Although animals eat all year round, there is no "all season" plant to use as forage. Knowing that some plants are C3 (cool season, temperate) and some plants are referred to as C4 (warm season, tropical) is a basic key to having quality forage all year long. But understanding the physiology (internal chemical changes) of both can even further improve the management of forages.

The science:
C3 and C4 plants both use the process of photosynthesis to convert light energy and atmospheric CO2 into plant food energy (carbohydrates).


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


C3 and C4 plants differ in the leaf anatomies and enzymes used to carry out photosynthesis. These differences are important with respect to their optimal growing conditions, N and water-use efficiency, forage quality, and seasonal production profile.

C3 plants (cool season)

The science:
C3 plants are called temperate or cool-season plants. They reduce (fix) CO2 directly by the enzyme ribulose bisphosphate carboxylase (RUBPcase) in the chloroplast. The reaction between CO2 and ribulose bisphophate, a phosphorylated 5-carbon sugar, forms two molecules of a 3-carbon acid. This 3-carbon acid is called 3-phosphoglyceric acid and explains why the plants using this chemical reaction are called C3 plants. The 3-phosphoglyceric acid molecules move out of the chloroplast to the cytoplasm and are used to make hexose, sucrose, and other compounds. The enzyme ribulose bisphosphate carboxylase also triggers a reaction where oxygen splits ribulose bisphophate into a 2-carbon acid and a 3-phosphoglyceric acid. The 2-carbon acid is respired to carbon dioxide via photorespiration and basically lost to plant function. As much as 15-40% of the light energy taken into the C3 plants is lost via photorespiration which rises with increasing temperatures.

Thus, C3 plants fix CO2 more efficiently in cooler environments.

The ramifications:
C3 plants have an optimum temperature range of 65-75 degrees F. Growth begins when the soil temperature is 40-45 degrees F. C3 plants become less efficient as the temperature increases, but they provide a higher percentage of crude protein than C4 plants. Cool temperatures of early spring also effect the activity of soil organisms which release nitrogen from organic reserves. Thus, C3 plants respond to nitrogen fertilizer during this season. Cool-season grasses are productive in the spring and fall because of the cooler temperatures during the day and night, shorter photoperiods, and higher soil moisture. During the summer, growth is reduced and dormancy is induced by high temperatures and low precipitation. However, in fall, when temperatures drop and moisture is more available, growth resumes.

There is evidence that summer dormancy is associated with mismanagement of seed heads. Timely removal of seed heads, at the late-jointing to early boot stages, triggers growth of the second cycle of tillers before the onset of hot, dry weather. It may be possible to increase summer productivity in this manner.

There is some evidence indicating that conditions necessary for floral induction in C3 plants are different from C4 plants. Cool-season grasses may require short days and/or low temperatures in the fall or early spring (a vernalization period) before the seedhead develops from the meristem (growing point). There also seems to be a need for the tiller (shoot, new plant) to reach a certain size before vernalization can commence. Timothy does not require this vernalization but requires long days to flower.

C3 plants can be annual or perennial. Annual C3 plants include wheat, rye, and oats. Perennial C3 plants include orchardgrass, fescues, and perennial ryegrass. The degradation of C3 grasses in the rumen of an animal is often faster than C4 grasses because of the thin cell walls and leaf tissue and are therefore often of higher forage quality.

C4 plants (warm season)

The science:
C4 plants are often called tropical or warm season plants. They reduce carbon dioxide captured during photosynthesis to useable components by first converting carbon dioxide to oxaloacetate, a 4-carbon acid. This is the reason these plants are referred to as C4 plants. Photosynthesis then continues in much the same way as in C3 plants. This type of photosynthesis is highly efficient and little fixed CO2 is lost through photorespiration.

The ramifications:
C4 plants are more efficient at gathering carbon dioxide and utilizing nitrogen from the atmosphere and recycled N in the soil. They also use less water to make dry matter. They grow best at 90-95 degrees F. They begin to grow when the soil temperature is 60-65 degrees F. Forage of C4 species is generally lower in protein than C3 plants but the protein is more efficiently used by animals. This efficiency may result because C3 plants contain a lot of non-protein nitrogen (NPN), very labile (changeable) in form, which pass into the gut or is absorbed directly into the portal vein leading to the liver and not incorporated into microbial proteins by rumen microflora. It is well established that NPN levels may exceed the liver's capacity to filter it out, thus it enters the systemic blood and causes ammonia intoxication.

Warm-season grasses are specifically triggered by daylengths so latitudes should be considered in selecting warm-season grass species. They are most productive during the warmer summer months. Often, cool-season and warm-season species are used in combinations to provide forage throughout much of the year. With ample soil moisture, warm season grassses may respond to nitrogen fertilizer but because irrigation is often expensive, supplemental nitrogen is seldom applied.

