre-measured rod to measure 1/1000 acre. For 30-inch rows, 1/1000 acre = 209 inches (17 feet 5 inches). For other row widths, divide 6273 by the row width. For example, for 36-inch rows: 1/1000 acre = 6273/36 = 174 in. = 14 ft. 6 in.
  4. Count number of ears in each 1/1000 acre sample (if 2 ears count both). But do not count “nubbin” ears that have less than 5% of the kernels of normal ears.
  5. Count kernels per ear on 3 ears from each 1/1000-acre sample. For example, take the 5th, 15th, and 25th ears. Do not use nubbin ears that were not included in the ear counts.
  6. Average the number of ears across all sample locations of the field.
  7. Average the kernels per ear across the 3 ears of each sample and across all the sample locations of the field.
  8. Multiply number of ears x kernels per ear x 1000.
  9. Divide the answer from step 8 above by number of kernels per bushel to get bu/acre (at 15.5% moisture).

Reference

Univ. of Illinois. 2005. Estimating corn yields (an online calculator). Illinois Agronomy Handbook.