The basic reaction in green plants that converts solar energy to chemical energy is called photosynthesis. This reaction is directly or indirectly responsible for all life on earth. It provides the energy (carbohydrate) for plant growth and maintenance as well as animal growth and maintenance.


Carbon dioxide (CO2) + water (H2O) -> carbohydrate (food) + oxygen (O2)
green plant material

The carbon of CO2 (a gas) is converted to the carbon of the carbohydrate (a solid). The carbohydrate is a chemical way to store the sun's energy as "food." Carbon dioxide in the air, a raw material for the photosynthetic process, is not very abundant. The atmosphere has approximately 0.03%. The success of a plant will depend on its ability to collect and use CO2 in the photosynthetic process. For perennial pasture grasses to remain productive, the photosynthetic process must first feed the plant before it can provide feed for livestock.

Cool- (C3) and warm-season (C4) Grasses

The C4 photosynthetic system found in warm season grasses is more efficient in gathering CO2 than the C3 system. Consequently, warm-season plants have the potential to be more efficient than cool-season plants when both are at optimum conditions.

Optimum temperature for the growth of C3 plants is around 65-75°F while it is 90-95°F for C4 plants. C4 plants may use up to 50% less water to produce a unit of dry matter. Water use efficiency and temperature optimums explain why warm-season pastures are more productive in hot, dry summer months and cool-season pastures are more productive in the cool, moist spring and fall months. Since cool-season plants start growth early in the spring, soil moisture is often depleted by early summer. In contrast, warm-season grasses which start their growth in late spring generally have a favorable soil moisture profile in early summer.

There is no "all-season" plant available. Producers must recognize the limitations of plant seasonality and take advantage of the variation in grasses. Incorporate both cool and warm-season pastures to provide a longer grazing in climates with a hot period during the summer.

C4 plants are also more efficient in nitrogen utilization. Warm-season plants recover more N from a given amount of available soil nitrogen than the cool-season plants due to increased soil microbial activity in the summer. In contrast, cool-season grasses have a high demand for nitrogen in the spring when there is little soil microbial activity. Consequently, nitrogen fertilization is essential to achieve satisfactory levels of production for cool-season grasses. Warm-season grasses generally respond better to fertilizer under humid climates.

These physiological factors help explain why cool-season grasses grow in the spring, mature by late spring or early summer, and become dormant during the hot summer months before resuming growth in the fall. Warm-season grasses mature during late summer and may become dormant early in the fall.

These physiological factors also have an effect on animal production. For example, the poor performance of livestock grazing tall fescue during the summer has been attributed to an endophyte fungus. Normal reduction in cool-season growth and quality during the summer is an important factor in grass utilization. The use of an endophyte-free, cool-season grass will only partially solve the problem of "summer slump."

Matching the seasonality of grasses with the season of livestock use is a good strategy.