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Why corn test weight makes a difference

gray leaf spot on corn leaf
DISEASE IMPACT: If this gray leaf spot takes off and isn’t controlled, it could set up plants for other issues that might impact test weight.
Corn Illustrated: Test weight has no correlation with yield, but that doesn’t mean it isn’t an important trait.

Despite use of foliar fungicide applications by several farmers, I’ve seen lots of disease pressure in many cornfields this year. Along with gray leaf spot, I’ve seen northern corn leaf blight.

Weather was favorable for these disease organisms, which affect yield and test weight. Disease lesions reduce the photosynthetic area of leaves, and when diplodia and gibberella organisms attack ears, kernels become chaffy. That can lower test weight.

Growers often discuss test weight, especially if it’s high. Grain moisture is important because of cost of drying, but why is test weight important? Does it just give the grower bragging rights, or does high test weight have some benefits?

Farmers can haul more bushels of high test-weight grain per trip to the elevator than low test-weight grain. Grain with a test weight lower than 56 pounds per bushel may be discounted by elevators, but no credit is given for higher test weight, even though grain with higher test weight generally is better quality.

Hybrid genetics plays an important role in test-weight determination and is usually mentioned in seed company literature. However, there is no correlation between test weight and yield potential of a hybrid. As the moisture goes down, test weight increases, since dry matter in grain weighs more than water.

High test-weight grain with deep-orange color has greater eye appeal. The integrity of the grain is maintained during shipping and is generally preferred by customers in foreign countries as compared to lower test-weight, chaffy grain.

Key factors
Here are seven factors that can affect test weight:

1. Planting date. Early planting helps hybrid maturity and leads to higher test weight. Our planting date studies for several years showed that earlier-planted corn had a test weight of 1 to 1.5 pounds per bushel higher than later plantings of the same hybrid. This can vary in any one year.

2. Hybrid genetics. Planting hybrids with genetics for higher test weight generally results in higher test weight. If the genetic potential for higher test weight is there, it usually shows up when kernels become fully mature.

3. Grain moisture. Lower-moisture corn will have higher test weight. This is a matter of simple physics. Kernel dry matter is heavier than water. Once a portion of the water is removed, the remaining dry matter contributes to higher test weight.

4. Drying. Drier grain shrinks, has higher test weight and packs better. This relates to the fact that you can pack more kernels into a bushel basket. That is the original definition of a bushel of corn: what fits in a bushel basket.

5. Stress. Heat and drought stress tend to decrease test weight. This happens because these stresses affect the photosynthetic process during grain fill and reduce kernel weight. In this case, these stresses also reduce yield.

6. Leaf diseases. Late-season leaf diseases such as rusts, anthracnose, tar spot and physoderma affect test weight. The effect can be dramatic. Disease interrupts the grain fill period and proper grain development.

7. Ear diseases. Grain and ear diseases such as aspergillus, diplodia, gibberella and ear smut hurt test weight. Hybrids differ in tolerance to various disease organisms such as these listed here, plus others. You can select hybrids with tolerance to these pathogens by paying attention to seed company demonstration plots.

Nanda is president of Agronomic Crops Consultants LLC. Email him at [email protected] or call 317-910-9876.

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