Corn Phenology
Dr. Saratha Kumudini, Plant Science Department, Rutgers University
Dr. Thijs Tollenaar, Department of Plant Agriculture, University of Guelph
Vintage: January 1998
An understanding of the developmental processes of a corn plant is important in evaluating its yield potential. As the plant develops from a seed to a vegetative plant it grows and puts forth more and more leaves. It then develops reproductive organs, which in corn, results in the emergence of a tassel and one or many ears. This process of vegetative and reproductive development is further subdivided into many stages. The importance of these various stages is evident when considering that plant requirements are different at different stages of its life cycle, just as a child's needs changes as it matures. Therefore, management practices and stresses impact the plant's ability to yield to different degrees at different points in its life cycle.
Computer simulations utilize mathematical calculations to make quantitative predictions of such values as maturity dates and yield. In fact, computer simulations of crop species usually rely heavily on the durations of developmental stages to accurately predict yield.
The objective of this web site is to provide an understanding of corn phenology. This site should help in your understanding of the importance of the correct timing of certain management practices and the reason why crop insurance adjusters need to evaluate developmental stage.
Temperature and Photoperiod
Roughly speaking, from 20 kg of seeds per hectare sown in the spring, 15 000 kg of seeds, leaves and stalk are produced by the fall. The production of dry matter by corn is dependent on its ability to capture radiation energy from the sun through the process of photosynthesis. Therefore, the longer the duration of the plant's life cycle, the greater is its potential to photosynthesize. The producer must try to maximize the duration of the hybrid's life cycle but also ensure that the crop is not unduly exposed to the risk of frost.
Temperature and photoperiod are the two most important factors controlling development of plants. Corn is a short-day plant and thus flowers faster under shorter days. Hybrids developed for Ontario are adapted to the daylength conditions in Ontario.
Corn develops slower under cool temperatures and faster under warm temperatures. There are a number of methods developed that attempt to quantify the effect of temperature on corn development. In Ontario, the Crop Heat Unit system is used to quantify hybrid relative maturity and hybrid relative maturity best suited for any region in Ontario (more information). Therefore, the producer can select the hybrid that is best suited for the farm based on the average crop heat units received in that area. This ensures that the crop will mature when grown on that farm while minimizing the risk of loss to frost.
Most Ontario corn hybrids produce from 16 to 20 leaves which corresponds to relative maturity regions of 2500 to 3100 CHU hybrids. These are general values and can change depending on hybrid, season, location and planting date.
Vegetative Phase
The vegetative phase begins at planting and ends with the initiation of reproductive primordia (tassel initiation). In corn, the end of the vegetative phase is marked by the initiation of the tassel primordia. However, leaves still emerge from the whorl even though the primordia is now reproductive.
Vegetative staging is defined by different people in different ways. Crop insurance adjusters, scientists and some herbicide companies use various staging methods. However, they all involve rating leaf appearance in some form. A leaf consists of three parts, (i) the leaf blade (i.e., the part of the fully expanded leaf that extends from the leaf tip to the stem), (ii) the leaf sheath (i.e., the part of the leaf that is wrapped around the stem), and (iii) the leaf collar, which joins the blade and the collar. Three different staging methods for the vegetative phase are described below.
Leaf Staging Methods
Leaf Tip MethodThe number of visible leaf tips is counted in the leaf-tip method. Temperature is the most important factor controlling the rate at which the leaves appear from the whorl. The relationship between temperature of the growing point and rate of leaf-tip appearence is constant from corn planting to the appearence of the topmost leaf (Tollenaar et al., 1979), which is the major advantage of this method over the other leaf-staging methods. Another advantage of the leaf-tip method is that leaf tips can be counted starting immediately after plant emergence. The plant in this picture is at the 5-leaf stage according to the leaf-tip method.
