Decimal growth scale of cereals
Accurate assessment of growth stage is important because the cereal plant's response to herbicide, growth regulator or fertiliser application depends on its stage of development.
The Decimal (or Zadoks) growth scale of cereals is recognised internationally for research, advisory work and farm practice.
In Europe the decimal scale is already used in farming publications and the labelling of agrochemicals. Its use in Australia has been endorsed by the Australian Weeds Committee.
The decimal growth scale is based on 10 principal growth stages labelled 0 to 9. These sub-divide the cereal plant life cycle into 10 major stages:
- 0 — germination
- 1 — seedling growth
- 2 — tillering
- 3 — stem elongation
- 4 — booting
- 5 — ear emergence
- 6 — flowering
- 7 — milk development
- 8 — dough development
- 9 — ripening.
Each primary growth stage is then sub-divided into 10 secondary stages extending the scale from 00 to 99.
Early growth stages of seedling growth, tillering and stem elongation — which are the most important commercially — are described exactly by counting the organs on the plant.
- a 'one-leaf' plant is scored as 11 and a 'three-leaf' as 13. Similarly, one tiller is 21, four tillers 24, one node or joint detectable in the stem 31.
Growth scores are concurrent. Because more than one growth process is going on at the same time (for example leaves emerging and tillers forming) more than one growth score may apply at the same time.
- a plant may be scored as 17 (7 leaves on the main shoot), 24 (four tillers) and 31 (one node detectable).
Another example is when drought affected plants flower before the head has fully emerged from the boot. Such a plant might be scored 53 (one quarter inflorescence emerged) and 65 (flowering).
Although confusing at first, the concurrent scores do accurately reflect the current growth stage of the plant.
Using the decimal scale
Like all growth scales the decimal scale includes certain conventions and requires some practice before the user becomes fully familiar with it.
The decimal scale is based on observation of an individual plant, not the general appearance of a crop. It requires the user to either choose a plant as representative of the crop — or to sample the crop, score the chosen plants and determine the average growth stage.
Seedling growth — 10 to 19
After crop emergence, seedling growth is scored by counting the number of emerged leaves. The rules for counting leaves are:
- count leaves on the main stem only
- a leaf is counted as emerged when either its ligule has emerged from the sheath of the preceding leaf, or when the tip of the next leaf is visible.
It is useful to sub-divide the scale further by scoring the youngest leaf in tenths by judging its size relative to the preceding leaf. A score of 11.5 corresponds to one-and-a-half leaves and 13.9 is very close to the four fully-emerged-leaf stage.
Tillering — 20 to 29
The first tiller usually appears in wheat at growth stage 13 to 14 — when the plant has 3 or 4 leaves emerged. Tillering is scored by:
- counting tillers as soon as they emerge from the sheath of the subtending leaf
- counting only tillers, not the main stem.
Tillers originate from small buds where each leaf joins the stem. These grow and eventually emerge from between the leaf sheath and the stem. Occasionally an additional tiller may grow from the seed — this is known as a tiller and should also be counted.
Stem elongation — 30 to 39
Stem extension or 'jointing' is the process by which the ear — originally formed as a microscopic structure between growth stages 14 and 16 — is pushed to the top of the crop canopy.
The stem originally consists of 'nodes' or 'joints' (where the leaves join the stem) and 'internodes' all closely pushed together and only a few millimetres long.
When stem extension begins, an internode in the middle of the stack expands to several centimetres long and the node above it swells and hardens to form the first joint. This process is repeated by other internodes above the first until eventually the ear emerges from the boot.
Score a node as present when a thickening can be felt 10 to 50mm from the ground.
As stem extension occurs before there is any significant thickening of the node, the node can only be detected by stripping back the leaves or splitting the stem with a blade. This is a major point of confusion and whether a node is detected by feel or dissection should be made clear.
