Winter Wheat Growers Calendar
A sound understanding of plant growth and development is an essential element of efficient, economic wheat management systems. The impact of frost, heat, drought, diseases, insects, and weeds can be more accurately predicted with a clear picture of the relationships between growth stage and plant response to stress. The optimum timing of fertilizer, irrigation, herbicide, insecticide, and fungicide applications are also best determined by crop growth stage rather than calendar date.
The ten major growth stages that the wheat plant progresses through during its life cycle are all familiar to farmers:
- Stem elongation or Jointing
- Flowering or Anthesis
Measures of Growth and Development
[ Heat Units | Thermal Time Requirements for Wheat Production ]
Wheat Growth and Development
[ The Wheat Kernel | Germination | Seedling Stage ]
[ Tillering Stage | Stem Elongation or Stem Jointing Stage ]
[ Booting Stage | Heading Stage | Flowering or Anthesis Stage ]
[ Milk Stage | Dough Development Stage | Ripening Stage ]
Critical Growth Stages
MEASURES OF GROWTH AND DEVELOPMENT
Several systems have been developed to provide numerical designations for growth and developmental stages. Among these, the Feekes, Zadoks, and Haun scales are used the most frequently (Table 1).
The Haun scale growth stages key on rate of development of the main shoot (Table 1). In the early stages, the description of each new leaf is related to the previous leaf that was produced. For example, a seedling with one fully extended leaf and a second leaf that is half as long as the fully extended leaf is at Haun stage 1.5. Similarly, a plant with five fully extended leaves on the main shoot and an emerging sixth main shoot leaf that is 30% as long as the fifth leaf is at Haun stage 5.3. The booting stage numerical designation starts at one more than the number of leaves produced by the main shoot, i.e., the flag leaf number plus one. This can cause confusion because the flag number is not a constant for all cultivars. For this reason, the Haun scale has been used mainly to describe the growth stages before the booting stage.
Stem elongation or jointing
Flowering of Anthesis
|01||Water uptake (imbibition) started|
|05||Radicle emerged from seed|
|07||Coleoptile emerged from seed|
|0.0||09||Leaf just at coleoptile tip|
|1||10||First leaf emerged|
|1.+||11||First leaf unfolded|
|1.+||12||2 leaves unfolded|
|2.+||13||3 leaves unfolded|
|3.+||14||4 leaves unfolded|
|4.+||15||5 leaves unfolded|
|5.+||16||6 leaves unfolded|
|6.+||17||7 leaves unfolded|
|7.+||18||8 leaves unfolded|
|8.+||19||9 or more leaves unfolded|
|20||Main shoot only|
|2||21||Main shoot and 1 tiller|
|22||Main shoot and 2 tillers|
|23||Main shoot and 3 tillers|
|24||Main shoot and 4 tillers|
|25||Main shoot and 5 tillers|
|3||26||Main shoot and 6 tillers|
|27||Main shoot and 7 tillers|
|28||Main shoot and 8 tillers|
|29||Main shoot and 9 or more tillers|
|4-5||30||Pseudo stem erection|
|6||31||1st node detectable|
|7||32||2nd node detectable|
|33||3rd node detectable|
|34||4th node detectable|
|35||5th node detectable|
|36||6th node detectable|
|8||37||Flag leaf just visible|
|9||39||Flag leaf ligule/collar just visible|
|8-9||41||Flag leaf sheath extending|
|9.2||10||45||Boot just swollen|
|47||Flag leaf sheath opening|
|10.1||49||First awns visible|
|10.2||10.1||50||First spikelet of head visible|
|10.2||53||1/4 of head emerged|
|10.5||10.3||55||1/2 of head emerged|
|10.7||10.4||57||3/4 of head emerged|
|11.0||10.5||59||Emergence of head complete|
|11.4||10.51||60||Beginning of flowering|
|11.5||65||Flowering half complete|
|11.3||91||Kernel hard (difficult to separate by fingernail)|
|93||Kernel loosening in daytime|
|94||Overripe, straw dead and collapsing|
|96||50% of viable seed germinates|
|97||Seed not dormant|
|99||Secondary dormancy lost|
The Feekes scale recognizes eleven major growth stages starting with seedling emergence and ending with grain ripening (Table 1). The Feekes scale is frequently used to identify optimum stages for chemical treatments, such as fungicide applications, that focus on the plant development period from the start of stem elongation (Feekes stage 6) to the completion of flowering (Feekes stage 10.53).
