Adaptive Grazing Management on the Santa Rita Experimental Range: Determining the Extent of Multiple Defoliation Events on Individual Grasses

Abstract In 2007, an adaptive grazing management process began on the Santa Rita Experimental Range (SRER) in southern Arizona with a primary management objective to reduce repeated defoliation of perennial bunchgrass plants during the summer growing season. In order to follow defoliation events, 1400 individual plants were identified in 6 pastures scheduled for summer grazing from June 15 to October 2, 2013. Plant height and diameter categories were measured before cattle entered each pasture and were re-measured daily or alternate days throughout the grazing period. The differences between heights and diameters were used to calculate any regrowth that occurred. Of the 1400 plants marked, 738 (52.7%) were not defoliated, 453 (32.3%) plants were defoliated once, 199 (14.2%) plants were defoliated twice, and 10 (0.7%) plants were defoliated three-times for a total of 881 defoliation events recorded. The majority of first defoliation events and first incidences of second defoliation occurred on day 3. Cattle began switching their grazing strategy from undefoliated plants to new growth of previously defoliated plants by day 9 with 48 of a total 161 previously defoliated plants being defoliated. The highest number of second defoliations were measured on day 11 when 115 plants were observed as having been defoliated for a second time and 5 plants were defoliated for a third time. Arizona cottontop (Digitaria californica) had a higher percentage of both single and multiple defoliation events when compared to the other perennial grass species. Distance from a permanent water source was not found to have a significant impact on the number of defoliations that an individual plant received. Plants defoliated the first time were taller and had a larger circumference (21.0±5.6 cm and 40.8±9.0 cm) than plants that were re-defoliated (15.5±5.6 cm and 31.4±9.0 cm). This study found that the planned 10 day SRER grazing rotation, when followed, would minimize multiple defoliations by cattle on perennial bunchgrasses