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Patch Burning Improves Forage Quality and Creates Grassbank in Old-field Pasture: Results of a Demonstration Trial
Devan Allen McGranahan, Charlotte B. Henderson, Jonas S. Hill, Gina M. Raicovich, W. Nathan Wilson, and C. Kenneth Smith

Southeastern Naturalist, Volume 13, Issue 2 (2014): 200–207

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Southeastern Naturalist D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 200 2014 SOUTHEASTERN NATURALIST 13(2):200–207 Patch Burning Improves Forage Quality and Creates Grassbank in Old-field Pasture: Results of a Demonstration Trial Devan Allen McGranahan1,2,*, Charlotte B. Henderson1, Jonas S. Hill1, Gina M. Raicovich3, W. Nathan Wilson3, and C. Kenneth Smith4 Abstract - Ecosystem benefits of heterogeneity-based rangeland management have been widely documented, but little research has explored ecologically based grazing systems from the livestock perspective. Fire and grazing management can advance conservation goals in old-field pasture stands in the southeastern US, but the viability of fire-based grazing for natural areas management remains unknown. Fire is a natural process in the southeastern US that can increase the forage quality of native vegetation. We report results from a patch-burn–grazing trial on a 16-ha pasture in eastern Tennessee, in which we predicted that fire would increase the crude-protein content of the stand and grazing would be concentrated in the burned patch. We measured crude protein content for the entire grazing season (April–September), expecting forage quality to decrease as forage matured. We also sampled fecal-pat density, tiller height, and frequency of herbivory in the burned and unburned areas in May and July to describe the spatial distribution of grazing before and after a four-week drought. Crude-protein content decreased as biomass increased following the fire, and in both sampling periods, fecal-pat density and frequency of herbivory were higher and tiller height was lower in the burned patch. Although the dominant native grass is widely perceived to have low forage quality, fire substantially increased crude-protein content in this study. We discuss how limited productivity between sampling events drove grazing in the unburned area, which acted as a grass-bank. Introduction Fire and grazing are important ecological disturbances in many rangelands, both historically and in the modern management era. Humans have long used fire to manipulate and manage ecosystems (Bowman et al. 2009), while grazing contributes to rangeland function and conservation (Allred et al. 2011a, Toombs et al. 2010). In fact, fire and grazing often operate as an interactive disturbance with ecological effects distinct from each alone (Fuhlendorf et al. 2009), creating heterogeneity in vegetation structure that is important to grassland conservation (Leis et al. 2013, McGranahan et al. 2013a). In the Southeastern United States, fire and grazing have historically been used in forest and range management (Duvall and Whitaker 1964, Lewis et al. 1982, Pearson and Whitaker 1973), but fire use has declined, and animal production has shifted from native range to improved pasture stands and confined 1Environmental Studies Program, The University of the South, Sewanee, TN 37383. 2Current address - Range Science Program, North Dakota State University, Fargo, ND 58108. 3The University of the South, Sewanee, TN 37383. 4Department of Forestry and Geology, The University of the South, Sewanee, TN 37383. *Corresponding author - mcgranah@ Manuscript Editor: Justin Hart Southeastern Naturalist 201 D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 feeding operations. As a result, common native grasses such as Andropogon virginicus L. (Broomsedge Bluestem) are widely perceived to have low forage quality and are thus managed as weeds (Butler et al. 2006). However, introduced pasture species such as Schedonorus phoenix (Scop.) Holub (Tall Fescue) provide poor wildlife habitat and reduce native plant diversity (Madison et al. 2001, McGranahan et al. 2012a). An ecological paradigm emphasizes the restoration of ecosystem processes and keystone structures through heterogeneity-based management (Fuhlendorf et al. 2012, Tews et al. 2004). Patch-burn–grazing simulates the fire–grazing interaction in a management context and has been employed in the US Great Plains to promote livestock production and enhance habitat diversity (Limb et al. 2011, McGranahan et al. 2013a). While preliminary evidence suggests patch-burn–grazing creates structural heterogeneity in old-field pasture in the Southeast (McGranahan et al. 2013b), it remains to be determined whether the ecological pattern of fire and grazing can satisfy forage quality and availability demands and thus serve as a feasible means to manage fire and grazing disturbance in these stands. We monitored a patch-burn–grazing demonstration trial to determine the effect of patch burning on forage quality and grazing patterns in a mixed, old-field pasture stand common in the Southeast. We hypothesized that as grass biomass increases post-fire, forage quality (measured here as crude-protein content) will decrease (Allred et al. 2011b, Sensenig et al. 2010). We also monitored the spatial and temporal pattern of grazing, and hypothesized that grazing would be concentrated in the burned patch versus unburned areas and would thus create and maintain spatial heterogeneity in vegetation height across the pasture (McGranahan et al. 2013a, Toombs et al. 2010). Field-site Description and Methods We conducted a patch-burn–grazing trial in the southern Cumberland Plateau region on the domain of the University of the South in Sewanee, TN. The pasture was dominated by native Broomsedge Bluestem and introduced Tall Fescue under scattered trees. One-sixth of the pasture—an