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Chapter 7 - Plant Development, No. 1, Spring 1987

Using Cereal Development to Estimate Optimum Planting Rates

Don Wysocki

The optimum population of plants per unit of area within a field depends on several factors. These include: plant variety, soil depth, stored and growing season moisture (climatic conditions), weed and disease problems and planting date. The planting rate for fall seeded cereals should be selected to achieve a plant population within the optimum range for a particular field. Knowing the optimum range for a field under a particular set of conditions is not easy, but the work of Ron Rickman, soil scientist, and Betty Klepper, plant physiologist, may help.

These STEEP researchers with the USDA Agricultural Research Service at Pendleton, OR, have developed a system to describe the pattern by which the wheat plant develops. This system is useful as a management tool in a number of ways, one of which is to match soil and climatic conditions with plant populations.

The Model

The leaves, tillers and roots of a wheat plant develop in a rigid, systematic pattern. A model devised by Rickman and Klepper describes the wheat plant by numbering each leaf, tiller and root with the number of the node from which it forms (Fig. 1). The entire crown of the wheat plant forms from nodes, which are numbered successively up the stem beginning at the base of the crown.

Fig. 1. Winter wheat plant identified according to the wheat development model (Rickman and Klepper, USDAARS, Pendleton),

Leaves and tillers formed in each node are labeled L1, L2, L3 and T1, T2, T3, etc. The first node in the crown bears the first leaf (Ll) and the first tiller (Tl) as well as four roots. Successive nodes bear leaves and tillers on alternate sides of the stem so that the second leaf (L2) and tiller (T2) are opposite from the first leaf and tiller. Below the crown are the coleoptile node and two nodes which produce the seminal roots. These are numbered node 0, -1 and -2, respectively.

Rickman and Klepper's work has also revealed that leaves, tillers and roots appear and develop in a regular pattern based on the amount of accumulated heat. Biologically the wheat plant is a clock with each structure of the plant appearing after a certain interval of "biological time. " These periods of biological time are called phyllochrons and are measured in units of accumulated heat called growing degree days (GDDs). The main stem of the wheat plant can bethought of as a stack of nodes, leaves and tillers, with each leaf and tiller appearing at a one phyllochron interval in the life of the plant. The first tiller appears in the axil of the first node as the fourth leaf is developing on the main stem. This tiller develops in synchronized step with the main stem so that it is always behind by about three leaves. After each phyllochron, subsequent tillers and leaves emerge synchronously from the respective nodes. If the plant is stressed during development it may abort tillers, but otherwise will continue in its pattern. This general scheme applies to all varieties of wheat, both winter and spring, and red and white.


Biological Time or Growing Degree Days

Measurement of the phyllochron interval or ''biological time" is by accumulated heat in units of "growing degree days, " Maximum and- minimum daily air temperatures are all that is required to obtain GDDs for a crop. GDDs can be calculated as follows:

1. Obtain maximum and minimum Fahrenheit air temperatures from planting to present at a location as close to the field as possible.

2. Calculate the average for each day by summing the maximum and minimum temperatures and dividing by two.

3.Convert daily Fahrenheit averages to Celsius averages by C = 5/9 (F – 32).

4. Discard all averages less than zero because growth does not occur below freezing (0°C = freezing).

5. Add together all daily averages to obtain the cumulative GDDs for the crop.


Generally winter wheat in the Pacific Northwest requires from 130 to 150 GDDs for seedling emergence in 90 to 100 GDDs for the elongation of each leaf. In other words the first phyllochron begins 150 GDDs after planting and each phyllochron is about 100 GDDs. If 500 GDDs have passed since seeding, plants would be expected to have about 3.5 leaves with a tiller in the first node.

Seeding Rates

The goal of selecting a seeding rate is to establish the correct plant population per unit area for the yield potential of a particular field. Yield potential relates closely to precipitation but is also influenced by soil and environmental conditions. According to Rickman, the wheat development model can be used to estimate optimum seeding rates for various planting dates, using average values of accumulated GDDs for the growing season, average annual precipitation and expected yield. Because plant development follows a rigid pattern, the time when tillers will appear can accurately be predicted from the number of accumulated GDDs. Knowing when tillers will appear can in turn be used to estimate the number of heads that will be present per unit of area.