The Zadoks scale provides the most complete description of wheat plant growth stages ( Table 1, Figure 1). It uses code based on ten major stages that can be subdivided, making it particularly suited for computerization. When using the Zadoks scale, the main growth stages, e.g., seedling versus tillering, should be identified before proceeding to a description of the secondary stages, e.g., seedling leaf number or tiller number.
Crop growth and development is often described in terms of time, e.g., 60-day barley, frost-free days, heading date, etc. However, a consideration of temperature is also important in these discussions because temperature determines the rate of growth and development. The time/temperature relationship that governs plant growth and development is known as thermal time and it is measured in heat units or growing-degree days. Heat units have been chosen as the measure of thermal time in this discussion to avoid confusion between growing-degree days and calendar days.
The thermal time required for crop production is determined by adding the daily heat units together for the period between planting and harvest. When the centigrade temperature scale is used, the heat units generated each day is determined by adding the minimum and maximum daily temperatures together and dividing by two. For example, a day with minimum and maximum temperatures of 10 and 20°C respectively, would generate 15 heat units [(20 + 10) / 2 = 15]. Days with average daily temperatures below 0°C do not contribute to the heat unit total.
Thermal Time Requirements For Wheat Production
The heat unit requirements to produce a mature crop are approximately 1550 for spring and 2200 for winter wheat. Translated into calendar days, this means that it would take 103 (103 x 15 = 1545) days to produce a spring and 147 (147 x 15 = 2205) days to produce a winter wheat crop if the average daily temperature was a constant 15°C. As we all know, there are large variations in temperature from day to day and growing season to growing season. The use of thermal time rather than calendar time takes this variability into consideration and provides an explanation for differences in crop maturity when observations from different years are compared. For example, we harvested Norstar winter wheat on July 20 in 1988 and August 24 in 1993 at Saskatoon. The 1988 growing season was much warmer with the result that the thermal time requirements to produce a mature Norstar crop were met five weeks earlier in 1988 than in 1993.
Figure 1. Wheat growth and developmental stages according to the Zadoks scale.
See Table 1 for comparisons with Feekes and Haun scales.
Figure 2. Wheat germination.
Figure 3. Wheat emergence. Zadoks stage 10.
Development of the roots, leaves, tillers, and spikelets on the head of the wheat plant takes place in an orderly, predictable pattern that is dependent upon thermal time. It takes approximately 105 heat units for a wheat plant to germinate and emerge from a seeding depth of less than one inch (2.54 cm). The appearance of each successive leaf on the main shoot and tillers then proceeds at a constant rate that is determined by cultivar, sowing date, and latitude. Most wheat cultivars require between 80 to 100 heat units to produce each leaf on the main shoot. After the requirements for leaf development have been met, another 650 heat units are normally required to complete the heading and maturation stages.
WHEAT GROWTH AND DEVELOPMENT
See Table 1 for Haun, Feekes, and Zadoks scale numerical designation for the following growth and developmental stages.
The mature wheat kernel (caryopsis) is composed of approximately 83 percent endosperm, 14.5 per cent bran, and 2.5 percent embryo. Once germination starts, the endosperm provides the developing plant with an energy source until its roots are established and newly expanded leaves allow it to harvest energy from the sun. The embryo of the mature wheat kernel has already undergone the first stages of plant development before the kernel is separated from the parent plant. In a mature kernel the embryo includes the coleoptile, which protects the first leaf as it pushes its way through the soil to the surface during germination, the radicle, which becomes the first root, and primodia, which develop into the first three leaves and seminal roots.
Germination starts with the uptake of water (imbibition) by a wheat kernel that has lost its post-harvest dormancy. Plant development is resumed once the embryo is fully imbibed. With the resumption of growth, the radicle and coleoptile emerge from the seed ( Figure 2 ). The first three seminal roots are produced and then the coleoptile elongates pushing the growing point toward the soil surface.
The seedling stage begins with the appearance of the first leaf ( Figure 3 ) and ends with the emergence of the first tiller. Up to six seminal roots and three leaves support the plant at this stage. The crown of the plant usually becomes noticeably distinct after the third leaf has emerged ( Figure 4 ).
Figure 4. Wheat seedling with three leaves and a developing crown.
Zadoks stage 13. Haun stage 2.6.
Figure 5. A wheat plant with five leaves, two tillers, and a well developed crown. Zadoks stage 22. Haun stage 4.7.
Crown formation is soon followed by the appearance of tillers and development of a secondary or crown root system ( Figure 5 ). The crown root system provides the plant with most of its nutrients and water during the growing season.
