Development

Development is orderly and predictable, following the pattern shown in Figure 2.3. Temperature and available soil water, both highly variable in the Great Plains, are the most important factors influencing development. While Table 2.2 gives the calendar date that various growth stages were reached, it is more accurate to use heat units (HU) or growing degree-days (GDD) to measure the time it takes for a wheat plant to reach a given developmental stage. In warm falls, more leaves and tillers are formed on the plant, and during cool springs, growth stages are delayed. This is because plants develop in response to temperature rather than time. Heat units or GDD, as estimates of thermal time, can be used to calculate when growth stages are reached and the rate of many growing point (shoot apex) developmental events, such as leaf and tiller appearance. One common way of calculating GDD is to average daily temperature (TAVG) and subtract a base temperature (TBASE). The TBASE, which represents the point at which wheat development stops, is usually set at 32°F and is subtracted from the result.

TAVG = daily maximum temp. + daily minimum temp.

GDD = TAVG - 32 (GDD ≥ 0)

Accumulated GDD (summed each day) can predict the occurrence of key developmental events, such as growth stages and the appearance of specific leaves and tillers. However, the required GDD for a given developmental event can vary among cultivars.

Predicting when growth stages should occur is straightforward if there is adequate soil water. Emergence of half the seedlings is expected to occur once 270 GDD have accumulated after planting or after adequate rainfall if you planted into a dry seedbed. The first tillers are expected to appear approximately 540 GDD after emergence. The remaining growth stages all occur after winter, when the vernalization requirement of winter wheat has been satisfied. By starting accumulation of GDD on January 1, the growth stages from jointing to maturity can be predicted (Table 2.3). Estimates are provided for both irrigated and dryland conditions because limited soil water tends to accelerate development, especially for flowering and maturity. The dryland GDD in Table 2.3 were obtained during two years of very low rainfall, so larger values should be used in years with greater rainfall.

Table 2.3 gives the basis for predicting growth stages for irrigated and dryland conditions. The Phenology MMS computer program is being developed to simulate the growth stages of different crops and determine how development is influenced by available soil water. Results of some of the most important growth stages for locations across the Great Plains are shown in Table 2.4. Soil water had little influence on early leaf number or the jointing growth stage, but flowering occurred about four days earlier under dry conditions and maturity about 12 days earlier.

Table 2.3 Growing degree days required by winter wheat to reach important growth stages, under irrigated and dryland conditions.

*Data are for means of 12 winter wheat varieties grown at two locations (Fort Collins and Akron, Colorado) for two years. Note that irrigation did not begin until just before this growth stage, so there is little difference between the two treatments. (From McMaster et al., 2005, Journal of Agricultural Science, Cambridge 143:1-14).

**(1-Dryland GDD / Irrigated GDD) x 100

Table 2.4 Average simulated occurence of key winter wheat growth stages at several western High Plains locations.

* Simulations were generated by the The Phenology MMS computer program. The number of historical years of weather data for each location is given in the 2nd column. Updated versions of the software are available at http://arsagsoftware.ars.usda.gov.

A uniform naming scheme for leaves and tillers allows us to communicate effectively about plant development and interpret how a wheat plant has responded to its environment. For instance, if a specific tiller is absent but a later-appearing tiller is present, then conditions likely were stressful for the plant for the period of thermal time when the absent tiller was to appear.

