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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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@
alumni.grinnell.edu.
Manuscript Editor: Justin Hart
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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.
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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
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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.
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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)
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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
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Team. S.B. Peterson and S. Evans reviewed early drafts. The manuscript was improved by
the comments of two anonymous reviewers.
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