Establishing Livestock Carrying Capacity From GIS and Range Science Research

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The Williamson Act is a tax assessment for agricultural land that basesproperty taxes on the use of the land of agricultural purposes, rather thanon its market value for the "highest and best use" of the land.This tax incentive to maintain land in agricultural production has had amajor effect in reducing the pressure to subdivide hardwood rangeland andother kinds of rangelands because of high taxes. The Williamson Act is oneof the major policies to conserve hardwood rangeland open space values,which are provided mostly by private livestock producers. In Tulare County,an estimated 2,301 parcels represent over 500,000 acres of grazing landcurrently under Williamson Act contract.

The carrying capacity ratings used today for rangeland enrolled in theWilliamson Act in Tulare County were established in the 1950s. In 1995,the State Board of Equalization reported that these ratings are 40% belowthe actual carrying capacities. The County Assessor contacted the IntegratedHardwood Range Management Program to determine estimates of livestock carryingcapacity through science-based methodology to ensure the sustainabilityof the Williamson Act program for grazing land within the County. Four landownersagreed to cooperate in this project by providing historic grazing informationand access to their land for field verification of various estimates. Theselected parcels are representative of the variety of canopy cover, slope,and elevation of grazing land under the Williamson Act within the County.

Maps the databases were developed using the ArcInfo geographic informationsystem (GIS). Existing resources were used to create slope and canopy covermaps for the pilot sample of parcels. Slope maps were created by using UnitedStates Geological Survey (USGS) Digital Elevation Models. Maps of oak, pine,and brush canopy cover were generated by using existing aerial photography,and manually mapping areas into canopy cover classes onto a mylar overlay.These canopy cover classes correspond to canopy cover classes in a "grazingcapacity socrecard" developed for Tulare County, and experience ofthe investigators (see Table 1 below). A zoom transfer scope was used tomatch the landscape features from the aerial photo mylar to those of a mylardeveloped from topographic maps.

Both the mylar aerial photo sheet and the mylar topographic maps sheetwere digitized to record the information. ArcInfo was used to combine thevalues in the cover class coverage (from aerial photos) and the slope classcoverage (from USGS Digital Elevation Models). ArcInfo was then used toanalyze the data to produce tables with carrying capacity estimates andmaps to graphically display the information. Carrying capacity estimatesgenerated through this procedure were referenced with parcels of known carryingcapacity based upon historic grazing and residual dry matter information.

Thirty-three parcels were analyzed during this project. These rangedin size from about 30 acres to over 775 acres, representing a total of over11,000 acres, or about 2% of Tulare County's Williamson Act grazing lands.Individual parcel analyses were generated detailing the animal unit months(AUMs) per acre, acres per class, and total AUMs by slope and canopy coverclasses present.

Tulare County currently rates each parcel based on the number of acresrequired to provide the forage needed to support one animal unit for oneyear (AUY). Existing ratings for the 33 parcel pilot sample range from 15to 50 acres. The estimates produced with the developed methodology rangedfrom 13 to 72 acres per AUY. These estimates fall within the range anticipated,based on field-0-derived forage production estimates, residual dry matterrequirements, and historic grazing rates.

A direct comparison of acres/AUY estimates with current ratings showedthat over 40% of the ratings were either similar to or lower than estimates,while approximately 60% were higher (lower ratings indicate more acres/AUYare required, higher ratings indicate that fewer acres are required). Whenestimates and ratings were within 10% of each other, they were consideredsimilar.

The old grazing capacity ratings for tax purposes averaged 27 acres perAUY. Estimates form the scorecard method on the same parcels averaged 20acres per AUY, about 25% higher than the old figures, but considerably lessthan the 40% difference claimed by the Board of Equalization.

There was no correlation between size of parcel and carrying capacityestimates, nor between the parcel size and whether estimates ere higheror lower than the current ratings. There also is no uniform adjustment tocurrent ratings that would bring them in line with our estimates. To accuratelyassess the carrying capacities of grazing capacities of grazing lands asenrolled in the Williamson Act, each parcel will have to be individuallyassessed.

prepared and edited by Richard B. Standiford and Pamela Tinnin