Effect of forage particle size on hindgut fermentation and the fecal bacterial community. Evaluation of fecal microbiota in Holstein dairy cows

Organic cattle farming encourages forage-rich diets, and the reduction of particle size has been suggested as a tool to improve forage utilization and enhance nutrient intake of cows. However, reducing forage particle size increases passage rate and the flow of potentially fermentable nutrients out of the rumen, and the consequences for hindgut fermentation have not been evaluated yet. This study evaluated the effects of decreasing dietary forage particle size on fecal pH and short chain fatty acids (SCFA), and the bacterial community of dairy cows fed forage-based rations. Twenty-one organically fed lactating Holstein cows (4 primiparous and 17 multiparous; mean and SD, 703 ± 65 kg BW and 135 ± 104 DIM) were divided in two groups and were fed 1 of 2 diets for 34 d. Diets were 20% concentrate and 80% forage, either as a control diet with a forage mean geometrical particle size of 52 mm (LONG; 11 cows), or a diet with the forage particle size reduced to mean geometrical size of 7 mm (SHORT; 10 cows). Fecal samples were collected every 8 h within the last 3 d of the experiment, fecal pH was immediately measured, and subsamples were frozen at -20 °C. Samples were analyzed for SCFA, and the fecal bacterial community was evaluated using a 16S rRNA sequencing. Data showed a reduced fecal pH (7.25 vs. 7.16 ± 0.025) for cows consuming SHORT. While the concentration of total SCFA was not affected, the proportion of propionate –a key glycogenic precursor in cattle- tended to be greater for SHORT (13.3 and 13.7 ± 0.17%, respectively). The predominant bacterial phyla including Firmicutes (58.0 ± 0.75%), Bacteroidetes (26.8 ± 0.42%), and Verrucomicrobia (4.0 ± 0.42%) were not affected by particle size. Family Lachnospiraceae increased in relative abundance when SHORT diet was fed (12.1 and 13.9 ± 0.59%); genera Acetitomaculum (1.1 and 1.8 ± 0.19%), Turicibacter (0.7 and 0.9 ± 0.07%), and Ruminobacter (0.1 and 0.4 ± 0.09%) increased in relative abundance when SHORT was fed (LONG and SHORT, respectively). Relative abundance of some bacterial taxa was correlated with major SCFA. In conclusion, reducing the particle size of forage-rich diets resulted in minimal drop of the fecal pH, but tended to enhance propionate with favorable modulation of bacterial communities. Thus, reduction of forage particle size represents an effective approach to optimize forage utilization while maintaining hindgut fermentation and fecal bacterial diversity in dairy cows fed forage-rich diets.