Anthesis (flowering) — 60 to 69
Anthesis means the opening of the floret to shed its pollen and be fertilised. Florets usually open in the early morning and then for less than 30 minutes. Anthesis is usually scored however, by the presence of the emerged anthers that have already shed pollen. This occurs first in the middle of the ear (stage 65) and spreads towards the top and base.
If moisture is lacking, head emergence may cease temporarily, or flower opening and anther extrusion may not occur.
Milk and dough development — 70 to 89
Grain development is not described well by the decimal or any other general growth scale and requires subjective assessment of the amount of solids in the milk and the stiffness of the dough.
Grain growth for seven to 14 days after fertilisation is mainly growth of the maternal pericarp — the ovary wall containing a watery fluid. This is scored as kernel watery ripe (71). Only then does starch deposition begin and the ratio of solids to liquid determines the early, medium and late milk stages. Dough development follows when no liquid remains.
Ripening — 90 to 99
Stage 92 can be taken as grain maturity and stage 93 as harvest ripe. These may be only a day or two apart where conditions are hot and dry, or several weeks apart in cool moist environments.
Stages 94 to 99 are not relevant to Australia.
Why the scale should be used
Use of the decimal growth scale may appear to be an unwelcome complexity to add to the problems of managing a crop. There are two vital reasons for introducing it:
- To improve and standardise communication between people in all sectors of agriculture. It is much more informative for the plant pathologist to know that the diseased crop was at growth stage 39 — flag leaf ligule visible — than to be told that it was at 'jointing'.
- more accurate description of growth stage is required as advice on crop management is increasingly given in terms of growth stage rather than calendar time. How much better is the advice available on herbicide tolerance when the growth stage is given as 24 or 31 rather than a vague description of 'mid to late tillering' or 'seven weeks from sowing'.
- 00 — Dry seed
- 01 — Start of imbibition (water absorption)
- 03 — Imbibition complete
- 05 — Radicle (root) emerged from caryopsis (seed)
- 07 — Coleoptile (shoot) emerged from caryopsis
- 09 — Leaf just at coleoptile tip
1 Seedling growth
- 10 — First leaf through coleoptile
- 11 — First leaf emerged
- 12 — 2 leaves emerged
- 13 — 3 leaves emerged
- 14 — 4 leaves emerged
- 15 — 5 leaves emerged
- 16 — 6 leaves emerged
- 17 — 7 leaves emerged
- 18 — 8 leaves emerged
- 19 — 9 or more leaves emerged
- 20 — Main shoot only
- 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
- 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
3 Stem elongation
- 30 — Pseudostem (leaf sheath) erection
- 31 — First node detectable
- 32 — Second node detectable
- 33 — Third node detectable
- 34 — Fourth node detectable
- 35 — Fifth node detectable
- 36 — Sixth node detectable
- 37 — Flag leaf just visible
- 39 — Flag leaf ligule just visible
- 41 — Flag leaf sheath extending
- 43 — Boots just visibly swollen
- 45 — Boots swollen
- 47 — Flag leaf sheath opening
- 49 — First awns visible
5 Inflorescence (ear/panicle) emergence
- 51 — First spikelet of inflorescence just visible
- 53 — Inflorescence one quarter emerged
- 55 — Inflorescence one half emerged
- 57 — Inflorescence three quarter emerged
- 59 — Emergence of inflorescence completed
6 Anthesis (flowering)
- 61 — Beginning of anthesis
- 65 — Anthesis half-way
- 69 — Anthesis complete
7 Milk development
- 71 — Caryopsis (kernal) water ripe
- 73 — Early milk
- 75 — Medium milk
- 77 — Late milk
8 Dough development
- 83 — Early dough
- 85 — Soft dough
- 87 — Hard dough
- 91 — Caryopsis hard (difficult to divide)
- 92 — Caryopsis hard (not dented by thumbnail)
- 93 — Caryopsis loosening in daytime
- 94 — Over-ripe straw dead and collapsing
- 95 — Seed dormant
- 96 — Viable seed giving 50% germination
- 97 — Seed not dormant
- 98 — Secondary dormancy induced
- 99 — Secondary dormancy lost