VOLUME 5 ISSUE 1 SPRING 1995

BASIC NATIVE GRASS PHYSIOLOGY

Managing native warm season grasses is not particularly difficult it just requires a different base of knowledge than managing cool season grasses. Many times you are told to apply a management technique at a specific time to correct a specific problem or to maintain a healthy, vigorous stand, but you really don't know why you are doing what you are doing and why at that time. You are just following recommendations. I hope by providing some basic information about warm season grass physiology you can gain some insight into what and why.
To start off lets make a quick review of grade school science and review the process of photosynthesis. Green plant material uses sunlight and carbon dioxide plus water to manufacture carbohydrates (food) and oxygen. This is an extremely simplified example but will suffice for this discussion. Keep this in mind as this discussion ensues.

There are several grass physiological facts that hold true no matter what. These are some points to remember:

Carbohydrates are only produced during the growing season.
During the growing season, native warm season grasses growth points are elevated.
When food production (carbohydrate) exceeds growth needs, carbohydrates are stored.
When plants use more energy than is provided by photosynthesis, stored carbohydrates are used.
Carbohydrates are used during the winter to maintain the roots.
Stored carbohydrates are necessary to initiate spring growth.
Given these facts there is one important rule to also remember: Growth always has priority over storage of carbohydrates. What effect do these items have on NWSG management? The tools available for NWSG management are haying, grazing, mowing, burning or no disturbance at all. With the exception of no disturbance, all of the options require the removal of vegetative growth of the grasses. This process is know as defoliation. Depending upon the timing and extent of defoliation, grass health and vigor will be more or less affected.
Carbohydrates are only produced during the growing season Duh! This is a no brainer. Obviously if the plant is not actively growing there is not any green plant material (remember photosynthesis above?) and carbohydrates cannot be produced. Leaf surface area is necessary for photosynthesis, if leaf surface area is removed (defoliation) during the growing season carbohydrate production is reduced. Whether that reduction is detrimental or not is dependent upon the extent and timing of defoliation.

Native warm season grasses typically grow from mid-April through late August or September. If you defoliate the plant before leaf surface area has developed sufficiently enough to produce enough energy for plant growth, carbohydrate stores will be used. That is why we recommend that native warm season grasses be allowed to reach 12 to 14 inches growth before haying or grazing.

During the growing season, native warm season grasses growth points are elevated With only a few exceptions, once native warm season grasses reach about 4 inches tall in the spring their growth points elevate (this is why we can burn in late spring, when grasses are up to 4 inches tall without adversely affecting the plant health or vigor) and become susceptible to removal under uncontrolled haying or grazing. This is one area where NWSG's differ from cool season grasses, whose growth points are maintained close to ground level.

Depending upon the warm season grass species and climate this is usually around the end of April or first of May. Removal of growth points, if adequate leaf surface area is not left, forces the plant to call upon stored reserves to initiate regrowth. Regrowth under these conditions is slower than if growth points or adequate leaf surface area remain intact. In addition, depending upon subsequent defoliation, root health and vigor may be compromised. A number of clipping studies have been conducted, all with similar results. In one, indiangrass was clipped at 2" and at 9", regrowth was then measured six days later. The clump that was clipped at 2 inches had barely regrown 5 inches, where the clump clipped at 9 inches had regrown well over 10 inches with several leaves reaching 12 inches. Weaver looked at the effect grazing pressure had on root systems by comparing little bluestem from excellent, good and poor pastures. The weight of the roots from each of the respective pastures was 4.70, 1.72 and .33 grams. This represents over a 1400% decrease in root mass from the excellent pasture to the poor. This is why we recommend leaving a 6 to 8 inch stubble height when haying or grazing. If adequate rest follows severe defoliation early in the season, mistakes in management can be compensated for. This also applies to haying. However, severe defoliation late in the season will reduce the production of crown tissue for next years growth, reducing next years production.

When food production exceeds growth needs, carbohydrates are stored April through June demand and food production are nearly equal. Lots of energy is available but the plants are in tremendous growth periods. June through the end of the growing season carbohydrates are stored. Severe defoliation during this time will result in energy to maintain growth, rather than for storage. It is critically important during this time to maintain adequate leaf surface area to keep up with the growth demands of the plant. Because carbohydrates are only produced during the growing season and stored carbohydrates are necessary to maintain plant productivity, this is why we recommend not haying or grazing later than 45 days before average first frost date.

When plants use more energy than is supplied by photosynthesis, stored carbohydrates are used Areas of storage are: stem bases, roots and rhizomes or stolons. Harvesting native grasses below their growth points, not leaving enough leaf surface area for photosynthesis, doing either too late in the growing season or too often causes plants to use stored carbohydrate reserves. Since the roots are the storage area for carbohydrates, this type of use affects root health and vigor. Continued tapping into this stored resource reduces root mass, creating an ideal environment for competition. This is why over grazing or cutting too close is discouraged.

Carbohydrates are used during the winter to maintain the roots From September through mid-April native warm season grass plants require very little energy, though some is necessary to maintain the roots. This energy requirement is very low, but if stored carbohydrates are low it will have an affect on the plants productivity during the next growing season.

Stored carbohydrates are necessary to initiate spring growth About mid-April native grasses break dormancy and begin their annual cycle again. If stored carbohydrates are low the productivity of the plant will be low. In addition it may not have enough energy to be competitive with other plants.

This just as easily could have been called the carbohydrate cycle. I hope that you can see how all of these things tie together in native grass management. If you are managing for forage production you will want to manage to make your grasses as productive as possible. I hope that by a basic knowledge of this carbohydrate cycle and how it is interrelated with grass growth you can make some management decisions, or at least know why a certain practice is recommended.

For the wildlife manager, managing for the most productive grasses is not always the objective. In most instances, once a native grass stand is established, it is in the best interest of wildlife to manage against the grasses and for diversity. I hope that you too, through an understanding of the grass physiology, can make management decisions without relying on "text book" recommendations.