Optimal Timing of Investments to Control Honey Mesquite

Abstract

A general economic model to analyze opthnal timing of brush control treatments and other range improvements was developed. The model was then applied to investments to control mesquite (Prosopisghdufosu Torr.) invading native rangeland in the Rolling Plains of Texas. Although a positive grass response from mesquite control would be anticipated for 5 years under average conditions, the economic optimum retreatment schedule is only 4 years. An increase in beef price shortens the optimal retreatment schedule, while an increase in treatment cost lengthens the optimal retreatment schedule. Implementing brush control treatments during a year favorable for a high rate of top kill is an important economic consideration. In an article that appeared in this journal, Ethridge et al. (1984) presented an economic analysis of chemical control of honey mesquite (Prosopis glandulosa Torr.) using 2,4,5-T (2,4,%trichlorophenoxy acetic acid). They developed an analytical framework to analyze the profitability of brush control based upon the standard economic criterion of net present value (NPV). If the discounted present value of additional income generated from mesquite control over an assumed 5-year treatment life was greater than the cost of treatment, i.e. a positive NPV, the authors considered the brush control treatment to be economically feasible. From the analysis presented by Ethridge et al. (1984) it is possible to consider an additional problem of determining when to retreat the mesquite canopy. When mesquite is sprayed, maximum grass yield generally occurs during the first 1 to 3 years after brush control, then production declines (Dahl et al. 1978). While a positive economic return can be obtained during later years of the treatment life, the deteriorating grass understory results in decreased beef production and annual returns. The implication is that it may be possible to make more money if the grass understory can be maintained in a more productive state through more frequent brush control treatments. The retreatment or replacement problem can be described as follows. Nonforage brush species compete directly with desirable forage species for available light, water, and nutrients. As rangeland is invaded by brush, forage production gradually deteriorates and the carrying capacity of the range is decreased. Chemical spraying, chaining, bulldozing, burning or other methods of destroying the invading brush will rejuvenate grass production on the site. The replacement problem involves determining the optimal frequency of these brush control practices. This paper investigates optimal timing of mesquite control when the rancher’s goal is profit maximization. A sensitivity analysis is presented that indicates how changing the level of initial mesquite infestation, the level of treatment success (degree of top kill), discount rate, beef price, and treatment cost would affect the optimal timing of mesquite control practices.