13.3 Small Grains Feekes/Zadoks Scales & Cotton Development

13.3 Small Grains Feekes/Zadoks Scales & Cotton Development

Understanding the growth stages of both small grains (like wheat, barley, and oats) and cotton is vital for effective agronomic management. While small grains follow precise numerical scales focusing on tillering, stem elongation, and heading, cotton development is characterized by its complex, indeterminate fruiting habit. This section covers the Feekes and Zadoks scales for small grains, followed by the developmental milestones of cotton.

Small Grains Staging: Feekes and Zadoks Scales

Accurate staging of small grains is essential for the precise application of fertilizers (especially top-dressed nitrogen), herbicides, and fungicides. Applying chemicals at the wrong stage can lead to crop injury or reduced efficacy. Two primary scales are used globally: the Feekes scale and the Zadoks scale.

The Feekes Scale (1-11)

The Feekes scale is a 1-to-11 numerical system that is widely used in the United States, particularly by agronomists and farmers. It is relatively simple and focuses on the visual milestones of the crop.

• Feekes 1-3 (Tillering):
  • Feekes 1: Emergence. The single shoot emerges from the soil.
  • Feekes 2: Beginning of tillering. Secondary shoots (tillers) begin to emerge from the crown.
  • Feekes 3: Tillers are formed. The plant has a prostrate growth habit (in winter wheat) and is building carbohydrate reserves. This is a common time for early nitrogen applications and weed control.
• Feekes 4-5 (Jointing and Sheath Elongation):
  • Feekes 4: Beginning of erect growth. The leaf sheaths begin to lengthen.
  • Feekes 5: Leaf sheaths are strongly erect. The growing point is still below ground, but the plant is preparing for rapid stem elongation.
• Feekes 6-7 (Stem Elongation):
  • Feekes 6 (Jointing): The first node is visible and can be felt above the soil surface. The growing point is now above ground and vulnerable to freeze damage. Herbicides must typically be applied before or at this stage to avoid crop injury.
  • Feekes 7: The second node is visible.
• Feekes 8 (Flag Leaf Emergence): The flag leaf, which is the final leaf to emerge, begins to unroll. The flag leaf is responsible for upwards of 75% of the carbohydrates that fill the grain, making its protection from disease critical.
• Feekes 9 (Ligule of Flag Leaf Visible): The flag leaf is fully expanded. This is the prime time for applying protective fungicides.
• Feekes 10 (Boot Stage): The head (spike) is swelling within the sheath of the flag leaf, creating a visible "boot."
  • Feekes 10.1-10.5: Heading. The head emerges from the boot.
  • Feekes 10.51: Beginning of flowering (anthesis). This is a highly critical stage for infection by Fusarium Head Blight (scab). Fungicides targeting scab must be applied during this brief window.
• Feekes 11 (Ripening):
  • Feekes 11.1: Milky ripe.
  • Feekes 11.2: Mealy ripe (dough stage).
  • Feekes 11.3: Kernel hard (physiological maturity).
  • Feekes 11.4: Harvest ready.

The Zadoks Scale (0-99)

The Zadoks scale is a more detailed, two-digit decimal code system preferred by researchers and used extensively in Europe. It provides a more granular description of the plant's architecture.

• 00-09: Germination
• 10-19: Seedling growth (number indicates the number of unfolded leaves, e.g., 13 = three leaves unfolded)
• 20-29: Tillering (number indicates the number of tillers, e.g., 22 = main shoot and two tillers)
• 30-39: Stem elongation (e.g., 31 = first node detectable, equivalent to Feekes 6; 39 = flag leaf ligule just visible, equivalent to Feekes 9)
• 40-49: Booting (e.g., 45 = late boot)
• 50-59: Inflorescence (head) emergence (e.g., 55 = half of head emerged)
• 60-69: Anthesis (flowering) (e.g., 65 = flowering half-way completed)
• 70-79: Milk development
• 80-89: Dough development
• 90-99: Ripening

Understanding the cross-reference between Feekes and Zadoks is essential. For example, Feekes 6 (Jointing) corresponds to Zadoks 31. Feekes 9 (Flag Leaf) corresponds to Zadoks 39. Feekes 10.51 (Flowering) corresponds to Zadoks 60-69.

Cotton Development and Physiology

Cotton (Gossypium hirsutum) has one of the most complex growth habits of any major row crop. It is an indeterminate perennial (managed as an annual) that produces vegetative and reproductive structures simultaneously over a long period. This requires careful management, often involving plant growth regulators (PGRs), to balance vegetative growth with boll development.

Vegetative Growth

Cotton emerges with two cotyledons (seed leaves). True leaves develop subsequently on the main stem. The plant produces two types of branches:

1. Vegetative Branches (Monopodia): These grow from the lower nodes on the main stem. They grow upward and function much like the main stem, producing their own fruiting branches.
2. Fruiting Branches (Sympodia): These develop from higher nodes on the main stem and the vegetative branches. They grow laterally and produce a flower at each node along the branch.

Reproductive Milestones

• Squaring: The first reproductive structures are called "squares" (flower buds). Squaring typically begins around 35-45 days after planting, starting on the lowest fruiting branches.
• Flowering (Bloom): About three weeks after the first square appears, the first flower opens. Cotton flowers are large and showy. They open as white or creamy yellow on the first day, turn pink or red on the second day, and eventually fall off, leaving a developing boll. Because cotton is indeterminate, it will continue to produce new squares and flowers as long as environmental conditions allow.
• Boll Development: The developing fruit is a capsule called a boll. Boll development takes about 45 to 60 days from flowering to opening. The boll goes through three phases:
  1. Enlargement: Rapid growth in size.
  2. Filling: Deposition of cellulose inside the cotton fibers (lint).
  3. Maturation: Drying and opening.
• Cutout: This is a crucial physiological stage in cotton management. Cutout occurs when the plant's demand for carbohydrates (to fill developing bolls) exceeds its photosynthetic capacity (to produce new vegetative growth and squares). The plant effectively stops producing new terminal growth and focuses entirely on maturing the existing bolls. Agronomists often measure cutout as the point when there are only 5 nodes above the uppermost first-position white flower (NAWF = 5).
• Defoliation and Harvest: Once the bolls are mature and open, chemical defoliants are applied to remove the leaves and prepare the crop for mechanical harvesting.

Cotton management is a delicate balancing act. Excessive nitrogen or water can lead to rank vegetative growth, delayed maturity, and reduced yield. Management relies heavily on monitoring square retention and boll development to make timely decisions.