ISSUES IN RISK/BENEFIT EVALUATION FOR PESTICIDE REGISTRATION

Agricultural and Food Policy,

CERTAINTY EQUIVALENCE FOR DETERMINATION OF OPTIMAL FERTILIZER APPLICATION RATES WITH CARRY-OVER

This note demonstrates that a certain class of stochastic problems for determination of optimal fertilizer application rates in the presence of fertilizer carry-over can be simplified to static, certainly equivalent problems. Conditions required for certainty equivalence to hold are: (1) fertilizer carry-over is agronomically equivalent to applied fertilizer; and (2) some addition of fertilizer is optimal in every decision period.Crop Production/Industries,

IMPLICATIONS OF CHANGING FARM POLICY FOR THE SOUTH: DISCUSSION

Agricultural and Food Policy,

A FLEXIBLE METHOD FOR EMPIRICALLY ESTIMATING PROBABILITY FUNCTIONS

This paper presents a hyperbolic trigonometric (HT) transformation procedure for empirically estimating a cumulative probability distribution function (cdf), from which the probability density function (pdf) can be obtained by differentiation. Maximum likelihood (ML) is the appropriate estimation technique, but a particularly appealing feature of the HT transformation as opposed to other zero-one transformations is that the transformed cdf can be fitted with ordinary least squares (OLS) regression. Although OLS estimates are biased and inconsistent, they are usually very close to ML estimates; thus use of OLS estimates as starting values greatly facilitates use of numerical search procedures to obtain ML estimates. ML estimates have desirable asymptotic properties. The procedure is no more difficult to use than unconstrained nonlinear regression. Advantages of the procedure as compared to alternative procedures for fitting probability functions are discussed in the manuscript. Use of the conditional method is illustrated by application to two sets of yield response data.Research Methods/ Statistical Methods,

DETERMINING OPTIMAL FERTILIZATION RATES UNDER VARIABLE WEATHER CONDITIONS

This paper presents a theoretical framework for incorporating the following sources of risk into the determination of optimal fertilization rates: (a) the influence of weather and other stochastic factors on the marginal product of fertilizer, and (b) uncertainty about the coefficients of the response function. The decision criterion considered is the maximization of profit subject to a risk constraint on the probability of not recovering the cost of the fertilizer. The theoretical framework is applied to the fertilization of dryland grain sorghum in the Texas Blacklands. Results indicate that the risk averse producer should substantially lower his fertilization rate if soil moisture at fertilization time is low.Crop Production/Industries,

REDUCTION OF STATE VARIABLE DIMENSION IN STOCHASTIC DYNAMIC OPTIMIZATION MODELS WHICH USE TIME-SERIES DATA

Statistical procedures are developed for reducing the number of autonomous state variables in stochastic dynamic optimization models when these variables follow a stationary process over time. These methods essentially delete part of the information upon which decisions are based while maintaining a logically consistent model. The relatively simple linear autoregressive process as well as the general case is analyzed and the necessary formulae for practical application are derived. Several applications in agricultural economics are discussed and results presented which quantify the relative amount of information sacrificed with the reduction in number of state variables.Research Methods/ Statistical Methods,

Nonlinear static and dynamic analysis of mixed cable elements

This paper presents a family of finite elements for the nonlinear static and dynamic analysis of cables based on a mixed variational formulation in curvilinear coordinates and finite deformations. This formulation identifies stress measures, in the form of axial forces, and conjugate deformation measures for the nonlinear catenary problem. The continuity requirements lead to two distinct implementations: one with a continuous axial force distribution and one with a discontinuous. Two examples from the literature on nonlinear cable analysis are used to validate the proposed formulation for St VenantKirchhoff elastic materials. These studies show that displacements and axial forces are captured with high accuracy for both the static and the dynamic case