In making an assessment of yield losses in corn due to defoliation and other types of plant injury, it is necessary to establish the stage of plant growth at the time when damage occurred. It's also important to know the stage of development in order to use postemergence herbicides effectively with minimum crop damage.
Several systems are currently used to stage vegetative corn growth. The "leaf collar" system is probably the method most widely used by Extension agronomists in the Cornbelt. With this method, each leaf stage is deﬁned according to the uppermost leaf whose leaf collar is visible. The ﬁrst part of the collar that is visible is the back, which appears as a discolored line between the leaf blade and the leaf sheath. The oval shaped ﬁrst leaf is a reference point for counting upward to the top visible leaf collar. This oval shaped leaf is counted as the #1 leaf when staging. Normally a plant at the four leaf stage will have parts of the 5th and 6th leaves visible, but only four leaves with distinct collars. A ﬁeld is deﬁned as being at a given growth stage when at least 50% of the plants show collars. Another widely used staging method is the "hail adjustor's horizontal leaf method" developed by the National Crop Insurance Service. Rather than using the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40 to 50% exposed and whose tip points below the horizontal. Typically a given "horizontal leaf" growth stage based on the hail adjustor's method will be 1 to 2 leaf stages greater than the collar method. One problem with this method is that it is often difﬁcult to identify the uppermost horizontal leaves in ﬁelds that have recently experienced severe leaf damage. Hail adjustors get around this problem because they usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.
At about the six-leaf stage of the collar method (V6) or 8-leaf stage of the hail adjustor's method, increasing stalk and nodal growth combine to tear the smallest lower leaves from the plant. Degeneration and eventual loss of the leaves results. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process.
To determine the leaf stage after this loss of lower leaves, split the lower stalk lengthwise and inspect for internode elongation. You can identify leaf location since leaf #5 is usually attached to the top of the ﬁrst noticeably elongated internode. (The ﬁrst node above the ﬁrst elongated stalk internode is generally the ﬁfth leaf node. This internode is about 0.4 inches in length.) Leaf growth stage can then be determined by counting from the ﬁfth leaf to the uppermost collared leaf, or the uppermost leaf that is 40-50% exposed from the whorl (depending on which growth staging system is being used).
Remember that prior to the six-leaf stage (collar method), the growing point is below or at the soil surface. As long as the growing point remains healthy and intact, severe defoliation during early vegetative stages seldom translates into signiﬁcant yield losses.