C4 plants can be annual or perennial. Annual C4 plants include corn, sudangrass, and pearlmillet. Perennial C4 plants include big bluestem, indiangrass, bermudagrass, switchgrass, and old world bluestems.

Practical Applications

In recent years, warm-season grasses have been recommended for seeding retired cropland. Efforts are also underway to improve rangelands by introducing species that have disappeared due to over grazing. To ensure persistence, pastures can be established using cool-season and warm-season grasses. Cool-season grasses could be utilized for fall, winter, and spring grazing and the warm-season grasses would flourish in the summer. In spring, the warm-season grasses should be protected until they can better withstand defoliation. To determine when that is, monitor the root system for the production of new tillers.

Warm-season grasses reach their peak of production about a month later than cool-season grasses. Although warm-season grasses produce less yield, their virtue is to provide superior midsummer grazing when cool-season grasses are semi-dormant. Both types can be stockpiled during late summer and fall to provide maintenance energy for livestock during the winter months.

Warm-season (C4) grasses normally contain less protein than is found in cool-season (C3) grasses. This might be expected because warm-season grasses are seldom fertilized with supplemental nitrogen. However, to achieve yield goals with cool-season grasses, they are often fertilized with some form of nitrogen. This increases the protein content of the grass, as nitrogen accounts for 16 percent of the protein molecule. Nitrogen that is not incorporated into proteins is temporarily stored in various forms: free amino acids, nitrates, amides, and amines, broadly classed as non-protein nitrogen (NPN). In chemical analyses of feedstuffs, these forms of nitrogen are commonly considered as being as nutritious as true proteins. This may not hold true if the NPN level is too high.

The protein in C4 grasses is used more efficiently by ruminant livestock. A higher percentage of the protein in C4 grasses is retained in the carcass and less is voided via the kidneys as urea. Cattle reach a higher degree of finish on C4 range grasses than on more lush C3 grasses. Why is this?

Research suggests that reduced efficiency in protein utilization in C3 grasses might be due to excessive levels of NPN. These NPN substances are rapidly deaminated (an amino group is removed) by enyzmes (chiefly urease) present in the rumen microflora. The ammonium (NH4) released from deamination can cause stress similar to the type which often occurs when feeding excessive amounts of urea to livestock.

There is evidence that livestock may suffer illnesses from lush pastures because the rapid release of ammonium N from the labile nitrogenonous substances in grasses.

High levels of soil nitrogen lead to rapid uptake of this element by plant roots. Some of it may be stored as NPN. If rumen microflora fail to incorporate the liberated ammonium N into microbial protein (this being the normal function of rumen bacteria), a significant portion may be absorbed through the rumen wall into the portal vein leading to the liver. Additionally, in a worst case scenario, some of the nitrogen-rich material may pass into the secum where it is degraded by bacteria rather than by the enzyme urease. This is known as intestinal putrefaction (proteins rot or putrify, whereas carbohydrates ferment). The ammonium N released via putrification is absorbed directly into the portal blood system leading to the liver. The liver is challenged to convert the the nitrogen in the portal blood system to urea so that will enter the general blood stream which nourishes the brain, kidneys, muscles, and other organs.

If the liver malfunctions, or its capacity to filter the ammonium nitrogen is exceeded, this toxic ammonium eventually reaches the brain via the general circulation and causes various forms of livestock disorders, broadly classed as ammonia intoxication.

The above interpretation suggests that cool-season grasses should not be heavily fertilized with nitrogen (>50 lb of actual N per acre per month). Nitrogen should be applied in split applications. As an extra precaution to maintain low levels of NPN in cool-season grasses, maintain ample levels of phosphorus and potassium in the soil. Potassium serves a catalytic function in protein synthesis thereby lowering the level of NPN and phosphorus is important in energy metabolism (ATP).

What about pasture supplements? Energy-rich supplements can be offered as additional insurance against NPN stress. For example, when urea is added to livestock rations, it is essential to supply grain or molasses to stimulate growth of rumen microflora, thereby creating a demand for the ammonium nitrogen released in the rumen. Additionally, livestock relish a mineral supplement which contains clay. The cation exchange properties (buffering capacity) of clay minerals promotes the absorption (and possible fixation) of ammonium ions.

Reference: Bessman, S.P. "Ammonia Metablolism in Animals: Symposium on Inorganic Nitrogen Metabolism." 1956. McElroy and Glass (eds). The Johns Hopkins Press.

The graphic that follows shows when different types of grasses are most productive. No species will work throughout the year, so other feed will be needed. It also indicates when to harvest abundant growth for hay or silage. Note the different growth patterns of warm-season and cool-season grasses.