The distance between the wheat kernel and the crown is determined by the length of the subcrown internode ( Figure 4 ). The subcrown internode can elongate several inches and, depending on soil temperature, usually positions the crown within 1.2 inches (3.0 cm) of the soil surface. At Saskatoon, crown depths of 0.7 inches (1.8 cm) and 1.2 inches (3.0 cm) have been recorded when soil temperatures were 18 and 11°C, respectively, at the time of seeding. The recommended planting depth for winter wheat is less than one inch (2.54 cm). Consequently, a clearly defined subcrown internode is not usually found on seedlings of properly managed winter wheat.
The roots, leaves, tillers, and spikelets on the head of the wheat plant develop from primodia at nodes. While the first tiller is not produced until the third leaf has fully emerged, the appearance of later tillers is usually synchronized with the emergence of each subsequent new leaf that develops on the main shoot. For example, emergence of the fifth leaf is normally accompanied by the appearance of the second crown tiller ( Figure 5 ) which originates from an auxiliary bud (primodium) located in the node at the base of the second leaf (leaf axil). Similarly, a tiller can start producing its own subtillers once it has three fully developed leaves.
Each tiller that is produced represents the potential for a wheat plant to develop an additional stem complete with its own leaves, roots, and head. Root and shoot development of the plant is synchronized so that the number of crown roots is related to the number of leaves produced. However, root production by a tiller is usually delayed until its third leaf has emerged. Consequently, tillers that do not produce at least three leaves are not competitive and usually die off once the stem elongation stage starts.
Coleoptile tillers can develop when environmental conditions are favorable ( Figure 5 ). The development of coleoptile tillers is not closely synchronized with the development of the rest of the plant, but their appearance often coincides with the emergence of the third leaf on the main shoot. They develop from a node at the base of the coleoptile and are separated from the main shoot by the subcrown internode. However, when wheat is seeded shallow the subcrown internode does not elongate and the coleoptile tiller will originate from a position adjacent to the crown of the main shoot.
A major change in the development of the wheat plant occurs at the end of the tillering stage. At this time, the growing points of the main shoot and tillers stop initiating new leaves and start producing reproductive structures. Conversion of the growing point signals the end of the vegetative and the start of the reproductive period.
Early maturing spring wheat cultivars change from the vegetative to the reproductive phase after seven to eight leaves have been initiated on the main shoot. However, many commercial wheat cultivars have a vernalization (growth at low temperature - see Chapter 7 ) or photoperiod (growth under long day length) requirement that extends the vegetative period allowing for the production of more main shoot leaves and a larger number of tillers. An extended vegetative period due to a vernalization requirement is the main reason why more heat units are needed to produce a winter than a spring wheat crop.
The nodes from which leaves develop are telescoped at the crown during the tillering stage. Once jointing starts, the internode region elongates, moving the nodes and the growing point upward from the crown to produce a long stiff stem that will carry the head. Appearance of the first node (Zadoks stage 31) can usually be detected without dissecting the plant by pressing the base of the main (largest) stem between your fingers.
Each successive tiller on a wheat plant normally has one less leaf than its predecessor. This synchronizes the start of the stem elongation stages of the main stem and tillers.
Synchronization of growth and development at this stage ensures there will be no more than a few days difference in the maturity of all heads on the plant.
Synchronization of growth and development at this stage ensures there will be no more than a few days difference in the maturity of all heads on the plant.
Spikelet development on the microscopic head is usually completed by the time the first node is 0.4 inches (1 cm) above the soil surface. The terminal spikelet is produced at about Zadoks stage 31. A rapid loss of younger, poorly developed tillers also normally starts at this stage.
The stem elongation or jointing stage comes to an end with the appearance of the last (flag) leaf.
The developing head within the sheath of the flag leaf becomes visibly enlarged during the booting stage. The booting stage ends when the first awns emerge from the flag leaf sheath and the head starts to force the sheath open.
The heading stage extends from the time of emergence of the tip of the head from the flag leaf sheath to when the head has completely emerged but has not yet started to flower.
The flowering or anthesis stage lasts from the beginning to the end of the flowering period. Pollination and fertilization occur during this period. All heads of a properly synchronized wheat plant flower within a few days and the embryo and endosperm begin to form immediately after fertilization.
Early kernel formation occurs during the milk stage. The developing endosperm starts as a milky fluid that increases in solids as the milk stage progresses. Kernel size increases rapidly during this stage.
Kernel formation is completed during the dough development stage. The kernel accumulates most of its dry weight during dough development. The transport of nutrients from the leaves, stems, and spike to the developing seed is completed by the end of the hard dough stage. The developing kernel is physiologically mature at the hard dough stage even though it still contains approximately 30 percent water.
