Calibration, validation, and application of the yak growth and development module in the SGM yak sub-model

Based on the Sustainable Grassland Model (SGM) model framework established previously, this study constructed a yak growth and development module in the SGM yak sub-model. This was conducted to provide research tools and decision-making support for the construction of a sustainable grazing production system on the Qinghai-Xizang Plateau. The modified GRAZPLAN animal model and the partial paddock simulation method were used for the secondary development of the SGM model on the MATLAB platform. Local sensitivity analysis was applied, and sensitive parameters for the simulation of grazing yak dry matter intake (DMI) and live weight change were selected by calculating the sensitivity coefficient (SC). Based on data published by others, the least squares method was used to construct a calibration model to fulfill the module calibration. Liveweight data from the published literature were used to statistically compare the difference between simulation data and observed data to conduct module validation. To demonstrate the application of the model, a typical yak farm with moderately degraded pastures was analyzed based on data collected in the alpine meadow area of Maqu County. The results showed that, after calibration and validation, the yak growth and development module in the SGM yak sub-model was able to mimic grazing yak DMI and live weight gain. Results from the application demonstration showed that the SGM model was able to mimic the effects of decreased stocking densities on grassland aboveground biomass, pasture composition, grazing DMI, and live weight changes in the alpine meadow yak production system under traditional management over longer timescales. This indicates that the SGM model is suitable for simulating the effects of stocking density on the grazing production system in alpine areas on the Qinghai-Xizang Plateau. This simulation method has strong potential for the development of a future grazing system model because it has the same functions as the two different sets of parameters for sheep and cattle. However, calibration needs to be conducted based on the simulation object, and further revision and development of yak lactation and grazing DMI are required.