Yield Components

Each yield component has a period during which it is most sensitive to environmental and management conditions. These periods correspond to the developmental stages in which the potential of a component is set and then realized. For example, number of plants per unit area is influenced by seeding rate, germination percentage, and seedling survival. Seedbed conditions, temperature, water content, soil-seed contact, etc., affect germination and seedling survival rates. Table 2.1 shows other examples of factors and management practices that affect specific yield components.

A triangle is a useful representation of how yield components interact to achieve a given yield (Figure 2.2). Number of spikelets per head and number of kernels per spikelet can be combined to create kernels per head. The triangles shown in Figure 2.2 represent the yield potential at the beginning of grain filling for two different growing conditions. Final yield is determined by the kernel number per unit area and the size of these kernels. Grain filling can be thought of as a pipe (the process of ) delivering material (carbohydrate) to fill the triangle (kernels). Under favorable conditions, all kernels fill to their potential, and yield is high. Under stressful conditions the flow of carbohydrate to the kernels is reduced because fewer carbohydrates are available or there is less time for filling and, as a result, yield is less than the potential.

In semi-arid production systems of the Great Plains, yield components related to number of plant parts (number of tillers per acre, number of kernels per head) generally are more important in determining yield than size of the parts (kernel size). This reflects the fact that the size of kernels tends to be more stable than number of heads or kernels. It follows that the number of heads per acre is the yield component most affected by environmental conditions, including management. In other words, management practices that promote good plant populations and tillering are critical for optimal yields (Table 2.1).

Figure 2.2 These yield components define the potential yield for the environmental conditions experienced during critical crop growth stages. Once kernel number is set, grain filling rate and duration define final yield by determining kernel weight.

Table 2.1 Environmental factors and management practices that determine yield components.

Yield components are determined throughout the orderly and predictable development and growth of the wheat plant. Most important developmental events occur at the growing point (shoot apex) and can be timed using the externally recognizable growth stages. All cultivars follow this developmental sequence, but they can vary in the rates and duration of developmental events. Table 2.2 shows how the timing of a growth stage is affected by cultivar and location. These variations are an important consideration in cultivar selection. The overall developmental sequence is also important in understanding why management practices frequently target certain growth stages for maximum efficacy.