Modeling diurnal rumen metabolism dynamics in dairy cattle: An update to a mechanistic model representing eating behavior, rumen content, rumination, and acid-base balance.

Eating and rumination activities influence rumen environmental characteristics, such as volume, digesta particle size, microbial activity, and the acidity of rumen fluid. The aim of this study was to update an existing mechanistic, steady-state rumen model to simulate diurnal patterns of rumen functioning, contributing to a better understanding of rumen metabolism. Daily patterns of eating behavior were defined as model inputs based on meal characteristics and feeding regimens. Mechanisms controlling the dynamics of rumen content and fluid volume, particle diminution of roughage, rumination behavior, lactic acid metabolism, and rumen fluid pH were mathematically represented. The integration of these mechanisms into the existing steady-state rumen model enabled simulation of the relationship between eating behavior and rumen function. Scenarios of frequent eating (i.e., 12 equal meals) resulted in small fluctuations of rumen conditions during the day, whereas scenarios of restricted feeding with large meals (i.e., 4 daily meals, with 1 meal larger than the others) resulted in pronounced diurnal fluctuations in simulated rumen volume, lactic acid concentration, and rumen pH but not in simulated overall rumination activity. Also, the simulated responses of rumen function to changes in eating behavior showed interactions with dietary characteristics. Short, intense meals moderately increased daily mean rumen volume in high-roughage diets but not in high-concentrate diets. In contrast, the simulation of short and intense meals in concentrate-rich diets induced lactic acid formation and reduced the minimum pH, although having little effect in high-roughage diets. The model was calibrated and evaluated using a database including 78 experimental treatments from 25 studies where TMR diets were fed to lactating dairy cows. The prediction performance was assessed for rumen volume, VFA concentration, mean and minimum pH, NDF digestibility, and daily rumination time both for the original model and the presented updated model. Incorporating diurnal dynamics in the updated model allowed for the explanation of variability of the experimental data, and further independent evaluation is required to confirm its adequacy in predictive performance. In summary, the updated model integrates multiple factors involved in the diurnal patterns of rumen functioning, advancing quantitative understanding of this complex system.