Importance of Fire
Fire is an important driver of many North American ecosystems, particularly grasslands. The influence of fire on the plant community is largely attributed to its removal of dead standing plant material, impact on forage quality, and the impact on grazing animals. Cattle production in undisturbed tallgrass prairie can be low due to the accumulation of dead standing plant material. However, fire can increase production over 75%. Historically, large bison herds followed fire as they were attracted to the lush regrowth that emerged. Today, managers use prescribed burning to capitalize on this change in forage quality for cattle production.
Forage quality is typically expressed as crude protein (CP) which is based on the nitrogen content of forage. This nitrogen content is critical for microorganisms in the rumen. Other measures of forage quality include palatability (typically associated with texture and moisture content) and digestibility (largely based on fiber and lignin). The primary influence of fire on forage quality is the removal of standing, dormant plant material. Forage quality is largely a function of time: as plants age (mature) they decrease in quality. This decrease in quality is due to the increase in fiber and lignin content, resulting in reduced digestibility and animal consumption. Prescribed fire remove dormant plant material, increasing the nutrition and digestibility of post-fire regrowth. While prescribed fires are commonly conducted during the late winter or early spring, growing season (summer) fires have the same effect and can boost forage quality during a period of the year where it is typically decreasing. Furthermore, the interaction of fire and grazing will impact below ground plant growth by increasing nitrogen mineralization and plant nitrogen availability, and improving the root tissue quality.
Benefits of Higher Forage Quality
Forage quality is a critical component of cattle production including reproductive efficiency, rebreeding, calf production, animal growth and milk production. Understanding the importance of fire to native rangeland plants and the potential benefits to cattle can be a useful tool for ranch managers.
Achieving optimum livestock production on rangelands many times can conflict with wildlife conservation strategies that require lower stocking rates to maintain suitable habitat. Traditionally, livestock producers try to maximize gain per-acre by uniformly managing vegetation in pastures. Often this is achieved either by heavily grazing, burning the entire pasture, or both. This management strategy can create suitable habitat for some wildlife species, but poor quality habitat for others.
Combining the spatial and temporal interaction of fire and grazing (patch-burning) a conservation-based approach to land management can be achieved. This practice can increase rangeland biodiversity by creating heterogeneous vegetation structure and composition that is beneficial to multiple wildlife species. However, for conservation strategies to be successfully implemented, they need to be both effective and economically sustainable. In both mixed-grass prairie and tallgrass prairie, cattle weight gain was compared in pastures with traditional fire and grazing management (continuous grazing, with periodic fire on tallgrass prairie and seasonal grazing without fire on mixed-grass prairie) and conservation based management (pyric-herbivory applied through patch burning), both at a moderate stocking rate. Stocker cattle weight gain, calf weight gain, and cow body condition score were comparable between the traditional and conservation based management at the tallgrass prairie site for the duration of the eight-year study, indicating that conservation management doesn’t decrease livestock production or profitability.
In the mixed-grass prairie pastures, stocker cattle gain was not different between traditional and conservation management for the first four years. However, stocker cattle in conservation based management out gained cattle in traditional management beginning in year five and remained 27% greater for the next six years of the study. Moreover, cattle weight gain under conservation management varied less year to year making profitability more consistent. Traditional management in mixed-grass prairie did not include fire, the process that improved range conditions and likely was associated with increased stocker cattle performance under conservation management. In conclusion, pyric-herbivory is a conservation-based rangeland management strategy that returns fire to the landscape without reduced stocking rate, deferment, or rest.
Figure 1. Cattle weight gain on traditionally managed and patch-burn managed tallgrass prairie.
Figure 2. Cattle weight gain on traditionally managed and patch-burn managed mixed-grass prairie. An * indicates years when patch-burned managed cattle gained more than traditionally managed cattle.
Bison selecting post-fire regrowth (foreground) over dormant, unburned vegetation (background). Photograph by Stephen Winter.
Fire is a primary driver of many ecosystems, but is particularly important in the establishment and maintenance of grasslands and savannas. Numerous research studies have shown the effects and importance of fire on several grassland ecosystem properties, including plant productivity, plant species composition, nutrient cycling, and woody plant encroachment. Many North American grasslands support large grazing animals, either native wildlife (e.g., bison, elk) or introduced livestock. The role of grazing is also critical to the development and maintenance of grasslands, influencing many of the same processes as fire.
Though often considered independent, fire and grazing interact and influence one another. This is referred to as the fire-grazing interaction or pyric-herbivory (defined as grazing driven by fire). This interaction occurs when fires are present within a landscape or pasture allowing grazing animals to choose between burned and unburned areas. Due to high forage quality of post-fire regrowth, many animals are attracted to recently burned areas, including bison, cattle, elk, deer, sheep, and many others. Because of the differences in forage quality, grazing distribution is altered as animals heavily graze recently burned areas and avoid areas with greater time since fire. The preference for burned areas is very strong, and will often be greater than selection or avoidance of other features (e.g., water, topography, etc).
As fires move around the landscape or pasture, animals will follow to consume nutritious regrowth. This interaction between fire and grazing creates a mosaic or gradient of recently burned and grazed areas to areas with greater time since fire and no grazing. Across a landscape, the differences among such areas increase the number of plant and animal species (diversity), affects invasive species establishment, and nutrient cycling rates also vary with time since fire. Using the interaction of fire and grazing as a management tool can help conserve or restore ecosystem processes while maintaining livestock production.
Patchy fire within the bison unit at the Tallgrass Prairie Preserve in northcentral Oklahoma; there are no fixed burn units. The response of grazing animals to the patchy distribution creates the fire-grazing interaction.
What is Patch Burning?
Patch burning (patch-burn grazing) involves the combined use of fire and grazing for ecological or agricultural goals within landscapes or pastures. The target area or pasture is subdivided using only needed burn lines (no cross fencing). Each year fire is used to burn a different portion of the pasture. Livestock can be added to the area at anytime. The grazers will be attracted to the burned area and spend most of their grazing time in that portion of the pasture. As fire moves around the pasture or landscape, grazing pressure will also change in synchrony. The focal area or patch will incurr heavy grazing creating a grazing lawn type structure. As grazing pressure is released, the plant community recovers and a shift mosaic is created.
Patch burning allows livestock to freely select the most recently burned part of a pasture. It has been found that livestock spend 75% of their time on these burned patches and, typically, evenly utilize all the palatable plants within the entire burned patch. This includes plants that are normally not considered desirable livestock forage. Then within 6-12 months another portion of the pasture can be burned. This will shift the focal grazing point to the new burn patch. After the heavy utilization (1 to 4 years post-burn depending on the chosen rotation) a transition state of bare ground, forbs, and small amounts of standing biomass and litter occurs. Within a couple of years post-burn, the patch receives very little grazing pressure which allows biomass and litter to accumulate (Figure 3). This rested patch is then ready to be burned and grazed again. This is all accomplished without fences. The total amount of hands on management is much less with this system than many other common grazing systems. For more information about patch burning go to the following link http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-4677/E-998survey.pdf
Patch burning (patch-burn grazing) is the purposeful grazing of a section of an landscape or management unit that has been prescribed burned, and then rotating fire through the managment unit to move the grazing pressure over time. This creates a shifting mosaic on the landscape or management unit.
Cattle spend 75% of the time grazing on the most recently burned patches. This allows the other patches to recover.