Small scale heterogeneity in a semi-arid North American grassland. II. Cattle grazing of simulated urine patches.
The effects of simulated urine deposition on the selectivity and intensity of cattle grazing on Agropyron smithii [Elymus smithii] tillers were studied in semiarid native grassland at Cheyenne, Wyoming in 1987. Swards were grazed continuously from 3 June to 8 Oct. at moderate (5 steers/12 ha) or heavy (5 steers/9 ha) stocking rates. Two litres of artificial urine supplying 51 g N/m2 were applied to 0.25 m2 circular microplots at the end of May. Tiller density was not affected by treatment during 1987 but it increased in urine patches 12 months after treatment. Up to July, more tillers were grazed in urine patches than in controls, although this difference disappeared later in the growing season. Tillers were grazed to a lower height, and blade length was shorter, when growing in simulated urine patches under both stocking rates. The mean height of grazed tillers and its variance were greater under moderate than under heavy stocking. Aboveground N concentration of tillers was higher in urine patches. The results suggested that the interactions among selective grazing, urine deposition, and grazing pressure increased the structural heterogeneity of the grassland canopy.