Avoid deception of test weight in estimating yield.
Agronomy Bulletin 73 ─ Corn Test Weight
Corn growers often mistakenly correlate test weight (TW) to yield. Understand why the two measures are only marginally related.
Factors to Consider
- What is TW?
- TW Influences
- Emotional connection to TW
- Corn yield equation
- Define TW. TW is a volumetric measurement that determines how many pounds of corn will fit into 1.244 cubic feet, the volume of a bushel. It is based on kernel density, kernel shape and size, slickness of the seed coat and more. TW heavily influenced selling decisions decades back when yields were measured by volume, not weight. Today, the United States Department of Agriculture dictates standards for bushel weight (56 pounds for No. 2 yellow corn) and moisture per kernel rate (15.5 percent). If moisture is higher, buyers will charge for drying costs.
- Understand TW Influences. TW may be reduced because of subtle or dramatic stresses that occur during the grain-filling period. These include drought, excessive soil moisture, nutrient deficiencies, lack of sunlight, temperature extremes, insect damage to leaf and stem tissue, frost and hail. TW also varies between seed hybrids and varieties. TW can be considered when selecting seed, but not at the expense of more important characteristics such as yield and disease resistance.
- Defuse emotional connection towards TW. Local grain buyers often reward growers whose grain has a higher TW. For instance, a grower might have a TW of 60, 4 pounds above the standard. In reality, growers are being rewarded for efficiency, not higher yield. Think in terms of a ton of feathers vs. a ton of bricks. The bricks are a more efficient use of volume, but the weights are equal.
- Instead, calculate your yield estimate. Note that TW does not factor into the following steps for determining yield:
a. Determine kernels per ear by multiplying the number of rows by the number of kernels in one row. Average at least five ears’ totals to increase accuracy.
b. Determine the number of ears per acre by subtracting 1,000 to 2,000 ears from the targeted plant population.
c. Using ears per acre and seed size, find the appropriate multiplier in Table 1.
d. Multiply kernels per ear by the identified multiplier.
• To demonstrate, an ear with 700 kernels (14 rows, 50 kernels per row) in a 28,000 population field (medium kernel multiplier rate: .311) would equal a yield of 217.7 bushels per acre.
This equation reflects that yield is a product of total kernels and weight per kernel, rather than pounds that fit into 1.244 cubic feet.
Contrary to popular grower belief, TW does not correlate to yield. If you have questions about calculating your yield, contact your local Mycogen Seeds customer agronomist or trusted agronomic adviser.
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