Naming of leaves is based on the order of their appearance on the stem, with the first leaf denoted as Ll, the second as L2, and so on until the last leaf, the flag leaf, is produced. Each leaf appears about 180 GDD after the previous one. The first seedling leaf is on the main stem and has a distinctive rounded tip; all other leaves have pointed tips. About 12 to14 leaves are normally formed on the main stem, with fewer leaves forming on tillers. Buds are formed at the base of each leaf where it attaches to the stem and produces tillers. Tillers appearing from main stem leaves are primary tillers. The primary tiller emerging from the first leaf (Ll) is called T1, and the primary tiller emerging from the main stem L2 leaf is T2, and so forth. Tillers appearing from leaves on primary tillers are termed secondary tillers, and identified with two digits, where the first digit refers to the primary tiller and the second to the leaf number. For example, T11 is the secondary tiller formed in the axil of leaf Ll on tiller T1. Because winter wheat leaves normally appear at 180 GDD intervals, the number of leaves can be estimated from weather records. For instance, the fourth leaf should appear when 630 GDD has accumulated:

90 (Ll) + 180 (L2) + 180 (L3) + 180 (L4) = 630

(Ninety GDD are used for the main stem leaf Ll because it appeared at seedling emergence and was already partially grown).

Thermal time also can be used to time management practices. For example, if an herbicide label says application should be at the 3.5 leaf stage, then the treatment should be made at 540 GDD after planting. Observed emergence date is more accurate than planting date to begin GDD accumulation. Figure 2.3 and Tables 2.3 and 2.4 can provide thermal time estimates for ideal application timing for a variety of growth stages.

A given tiller appears only during a specific window of time and only if conditions are suitable. This tiller production window generally occurs after 1.5 to 2.5 leaves have appeared above, or roughly 270 GDD after the corresponding leaf appears. This knowledge can be used to evaluate management practices and yield production. For example, if the T2 tiller is absent but the T3 tiller is present, stress likely occurred during the T2 window, knowing that T2 and T3 should appear at about 630 and 810 GDD after emergence. Weather data can be used to calculate stress and management practices available to alleviate the stress.

Use this knowledge of wheat plant development, thermal time, and leaf and tiller appearance to maximize the most important yield component, number of heads per acre. The number of heads per acre depends on how many seedlings emerge, how many tillers appear on each plant, and how many tillers survive to produce a head. However, the tillers that contribute the most to final yield are T1, T2, and T11, so planting times should be adjusted to insure that these tillers are produced in the fall. This requires the accumulation of at least 540 GDD before fall growth stops (the T2 tiller appears at about the 3.5 leaf stage of the main stem).

If these tillers appear in the fall, they can grow sufficiently and produce a head. Typically, more tillers than heads are produced by a wheat plant. Shortly before jointing the plant begins aborting tillers unlikely to form heads; generally, those with fewer than four leaves. A few additional tillers may be lost up to anthesis, but the tiller number per acre is essentially set by the time of jointing. Consequently, the later emergence occurs, the more likely it is that a tiller will have insufficient leaves and will be aborted. This reduces the heads produced per acre, unless seeding rate is increased to compensate for this lower tiller production.

The key growth stages for determining wheat yield are emergence, jointing, and flowering. Patchy emergence reduces the number of heads per acre. Delayed emergence reduces the heat unit accumulation that is critical in leaf and tiller production. The survival of most tillers is determined during the jointing growth stage, and many of the developmental processes related to head and kernel number occur at or near jointing. Flowering is when the number of kernels per head is set, which in turn determines how much of the potential yield will be realized. Hot and dry conditions at flowering can severely reduce grain set and initial kernel development and growth.

We have briefly discussed how the wheat plant develops from planting through maturity and when yield components are determined. The developmental sequence is followed by all wheat plants. However, the timing of developmental events differs among cultivars, fields, and years. Thermal time (GDD) can be used to time events accurately and predict the occurrence of future crop growth stages when combined with forecast temperatures. Although GDD are extremely valuable for predicting the timing of future events, these are only estimates and not exact predictions. As shown in Tables 2.3 and 2.4, soil water availability has a strong influence on the accuracy of these estimates, particularly for the timing of anthesis and maturity. Variations in GDD between growth stages for different cultivars can be used to aid cultivar selection for your specific needs. Using the GDD approach to predict growth stages also can help in planning the optimal timing of management practices (e.g., planting date and herbicide applications).