The Effects of Continuous and Rotational Livestock Grazing on Forb Quality and Quantity: Implications for Pronghorn Habitat Management

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Abstract

Pronghorn (Antilocapra americana) evolved in grasslands with a diet composed of highly nutritious forbs. However, pronghorn habitat throughout North America has been lost to fragmentation and degradation. Additionally, the effects different cattle grazing regimes have on forb biomass, protein, and energy production for pronghorn are not well known in West Texas. We sampled forbs during the growing season in the months of September 2018 and 2019 to assess the effects of different cattle grazing regimes on forbs. We hypothesized rotational grazing would increase the nutritional quality of the forb community and overall forb production, compared to continuous grazing and no grazing. We randomly sampled pastures subject to continuous and rotational grazing, as well as ungrazed exclosures using 100, 96, and 64 1 m2 plots, respectively. We collected all forbs in each plot and analyzed differences in nutritional composition and biomass production using redundancy analysis. We found that the effects of grazing varied by year. In wetter conditions, rotational grazing exhibited higher forb quality and biomass, while exclusion from grazing exhibited these results under drier conditions. The knowledge gained from this study helps resource professionals and landowners understand how cattle grazing affects forbs for pronghorn. This knowledge may be used to improve the suitability of pronghorn habitat through cattle grazing regimes.

Introduction

Pronghorn (Antilocapra americana) populations have declined throughout their range in New Mexico, Texas, Arizona, and Mexico (Bender, Boren, Halbritter, Cox, 2013, DeVos Jr., Miller, 2005, Lucia, 2004). The declines in pronghorn numbers are generally attributed to habitat loss through degradation and fragmentation. Habitat loss due to brush encroachment is one of the main factors affecting struggling pronghorn populations in Texas, Arizona, and Mexico (DeVos Jr., Miller, 2005, Schmidly, 2002). Pronghorn avoid brush-encroached areas in order to spot and evade predators (Goldsmith, 1990). Habitat degradation is a major past and present influencer in the decline of pronghorn populations in New Mexico and Arizona. In New Mexico, limited quantity and quality of forage have negatively affected pronghorn populations by reducing nutrient availability (Bender et al., 2013). Habitat degradation also results in low fawn recruitment. For example, low vegetation height produced insufficient fawning cover in Arizona, leading to reduced fawn survival (Neff, Smith, Woolsey, 1985, Neff, Woolsey, 1979). The interspersion of pronghorn habitat components on the landscape also influences the success of pronghorn populations (Gates et al., 2012). The high variation in precipitation suggests that pronghorn demographics are more susceptible to drought conditions than other populations of pronghorn (Simpson et al., 2007).This juxtaposition is particularly important with respect to quality forage and fawning cover (Loeser et al., 2005). Management efforts to restore pronghorn should focus on understanding the processes that affect the quality and quantity of pronghorn habitats.

Pronghorn life history is adapted to grassland habitats. They rely on their vision to detect predators and their speed to avoid them (Goldsmith, 1990). Because of this, they require open habitats with little woody vegetation (Goldsmith, 1988). Pronghorn exhibit a strong preference for high quality forbs, with secondary use of shrub species (Beale, Smith, 1970, Beasom, LaPlant, Howard, 1982, Buechner, 1950, Koerth, Krysl, Sowell, Bryant, 1984). These critical habitat factors are all shaped by the distribution and intensity of grazing by larger herbivores (Loeser et al., 2005). Thus, grazing was, and continues to be a dominant force shaping these aspects of pronghorn habitat.

Pronghorn evolved sympatrically with American bison (Bison bison) on the grasslands of North America (Buechner, 1950, McCullough, 1980, Seton, 1937). Bison grazed in large herds, moving between areas after short periods of time, while periodically resting the rangeland (Knapp et al., 1999). Bison grazing also increased forb production, and grazed less forbs leaving them available to pronghorn (Catchpole, 1996, Damhoureyeh, Hartnett, 1997, Fahnestock, Knapp, 1993). However, bison were nearly extirpated in the 1800s due to the combination of overhunting and habitat loss (Knapp et al., 1999). Following the decline of bison, cattle (Bos taurus) became the primary large grazer on North American rangelands (Allred, Fuhlendorf, Hamilton, 2011, Yoakum, 1975).

