Nutritional influences of granulometry of ruminant diet

Abstract

This review is aimed at extracting the major quantitative influences of variations of granulometry of the ruminant diet. Granulometry is assessed by successive screens and summarised by mean particle size (MPS). Other parameters can be used, such as the proportion of particles retained by a screen with a 2 or 4 mm aperture. These various parameters are not linked linearly. A 1 mm decrease of the MPS of the diet under a threshold of 3-4 and 1 mm induces a drop of chewing time of about 100 and 500 min/d in cattle and sheep respectively. Duration of rumination is more influenced by MPS than the eating one. A lower MPS allows to increase the forage ingestibility in sheep, however for cattle the results are less pronounced. The rumen pH (if initial pH < 6.2) and acetate/propionate ratio are decreased, by 0.5 point, when MPS is decreased by 1 mm under a threshold of 3-4 mm in cattle. The in sacco degradability of starch and protein is significantly influenced by MPS. The mean rate is about 12 points of that of effective degradability (outflow rate = 6 %/h) per mm of MPS. Data related to protein and energy metabolisms are still too rare and dispersed to permit the calculation of any metabolic influence of MPS. Milk fat content is however significantly decreased by bout 2 g/kg per mm of MPS under a threshold of 4-5 mm.