Calibrating and validating the Texas A & M forage, diet, and sheep simulation models for the Edwards Plateau

Abstract

The objective of this study was to calibrate and validate the Texas A&M forage dynamics, diet selection, and sheep production models for the Edwards Plateau region of Texas. Field data from three sites, one near Brady and two near Sonora, Texas, were used to validate model output for forage standing crop and quality, diet composition and quality, and sheep intake and production. Simulation experiments were conducted to test the sensitivity of the models to changes in the initial partitioning of leaf and stem and live and dead in forage standing crop and in the palatability, harvestability, and avoidance values of forage pools. Simulated trends for forage dynamics, diet selection, and sheep production generally followed observed patterns for each of the three sites. Forage dynamics were driven by growth and showed greater sensitivity to the division of leaf and stem within the initial standing crop than to the division of live and dead. Diet selection was highly responsive to changes in forage availability. Trends in diet selection closely followed the seasonal growth patterns of warm- and cool-season grasses and forbs. Palatability, harvestability, and avoidance parameters were most sensitive when live biomass was limiting. Avoidance affected the preference of a forage pool, while palatability and harvestability affected intake. Responses to the diet search algorithm suggested a need for more precise measures of the relation between forage availability and intake, and a better understanding of forage dynamics, particularly for forbs. The linked models provide a working hypothesis for evaluating the dynamic interaction between available forage and diet selection.