Posted on Jun 08, 2018
In the life of any crop, certain growth stages negatively impacted by weather or cultural practices suffer a greater effect on yield than others. The first critical stage in the life of a sorghum crop is obtaining a healthy, uniform stand at the correct plant population for a given environment. Once the crop is established, a grower should know the importance of recognizing the different vegetative stages and understand how management during each specific stage can impact yield. Although infrequent, cutworm damage can occur during the first few days after emergence. An insecticide application should be considered if the majority of the cutworms are less than 1/2 inch long.
Early growth stages are determined by counting fully expanded leaves with the presence of a leaf collar. The first leaf is the coleoptile leaf, which is shorter than the later-emerging leaves and has a rounded leaf tip. Usually within 10 to 14 days after emergence, the sorghum plant has three fully expanded leaves and is only 3 to 4 inches tall. If a preemergence herbicide, such as Dual or Outlook, was not applied prior to emergence, the herbicide should be applied during this stage. Atrazine also can be applied without any risk of sorghum injury. Because the plant is very small, competition from weeds over the next 20 days can greatly impact yield. If weeds have emerged, growers should control them with a postemergence herbicide. This stage is also the time to make stand counts prior to the appearance of tillers, which will occur soon.
A sorghum plant typically reaches the five-leaf stage 20 to 25 days after emergence, when the plant is approximately 8 inches tall (Figure 1). The roots should be rapidly expanding, and basal tillers should be present. The growing point is still below the ground surface but not for much longer. If a grower plans to apply 2,4-D or dicamba, the best application time is between the third- and fifth-leaf stages. After the fifth-leaf stage, labels of other herbicides should be carefully examined prior to application to determine if and how the herbicide can be applied safely. If using a nitrogen fertilizer as a side-dress treatment, it should be applied within 10 to 15 days.
At 30 to 40 days after emergence, the crop reaches the critical growing point differentiation (GPD) stage, where the plant shifts from producing only leaves to beginning the formation of the grain sorghum head. The GDP stage occurs at the seven- or eight-leaf stage, when the plant is 12 to 15 inches tall. If one or two of the lowest leaves fall off of the plant, a grower may find it difficult to determine the actual leaf stage. To determine if the plant has reached the GDP stage, the grower can slice the stalk open and look for the growing point, as shown in Figure 2. If the growing point is visible above the ground, the plant is in the GPD stage. Prior to the GPD stage, the plant can withstand considerable stress, including hail damage, with minimal effect on yield. Herbicide application becomes much more risky at the GDP stage, and any stress on the plant can reduce head size and potential seed number per head. The GDP stage means the crop is entering into a phase of rapid growth with accelerating uptake of water and nutrients.
The last key growth stage before head emergence and flowering is the boot stage. At this stage, collars of all leaves are visible, and the sorghum head or panicle is located just below the flag leaf collar, enclosed in the flag leaf sheath. Just below the flag leaf, the stalk appears swollen. The sorghum head emerges within as few as three days, which is the beginning of a critical time in the life of the sorghum plant. Moisture and heat stress over the next 20 days can greatly influence final yield, and water and nutrient use is at its peak. Growers should closely monitor sugarcane aphid infestation levels prior to and throughout the boot stage. If sugarcane aphid populations increase above the established regional threshold, an insecticide should be applied immediately.