Evaluation of Average Daily Gain Predictions By the Integrated Farm System Model for Forage-Finished Beef Steers

Representing the performance of cattle finished on an all forage diet in process-based whole farm system models has presented a challenge. To address this challenge, a study was done to evaluate average daily gain (ADG) predictions of the Integrated Farm System Model (IFSM) for steers consuming all-forage diets during the finishing period. Animal performance data from warm-season annual forage grazing trials conducted in 2014 and 2015 at the Eatonton Beef Research Unit were compared to model predictions. Treatments representing the highest (BMR14), average (BMR15), and lowest (PMCG15) forage yields were simulated. Forage quality for each treatment was provided to the model, and predicted forage yield was calibrated within 10% of observed values. With initial settings based upon recommended nutritional requirements, predicted ADG was 71, 12, and 44% lower than that observed for BMR14, BMR15, and PMCG15, respectively. These discrepancies may be related to assumptions about forage quality and intake, or the animal’s nutrient requirements. The use of variable stocking rates to maintain forage quality in the grazing trial precluded the collection of intake data, which would have enabled a more conclusive evaluation of these differences. Two parameters representing feed efficiency (FE) and feed digestibility (FD) were explored for their impact on ADG predictions. FE modified the amount of net energy available per unit of feed consumed, while FD modified fiber digestibility, which essentially increased the rate of passage. A sensitivity analysis showed that ADG was highly sensitive to each of these model parameters. A 37% increase in FE or 39% increase in FD was required to predict ADG within 10% of observed values. Predicted forage intake decreased from 9.9 to 9.7 kg DM/d when FE was increased, and it increased to 15.4 kg DM/d when FD was increased to meet nutrient demands. No change in manure output was observed in either scenario, though manure N and P content decreased by 26 and 13% when FE was increased, and by 35 and 7% when FD was increased. These values remain within biologically reasonable bounds, and suggest that these parameters may be utilized to improve representation of finishing animals on all-forage diets. This improvement will enable utilization of IFSM in a life cycle assessment of forage-finished beef production systems.