Section 2 - Crop Development

Evaluating Corn Growth Stages

The growth and development of corn is largely regulated by temperature accumulations and not calendar days. In fact, corn development can accurately be predicted from corn growing degree day accumulations. Corn growing degree days are calculated using a base temperature of 50 degrees and can readily be obtained from the NDAWN web site (http://ndawn.ndsu.nodak.edu) by going to “corn degree days” under the “applications” section on the left hand section of the home page.

At the five leaf stage, the corn plant switches from vegetative growth to reproductive growth as the growing point stops initiating leaves and begins initiating the tassel. At the 6 leaf stage, ear shoots begin to form. The number of kernel rows on the developing ear is determined relatively soon after it begins development and is largely determined by genetics, and less so by the environment. The length of the kernel row and therefore the total number of potential kernels, however, is determined during a longer period of time (6 leaf stage through one to two weeks before tasseling). Severe stress during this stage can shorten the length of the cob and reduce yield potential. Nevertheless, stress during ear development is far less damaging to yield potential than stress during and shortly after pollination, as the potential number of kernels developed typically exceeds the number that can be pollinated filled.

Certain management practices are growth stage dependent; therefore, properly identifying the growth stage of your corn crop will be important to ensuring that management practices are applied at the appropriate time. This is particularly true of the application of herbicides. When growth staging a crop you should begin by obtaining a representative sample of plants from the field or part of the field of interest. Remove any soil attached to the plants so that you are able to observe the roots and crown. Vegetative growth stages of corn are defined by the number of leaves. Counting leaves in corn is fairly straight forward as the process is not encumbered with tillers and leaves on tillers as is the case in small grains. However, care must be taken to ensure that the earliest leaves are included when counting leaf numbers. The first leaf is small and often dies and is torn from the plant early in the growth of the plant. The first leaf has a blunt tip. Look for sheath remnants at the crown of the plant if you suspect that the first leaf (or second for that matter) is missing.

Include only those leaves that have a collar. Include all leaves, even those that have been damaged by hail or frost. The total number of leaves that a plant will develop is more or less fixed for a given hybrid; leaves that are stripped from the plant will not be replaced by additional new leaves. In order to determine the growth stage of older plants that have lost their lower leaves, uproot the plant and split the stem with a knife through the root ball. At the very base of the stem, identify the first visible internode. Internodes are the white area between the more yellow bands of the nodes. The first obviously visible internode should about to 3/4 inch in length. The node directly above this internode will be the fifth node, and the leaf arising from this node will be the 5th leaf. Find that leaf and continuing counting leaves from that point.

 In corn, management recommendations can also refer to the height of the plant, rather than leaf stage. For example, certain herbicides can only be applied to corn less than 12 inches tall. The plant height in this case is measure from the base of the plant to where the upper most leaf reaches without stretching it out.

Vegetative Stages

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