A Test of Grazing Compensation and Optimization of Crested Wheatgrass Using a Simulation Model

We developed a simulation model based on tiller population processes to test grazing compensation and optimization in crested wheatgrass (Agropyron desertorum (Fisch. ex Link) Schult.). Model functions describing tiller dynamics and growth were derived from field observations in west-central Utah. Predicted tiller growth and new tiller production following defoliation were verified against additional data from the same site; total production was validated against a 30-year-old data set from a different site. We then simulated 2 grazing experiments. First, grazing compensation was determined as a function of the timing of a single defoliation during the growing season. Response variables included tiller density, plot growth rates, standing crop, and seasonal production. Second, grazing optimization, a combination of grazing frequency and intensity that increases primary production above that of ungrazed plants, was assessed by the systematic variation of these defoliation parameters under simulated dry, average, and wet winters (September-May). Results of the first experiment indicated that compensation depended mainly on the timing of defoliation, presumably because of phenological constraints to regrowth and the short growing season in this cold-desert region. Overcompensation only occurred when plants were defoliated before the traditional start of the grazing season. Although defoliation increased tiller growth rates, the second experiment failed to reveal an optimum combination of defoliation frequency and intensity resulting in maximum biomass production except after a dry winter. Results from the second experiment indicated that implementing intensive rotational grazing systems will seldom increase crested wheatgrass production in these cold-desert systems.