The seed loses moisture, and any dormancy it may have had, during the ripening stage.
CRITICAL GROWTH STAGES
Successful adaptation of a crop species is dependent upon the programming of critical growth stages so that the plant can capitalize on favorable weather periods during the growing season. Plants have evolved a variety of adaptive mechanisms that allow them to optimize growth and development while coping with environmental stresses. Plant breeders have selected and recombined the variability that exists in natural populations to produce cultivars with increased production potential and adaptation to a wide range of environments. An understanding of how plants respond to environmental stresses at different growth stages can assist in the assessment of crop condition and production potential throughout the growing season.
Winter wheat plants must survive the many stresses of winter (see Chapter 12 ). Roots and leaves that develop in the fall are often killed off during the overwintering period. However, as long as the crown remains alive, new roots and leaves can be regenerated. Therefore, plants that enter the winter with well developed crowns have the best chance of winter survival.
Grain yield can be expressed as the product of three variables (yield components):
(number of heads)
(kernels per head)
The impact of each yield component on final grain yield is determined at different stages during the growing season.
The number of viable seeds planted and the number of tillers produced per plant sets the upper limit on the number of heads that can be produced by a wheat crop. Tiller production is favored by moist, warm weather and good soil fertility, especially nitrogen fertility, prior to the stem elongation stage.
Tillers produced during the tillering stage must survive to maturity to contribute to grain yield. The developing head and elongating stem start making large demands on the plants' resources once stem elongation starts and younger, poorly developed tillers that are unable to compete are quickly lost. Tiller mortality level is especially dependent on environmental conditions immediately after terminal spikelet formation (Zadoks stage 31).
Drought and heat stress during the stem elongation and booting stages increase the rate of tiller mortality by placing added restrictions on resource availability. For example, nearly 250,000 tillers per acre per day were lost from winter wheat stands subjected to extreme drought and heat stress in the late spring of 1988 at Saskatoon. Only the main stem of each plant was left to set seed after this period of extreme environmental stress. If a drought is broken or a late application of nitrogen fertilizer suddenly becomes available during this period, the developmental synchrony of the plant may be disrupted producing a flush of later maturing heads. A dry spring followed by cool damp weather during the last half of June in Saskatchewan in 1993 produced many examples of this type of maturity problem ( Figure 6 ).
Environmental stress prior to flag leaf appearance can result in a loss of spikelets on the developing head ( Figure 7 ). As many as twelve florets per spikelet can be initiated under favorable conditions for development. However, later forming florets abort and normally only two to four florets actually set seed in each spikelet ( Figure 8 ). Floret initiation starts in the lower central region and progresses toward the base and tip of the head. Under extreme environmental stress, all of the florets in the spikelets at the top and bottom of the head may abort prior to flowering.
The number of tillers and florets initiated by the wheat plant is usually far in excess of the number of heads and kernels that can be supported through to maturity. As we have seen, a downward adjustment in yield potential normally starts with tiller loss at the beginning of stem elongation and continues with floret abortion prior to flowering. Environmental conditions experienced during these developmental stages determine the magnitude of the loss in yield potential. The final adjustments in yield potential are made during the grain filling period when kernel size is determined.
Asynchronous tiller development caused by a dry spring followed by cool, damp weather during the booting stage. Note the large differences in stage of tiller maturity.
Drought stress immediately before flowering can cause floret abortion (blasting).
Under favorable conditions of moisture and temperature, winter wheat can produce as many as five kernels per spikelet.
Bauer, A., D. Smika, and A. Black. 1983. Correlation of five wheat growth stage scales used in the Great Plains. USDA-ARS, Peoria, Ill.
Cook, R.J. and R.J. Veseth, 1991. Wheat health management. Amer. Phytopath. Soc., St. Paul, Minn.
Haun, J.R. 1973. Visual quantification of wheat development. Agron. J. 65: 116-119.
Large, E.G. 1954. Growth stages in cereals: Illustration of the Feeke's scale. Pl. Path. 3: 128-129.
Nelson, J.E., K.D. Kephart, A. Bauer, and J.E. Connor. 1988. Growth staging of wheat, barley, and wild oat. Montana State Univ. Coop. Exten. Service, Bozeman, and Univ. Idaho Coop Exten. Service, Moscow.
Zadoks, J.C., T.T. Chang, and C.F. Konzak. 1974. A decimal code for growth stages of cereals. Weed Res. 14: 415-421.