Kinetics of In Vitro Cell‐Wall Disappearance and In Vivo Digestion

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Abstract
Understanding the dynamics or kinetics of digestion and passage may suggest ways to manipulate forages to improve the diet, and consequently the production, of domesticated ruminants. The methodology of fitting exponential curves was tested using simulated cell‐wall‐disappearance curves with both an exponential and a gamma‐distributed exponential driving function and normal‐random variation superimposed. Nonlinear regression gave more precise estimates of the true rates of digestion and lag times than linearization. The gamma distributed exponential gave lower residual sum of squares but estimates of digestible and indigestible fractions as well as lag times and disappearance rates were not improved over the simpler exponential function. Estimates were less precise when the gamma‐distributed exponential was the driving function for the disappearance curve and the gamma function was skewed. The use of the exponential decay constant (k) in studies of cell‐wall composition is confounded with extent of disappearance as a result of plant anatomy. A surface area fermentability index (SAFI) is proposed as (kCd)/(1000 − Ci) where Cd and Ci are the digestible and indigestible fractions, respectively, of the cell wall. The SAFI was used to adjust literature values for k as well as to partially explain observed negative correlations between extent and rate of cell‐wall disappearance (k) in bromegrass (Bromus inermis Leyss.) that had been selected for high and low in vitro dry matter disappearance. Study of in vivo kinetics is complicated by variable rate of passage, particle size, and extent and rate of particle size reduction. Models of in vivo kinetics offer an opportunity for synthesis of understanding of the animal system. However, the many interactions make interpretation and incorporation of in vitro data into models of in vivo ruminant digestion difficult.

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