A simulation model of draught animal power in smallholder farming systems. Part II: Model evaluation and application

Abstract

Draught Animal Power Simulator (DAP-Simulator) is a computer simulation model developed for smallholder farming systems in which animals are the main source of draught power and nutrition is the main constraint. The description of the model was presented in an earlier serial paper. The objectives of this study were to evaluate the adequacy of the DAP-Simulator model and to demonstrate its application as a decision support system. The predictive performance of the model was evaluated using independent data sets from literature. Linear regression analysis showed ( P<0.05) that force requirements when carting ( r 2=0.72; constant=−0.07 and slope=1.2) and when ploughing ( r 2=0.57; constant=0.01 and slope=0.78), area ploughed per day ( r 2=0.97; constant=0.01 and slope=1.03), daily energy requirements for maintenance ( r 2=0.99; constant=−3.1 and slope=1.09) can be described by this model. The model over-estimated daily weight changes (16%) and also over-estimated feed intake, energy intake and dry matter (DM) digestibility, ranging from 0.4 to 7.8%, for working animals. While the fits obtained were satisfactory, the results emphasised the importance of estimating parameters by more detailed experimentation. Sensitivity analysis was used to examine the response of the model variables to specific changes in the values of the parameters. This is particularly useful as it can be indicative of parameters that management can manipulate so as to increase productivity and reduce costs. The sensitivity analysis indicated that ploughing width (sensitivity index=0.49), ploughing depth (0.58), fraction of soil moved by implement (0.59) and specific soil resistance (0.60) were of limited importance in the estimation of force requirements for ploughing. The force requirements for pulling carts were most sensitive to the weight of the cart (1.76) and the frictional resistance force (1.24). The weight of the load (0.32) and the horizontal component of the pull angle (0.04) had very small effects on the forces for carting loads. It is, therefore, not important to estimate the angle of pull accurately as it has little effect on the output. Area ploughed per day was mainly affected by speed (5), number of passes (4.76), and ploughing width. The most important parameters in simulation of feed intake were fractional rate of passage (3.31), and fractions of degradable (2.5) and indegradable (2.3) fibres. Rate of digestion was most sensitive to fraction of degradable fibre (4.1), with relatively small effects of fractional rates of passage (1.4) and degradation (1.16). A situational analysis of draught power management options and strategies showed that the computer-based simulation model can aid decision-making in complex and dynamic smallholder agricultural production systems.