Date of Award:

1986

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Ecology

Department name when degree awarded

Range Science

Advisor/Chair:

John P. Workman

Abstract

The purpose of this study was to investigate analytical approaches for estimating the optimum (profit maximizing) rate of initial overstory kill for increasing seasonal forage availability on a specific ranch. The models were developed using the big sagebrush (Artemisia tridentata) - crested wheatgrass (Agropyron cristatum, A. desertorum) vegetation type on a Utah cow-calf-yearling operation as an example. The models should be applicable on any ranching situation where understory forage production is constrained by the establishment and growth of undesirable overstory vegetation.

Three model forms were developed and compared for their applicability to on-the-ground investment decision making. Single treatment period (STP) static and dynamic models and a multiple treatment period (MTP) dyne.mic model were theoretically developed. Ee.ch model we.s formulated using e. biological production function relating under story production to ini tie.l kill percentage, e. cost of overstory kill function fore. specific overstory reduction method, and a derived demand function for seasonal forage availability. The production function was estimated in both static and dynamic forms for use in the different models. The remaining functions were used in the same form for a11 model applications.

Results indicated that the STP static optimization model holds the most promise for an on-the-ground decision making tool at the present time. Data to estimate dynamic production functions over a variety of vegetation types and grazing management situations essentially do not exist. This deficiency makes the use of long-term dynamic models less feasible for current decisions. It does not, however, alleviate the need to recognize long-term biological and economic relationships in making such decisions. The STP static model indicated that for the example Utah ranch a target big sagebrush kill rate between 89 and 100 percent would be optimal depending on the derived demand and cost of kill functions used. Target kill rates other than the optimum would represent potentially significant opportunity costs to this particular operation. The optimal solution obtained by this method is expected to change depending on the situation being analyzed. The model must be applied on a specific ranch basis and was not designed to make general recommendations.

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