To demonstrate use of the model let's assume the following generalizations: (1) yield potential is a field condition determined by available water, (2) a crop is a population of heads (one or more per plant), (3) seedling survival is 80 percent, (4) heads average 36 kernels and (5) average kernel weight is 40 mg. Table 1 shows the number of heads and kernels per square foot required to produce different yields assuming a bushel weight of 60 pounds. If each plant produced a single tiller, 19 seeds per square foot would have to be planted to achieve 15 heads per square foot (19 X 0.8 = 15). Based on the previous generalizations 15 heads per square foot produce a yield of 35 bushels/acre. In winter wheat, most plants will have some tillers and there will be more than a single head per plant. This is where the model is critical. It can be used to predict the number of tillers per plant and thus the number of heads.

Fig. 2 shows the accumulated GDDs for the 1984-85 growing seasons, beginning September 1, at Pendleton, OR. Using figures like this for GDDs, the stage of development of a plant can be predicted from the number of GDDs that will elapse between planting and jointing. Jointing is critical because only tillers with three or more leaves at this time are likely to produce heads. As a general guide, jointing occurs about 400 GDDs after January 1. At Pendleton, this is approximately April 1. For a September 15 planting date, 900 GDDs elapse by April 1 (Fig. 2). The wheat model predicts that emergence occurs 150 GDDs after planting, about October 1. At jointing (900 GDDs after planting and 750 after emergence), plants should have about 7.5 leaves (7 leaves are fully extended and the 8th is elongating) and 4.5 tillers. Coleoptile tillers (TO's) usually do not develop unless seedbed conditions are ideal, so tillers T 1 through T5 are those that should be present. Of the five tillers, T1, T2 and T3 will have reached the three-leaf stage and have a high probability of surviving to produce a head. Thus a September 15th seeding date at Pendleton should produce plants that bear four heads — three on tillers and one on the main stem.

Table 1. Kernels and heads required per square foot to achieve given yields (Rickman, ARS, Pendleton).



Heads/ft2 Kernels/ft2
35 15 547
70 30 1,093
105 45 1,639
14 60 2,185

The head population must be matched to the yield potential for a field to obtain a plant population that will produce the correct number of heads per square foot for a particular date. For instance if the yield potential at Pendleton is 70 bushels/acre, the proper head density is 30 per square foot (Table 1). A September 15 seeding date will allow plants to develop four heads per plant. Assuming eighty percent survival for seedlings, 10 seeds must be sown per square foot to obtain a plant population of 7.5 plants that produce thre quired 30 heads. This works out to a seeding rate of about 40 pounds/acre. Table 2 shows accumulated GDDs, predicted heads/plant and estimated seeding rates for a 70 bushels/acre yield using the climatic data shown in Fig. 2. Keep in mind that the data shown in Table 2 are based on assumptions of certain head size, kernel weights, seeding survival, moisture level and unstressed plant development, Drought, weeds or disease could drastically alter tillering, seed and head size, or seedling survival, Nonetheless these data demonstrate importance of selecting proper seeding rates and the utility of the plant development model.

Fig. 2. Accumulated growing degree days from September 1 for Pendleton, OR, 1929-85 average.


The wheat plant follows a rigid development pattern. The appearance of leaves and tillers is regulated by the amount of accumulated heat, measured in growing degree days. The population of head per unit area is set by the planting rate and the number of tillers per plant. Planting rates should be selected to achieve a head population that matches the potential yield with available water. The plant development model is a useful management tool that permits reliable estimates of planting rates.

Table 2. Growing degree days, predicted tillers per plant and seeding rates for three different planting dates.

Planting Date GDD's1 Heads/plant Seeding rate(lb/acre)
Sept. 15 900 4 36
Oct. 1 675 2 72
Oct. 15 525 1 140

lGrowing degree days accumulated from time of planting to April 1 at Pendleton, 1984-85 (Fig. 2).


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