Photosynthesis is the process by which plants combine solar energy, atmospheric CO2, and water, within green leaf tissue (chlorophyll) to produce carbohydrates. Plants use these carbohydrates as a source of energy to carry on basic metabolic processes. In short, while overly simplistic, plants can create their own food using the simple ingredients of sunlight, water, and CO2. Plants do require, however, adequate green leaf (photosynthetic tissue) in order to carry out photosynthesis. Without these four main ingredients plants cannot survive. As managers there is not a lot we can do regarding CO2; there is adequate quantities in the atmosphere. We can, however, have an impact on water, sunlight, and the amount of green leaf involved in the photosynthesis process. Let’s take a look at what impacts we can have on the important aspect of plant production.
Some might argue we have little control over the amount of sunlight plants receive. The truth is we have a substantial influence on the amount of sunlight reaching the plant leaves. If we do not control weeds or if we do not appropriately manage winter pasture as warm-season grasses are breaking dormancy, a canopy (think umbrella) of weeds or winter annuals results and intercepts/uses most of the sunlight with very little reaching the forage plant below. Since sunlight is a crucial component of photosynthesis, the forage below the canopy suffers. Without adequate sunlight, photosynthesis is reduced, root growth and development is decreased, and overall vigor and production of the forage plant declines. As managers we have direct control over the amount of sunlight reaching the desired forage by either removing weeds with the appropriate herbicide at the appropriate time or by removing winter annuals prior to the time warm-season grasses begin to make active growth. When night time temperatures consistently reach 60⁰F, warm-season grasses begin making active growth and cool-season annuals should be removed prior to this time.
The same argument could be made regarding precipitation; obviously, managers have no control over the amount of precipitation occurring during the year. Managers do, however, can control how much of the precipitation remains on their property. When pastures are routinely grazed short, overland flow of runoff during precipitation events is increased and more water runs off the pasture rather than into the soil (infiltration). Consider making your property a “sponge” to capture and keep as much moisture as possible. This concept of slowing down runoff velocity and increasing infiltration also protects the soil from loss (erosion), keeps expensive fertilizer nutrients, pesticides, and bacteria in the pasture, and protects soil organic matter from being lost as a result of soil erosion. Water + topsoil + nutrients + organic matter = a much healthier pasture environment and will pay dividends for years to come. Conversely, loss of water, topsoil, organic matter, etc., results in a pasture system that is low in productivity.
Finally, as managers we have a direct control over how much green leaf remains in the pasture. Research data indicates up to 50% of the forage leaf may be removed without deleterious effects on the root system. Once >50% of the leaf is removed root growth and development is reduced significantly. As root growth and development is reduced there is a negative feedback to the top growth; this further exacerbates an already bad situation. As managers we primarily control the amount of green leaf taken by using the appropriate stocking rate and possibly some type of grazing method. However, other “grazers” such as grasshoppers, fall armyworms, and the new bermudagrass stem maggot can also remove large amounts of green leaf, thus depriving the plant of the ability to produce roots. The ability for good root growth and development is always important, but critically so during drought.
The bottom line? Maintain an adequate amount of forage residue in the pasture at all times. Different species have different thresholds. Bermudagrass and bahiagrass, for example, may be grazed repeatedly to a height of 4” with little detrimental effect. Kleingrass, Old World bluestems, intermediate native species should not be grazed below 6”-8”. Our native tall grasses such as little bluestem, big bluestem, indiangrass and others should not be grazed below 12”-14”. Keeping some of these thresholds in mind, scouting the field to check on the key species and for potential weed and insect problems, and sometimes making the hard decisions regarding stocking rate will help maintain plant vigor and production by allowing optimum photosynthesis to take place. These same actions will also enhance animal performance, protect the environment, and increase profit potential for your operation.
Dr. Larry Redmon
Professor and Associate Department Head & Extension Program Leader
Soil & Crop Sciences Department
Texas A&M AgriLife Extension