The shift from bison to cattle as the dominant grazer on North American prairies altered the frequency and intensity of grazing (Plumb and Dodd, 1993). Fences associated with cattle grazing permit management of timing, frequency, and intensity (Mather and Hart, 1954), which led to various approaches to grazing management. Continuous grazing is the simplest and most common grazing method, requiring little labor, infrastructure, or maintenance (Gillespie, Wyatt, Venuto, Blouin, Boucher, 2008, Holechek, Pieper, Herbel, 2004). This strategy applies grazing to a specific pasture year-round or while grazing is feasible (Driscoll, 1969). Rotational grazing, on the other hand, consists of moving cattle across different pastures throughout the year (Hart et al., 1988). Because this strategy requires regular movement of cattle and additional fencing, it is more labor intensive and costly than continuous grazing (Gillespie et al., 2008). However, rotational grazing allows rest for forage recovery, improves water infiltration into soils, and increases mineral cycling (Savory, 1999). Rotational grazing strategies emulate the historic relationship between pronghorn and bison in order to achieve similar benefits (Knapp, Blair, Briggs, Collins, Hartnett, Johnson, Towne, 1999, Krausman, Naugle, Frisina, Northrup, Bleich, Block, Wallace, Wright, 2009). How grazing affects forb abundance and quality may make it a valuable tool for improving pronghorn habitat.

We investigated the effect of grazing strategies on forb abundance and quality to determine the utility of alternative strategies for improving pronghorn habitat. Literature suggests the implications of different grazing systems for forb production may be complex. Moderate to heavy continuous grazing produces a higher quantity, but lower quality of forbs compared to rotational grazing (Heitschmidt, Dowhower, Walker, 1987, Pieper, Parker, Donart, Wallace, Wright, 1991, White, Pieper, Donart, Trifaro, 1991). However, rotational grazing could increase pronghorn forb utilization by restricting cattle’s range during the growing season (Holechek et al., 2004). Grazing exclusion is unlikely to increase forb richness or cover due to the positive effects disturbance from grazing has on forb communities, suggesting some degree of grazing is important in maintaining pronghorn habitat (Loeser et al., 2005). We expect the effect of grazing systems on forb production to depend on annual conditions, due to the complicated nature of plant community responses to disturbance, and the influence of precipitation on those processes. We compared biomass, protein, and energy of forb communities under moderately stocked continuous and rotational grazing regimes to those in ungrazed exclosures, for two years that had variable precipitation levels.

Section snippets

Study area

This study took place in Presidio County on the 4,391-ha Mimms Ranch, north of Marfa, Texas (Fig. 1). The ranch is bounded by US Highway 90 to the south and State Highway 17 to the east, and is located within the Trans-Pecos ecological region (Gould, 1975). Temperatures of the Marfa grasslands range from an average high of 23 C in the summer to an average low of 5 C in the winter (National Oceanic and Atmospheric Administration (NOAA), 2018). The study area ranges between 1,371.61,981.2 m in

Results

We found there is variability in biomass and nutritional values of forbs across grazing regimes (Appendix). Our biomass response variable was not linearly correlated with protein or TDN (R = 0.16, and 0.23, respectively), but the nutrition measures were strongly correlated (R = 0.88; Fig. 2). These correlations allow us to interpret the directions of the original nutrition variables as a single quality (nutrition) axes, which is approximately orthogonal to the original quantity (biomass) axes.

Discussion

While we detected small differences in means between the two grazing systems and no grazing, observed differences in the tails of the distributions may be of more interest. Results suggest quality measures were strongly colinear, thus they reduce to a single axis. This reduces the distribution to two dimensions representing quality and quantity, respectively. We also found evidence of a non-linear trade-off between these axes that shows a majority of the plots exhibited either high-quality or

Implications

Our results suggest rotational grazing facilitates a higher frequency of high-quality forb production for pronghorn in years of adequate rainfall. However, exclusion from grazing likely produces similar outcomes in drier years with late rains. While continuous grazing did show a slight lower frequency of plots with higher quantity and quality of forbs. It should be considered when managing for late summer pronghorn forage production as we found it to provide equivalent results to systems

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to thank the Dixon Water Foundation for allowing us to conduct this work on their property and all of their support throughout the study. We greatly appreciate Katherine Haile for all of her hard work assisting with data collection throughout the course of the project. We would also like to thank our funding sources including the Texas Parks and Wildlife Department, Texas Parks and Wildlife Foundation, U.S. Fish and Wildlife Service (USFWS), and San Antonio Livestock Exposition.

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