Leaf Collar Method
One of the most common staging methods is the leaf collar method (Ritchie et al., 1992). Leaf stages are referred to as V (for vegetative) stages. The first leaf is smaller and has a rounded tip. This leaf is counted as leaf 1 when staging by this method. A single plant is staged by counting the number of visible leaf collars. If a plant has "n" number of visible leaf collars, then it is defined as being at leaf stage Vn. (e.g if a plant has 3 visible leaf collars, then it is at stage V3). A field is defined as being at a specific leaf stage when at least 50% of the plants are at the given stage or beyond. The plant in this picture is at 3-leaf stage according to the leaf-collar method.
Horizontal Leaf Method
Crop insurance adjusters use a slightly different method. This method as described by Vorst (1990), also starts counting at the first leaf, but continues counting leaves to the uppermost leaf that is 40-50% exposed out of the whorl. This last leaf is called the "indicator" leaf. The tip of the indicator leaf is typically pointing downward. For this reason this method is sometimes known as the droopy leaf method. This method is harder to use as it depends on hybrid differences in leaf angle. The ability to determine the percentage of the leaf exposed is dependent on an individuals subjective view of the potential size of the developing leaf. Therefore, individuals may differ in opinion as to which leaf should be defined as the indicator leaf. The plant in this picture is in the 4-leaf stage according to the horizontal-leaf method.
Degeneration or loss of lower leaves can occur naturally after about V5. In such cases it is necessary to split the lower stalk and check internode number in order to assess the vegetative stage.
Germination and Emergence The first observable stage on the soil surface is emergence (VE). Emergence is achieved by rapid growth of the mesocotyl which pushes the growing coleoptile to the soil surface. Mesocotyl and coleoptile elongation terminates at emergence. At this point the growing point is below the soil surface. The embryonic leaves then begin to grow through the coleoptilar tip (Fig 1). As the growing point is below the soil surface, destructive hail, wind or frost which destroys above ground leaves have little or no effect on final yield.
In typical Ontario hybrids, the growing point remains below the soil surface until about the sixth leaf stage (leaf tip method). In this figure note that the stem apex (growing point) is below the soil surface (Fig. 2). The growing point ascends above the soil surface before the primordia becomes reproductive (Fig 3). At the appearance of about the eighth leaf tip from the whorl (leaf tip method), a tassel primordium is initiated at the stem apex. The beginning of tassel initiation marks the end of the vegetative phase. After this point no more leaves are initiated from the stem apex although, externally, the leaves are still appearing from the whorl. At the time of tassel initiation, approximately 50% of the final leaf number have emerged.
Critical Period of Weed Control in Corn
Efficient use of herbicides can reduce the environmental and financial cost of herbicide application. The critical period of weed control is defined as the time in the life cycle of the crop when weeds need to be controlled to prevent yield loss. Weeds present before or emerging after this period do not cause significant yield loss. A critical period for weed control in corn grown in southern Ontario was identified by Hall et al. (1992). They found that the onset of the critical period was generally around the 3-leaf stage and weed control measures to prevent yield losses were found to be only necessary until the 14-leaf stage (leaf-tip method).
Reproductive Phase
The reproductive phase begins with tassel initiation and ends at physiological maturity (black layer). The growing point and tassel initial are above the soil surface and undergo a period of greatly ncreased elongation. Stalk elongation occurs through elongation of its internode. Each internode will elongate before the internode above it on the stalk. Ear shoots (potential ear) develop in the leaf axils of every node with a delay of 5 to 7 plastochrons (time between the initiation of two successive leaf initials) from the initiation of the most recent leaf initial. Consequently, when the topmost leaf is initiated the topmost axillary bud (i.e., topmost ear) is positioned 5 to 7 leaves below the topmost leaf . The potential for a plant to produce more than one harvestable ear on the main stem will increase with low plant density.
Ear development proceeds as the last few leaves expand before tassel emergence (VT). Stress during this period is more inhibitory to ear development than tassel development. Therefore, stress during this period can affect yield by reducing ovule number. Tassel emergence (VT) is the point at which the last branch of the tassel is completely visible. Tassel emergence occurs shortly after the last leaf appears. The period between tassel emergence and silking can vary from a few days to a week.
