Rotational grazing management has little effect on remotely-sensed vegetation characteristics across farm fence-line contrasts

Abstract

The consensus from world-wide experimental studies is that rotational grazing management has little effect on enhancing vegetation cover and animal productivity relative to conventional management. Critics of these studies claim that experimental trials fail to capture the complexity of adaptive management decisions that bring about long-term vegetation changes at the landscape scale. Thus, we surveyed 48 working farms nation-wide in South Africa to test the hypothesis that rotational grazing sustains higher animal numbers while increasing grass cover and reducing bare ground and woody plant cover. A subset included 23 fence-line comparisons between farm neighbors or paddocks with a similar fire regime but with grazing management varying from continuous to ultra-high density grazing, and we coupled this with remotely-sensed vegetation indices.Results from 48 farms under consistent management for 15 ± 0.8 years (mean ± standard error), revealed that farm stocking rates were relatively higher than those recommended by agricultural extension services at 59 ± 12% (Farms-RecommendedFarms*100 ± standard error) and that adherence to high density rotational grazing management did not affect this. For fence-line contrasts, the change (Δ) in each management or explanatory variable (stocking rate, stock density and proportion of grazers versus all livestock) across the fence was expressed using Hedge’s g effect size as Δ=a-ba ×100 where (a) is the farm with the highest grazing density and (b) is its neighbour. As intended, the comparisons yielded a 85 ± 5% relative difference in grazing densities but revealed non-significant differences in normalised difference vegetation indices (NDVI), fractional bare ground, grass and woody plant cover at 82% of fence-lines. The absolute magnitude of fence-line differences in stocking rate (30 ± 7%) and grazer percentage (55 ± 16%) also had no consistent effect on vegetation cover. This regional analysis of working farms corroborates findings from multiple experimental studies on rotational grazing and adds weight to it by including a diversity of rotational grazing intensities. Evidence in this study was not compatible with commonly observed negative effects of high stocking rates on vegetation cover, implying that the relatively high stocking rates were within the carrying capacity of farms studied. Further, the previously untested hypothesis that rotational grazing alters woody plant cover was not supported in our study. Based on these and the findings of others, continued advocacy for extreme forms of rotational grazing management is unfounded.