Combustion Dynamics of Cattle Fecal Pats Ignited by Rangeland Fires

The Great Plains of North America, as well as other grasslands across the world, constitute an enormous land area in which both wildfire and prescribed fire occur. Most of the grasslands in the Great Plains are privately owned and grazed by large herbivores, mostly beef cattle, but other wild and domestic species can be locally important. The role of cattle fecal pats (“cow pies”) in wildland fire has been rarely studied despite their dominant presence throughout this fuel type. Cattle fecal pats can readily ignite, release tremendous amounts of energy, and become a source of spot fires. The combustion phase can last long after the head fire has passed, with active flaming going on for hours thereafter (up to over two hours active flaming was observed in the field study to be discussed) and the smoldering phase for much longer periods after that.

This presentation will focus on a multi-year field study involving ten prescribed fires on grazed rangeland during 2011 to 2013. Four grazed pastures, ranging in size from 12 to 46 hectares, at the Oklahoma State University Range Research Station in north central Oklahoma were utilized for the field study. All cattle were black Angus and the vegetation was dominated by warm-season perennial grasses characteristic of tallgrass prairie.

Prior to each burn, 50 fecal pats were spatially located and then rated for percent combustion about 30 days after each fire. Input variables considered for the regression model included: relative humidity, air temperature, wind speed, 1-hr dead fuel moisture (DFM), 10-hr DFM, 100-hr DFM, 1000-hr DFM, fecal pat age index, fecal pat volume, and herbaceous fine fuel load. Forward stepwise regression eliminated five variables, with the remaining five variables resulting in a model with an r2 of 0.987. Fecal pat age index explained the greatest variation (partial r2 of 0.748), followed by 10-hr DFM (partial r2 of 0.124), fuel load (partial r2 of 0.068), 100-h DFM (partial r2 of 0.029), and fecal pat volume (partial r2 of 0.018).

The presentation will cover the range of conditions (weather, DFM, fuel loads) encountered during these burns, the model methodology and results, and combustion observations as functions of 10-hr DFM, 100-hr DFM, and fuel load grouped according to fecal pat age index, which turned out to be the most important independent variable in the regression model. In addition, fire management implications will be discussed, including conditions under which to minimize fecal pat combustion (so as to minimize spot fires) as well as, conversely, conditions under which to maximize combustion so as to kill cattle parasitic organisms that utilize fecal pats for part of their life cycle.