Global Rangeland Primary Production and Its Consumption by Livestock in 2000–2010

Abstract

Livestock grazing occupies ca. 25% of global ice-free land, removing large quantities of carbon (C) from global rangelands (here, including grass- and shrublands). The proportion of total livestock intake that is supplied by grazing (GP) is estimated at >50%, larger than the proportion from crop- and byproduct-derived fodders. Both rangeland productivity and its consumption through grazing are difficult to quantify, as is grazing intensity (GI), the proportion of annual aboveground net primary productivity (ANPP) removed from rangelands by grazing livestock. We develop national or sub-national level estimates of GI and GP for 2000–2010, using remote sensing products, inventory data, and model simulations, and accounting for recent changes in livestock intake, fodder losses and waste, and national cropland use intensities. Over the 11 study years, multi-model average global rangeland ANPP varied between the values of 13.0 Pg C in 2002 and 13.96 Pg C in 2000. The global requirement for grazing intake increased monotonically by 18%, from 1.54 in 2000 to 1.82 Pg C in 2010. Although total global rangeland ANPP is roughly an order of magnitude larger than grazing demand, much of this total ANPP is unavailable for grazing, and national or sub-national deficits between intake requirements and available rangeland ANPP occurred in each year, totaling 36.6 Tg C (2.4% of total grazing intake requirement) in 2000, and an unprecedented 77.8 Tg C (4.3% of global grazing intake requirement) in 2010. After accounting for these deficits, global average GI ranged from 10.7% in 2000 to 12.6% in 2009 and 2010. The annually increasing grazing deficits suggest that rangelands are under significant pressure to accommodate rising grazing demand. Greater focus on observing, understanding, and managing the role of rangelands in feeding livestock, providing ecosystem services, and as part of the global C cycle, is warranted.

Global trends in grassland carrying capacity and relative stocking density of livestock

“…Global studies dealing with the NPP partitioning (dividing the NPP into belowground biomass and AGB) are rare, but a recent study (Sun et al, 2021) supports our conclusions that high AGB values appear in Central Africa, Brazil and Northern Australia, whereas AGB decreases with increasing latitudes. For more insights, we calculated country‐specific total AGB estimates (Figure S6d in the Appendix) and compared those with total biomass estimates reported by Sun et al (2021) and a global study by Wolf et al (2021). Compared to those studies, our total AGB estimates are equal in size in Argentina, Brazil, and Mexico, but divergent especially in Canada, China, and Russia, where our estimates are lower (Figure S5 in the Appendix).…”

“…Large dairy industries in Saudi Arabia, Syria, and Jordan that are not visible in our maps, as those are not located in grassland areas, provide examples of this (Alqaisi et al, 2010). According to Wolf et al (2021), the amount of total livestock intake requirement supplied by grazing remained around 63% over 2001–2010. Although regional differences in this intake are notable (Sandström et al, 2021; Wolf et al, 2021), supplementary feed generally covers around one‐third of the total livestock intake.…”

“…We assumed that the derived sub-canopy vegetation (Equation 2) also includes consumable leaves and shrubs while acknowledging that this might not be the case in all the ecosystems (see Appendix S2.4). Nevertheless, this captures the available biomass in the majority of the grasslands and although there are still uncertainties, it is a considerable improvement to the currently used methods which either do not include woody areas in the analysis (Petz et al, 2014;Wolf et al, 2021) or use a single reduction factor despite the canopy cover density .…”

“…The method we developed, applied to both MODIS-based and modelled NPPs, might partly explain the differences between our results and existing literature. While we used temperature as a predictor when allocating a fraction of total NPP to AGB, other global grazing studies, either modelled or MODIS-based (Petz et al, 2014;Wolf et al, 2021)…”

“…Second, we developed a relationship between forest cover and available biomass, based on a meta‐analysis, to estimate the AGB. This approach contrasts with existing methods that either exclude woody areas from the analysis (Petz et al, 2014; Wolf et al, 2021) or use a single reduction factor for all areas that include trees (Fetzel, Havlik, Herrero, & Erb, 2017). Finally, we included a detailed uncertainty assessment in our analysis to capture some of the uncertainties associated with our estimates in the different world regions.…”