ReviewSustainable grazing management in rangelands: Over a century searching for a silver bullet
Introduction
Human population growth has led to an increase in the demands of food, fiber and fuel (Herrick et al., 2012), which translates into the expansion of agricultural and livestock production frontiers into natural habitats. This expansion usually generates land degradation and biodiversity loss in these natural ecosystems, reducing their long-term ecological and socio-economical sustainability (Foley et al., 2011; Green et al., 2005; Tilman, 1999). Moreover, in some regions of the world the increases in demand are not corresponded with increases in food production (De Haan, 1999). In the case of livestock production, this expansion led to a more intense use of natural plant communities for domestic herbivores grazing (Van Zanten et al., 2018). These natural ecosystems stocked with domestic herbivores are called rangelands (Holechek et al., 1989) and represent 91% of the surface devoted to livestock production (Reed, 2008). While the use of rangelands for livestock raising began thousands of years ago (Walker, 1996), their relevance for the conservation of natural resources started to be recognized by ranchers, scientist and governments in the last few decades. There is a pressing need to find management strategies that maximize productivity per area and, at the same time, maintain the socio-ecological long-term sustainability of rangelands.
A large proportion of rangelands around the world are exposed to some sort of degradation, and this trend is increasing and expanding to all type of rangeland systems (see review in Archer and Stokes, 2000). Although rangeland degradation depends on many factors such as precipitation and nutrient cycling among others, managing grazing was the main focus for the past 80 years or so. For this reason, most research has been aimed at assessing the effects of different grazing management strategies on different aspects of rangeland sustainability (e.g., vegetation dynamics, soil properties, livestock productivity and grazing distribution). The set of grazing strategies ranged from grazing exclusion to high stocking densities for very short periods of time (e.g., Holechek, 1983; Fig. 1). For many years, the issue of which grazing system to adopt was more important than any other management factor (Grice and Hodgkinson, 2002),with most of the studies focusing mainly on the effects of different grazing systems on vegetation trends, animal performance and soil properties without considering other management factors such as rangeland type, landscape composition, animal type or paddock sizes.
Although several reviews discussing grazing schemes are available and were very helpful to synthetize the extensive volume of scientific data available, they only provide a qualitative assessment of their performance (e.g., Holechek et al., 1999; Briske et al., 2008; Teague et al., 2008a,b; but see Hawkins, 2017). Generally, these reviews were focused on a limited number of management strategies (e.g., Holechek et al., 1999 [short duration grazing]; Holecheck et al., 2000 [continuous vs rotational grazing] ; Hawkins, 2017 [holistic vs continuous grazing]) and most of them considered their performance in terms of vegetation and livestock productivity trends. Several of the available reviews highlight the inconsistency of empirical evidence in support of theoretical positive effects of the evaluated strategies (Briske et al., 2008; Holechek et al., 1999; Teague et al., 2013). Moreover, considering that rangelands are distributed worldwide, a quantitative study of the most common grazing schemes is timely and necessary to set the basis for future directions of range science and management. Therefore, our goals were: 1) to summarize the available information about fenced grazing schemes in literature; 2) statistically assess the performance of grazing schemes on rangeland sustainability indicators; 3) assess the influence of terminology controversy: and 4) the inclusion of gray literature on grazing strategies performance. Particularly, we evaluated the impact on vegetation dynamics, soil properties, livestock productivity and grazing spatial distribution in the four main types of rangelands worldwide: grasslands, deserts, woodlands and forests (Holechek et al., 1989). Then, we focused on those grazing strategies that were more frequent in literature to statistically assess which other management factors (e.g., grazing level, paddock sizes, and precipitation, among others), affect the different indicators of rangeland sustainability. Finally, we aimed at using our quantitative results to detect any general trends and to make recommendations for livestock producers and range scientists.
Section snippets
Data collection
We first identified in literature all the grazing management strategies used in rangelands around the world, obtaining a total of 14 grazing strategies that included unfenced (e.g., Pastoralism) and fenced grazing schemes (see Fig. 1). This work is focused on fenced schemes, which can be grouped according the number of paddocks (i.e., pastures) that are being managed: single-paddock schemes which include complete destocking (i.e., grazing exclusion; CD), seasonal grazing (SG) and continuous
Results
The literature search returned 427 experimental publications (i.e., excluding reviews, synthesis and opinion articles) about grazing management strategies from 1914 to 2017 (see Appendix A), and 65% of them were available online. From those publications that we were not able to obtain, 46% were technical reports or bulletins, and 22% corresponded to conference proceedings. This resulted in 278 publications of which 9% of them (N = 27 studies) were grey literature (such as conference
Discussion
Although there are studies focused on summarizing the performance of different grazing management strategies (e.g., Holechek et al., 1999), our work is the first that statistically assess the performance of the different grazing schemes, and the potential factors that could influence their outcomes, in rangelands around the world. We highlight that, despite the large number of publications about different grazing schemes, we did not find statistical evidence showing positive effects of any
Funding
No specific funding was received to conduct this work.
Acknowledgements
We want to thank Dr. P Cipriotti for his comments that encourage the authors to perform this work and the reviewers that contributed with their very valuable comments to improve this manuscript. We also want to thank Universidad Nacional del Comahue project 04/B227, Fulbright - Bunge & Born (Fellowship from Fulbright Scholar Program), CONICET, and PICT 2015 0815.
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