Development of a mechanistic model of intake, chewing and digestion in cattle in connection with updated feed units

Abstract

Recent proposals have updated systems of feed units of energy, protein, structure and rumen fill. However, consistency between the largely independently built units has not been checked, even though energy, structure and fill values of feedstuffs are linked throughout feeding in ruminants. The target of this work was to build a mechanistic model to ensure consistency across equations related to intake, mastication, fill value of feeds and diets, digestion of organic matter and transit times. The first step was to build a large database of published results on digestion, intake and chewing in cattle. This database was studied by meta-analyses to calculate more than 100 empirical regressions; about half of these equations were used to build the new French systems of net energy and metabolisable protein. The other equations were mainly related to mastication (per day or kg dry matter intake, DMI), as well as the sizes and adjacent flows of 12 compartments inside the rumen: neutral detergent fibre (NDF), degradable (D) or not (U), in large or small particles; protein, soluble (S) or D and U; starch, S or D; water; volatile fatty acids; and microbes. The rumen digesta load (RL) was closely linked with the forage NDF intake, but in this dataset, there was no maximum RL, so it does not seem to limit DMI. Close relationships were observed between long-forage intake (NDFI, % of liveweight) and dietary forage NDF (%DM) and between mastication (min/day) and dietary index of mastication (min/kg DMI). Unlike RL, each of these two relationships exhibited obvious asymptotic maximum values for the duration of mastication per day (~1000 min/day) and for the level intake of NDF from long forages (1.35% of liveweight). In these two equations, the slope represented the level of DMI. The two equations were added to the other structural equations. It is therefore possible to use a mechanistic model with 12 compartments to consistently integrate aspects dealing with fill, chewing, DMI and energy value of rations in cattle.