Reasons for Variable Corn Yields

• Field-to-field yield differences can often be explained by better understanding what yield-limiting factors were dissimilar across fields.
• Grouping hybrid yield data by specific yield-limiting  factors can help with future hybrid selection and placement.

Yield data can be a fantastic tool for understanding how well a hybrid did or did not perform and ultimately in helping make decisions on what to plant next year. However, in some years, corn yield can be highly variable, making data-driven decisions difficult unless you are able to gather performance results into groups that best represent why performance varied. 

Temperature and precipitation are 2 of the main drivers associated with fluctuations in year-to-year yield potential. Temperature changes tend to span across bigger geographies and explain large yield trends. However, they may not explain field-to-field yield variability within smaller geographies. Conversely, rainfall patterns may cover big swaths or vary greatly within a few miles, which could sometimes explain field-to-field performance variability.

In addition to rainfall and temperature, many different soil parameters can influence plant available water capacity and lessen or worsen the effect of drought conditions. The following paragraphs will help better understand how precipitation, temperature and soil parameters interact and result in high yield variability.

Temperature Variability 

Corn is well adapted to warmer days to maximize growth and cooler nights, which help plants recover. Analysis of state-county yield data across years has shown a trend for yield penalties ranging from 2.8-4.7 bu/A for every 1°F increase in July and August average night temperatures. A lack of cooler nights leads to a decline in physiological efficiency that can either reduce kernel set or kernel size depending on when the warm nights occur.  Heat stress as pollination begins is known to reduce kernel set, whereas heat occurring later in grain fill stages can reduce kernel size, weight and grain fill duration as shown in Table 1. Drought and heat often occur simultaneously. However, areas experiencing excessive heat in combination with timely rainfall may still encounter significant ear tip back. 

Precipitation Variability 

Drought conditions can reduce nutrient uptake and grain fill period as well as cause premature plant death. The crop stage and duration of the drought stress play a critical role in how the crop may be impacted due to changes in daily plant water use (Graph 1). Research shows that drought stress during pollination (peak daily water use) can cause up to 50% yield loss, whereas stress prior to or after pollination only resulted in 20%-25% yield loss.² 

In extended dry periods, corn plants have reduced nutrient and water uptake from the soil, resulting in reduced grain fill (shallow kernels) and subsequent yield loss. Dry conditions slow root growth, limiting the ability to access nutrients within the soil. Insufficient soil moisture also reduces the ability for nutrients such as N and K to flow freely in soil solution to roots, further reducing uptake.

The combination of reduced root growth and nutrient movement in soil often results in plant deficiencies, which may require the plant to reallocate nutrients from other areas of the plant to complete  grain fill. Nutrients are generally stolen from the lower stalk, making plants more susceptible to late-season lodging. 

Soil Factors Causing Spatial Yield Variability  

Soil texture, structure, depth and organic matter all interact to determine plant-available water content. Often these soil parameters can help explain field-to-field yield variability where precipitation and temperature were similar.  

Soil texture is characterized by its proportion of sand, silt and clay, each of which has varying particle sizes. Spatial changes in texture alter both soil nutrient and water-holding capacity, greatly influencing variability of corn yields.

Soil structure is a description of how individual sand, silt and clay particles are assembled into what is more commonly referred to as aggregates.  Soil structure is important because it influences water infiltration and retention rates as well as oxygen availability in the soil. Soils with poor structure will tend to be poorly drained and anaerobic, limiting the oxygen needed for metabolic processes in addition to restricting root elongation and penetration into the soil. Soil structure is commonly degraded from excessive tillage or compaction from heavy machinery.

Soil depth is the depth in which the root system can physically penetrate, greatly influencing the amount of nutrients and water plants can absorb.  Root depth could be limited by how shallow bedrock or impenetrable subsoils are or by the formation of a plow pan from tillage and compaction. 

Organic matter is anything that contains carbon compounds formed by living organisms. Increased soil organic matter provides many benefits such as managing storage and release of nutrients, which provides aggregation for improved soil structure, moisture retention and water infiltration and reduced compaction and surface crusting. 

Factors beyond plant water availability, such as soil pH and nutrient levels, should not be overlooked as potential explanations for yield variability. Any differences between management practices, such as planting dates, nitrogen application, or loss and presence of disease, can contribute to field-to-field yield variation. 

Select the Best Products for Your Fields 

Continue scouting and monitoring fields throughout the growing season to set yield expectations and avoid being caught off-guard by variable yield. The most important way farmers can manage increasingly variable conditions is to plant the right hybrid for the right acre. Work with your Golden Harvest sales representative or Seed Advisor for hybrid-specific field placement recommendations that are designed around the unique conditions you anticipate seeing in future growing seasons.