Light Interception and Crop Yield
DR. DENNIS B. ELGI
LEXINGTON, KENTUCKY
The Golden Rule of Crop Physiology states that ‘the crop must intercept all of the solar radiation that it receives to produce maximum yield’. We cannot control how much sunlight (solar radiation) reaches the surface of our fields, that’s controlled by location (latitude), time of the year (maximum levels on the summer solstice, June 21st), and the condition of the atmosphere (cloudy or clear). But we can do something about the proportion of the sunlight intercepted by our crops.
Crops grow because photosynthesis by the leaves uses CO2 from the air to produce the simple sugars that serve as the building blocks for all plant tissues. Sunlight intercepted by leaves provides energy to drive this process. The energy available for photosynthesis is maximized when the leaves provide complete ground cover. Maximum photosynthesis means maximum yield. The other inputs and conditions needed for crop growth (appropriate temperatures, water, fertilizer nutrients, absence of diseases and insects, etc.) mostly serve to facilitate photosynthesis. Sunlight that is not intercepted by the crop’s leaves is wasted, actually it is worse than wasted because it helps weeds grow.
Light interception by the crop is very low when the seedlings first emerge from the soil, but it increases as the crop grows, eventually reaching nearly 100% (actually the crop will never intercept all of the sunlight, so 95% is usually taken as the maximum) when you can no longer see the soil when you look at the crop (i.e., the leaves completely cover the soil). The maximum crop growth rate occurs when the crop reaches maximum light interception. There will be no further increase in crop growth rate if the plant community continues to produce leaves after it reaches complete ground cover (‘extra’ leaf area) because sunlight interception cannot increase above 95%.
This ‘extra’ leaf area is useful because it provides a cushion against reductions in leaf area caused by stress, such as water stress or insect feeding. The crop can tolerate loss of this ‘extra’ leaf area without reducing crop growth and yield because its loss does not reduce light interception.
How soon the crop reaches complete ground cover depends upon row spacing, plant population, growing conditions, and the characteristics of the crop (does it have mostly horizontal or vertical leaves?). Horizontal leaves, narrow rows or high populations encourage earlier complete ground cover than vertical leaves, wide rows, or lower populations. In a worst-case scenario, a crop in wide rows or one with restricted vegetative growth (for example, a late-planted crop) may never reach complete ground cover. In fact, one of the reasons that narrow rows often increase yield is that they make it possible for the crop to reach complete ground cover by flowering.
Is there any advantage from reaching complete ground cover before the crop begins reproductive growth? In a perfect world, the answer is no – yield is determined by sunlight intercepted during reproductive growth, so sunlight intercepted before reproductive growth does not contribute directly to yield. However, we seldom grow our crops in a perfect world, so reaching complete ground cover before flowering can affect crop growth and yield.
As mentioned previously, light not intercepted by the crop can support weed growth. Reaching complete ground cover early in vegetative growth helps control weeds which can add to yield, or at the very least, reduce the number of herbicide applications.
Reaching complete ground cover early can also have negative effects on the crop by increasing water use. The surface of the soil is usually dry, so soil evaporation is low. Water use will be higher when the soil is completely covered with leaves because the roots are in moist soil which provides water for transpiration. Water use during early crop growth usually tracks the increase in leaf area until the crop reaches complete ground cover unless, of course, rainy weather or frequent irrigation keeps the soil surface wet and then water use will equal the use with complete ground cover. This ‘extra’ water use early is only important if it reduces the soil water supply later during reproductive growth.
The benefits of early canopy closure for weed control are usually greater in humid climates than the possible negative effects of higher early water use.
Crop production systems must be designed to provide maximum sunlight interception by at least the beginning of reproductive growth. Complete ground cover is needed during reproductive growth to maximize photosynthesis and yield. Narrow rows and high populations will provide complete ground cover well before the beginning of reproductive growth for many crops. Early interception has no direct effect on yield, but it will help control weeds at the expense of a little extra water use. ∆
DR. DENNIS B. ELGI: University of Kentucky