Rumen fermentation and microbiota are very important for the ruminants, different carbohydrates have different effect on both of them, but only a few people know that dissolved ruminal hydrogen plays a critical role in the rumen and associated with them. Carbohydrate fermentation in the rumen leads to the formation of volatile fatty acid (VFA), carbon dioxide, hydrogen, and microbial biomass. The hydrogen can be used by methanogens to produce methane, which is an important greenhouse gas, hydrogen is also an important intermediate that is linked with VFA production pathway and methanogenesis. But how dissolved hydrogen concentrations act in the rumen?

Professor Wang Min and his group did an experiment to test the hypothesis that carbohydrates ingested greatly alter the rumen environment in dairy cows, and that dissolved hydrogen concentration is associated with these changes in rumen fermentation and microbiota. They chose 28 lactating Chinese Holstein dairy cows and randomized complete block design to investigate effects of 4 diets differing in forage content (45% compared with 35%) and source (rice straw compared with a mixture of rice straw and corn silage) on feed intake, rumen fermentation, and microbial populations. Each dietary treatment had 7 replications (cows), the 2 forage content treatments were 45% [high forage content (HF)] and 35% [low forage content (LF)] forage, whereas the two forage source treatments were rice straw (RS) and a mixture of rice straw and corn silage (RC).

They found that the cows fed the LF-RC diet had greater ruminal dissolved hydrogen and dissolved methane concentrations than cows fed the HF-RS diet. Hydrogen mainly generated during fermentation of dietary fiber and starch to VFA, dissolved hydrogen concentration was greatly affected by dietary forage content and source, starch is hydrolyzed to glucose more rapidly and efficiently than fiber in the rumen. Ruminal dissolved hydrogen concentration was positively correlated with rumen dissolved methane concentration and dietary starch intake, but negatively correlated with the ingested NDF-to-starch ratio. Cows fed the HF-RS diet had a greater molar proportion of acetate and a lower total VFA concentration than cows fed the other 3 diets, ruminal dissolved hydrogen concentration was positively correlated with total VFA and molar proportions of propionate, but negatively correlated with the molar proportion of acetate and acetate-to-propionate ratio.

Cows fed the HF-RS diet had smaller numbers of protozoa and greater numbers of fungi, but LF-RC diet had greater numbers of methanogens. When more dietary starch was included, fibrolytic microorganisms were replaced by amylolytic bacteria, cows fed the starchy LF-RC diet had a greater relative abundance of amylolytic bacteria (such as S. ruminantium and Prevotella spp). Changes in the rumen microbiota were closely associated with ruminal dissolved hydrogen concentration, lower ruminal dissolved hydrogen was associated with greater numbers of fibrolytic microorganisms that enhance fiber degradation and hydrogen generation through acetate production, whereas greater ruminal dissolved hydrogen was associated with increased amylolytic microorganisms that are capable of disposing of electrons derived from fermentation through propionate production.

The gaseous methane and hydrogen emissions were greater in cows fed the fibrous HF-RS diet. Methane emissions were influenced by complicated interactions between factors, including supersaturation of dissolved gases, rumen passage rate, feed digestibility, fermentation pathway, population of methanogens and so on, if we want to understand how to control the gaseous methane emission, we should have a better understood of these complex interactions.

This research entitled "Shifts in Rumen Fermentation and Microbiota Are Associated with Dissolved Ruminal Hydrogen Concentrations in Lactating Dairy Cows Fed Different Types of Carbohydrates" has been published on The Journal of Nutrition(Doi: 10.3945/jn.116.232462), details could be found at http://jn.nutrition.org/content/146/9/1714 

Contact: TAN Zhiliang

E-mail: zltan@isa.ac.cn

Institute of Subtropical Agriculture, Chinese Academy of Sciences