Silking (R1) begins when silks are visible outside the husks. Generally 2-3 days are required for all silks on a single ear to be exposed. The ovule at this stage is white in colour on the outside, it is clear on the inside and contains very little fluid (Fig. 4). The embryo is not yet visible at this stage. The two weeks prior to and after silking mark the period in the corn plant's life cycle in which it is the most sensitive to environmental stresses. Kernel abortion is common under stress conditions during this period. Reductions in kernel number due to abortion can seriously reduce yield.
Blister (R2) stage is characterized by a white kernel that resembles a blister in shape. The endosperm within contains a clear fluid and the embryo is small but distinguishable. The four images of the kernels represent (from left to right) the R2 kernel.
- with the surrounding material
- the intact kernel
- the kernel sliced longitudinally to reveal the front of the embryo
- the longitudinally sliced through the centre.
The silks have completed their function and are beginning to dry (Fig. 5).
Milk (R3) stage is distinguishable when the kernel displays a yellow colour on the outside. The inner fluid is now milky as starch is beginning to accumulate in the endosperm. The embryo is now easily seen upon dissection. This figure shows the R3 kernel (left to right);
- with surrounding material
- intact kernel
- sliced longitudinally to reveal the front of the embryo
- sliced longitudinally through the centre.
Kernel abortion is rare at this stage, therefore, kernel number is more or less set. Yield is now dependent on accumulation of dry matter in the kernel (kernel weight).
Dough (R4) stage occurs when the accumulation of starch within the endosperm causes the milky inner fluid to thicken and produce a doughy consistency. Usually four or five embryonic leaves have formed by this time. By R4, the corn grain has accumulated almost half of its mature dry weight (Fig. 6).
Dent (R5) stage begins when the top of the kernel dries and collapses forming a ridge around the horny endosperm. The plant is said to be at the R5 stage only when all the kernels on the cob are dented. A hard white layer of starch is formed from the top of the kernel as the kernel dries down. The hard starch line will advance toward the base of the kernel (toward the cob) as the kernel matures. The line at which the hard starch line and the milky layer meet is referred to as the milk line. It can be identified by pressing with the thumbnail. When the milk line is 50% between tip and base of the kernel (i.e., half milk line), kernels are at 40-45% moisture and have reached 95% of their potential final dry weight. An early frost can reduce yield by reducing kernel weight. Delays in drying may also occur as the frost-damaged corn takes longer to dry down.
Physiological maturity (R6) is reached when a black or brown abscission layer has formed at the base of the kernel. Black layer is found first on tip kernels and progresses to the kernels at the base of the cob. The kernels are now at their maximum dry weight and the hard starch layer has advanced to the base of the kernel. The average kernel moisture at black layer is 30-35%, however, this can vary with environmental conditions. Safe storage requires 13-15% moisture levels for shelled corn.
References
Hall, M.B. , C.J. Swanton, and G.W. Anderson. 1992. The critical period of weed control in grain corn (zea mays). Weed Sci. 40:441-447.
Ritchie, S.W., J.J. Hanway, and G.O. Benson. 1992. How a corn plant develops. Sp. Rpt. #48. Iowa state university of science and technology. Cooperative extension service. Ames, IA.
Tollenaar, M., T.B. Daynard, and R.B. Hunter. 1979. Effect of temperature and rate of leaf appearance and flowering date in maize. Crop Sci. 19:363-366.
Tollenaar, M., and R.B. Hunter. 1983. A photoperiod and temperature sensitivity period for leaf number of maize. Crop Sci. 23:457-460.
Vorst, J.J. 1990. Assessing hail damage to corn. NCH-1. Purdue university cooperative extension service, W. Lafayette IN 47907.
Salvador, R. "How a Corn Plant Develops. Special Report No. 48", Iowa State University of Science and Technology, Reprinted June 1993. http://www.ag.iastate.edu/departments/agronomy/corngrows.html
Nielsen, R.L. "The Corn Growers Guidebook", Purdue University. http://www.agry.purdue.edu/ext/corn/
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