Ruminants, like sheep, goats, cows, deer, moose, etc., have a four-chambered stomach, the largest of which is called the rumen. The rumen houses symbiotic microorganisms which break down plant fibers that the animal can’t digest on its own. It’s estimated that up to 80% of a ruminant’s energy need is met from the volatile fatty acids (also called short-chain fatty acids) that bacteria produce from digesting fiber, and that up to 85% of a ruminant’s protein need is met from microbial proteins.
A lot of factors can be manipulated to help get the most out of one’s diet, including adjusting ingredients for water content, palatability, ease of chewing, and how easy the ingredients are to digest. For example, highly fibrous foods with larger particles/pieces require more chewing, as well as a longer time spent in the rumen digesting so that microorganisms have plenty of time to break the chemical bonds of large molecules. Smaller food particles can reduce the time and effort spent chewing, allow for more surface area on plant fibers for microorganisms to attach to and digest faster, and speed up the movement of food through the digestive tract. On the other hand, moving food too quickly could reduce the amount of time microorganisms can spend digesting, or time the ruminant can absorb nutrients across their GI tract lumen, or cause slow-growing microbial species to wash out.
Surprisingly, almost no work has investigated the effect of diet particle size on the community, despite knowing that microbial digestion is contingent on the ability to attach to and process complex nutrient structures. In this study, we observed the effect of particle size on rumen bacteria, by feeding long-stem (loose) alfalfa hay compared to a ground and pelleted version of the same alfalfa in yearling sheep wethers.
The pelleted-hay diet group had a greater increase in bacterial richness, including common fibrolytic rumen inhabitants, which may explain the increase in average daily gain and feed efficiency in this group.
Ishaq SL, Lachman MM, Wenner BA, Baeza A, Butler M, Gates E, et al. (2019) Pelleted-hay alfalfa feed increases sheep wether weight gain and rumen bacterial richness over loose-hay alfalfa feed. PLoS ONE 14(6): e0215797. Article.
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.
Feature image credit: Pellet Mill