approximately 3-ha patch—was burned mid-March 2012, and the pasture was stocked with twenty 225–320-kg Bos taurus L. (Domestic Cattle) steers in mid-April until mid-October. The cattle had free access to water, and the pasture had no interior fences. Stocking was targeted to remove no more than 50% of expected primary production based on 2011 endof- season aboveground biomass. For more information about the study location and fire behavior, see McGranahan et al. (2013b). After the burn, we erected three 2-m2 exclosures within the burned patch to restrict access by large herbivores. Within the exclosures, we clipped all aboveground biomass from within a permanently placed 0.25-m2 quadrat weekly from one week following the fire (late March) through mid-August. To determine crude-protein content of forage at weekly post-fire intervals, we also clipped an additional, previously unclipped quadrant within each exclosure in the burned patch. Woody material (Rubus spp.) was removed. Southeastern Naturalist D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 202 To compare forage samples from the exclosures to forage actually available to grazers, we collected three composite forage samples from the grazed area of the burned patch outside of grazing exclosures—the grazing lawn (McNaughton 1984)—once prior to the fire (March 20) and three times after the fire during the grazing season (May 28, July 28, and September 15). We dried all samples at 45 °C for 48 hrs and weighed them to determine forage production prior to crude-protein– content analysis. To compare the spatial distribution of grazing across burned and unburned patches, we placed four 25-m transects in each patch type and measured tiller height, frequency of herbivory, and fecal-pat density twice during the grazing season (late May and late July). At 1-m intervals, we selected the nearest two grass tillers on either side of the transect, regardless of species, and measured tiller height to the nearest centimeter. We classified each tiller as grazed or un-grazed as indicated by at least one leaf with the flat defoliation pattern of an ungulate bite. Finally, we counted the number of fecal pats within the 250 m2 around the transect (Limb et al. 2010). Although a lack of replication precluded hypothesis testing via analysis of variance in this pilot study, we calculated an effect size and associated 95% confidence interval for each response variable. We used Cohen’s d, which is calculated as the difference between two data means (here, transects within burned and unburned patches) divided by the square root of the pooled variance (here, standard deviation among transects). Cohen's d is a concise method to demonstrate the effect of patch-burn grazing on spatial heterogeneity among patches; to highlight the effect of patch-burn grazing, we report the absolute value of each effect size (McGranahan et al. 2013a, b). Results and Discussion Fire increased crude protein content and created spatially heterogeneous vegetation that functioned as a forage reserve in our old-field demonstration pasture; thus, the fire–grazing interaction might be a viable management option for old-field pastures in the southeastern US. While managers must pay careful attention to stocking rate (e.g., McGranahan et al. 2012b), patch burning might actually buffer against fluctuations in productivity by creating a forage reserve in unb urned areas. As biomass increased in previously unclipped quadrats, crude-protein content decreased (Fig. 1). Although biomass remained consistently low within the weekly-clipped quadrat, crude-protein content varied widely (Fig. 1) and generally decreased through the growing season (Fig. 2). Samples from the grazing lawn confirm that data from the exclosures accurately represented available forage (Fig. 2). When we analyzed native and exotic grasses separately (primarily Broomsedge Bluestem and Tall Fescue, respectively), we found no substantial differences in crude-protein content between these two categories (data not sh own). These results dispute conventional wisdom that Broomsedge Bluestem is a low-quality forage with crude-protein content between 6–12% (Butler et al. 2006). Our 10–25% range corroborates previous work on prescribed-fire effects on native Southeastern Naturalist 203 D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 grass stands in Tennessee that reported crude-protein content up to 18% (Mathenia 2011). In all studies that have accounted for post-fire growth (e.g., Lewis et al. 1982, Mathenia 2011, Sensenig et al. 2010), crude-protein content declines with time-since-fire as plants invest in a greater proportion of cellulosic structures as they mature (O’Reagain and Mentis 1989). Our data also show that prescribed fire can drive the spatial distribution of grazing in this old-field pasture stand. All three measures of grazing activity indicate increased grazer activity in the unburned areas in July, but overall, the effect of patch-burn grazing was positive for all three variables across both sampling periods (Fig. 3). Tiller height was lower and fecal-pat density was greater in the burned patch Figure 1. Crude-protein content (measured as percent of forage biomass) of all herbaceous aboveg r o und biomass for two simulated grazing regimes plotted against sample biomass. Data collected from a patch-burn– grazing demonstration pasture in Sewanee, TN, mid-March—mid-August 2012. Figure 2. Crude-protein content (measured as percent of forage biomass) of all herbaceous aboveground biomass for two simulated grazing regimes plotted against time. Open squares represent samples taken from the previously burned, grazed area outside of the exclosures (grazing lawn) to reflect actual forage available to cattle. Data collected from a patch-burn– grazing demonstration pasture in Sewanee, TN, mid- March—mid-August 2012. Southeastern