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Identifying key sorghum plant structures is the first step to better understanding sorghum growth and development. The first leaf that emerges from the ground is the coleoptile. The coleoptile is different than other leaves because it has a rounded leaf tip. The leaf collar is visible only when the leaf has completed development, and occurs where the leaf blade and leaf sheath meet. This intersection occurs at a node. The flag leaf is the final leaf formed on the plant and tends to be somewhat smaller than the other leaves. Lastly, the panicle (head) emerges from the flag leaf sheath supported by the peduncle (stem holding the head).1
The exact timing between sorghum growth stages can vary depending on sorghum products, plant population, environmental conditions, growing conditions, and region. When determining the leaf stage of a sorghum plant, count the number of leaves that have the leaf collar showing. If the lowest leaf does not have a rounded tip, then a minimum of one leaf has been lost.
Stage 0. Emergence may occur 3-10 days after planting depending on soil temperature, moisture, planting depth, and seed vigor. Growth relies on seed nutrient reserves. Cool and wet conditions may increase incidence of disease.
Stage 1. The collar of the third leaf is visible and the growing point remains below the ground. Speed of growth depends on air temperature. Good weed control during this stage will help small seedlings grow faster.
Stage 2. The collar of the fifth leaf is visible, and the plant is entering a period of accelerated growth. The root system is developing quickly and the first leaf may slough off. Proper management of weeds, insects, moisture, and nutrient availability can greatly help to realize potential yield.
Stage 3. The growing point differentiates from vegetative growth to reproductive growth. New leaves will no longer be produced, but existing leaves will continue to grow. The stalk will grow rapidly. Nutrient uptake will greatly increase as head development begins. Nutrient and water availability is critical at this point. Canopy closure reduces the need for weed management.
Stage 4. The flag leaf can be seen in the whorl. Head continues to develop and about 80% of the total leaf area has formed. Nutrient and water uptake requirements continue.
Stage 5. Leaves have fully expanded and are at maximum light interception. The head is almost full size and can be found enclosed in the flag leaf sheath. Moisture stress or injury from herbicides can reduce complete head exsertion from the sheath, which can prevent pollination.
Stage 6. A plant is in bloom when the peduncle rapidly grows pushing the head through the flag leaf sheath. A crop is in half bloom when more than half of the plants in the field are in some stage of bloom. Grain begins to fill, making the crop susceptible to seed set losses during hot and dry weather.
Stage 7. Grain continues to fill during the soft-dough stage, accumulating about half of its final dry weight during this stage. Leaves will continue to slough off.
Stage 8. About 75% of final dry weight has been accumulated by the hard-dough stage. Nutrient uptake is considered complete. Severe moisture stress may contribute to light, chaffy grain.
Stage 9. Sorghum plants have reached physiological maturity. Black layer is formed, visible by a dark spot on the kernel opposite the embryo. Grain moisture typically varies between 25 and 35% depending on sorghum product selection and growing conditions.2,3
The nutrient requirement of sorghum between growth stages 2 through 6 is critical for crop development. At growth stage 6, sorghum plants have used roughly 60% of phosphorous, 70% of nitrogen, and 80% of potassium required for production, but have only produced 50% of their final dry weight. As sorghum plants grow toward maturity, large amounts of nitrogen, phosphorous, and potassium are translocated from older leaves to the grain. Limited nutrient availability during grain fill may result in increased leaf loss, which may lead to a reduction in yield potential.2
As the time between sorghum growth stages and expected maturity date depend on so many variables, growers may want to more accurately determine crop development. To do this, growers can calculate how many growing degree units (GDU) the crop has received.
The lowest temperature for sorghum development is 50° F, and the highest temperature that may be entered is 100° F. The cumulative GDUs required to reach certain sorghum growth stages differs between short season and full season sorghum products (Table 2).4