A model of diurnal grazing patterns and herbage intake of a dairy cow, MINDY: Model description

Estimates of herbage intake and parallel measurements of ingestive and digestive behaviors of grazing ruminants pose considerable experimental and technical difficulties, owing to dynamic interactions between the plant, the rumen and the animal. As a consequence, advances in the area have been slow and costly. Model simulations that capture such interactions are critical for research and management decisions involving the grazing process. This work describes MINDY, a mathematical, mechanistic and dynamic simulation model of the diurnal grazing pattern of a dairy cow. MINDY is based on a cluster of three models: (1) Molly (Baldwin, 1995), a model of ruminant digestion and metabolism; (2) a model representing feed consumption as a function of diurnal fluctuations in the internal state of the animal; and (3) a sward structure, herbage quality and grazing behavior model. The objective of the work was to describe the diurnal grazing pattern, including ingestive actions and rumination behaviors, herbage intake, and nutrient supply to the animal in response to the animal's internal state and grazing environment. The model was coded in ACSL and simulations were conducted using ACSLXtreme. In addition to dietary nutrient composition required by Molly, MINDY requires sward surface height and mass, and grazing area offered to the cow. Key sub-model parameters were identified by sensitivity analyses and parameterized using two data sets from mid-lactation Friesian and late lactation Holstein dairy cows breeds under set stock conditions. The parameterized model predicted realistic estimates of ingestive behavior for different cow genotypes managed under set stocking and rotational grazing. It also predicted a realistic number of steps taken while eating and searching and sward defoliation dynamics as well as diurnal fluctuations of digestion and metabolism. Additional evaluations are required and further data may be needed to better define some parameters, but the model offers promise as an heuristic tool for feed intake and grazing process research and as an informative tool for grazing and cow management decisions.