Abstract
Prescribed cattle grazing has shown promise as a fuels management tool to reduce herbaceous fuels and change fuel composition. Early season (during the boot stage prior to seed production) and late season (while perennial grasses are dormant in late fall or winter) grazing have been shown to reduce annual grasses. Few studies have compared fuel treatments among different grazing seasons, focusing instead on comparing grazed and ungrazed treatments. Evaluating grazing effects by cattle is further complicated by the presence of shrubs because cattle grazing has limited influence on woody fuels, and shrub-derived fuels may override the effects of grazing on fire behavior. This study compared four grazing treatments (early, late, multi-season, and no-graze) on composition of grass fuels, fuel loading, and modeled fire behavior. Three years of vegetation and fuels data were collected from 2021-2023 on two sagebrush steppe sites in southern Idaho. Fuel estimates were subsequently evaluated with Fuel and Fire Tools modeling software to examine potential fire behavior. Regression analysis showed that grazing season effects varied annually and across sites. Proportions of annual and perennial grasses did not vary by grazing season. Under some conditions, spring and multi-season grazing decreased grass fuel loads by similar magnitudes to annual variance. On the site with higher shrub cover, grazing treatments increased total fine fuel load through cumulative increases in forbs, herbaceous litter, and woody litter. Predicted fire rate of spread was reduced by multi-season grazing, which likely reflected greater grazing intensity of this treatment. Predicted flame length did not differ among grazing treatments. Predicted reaction intensity increased with multi-season and late season grazing, but results were subject to the assumption that shrub fuels were unaffected by cattle grazing. Increases in woody litter challenged the widely held belief that cattle grazing has minimal effect on shrub fuels, highlighting a need for comprehensive fuel sampling in shrub ecosystems. Overall, this study provides support for grazing as a management tool for fuels and fire behavior, while highlighting complex relationships with shrubs and variability of annual and site conditions.