COMPUTER SIMULATION OF ANIMAL AND PLANT GROWTH AND THEIR INTERACTIONS IN A GRAZING SITUATION

Models simulating animal growth (COWCALF), plant growth (GROWIT), and animal performance on pasture were developed and validated, with the objective of understanding and quantifying animal and plant growth and their interactions. Within COWCALF, a breed/sex specific weight-age relationship representing gastrointestinal tract capacity is utilized to predict feed intake. NRC equations are used to predict maintenance and daily gain. In GROWIT, prediction of plant growth is based upon genetic potential, air temperature, leaf area, soil water balance and nitrogen fertilization rate. Change in sward dry matter digestibility is a function of plant material present, daily plant growth and environmental temperature. COWCALF and GROWIT were interfaced with the objective of predicting animal performance and forage measures for cattle grazing smooth brome (Bromus inermis). For the grazing model, GROWIT predicts forage production and quality. Diet quality is a function of forage quality and availability. Trampled forage is also predicted. COWCALF predicts feed intake and daily gain. Feed intake and forage losses feedback to influence the next simulated days forage production, availability and quality. For validation of the three models, prediction equations, and R(\u272) were developed between observed and model estimates. Means and standard deviations of observed and model estimates were also compared. A prediction equation intercept near zero with a slope near 1.0 with similar means and standard deviations between observed and model estimates indicates that the model is accurately simulating observed values. COWCALF accurately predicts feed intake of forage based diets (R(\u272) = .92) while the prediction of feed intake for high energy diets was not as strong (R(\u272) = .80). COWCALF provided on accurate estimate of daily gain (R(\u272) = .98) and gain/feed (R(\u272) = .92). GROWIT gives an accurate estimate of plant production (R(\u272) = .98) and changes in forage quality (R(\u272) = .90). The grazing model provides an accurate estimate of animal performance on pasture (R(\u272) = .93)