Management of drought stricken rangelands

Michael A. Smith, Professor and Rangeland Extension Specialist, Renewable Resources Dept., University of Wyoming, Laramie, 82071

Prepared for WSGA - Cow Country Fall 02 issue

Widespread concerns over drought conditions of the last three years have lead many to question the impacts of grazing, the necessity for more intense grazing management on rangelands, or whether range can be safely grazed without sacrificing future productivity. An examination of plant growth characteristics, plant responses to seasonal grazing, and the nature and extent of drought conditions suggest that effective management strategies are available to cope with these issues.

Grasses begin growth in spring when soil and daytime temperatures reach thresholds that trigger initiation of growth if soil moisture is available. The threshold temperature for growth initiation is somewhat species specific, but generally lower for cool season compared to warm season species. Growth involves the capture of solar energy by leaves and the production of carbohydrates, including structural materials, and proteins. A basic principle of growth is that sufficient leaf material must be present to capture sunlight. Initiation of growth is from stored energy but most growth is based on photosynthates produced in leaves nearest the growth meristems including the stem nodes and base of leaf sheaths and blades. Optimal leaf area maximizes growth and available animal forage while providing cover for protecting the soil surface.

The available moisture rather than soil fertility generally limits the amount of plant growth potential. Residual soil moisture from fall and winter usually results in a modest fairly consistent amount of plant growth. Subsequent moisture inputs generally determine total growth amount and the length of the green period. Plant maturation, usually expressed by seed stalk production, occurs from near the end of May through late June in most plains environments of Wyoming. Subsequent growth after maturation of cool season grasses is negligible although adequate soil moisture maintains green leaves and forage quality later in the season.

Grazing has been shown to have a distinctly seasonal effect on grass plants. Spring grazing before seed stalk elongation or grazing after seed production have little effect on subsequent year’s plant health or growth potential. Grazing during seed stalk growth will reduce subsequent year’s plant productivity when viewed at the single plant level. Moderate use at this time period in a plant community would affect various plant species differently depending on their level of maturity and would not affect every plant as many are not grazed. Grazing use before or after seed stalk elongation only reduces the standing crop of that season by the amount used. Heavier levels of grazing use may impact soil surface protection and has been shown to reduce animal performance.

Most areas of Wyoming have had below normal annual precipitation over the last three years. The season of drought conditions have an important bearing on the resources that may be impacted. Winter snow pack moisture has had the most consistent and widespread shortages. Most areas of the state have not had consistently below average spring precipitation and summer precipitation has been erratic over the last three years.

From the perspective of rangeland, below normal spring and summer precipitation are most important in determining the level of plant growth and the length of the green forage season. Research in southern Carbon County over the last 15 years indicates that April precipitation is the most influential month in determining the amount of forage produced. May precipitation will add to the amount produced and extend the season of green forage. Later months precipitation will help to maintain forage greenness and quality but adds little to productivity. Summer precipitation stimulates the commonly observed regreening of plants but provides little additional growth.

Plant health and future growth potential are not significantly affected by short term drought conditions. The predominantly cool season plant communities in Wyoming are well adapted to withstand seasonal or annual drought. Long term multi year droughts of growing season precipitation may have the potential to cause plant mortality and have extended range recovery periods, but these effects have only been demonstrated in southwestern US areas where precipitation is much more erratic than experienced in recent years in Wyoming.

The above information can be summarized in the following recommendations. Expected summer forage amounts on rangelands can be predicted by late April-early May. This time is a decision point for estimating levels of stocking compared to normal years or planning for alternative forage resources. Drought conditions dictate that fewer animals or a shorter season should be used unless the range has been very conservatively stocked. Rotational grazing is desirable to limit the exposure of plants to grazing during the short period when grazing may reduce subsequent years production but moderate levels of grazing use even with drought will not affect the future sustainability of the forage resource. Careful observation of forage remaining during grazing in any pasture unit should be used to insure adequate forage is present to sustain animal production, assist in determining when to move animals, and provide for the health and protection of soils. Improving the usually limited amount and locations of drinking water will improve the distribution of grazing into previously unavailable areas or areas where dugouts, springs. or streams have dried up, thus extending the grazing resource and limiting concentration or high use areas.