ISSUE 7   June 21, 2007


Corn growing degree day (GDD) accumulations since the first of May are running ahead of normal in North Dakota expect for the northwest corner of the state, where temperatures are slightly below average. Corn growth and development is starting to accelerate now that the weather has warmed. Depending on the planting date and the region of the stage, corn is now somewhere between the fourth and seventh leaf stage (using the leaf collar method of staging). Conditions have been conducive for corn growth, except in areas that have received excessive rainfall and where soils have become saturated or flooded. In this article, I will briefly review the key developmental stages that impact corn yield potential.

The first component of yield is plants per acre. This component was more or less fixed at emergence. Crusting and/or excessive moisture during the emergence process impacted plant stands in some areas of the state this spring. Optimum yield is obtained when emergence is uniform and the plant stand matches that recommended for the environment.

Ears per plant is the next component of yield to develop. Ear shoots start to develop during the 4-5 leaf stage at nodes on the stalk, so each corn plant has the potential for producing many ears. Using current hybrids at recommended plant populations, only one or two ears will actually develop, however. Plants that emerge two or more leaves later than the rest of the crop or that become infested with smut will likely not develop a harvestable ear. These plants will effectively compete with the rest of the crop and can adversely affect the overall yield. Tillers can also arise from nodes and have the potential for producing an ear, but rarely do. Tillers in corn are not considered undesirable and most hybrids will not produce tillers under normal growing conditions.

Shortly after the 6-leaf stage, the growing point emerges from below the surface of the soil and the number of rows per ear is determined. Row number is largely controlled by the genetics of the hybrid, and less so by the environment, so stress during early vegetative development will seldom reduce the number of rows per ear. The number of kernels in a row, therefore, is a more important determinant of yield than row numbers. This number is established from the 6-leaf stage to about 2 weeks prior to tasseling. It is determined more by the environment than by genetics. The corn plant is fairly tolerant to stress during these early growth stages; far more potential kernels develop in an ear than the plant will ultimately be able to fill. The actual number of kernels that fill and contribute to yield will depend on successful fertilization and sufficient carbohydrate flow during grain filling. The growth stage most sensitive to stress is just prior to silking through early grain filling. Kernel weight is the final yield component. Kernel weight can be reduced by stress during grain filling and in North Dakota most commonly by frost damage before physiological maturity.

The key to optimizing yield potential is to have healthy leaves that are actively photosynthesizing. Nutrient deficiencies, water logging, physical damage by hail or insects can all reduce the effective leaf area. Furthermore, water stress can cause stomata to close and restrict photosynthesis. Cloudy days also limit the rate of photosynthesis. Roots play a critical part in ensuring that there is sufficient water and nutrient supply to the plant. The seminal root system enables the new seedling to obtain the needed nutrients and water after germination, but it is the nodal roots that sustain the growing plant. Nodal roots initiate at the crown shortly after emergence and become visible at the first leaf stage. Nodal roots will be the dominant root system by the 6th leaf stage. Most nodal roots develop from nodes that are below the surface of the soil, but can also develop from nodes above the ground if conditions are favorable. These brace roots, as they are called, can help anchor the plant and reduce lodging, but rarely penetrate deep enough into the soil to be a major source of water and nutrients for the maturing plant.

Between now and about two weeks after fertilization is a critical period in the development of yield. Making sure that weeds are controlled in a timely manner and that there is sufficient N for the crop's need are a couple of management practices that can help realize the yield potential that will be associated with this year's weather and the hybrids that were planted.

Joel Ransom
Extension Agronomist - Cereal Crops

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