Date Published: June 5, 2019
Publisher: Public Library of Science
Author(s): Suzanne L. Ishaq, Medora M. Lachman, Benjamin A. Wenner, Amy Baeza, Molly Butler, Emily Gates, Sarah Olivo, Julie Buono Geddes, Patrick Hatfield, Carl J. Yeoman, Garret Suen.
Diet composed of smaller particles can improve feed intake, digestibility, and animal growth or health, but in ruminant species can reduce rumination and buffering–the loss of which may inhibit fermentation and digestibility. However, the explicit effect of particle size on the rumen microbiota remains untested, despite their crucial role in digestion. We evaluated the effects of reduced particle size on rumen microbiota by feeding long-stem (loose) alfalfa hay compared to a ground and pelleted version of the same alfalfa in yearling sheep wethers during a two-week experimental period. In situ digestibility of the pelleted diet was greater at 48 h compared with loose hay; however, distribution of residual fecal particle sizes in sheep did not differ between the dietary treatments at any time point (day 7 or 14). Both average daily gain and feed efficiency were greater for the wethers consuming the pelleted diet. Observed bacterial richness was very low at the end of the adaptation period and increased over the course of the study, suggesting the rumen bacterial community was still in flux after two weeks of adaptation. The pelleted-hay diet group had a greater increase in bacterial richness, including common fibrolytic rumen inhabitants. The pelleted diet was positively associated with several Succiniclasticum, a Prevotella, and uncultured taxa in the Ruminococcaceae and Rickenellaceae families and Bacteroidales order. Pelleting an alfalfa hay diet for sheep does shift the rumen microbiome, though the interplay of diet particle size, retention and gastrointestinal transit time, microbial fermentative and hydrolytic activity, and host growth or health is still largely unexplored.
It has been well established that the nutrient composition of a diet affects the gastrointestinal tract (GIT) microbiota [1,2], yet the physical structure and complexity of the diet may also alter its interactions with the GIT microbiota and host. In cattle, longer fiber particles have been shown to improve rumination  and ultimately fiber digestibility . However, shorter or smaller diet particles can change the dynamics of digestion and produce a number of favorable outcomes. Specifically, reductions in particle size associated with mastication or mechanized processing correspond to increases in feed surface area and thus allow for greater microbial attachment and relative fibrolytic and fermentative activities as has been shown in vitro . Mechanical breakdown during diet preparation can also physically disrupt waxy plant cuticles and cell walls that can otherwise impede microbial attachment and degradation, thus making plant carbohydrates more available  and decreasing the potential confounding effect of forage fragility within the rumen . Hydrolytic activities are also likely further enhanced by the reductions in buoyancy and increased functional specific gravity  associated with reduced particle size, that would allow smaller particles to sink beneath the dorsally-located rumen mat and into the microbe-rich rumen liquor [9,10].
Despite the known effect of diet particle size on physical egress through the GIT, chemical accessibility and digestibility, and host absorption, the explicit effect of feed particle size has never been examined within the context of rumen microbial diversity. Differences in the structural fiber composition and resulting bioavailability of nutrients that occur among plant species can select for distinct microbial communities . In the present study, pelletization of an alfalfa diet increased bacterial diversity and sheep performance, presumptively via a smaller particle size which thereby improving protein digestion of feed.