Size and density influence of concentrates to increase by-pass protein fraction in dairy cows’ diet
Résumé
Increasing by-pass protein is one of the challenges of ruminant nutrition to both cover the protein needs of animals and reduce the nitrogen excretion in the environment. Industrial process such as heat treatment, formaldehyde tanning and vegetable extract are focused on the protection of proteins against ruminal fermentations but show some limits (low intestinal digestibility, toxicity, low or variable efficiency). The reduction of the mean retention time (MRT) by varying size and density of particle is another way to increase by-pass protein fraction of concentrates. Plastic particles of four sizes (0.5, 1, 2 and 3 mm) and four densities (0.9, 1.1, 1.3 and 1.5) were used to study the combined effect of size and density on MRT of particles without microbial fermentation interactions. The kinetics of faecal excretion of particles (17 sampling over 106 hours) were studied in a Latin square experiment including four lactating cows. Data were adjusted to a double exponential model. Density has a quadratic response with the fastest escape for densities 1.1 and 1.3 in the total digestive tract (TMRT=29.5 and 31.2 hours respectively) as well as in the compartments isolated by the model compared to densities 0.9 and 1.5 (TMRT=64.0 and 51.2 hours respectively). Size has a linear effect on the total digestive tract transit time (+12.9 hours for the 3 mm size compared to the 1 mm) and in the time of first appearance of particles but no effects were observed in the others compartments. A combined effect of size and density is observed: particle size has no effect on TMRT when density is between 1.1 and 1.3 but
outside this range, an increase of particle size induces an increase in the TMRT. In conclusion a density in a range 1.1 to 1.3 will be optimal for the by-pass of particles of concentrate. These particles will be submitted to ruminal fermentations, and their optimal size must be small enough to pass the reticulo-omasal orifice but large enough todelay the start of fermentation on their surface and thus the loss of density by fermentation gases production. Size 3 to 4 mm would be a good compromise between these two constraints to allow the shortest escape from the rumen.