Influence of particle size and density on mean retention time in the rumen of dairy cows.

Increasing rumen-undegraded protein is one challenge of ruminant nutrition to both meet protein requirements of animals and reduce nitrogen excretion in the environment by increasing nitrogen efficiency. Industrial processes using heat or tanning to reduce rumen protein degradation have certain limitations, such as difficulty in balancing low ruminal degradation and high intestinal digestibility. Reducing the mean retention time (MRT) in the rumen by varying the size and density of particles may be another promising way to increase the rumen-undegraded protein proportion of concentrate feeds and improve the effectiveness of industrial processes. Spherical plastic particles of 3 mean diameter sizes (1, 2, and 3 mm) and 4 densities (0.9, 1.1, 1.3, and 1.5) were used to study the combined effect of size and density on the MRT of particles without interactions with microbial fermentations. Dynamics of fecal excretion of particles were monitored over 106 h (17 sampling times) in a Latin square experiment with 4 lactating cows. Cumulative particle excretion curves were fitted to a double exponential model to calculate total MRT in the digestive tract (TMRT), MRT in 2 compartments (MRT1 and MRT2), and retention time in the intestines' tubular section (TT). Differences in density had a quadratic effect, with densities of 1.1 and 1.3 yielding lower TMRT (29.5 and 31.2 h, respectively) than the densities of 0.9 and 1.5 (TMRT = 64.0 and 51.2 h, respectively). Similar responses were observed for MRT1, which was assumed to be the ruminal MRT for densities 1.1 and 1.3 (8.9 and 10.5 h, respectively) compared with densities 0.9 and 1.5 (39.6 and 22.6 h, respectively). Differences in diameter had a linear effect on TMRT (12.9 h longer for 3 mm than for 1 mm) and on TT. A combined effect of size and density was observed and particle size had no effect on TMRT when density was 1.1 to 1.3; however, outside this range, an increase in particle diameter increased TMRT. Consequently, a density of 1.2 to 1.3 is optimal for the escape of particles. As smaller particles of concentrates lose functional specific gravity more rapidly than larger particles due to their higher fermentation rate, our results, obtained with plastic particles, suggest that a diameter slightly greater than 3 mm seems a compromise to delay the start of fermentation and allow for rapid passage through the reticulo-omasal orifice.