E. Paul and Helen Buck Waggoner Foundation, Inc., USDA-NRICGP Agricultural Systems Grants 9404256 and 98-35108-6491, Texas Experimental Ranch Committee, and Texas AgriLife Research.
September 1995 – August 2002
Rangeland ecosystems of the Southern Great Plains have dramatically shifted from predominantly grasslands to mixed woody plant, grass and broadleaf plant associations over the last 200 years. This has been variously attributed to continuous grazing, inappropriate stocking rates and fire suppression. Southern Plains rangelands are predominately used for grazing of domestic livestock, primarily cattle, and will be for the foreseeable future. The encroachment of woody plants and prickly pear cactus on these lands severely impacts the productivity of the livestock industry by reducing forage availability and limiting management activities such as animal movement, health monitoring and supplemental feeding.
Recognizing that failure to maintain low levels of mesquite encroachment results in decreasing livestock carry capacity and increasing dependence on more expensive restoration practices and increasing levels of ecological impacts will lead to crossing thresholds which may make it be impossible to restore, we chose to focus on the reintroduction of grazing and fire regimes to these ecosystems. We wanted to determine how effective prescribed fire could be in reducing mesquite and cactus since it was believed to be a significant factor reducing woody plants and cactus in the past. The purpose of using prescribed fires was to reduce encroachment impacts of honey mesquite and cactus cover to restore herbaceous cover, production and diversity. Rotational grazing was chosen to regularly provide sufficient herbaceous biomass as fine fuel to conduct prescribed fires and provide adequate post-fire deferment. In the process, we wanted to determine what grazing management actions were necessary for prescribed fire to be an effective tool.
Our objective was to determine the effectiveness of integrated grazing and fire strategies utilized at an operational ranch scale to reduce mesquite encroachment related impacts and sustain livestock production at economically acceptable levels in the Rolling Plains region of the Southern Great Plains. We structured the research so we could provide answers to the following questions:
- What are the advantages of fire as a management option?
- Under what conditions is fire likely to be successful?
- How to manage to make fire work as well as possible?
The study includes three experimental treatments with two replications covering an area of about 34,000 acres. Replicate sizes range from 3,000 to 5,000 acres so results will apply to ranches of this size as well as larger ranches. A similar mixture of soils is present between treatments. All treatments were stocked with Hereford cows having the same age composition in September of 1995 at levels consistent with NRCS moderate stocking rates. These were calculated according to the acreage of the 3 major range sites in each pasture and the range condition in each.
Three different rotational grazing systems are being compared with continuous grazing. These three different systems represent different levels of management intensity and are being compared to provide the points for and against each, since different ranchers are likely to be comfortable with different intensities of management. The treatments include:
- Two – Continuously grazed pastures at a moderate stocking rate (control with no burn).
- Two – 4-pasture, 1 herd systems where all pastures will receive about 45 days rest during the rapid growth and 90 days at other times. One fourth of the system or one pasture was rested to provide sufficient fuel for burning in February and March.
- Two – 8-pasture, 1 herd systems where all pastures will receive about 45 days rest during the rapid growth and 90 days at other times. One fourth of the system or two pastures were rested to provide sufficient fuel for burning in February and March.
What we measured
Field and animal production data from the experiment were used to calculate rates of brush recovery and forage and livestock production after treatment or with no treatment. In order to determine the potential of fire to reduce brush and pear and how to manage for fire to be as effective as possible we needed to determine:
- Effect of brush on forage production and composition
- Increase in brush and cactus and treatment longevity
- Effect of grazing management on herbaceous vegetation
- Animal performance
- Economic scenarios
How to manage to get the greatest benefit using fire
In this study mesquite cover increased from 18% to > 40% in 7 years. This was much faster than we were expecting and resulted in a significant decrease in forage production over these 7 years. Mesquite not only decreases herbaceous production markedly but it also increases the amount of winter growing grasses. Both of these factors reduce the effect of winter controlled burns. This can be partly offset by burning in late summer since summer fires are more effective in reducing both mesquite and prickly pear and in summer the winter grasses are dry and provide excellent fuel.
Fire is by far the least expensive means of reducing brush, and it should be used whenever possible to minimize the use of more expensive treatments for reducing brush. In the Rolling Plains of Texas the use of fire provides an economic advantage when the canopy cover of mesquite reaches 10-15% after the application of root-killing herbicide. At the rate of increase of mesquite we measured in this study, if a manager starts using fire at greater than 15% brush cover the brush cover will be far too high after 5 or 6 years for fire to be effective. As mesquite cover increases beyond 15% fire becomes less effective so more expensive means of reducing the brush have to be used to restore the productive capacity of these rangelands.
To ensure effective burns it is necessary to have approximately 2,000 lbs per acre of fuel for each burn. To do so where the rainfall and hence herbaceous production varies markedly each year, it is of prime importance that a stocking rate is chosen to allow for sufficient buildup of fuel. Our experience from this study suggests it is necessary to plan to:
- use fire at low levels of mesquite (< 15% cover);
- burn 12% of the unit each year; and
- stock at 12% lower than the moderate NRCS level for the range type and range condition.
Our economic assessments indicate that stocking rates can be lowered by 25% and still be economically competitive with alternative brush treatments if, and only if, fire is used regularly as a consequence.
Using a rotational grazing strategy to defer areas for the entire growing season before burning will provide:
- the best fuel load for the precipitation received and better continuity of fuel;
- improved litter and herbaceous ground cover which reduces soil temperature, runoff and erosion and increases soil carbon;
- post-fire deferment to ensure adequate recovery of palatable grasses and litter cover; and
- comparable animal performance to stocking continuously at the same stocking rate.
Our study suggests that fire can only be used for maintenance of low mesquite cover. At levels of mesquite above 15-20% using a more expensive means such as herbicides is indispensible. Where mesquite cover is low enough to use fire effectively, the use of fire as a follow-up to other treatment such as herbicide would be economically superior to using herbicide with no follow-up burn.