Study FSRM 1706-25 - Santa Rita Rotational Grazing System

Study FSRM 1706-25: Benefits of Rest March-October Two Years Out of Three

S. Clark Martin designed this study to evaluate the response of vegetation to a three-pasture grazing rotation design where no grazing occurred during two of three summer growing seasons. It expanded the scale of the earlier seasonal grazing research developed in “Forest Service Study FSRM 1706-09: A comparison of vegetation responses to grazing yearlong, in the growing season, and in the dormant season on low-value semidesert grass-shrub ranges in southern Arizona” (described above). This larger scale study included 12 pastures compared to only six pastures in the earlier study. This three-pasture grazing design became widely applied by land management agencies in Arizona and was referred to as the Santa Rita Grazing System.

The 12 pastures used in the study were arranged in three sets of four pastures each, where three of the pastures were included in the seasonal rotation and the fourth was grazed yearlong (see Study Plan ASCII text file below). The yearlong pastures served as the comparison for the grazing rotation treatment. Two grazing seasons were defined in the rotated pastures: a summer season from March through October and a winter season from November through February. The grazing sequence for a pasture was winter rest > summer graze > winter rest > summer rest > winter graze > summer rest over a 3-year period. 
The study used 145 permanently established transects, with 10 transects in each of 10 pastures (Pastures 1, 2N, 2S, 3, 5N, 5S, 8, 12B, 21, and 22), 20 transects in Pasture 6B, and 25 transects in Pasture 6A. Remarkably, 115 of these study plots were used in previous studies, and the continued measurement of vegetation attributes extended the long-term record another 12 years, in most cases from the 1950s to the mid-1980s. Sixty plots that were measured between 1957 and 1971 in Pastures 2N, 3, 5N, 5S, 6B, and 12B were continued from “Forest Service Study FSRM 1706-09: A comparison of vegetation responses to grazing yearlong, in the growing season, and in the dormant season on low-value semidesert grass-shrub ranges in southern Arizona.” Twenty plots that were measured between 1953 and 1971 in Pastures 1 and 8 were continued from “Forest Service Study FSRM 1706-12: A comparison of vegetation and grazing capacities on mesquite-free and mesquite-infested semidesert ranges in southern Arizona.” Thirty-five plots that were measured from 1959 to 1966 in Pastures 6A and 6B were continued from “Forest Service Study FSRM 1706-15: A demonstration to determine the feasibility of achieving rotation-deferred grazing within a single pasture by controlling access to water.” The remaining 30 study plots were first measured in 1972 for this study.
Estimates of cover by species were made along 100-foot transects by stretching a graduated tape between permanently placed rebar stakes in the ground at either end of the transect. Basal intercept for perennial grasses and canopy intercept for shrubs and trees were measured to the nearest 0.01 foot. Estimates of perennial plant density were made in a 100-foot by 1-foot belt transect, where the 100-foot dimension was provided by the permanent line intercept transect location, and the 1-foot width was on the right side of the transect as viewed from the 0-foot marker. 
Estimates of grass biomass production for each species were made at the end of each summer growing season using a double-sampling method. Biomass was estimated to the nearest gram on six 9.6-foot by 1-foot plots along each of the 100-foot transects. Clipped 9.6 square foot plots used for the double-sampling estimates for each grass species were not located on the permanent transect.
Based on the transect locations marked on aerial photographs, estimates of these locations were placed on mylar overlays of USGS 7.5' (1:24000) topographic quad maps of the SRER, and then digitized from the overlays. The UTM coordinates represent the digitized points reprojected to the NAD83 datum using the GRS1980 spheroid.

The results of the study are described in one publication. Cover and density measurements are continued on 132 Ongoing Vegetation Transects measured on the SRER since 1991.

This page provides the following contents related to the Study FSRM 1706-25: