Primary symptoms

Low radiation: Cloudy conditions lower yield by reducing the number of grains.

Summary

Crop growth is primarily determined by the amount of sunlight (solar radiation) that the crop can intercept and use during its life. Too much radiation is rarely a problem as long as water and nutrients are in adequate supply. To get high yields, leaves should expand to cover the ground surface as soon as possible after planting. With delay, solar radiation is wasted just heating up bare soil and evaporating soil moisture. Solar radiation is particularly important to yield during the period from late stem elongation to one week after anthesis (Z3.3 to 7.05). Low solar radiation accompanying hot weather during this period reduces the numbers of grain sites and severely limits potential yield. Crop growth rate increases linearly with absorbed sunlight.

Has the sun been hidden by clouds or fogs?

• During early tillering check for numbers of tillers. Are they what you would expect for a normal crop? Compare your counts for main shoot leaf number and tillers per plant with those on the graph. Tillering is suppressed if the weather is dull but hot. Tillering is also suppressed when it is hot and plants are short of water.
• Look for weak shoots during the late tillering phase (Z2.4 - 3.5).
• Following anthesis, are there fewer grains per spikelet than you would expect? Was it overcast or misty but warm during the two weeks or so before anthesis? During this time, percent reduction in grain numbers almost equals percent reduction in solar radiation.
• After anthesis are florets still gaping with the anthers pale coloured or misshapen? Was solar radiation low between Z3.9 and Z5.0 because of clouds or fogs, though it was warm? If your field is marginally low in boron this sterility is probably due to that. Soil-based boron effects are exacerbated by dull, low-light conditions and high humidity at this stage. These conditions reduce transpiration rate and associated uptake of boron from the soil (see Rawson and Subedi 1996 for more detail).

Can you do anything about solar radiation?

• Avoid moisture stress: Irrigation should be managed to avoid water stress during periods of high solar radiation and high temperature. Then those weather variables will be used positively in the production of biomass and yield.
• Apply boron: If it is likely that radiation will be very low between the time that the flag leaf collar appears and anthesis and your soil is low in boron, avoid low-boron sterility by applying boron at sowing (1 kg/ha). This is good insurance. Additionally, plant a variety that is tolerant of low boron.