In July, it appeared there would be a great corn crop. However, Mother Nature had different plans for some of you. Late July and August was hot and dry. Some farmers wondered if we would have lower test weight and yield.

Yes, those farmers may be right, and we may see both lower test weight and yield in some cases. However, lower yield does not always mean lower test weight. Some of the highest-yielding hybrids have average test weights, and some of the lowest-yielding hybrids may have high test weights.

Hybrid genetics plays an important role in test weight determination. There is no correlation between test weight and yield potential of a hybrid. As grain moisture goes down, test weight increases because dry matter of the grain weighs more than the water.

In this year, despite foliar fungicide applications, corn had a lot of disease pressure. Gray leaf spot, northern corn leaf blight, tar spot and even southern rust were all active in parts of Indiana. Drastic changes in weather patterns during the season shifted which disease was favored at any one time. Later, some fields were hit by anthracnose stalk rot, as well as diplodia and gibberella ear rots.

Since weather was favorable to disease organisms, the result affected both yield and test weight. Some fields that ran short on nutrients — like nitrogen due to too much water early in the season, causing N losses — may have cannibalized stalks to help finish kernels, leading to loss of stalk integrity.

2021’s situation

To help explain the relationship between test weight and yield, here are my answers to a colleague’s questions about this year’s conditions:  

If the grain fill period is stressed by dry weather, will both test weight and yield be lower? Or is test weight enough related to genetics that yield could be hurt by smaller kernels without affecting test weight? Genetics plays a strong role in test weight determination if the kernels reach maturity. However, if kernels are not fully mature, they could be chaffy, and both yield and test weight may be negatively impacted.

If kernels reached black layer before drought stress occurred, as was the case in some areas this year, kernels would be smaller, and yield will be reduced but test weight may be normal for that hybrid.

If dry weather, disease or nutrient deficiency stress causes smaller kernels, does it follow that test weight will also be low? If it does, how do you convince someone that yield and test weight aren’t related? Or are they related if the cause of low test weight and yield is stress, not genetics? If the reduced kernel size is because of diseases and occurred before black layer, it will likely negatively affect both yield and test weight.

However, nutrient deficiency or drought stress could produce smaller, fully developed kernels with 

lower yields and higher test weights. It all depends upon the timing and environmental conditions in the field during grain fill.

Nanda is director of Genetics for Seed Genetics Direct, Jeffersonville, Ohio. Email [email protected] or call and leave a message at 317-910-9876.