| Higher stocking density under seasonal-rotation grazing is expected to increase plant abundance because expanded animal distribution and reduced selective grazing on forage species will reduce the spatial variation and competitive advantage of non-forage species compared to yearlong grazing. Rangeland scientists struggle with how long rangeland experiments must continue in order to detect grazing treatment effects, particularly in semi-arid ecosystems with slow responses and high spatio-temporal variability. My first study investigated grazing system effects on plant abundance (cover and density) over the short-term (12yrs) or long-term (22 or 34yrs) after accounting for covariates (mesquite and precipitation gradients). My second study assessed how grazing systems affected spatial variation in grass abundance over 22 or 34 years after accounting for precipitation gradient. The first study was a course resolution approach, looking at grazing impacts on plant abundance. The second study was a finer resolution assessment of the underlying assumption that rotational grazing systems reduce selective grazing. Using split-plot analysis of variance, with year as the split, changes in mean plant abundance and variance in grass abundance were compared between two grazing systems (yearlong vs. seasonal rotation), after accounting for covariate(s). Variance of grass abundance among sample locations within an experimental pasture was the measure of spatial variability and was expected to increase with selective grazing. Grazing systems did not influence plant abundance or spatial variation of grasses. The absence of grazing effect may be due to overriding influences of grazing intensity, large pasture sizes, temporal variation in precipitation, and few replicates. Specific to spatial variation, the absence of grazing system effect may be due to discrepancies in transect representation across ecological sites and spatial variation of grasses occurring at scales different than the 30-m transect size. Like earlier research, my studies failed to substantiate the fundamental premise for implementing rotational grazing systems. This exposes challenges that confront rangeland scientists when implementing grazing studies in spatio-temporally heterogeneous ecosystems. I recommend that research shift from comparing rigid schedule-driven grazing systems to more adaptive management approach where there are comparisons between different levels or different designs of flexible systems. |