Naturalist D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 204 than in the unburned areas in both sampling periods; likewise, a greater proportion of tillers were grazed in the burned patch in both sampling periods (Table 1). Stocking rate regulates heterogeneity outcomes of the fire–grazing interaction: when the burned patch fails to provide sufficient forage, herbivores turn to grazing in the unburned area and reduce the vegetation height (McGranahan et al. 2012b). According to forage-demand and stocking-rate calculations from Redfearn and Bidwell (2004), our burned patch produced less than 70% of the weekly forage demand for 20 steers at the beginning of the study, and fell below 30% by the end of a four-week drought (mid-June–mid-July). Increased grazing activity in the unburned area in July indicates cattle turned to the unburned area to compensate for decreased productivity (Table 1). With these resources, cattle maintained condition without supplementary feed or increased pasture area. Despite the increase in grazing activity in the unburned area, the effect of patch-burn grazing on spatial heterogeneity remained positive (Fig. 3). Figure 3. Effect sizes and associated 95% confidence intervals demonstrate the response of three variables (grasstiller height, fecal-pat density, and proportion of tillers grazed) to patch-burn grazing in an old-field pasture on the Cumberland Plateau, Sewanee, TN. Effect size measured as Cohen’s d (McGranahan et al. 2013b, 2013a), which for each variable compares the mean and standard deviation of three transects in the burned patch to five transects in the unburned area to determine the effect of patch-burn grazing in creating patch contrast (McGranah an et al. 2012a) for each variable. Table 1. Mean (± standard error) values for three measures of cattle-grazing activity across burned and unburned patches at two sampling periods from a single patch-burn–grazing demonstration trial on an old-field pasture in Sewanee, TN. Sampling period Variable Patch May July Grass height (cm) Burned 17 (± 2) 11 (± 1) Unburned 33 (± 4) 24 (± 3) Grazing frequency (% tillers grazed) Burned 43 (± 4) 66 (± 6) Unburned 15 (± 3) 46 (± 5) Fecal pat density (pats/m2) Burned 0.04 (± 0.01) 0.08 (± 0.01) Unburned 0.02 (± 0.01) 0.05 (± 0.01) Southeastern Naturalist 205 D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 Enhanced spatial heterogeneity of forage resources is one strategy for managing temporal variability in forage productivity (Fynn 2012). In pasture management, grass-banking and forage stockpiling are two common practices to ensure seasonal forage availability. A grass-bank is a forage reserve area temporarily leased to managers who must move livestock when forage is unavailable on their own land (Gripne 2005, White and Conley 2007). Forage stockpiling consists of restricting access to some grazing resources to accumulate forage that is later made available to grazers, often after the growing season (Riesterer et al. 20 00). We suggest that patch-burn grazing combines the concepts of grass-banking and forage stockpiling into a previously undescribed pasture management practice based on ecological principles. In the patch-burn–grazing system, grazing activity is focused on the burned area, which allows unburned areas to accumulate forage without the need to restrict access to this forage stockpile with fences. In the event of limited forage production in the burned patch, livestock are free to shift grazing to the spatially contiguous unburned areas, without the added transportation and lease costs of conventional grass-banking. In our pasture, unfenced access to this emergency forage reserve brought cattle through the drought without additional management intervention or inputs. These observations support predictions of functional resource heterogeneity theory (Fynn 2012; D.A. McGranahan et al., unpubl. manuscript; Owen-Smith 2004). Thus the unburned area buffered against limited forage productivity, which in this study resulted from a combination of drought and insufficient patch size. (We initially burned 1/6 of the pasture in spring with the intention to burn another 1/6 in late summer, to initiate a 3-year fire-return interval—1/3 of the pasture burned annually— and create a “green flush” of high-protein forage at the end of the grazing season, but the second burn did not occur.) Using an ecological pattern of fire instead of fences makes patch-burn grazing an attractive means to manage grazing disturbance on natural areas without costly agricultural inputs that might detract from conservation and recreation goals. From a forage standpoint, fire removes moribund vegetative material with low nutritional value (Anderson 2006) and increases the crude-protein content of post-fire vegetation (Allred et al. 2011b, Lewis et al. 1982). Although we have not yet surveyed the wildlife community itself, our data suggest that patch-burn grazing can create the spatial heterogeneity in vegetation structure associated with grassland biodiversity (Fuhlendorf et al. 2012, McGranahan et al. 2013a, Tews et al. 2004). Under this paradigm, the patch-burning scheme—burning a fraction annually vs. burning entire pastures every second or third year—might enhance both forage quality and availability while creating spatially heterogeneous vegetation structure. As such, we suggest further research to develop patch-burn grazing for ecological grassland management in the Southeastern United States. Acknowledgments We recognize financial support from the Sewanee Environmental Institute and the contributions of R. Petropoulos, S. Gilliam, D. Carter, and the 2012 Sewanee Summer Farm Southeastern Naturalist D.A. McGranahan, C.B. Henderson, J.S. Hill, G.M. Raicovich, W.N. Wilson, and C.K. Smith 2014 Vol. 13, No. 2 206 Team. S.B. 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