MANAGEMENT OF IMPROVED PASTURES

Because of the potential seriousness of a drought whenever and wherever it occurs, landowners and managers need to be aware of the effects of drought on range plant growth. Obviously, lack of soil moisture restricts plant growth, both in terms of amount and time in which it is produced. The extent to which forage production is decreased by weather varies with the site, climate, vegetation type, and current and past grazing management. Every situation is different and it is impossible to present management guidelines something that will be universally applicable even for Texas.

Productivity of annual plants generally will be reduced by drought more than will that of perennial plants. In a drought, annuals produce little or no forage. Annuals are not as deeply rooted as perennial grasses and woody forbs or shrubs and trees. For the annual plant to survive, it must put out a seed stalk with some viable seed. In a drought, annuals will be very short with fewer leaves present and will use available water to produce flowers and a viable seed crop. Typically there are two peak germination periods for annuals in Texas. Germination in the fall (September to December) will produce cool season annuals that grow roots during winter and spring, and mature seeds in the spring or early summer before the plant dies. With a lack of fall moisture, some cool season annuals may germinate later as warmer temperatures are encountered. Annual species are well adapted to dry years where they can escape periods of drought by remaining in the seed stage. Warm season annuals typically germinate in the spring as warm conditions persist. Annuals such as one-seeded croton, woolley croton and spurges germinate under favorable moisture conditions and by-pass much of the rosette type growth of cool season annuals. Warm season annuals may not geminate but remain dormant in the soil until more favorable conditions return.

Typically warm season, perennial sod grasses and bunchgrasses support above ground growth for eight to nine months out of the year. The plant must support its root system and its meristematic or bud tissue, which will produce next years' growth. To survive during this period, the plant must draw on carbohydrates or food reserves that were produced the previous growing season and stored in the roots or crown of the plant. These reserves furnish the necessary energy for growth initiation each spring. About 20 percent or more of the years growth will occur using these stored reserves before the plant stops using reserves and maintains itself on mature leaves produced that season.

In a drought the plant has to rely on the stored reserves for a longer period of time, thus reducing stored nutrients for future use and increasing the plant's susceptibility to damage in extended periods of drought and grazing uses. A healthy root system is of paramount importance to the growth of a range plant when we realize that 50% to 80% of the plant exists below the soil surface.

The lack of available moisture usually reduces the length of the growing season. Warm season perennial grasses will initiate growth in the spring, but produce less forage and go dormant sooner under drought conditions. During drought we would expect plant growth to slow before carbohydrate reserves (sugars and starches) are replaced. Because of this, grasses may enter a longer than normal dormant period with less reserves. If rains come, the plant is slower to respond. If heavy grazing has occurred, this may hinder the accumulation of new reserves. A perennial grass that is heavily grazed during the growth period could stop growth altogether. If soil moisture were declining rapidly at the same time, the grazed plant would not have opportunity to recover. In drought years, grazing should be light at this period to enhance the plant's ability to make maximum use of soil moisture available. Plant loss or death occurs in periods with several growing seasons with below normal precipitation.

The effect of drought on range plants is a function of both the intensity and duration of drought and the general health and vigor of the vegetation before the drought. Plants with healthy root systems and adequate carbohydrate reserves will fare much better during and after drought than plants that have been struggling to maintain themselves continuously.