Forages and pastures symposium: forage biodegradation: advances in ruminal microbial ecology

The rumen microbial ecosystem provides ruminants a selective advantage, the ability to utilize forages, allowing them to flourish worldwide in various environments. For many years, our understanding of the ruminal microbial ecosystem was limited to understanding the microbes (usually only laboratory-amenable bacteria) grown in pure culture, meaning that much of our understanding of ruminal function remained a "black box." However, the ruminal degradation of plant cell walls is performed by a consortium of bacteria, archaea, protozoa, and fungi that produces a wide variety of carbohydrate-active enzymes (CAZymes) that are responsible for the catabolism of cellulose, hemicellulose, and pectin. The past 15 years have seen the development and implementation of numerous next-generation sequencing (NGS) approaches (e.g., pyrosequencing, Illumina, and shotgun sequencing), which have contributed significantly to a greater level of insight regarding the microbial ecology of ruminants fed a variety of forages. There has also been an increase in the utilization of liquid chromatography and mass spectrometry that revolutionized transcriptomic approaches, and further improvements in the measurement of fermentation intermediates and end products have advanced with metabolomics. These advanced NGS techniques along with other analytic approaches, such as metaproteomics, have been utilized to elucidate the specific role of microbial CAZymes in forage degradation. Other methods have provided new insights into dynamic changes in the ruminal microbial population fed different diets and how these changes impact the assortment of products presented to the host animal. As more omics-based data has accumulated on forage-fed ruminants, the sequence of events that occur during fiber colonization by the microbial consortium has become more apparent, with fungal populations and fibrolytic bacterial populations working in conjunction, as well as expanding understanding of the individual microbial contributions to degradation of plant cell walls and polysaccharide components. In the future, the ability to predict microbial population and enzymatic activity and end products will be able to support the development of dynamic predictive models of rumen forage degradation and fermentation. Consequently, it is imperative to understand the rumen's microbial population better to improve fiber degradation in ruminants and, thus, stimulate more sustainable production systems.

Impact of in vitro fermentation techniques upon kinetics of fiber digestion

Three in vitro fermentation experiments were conducted to examine the impact on kinetics of fiber digestion of microminerals and tryptone addition, media reduction, fermentation vessel, CO2 gassing regimen, and buffer type. Alfalfa and bromegrass hays were incubated for 0, 4, 8, 12, 18, 24, 30, 36, 48, 72, and 96 h and analyzed for NDF. Kinetic measures of fiber digestion were estimated using nonlinear regression with iteratively reweighted least squares. In Experiment 1, continuous CO2 gassing increased rate and decreased lag time prior to NDF digestion compared with purging a non-CO2-saturated buffer at inoculation. Vessel type (50-ml polypropylene tube, 125-ml pyrex Erlenmeyer flask), use of additives (microminerals, tryptone), and media reduction had no effect on kinetics of NDF digestion. In Experiment 2, elimination of both media reduction and nutritive additives increased the lag time prior to NDF digestion. In Experiment 3, continuous CO2 gassing of buffer in 125-ml Erlenmeyer flasks resulted in faster rates of NDF digestion than CO2-saturated buffer in 50-ml screw-cap polypropylene tubes. The method that yielded the fastest rates and shortest lag times of NDF digestion consisted of continuous CO2 gassing, reduction, and use of additives to ensure that no nutrient limited fiber digestion.

Effects of amino acids and peptides on rumen microbial growth yields

Experiments were conducted using mixed rumen bacterial cultures to determine which amino acids limited growth. Complete amino acid mixtures stimulated microbial growth alone and when added to casein. Amino acid subgroups did not stimulate growth alone or when added to casein or casein hydrolysates. Results were similar whether growth was limited by periodic addition of low amounts of carbohydrate or when higher amounts were added to batch cultures. Little growth occurred with ammonia as sole N source. Addition of 100 mg/L of amino acids and peptides quadrupled growth, peptides at 10 mg/L resulted in higher growth than the corresponding amount of free amino acids. Apparent saturation of growth occurred when 10 mg/L of a complete amino acid mixture or trypticase was added to cultures. The Michaelis constant values for amino acids and trypticase were determined to be .5 and 1.0 mg/L, respectively. Growth was a linear function of amount of carbohydrate fermented with the coefficient of slope increasing with increasing amino acid concentrations. These experiments demonstrate that growth stimulation from amino acids and proteins is due to the number of amino acids provided in a given mixture rather than specific growth limiting amino acids. Rumen bacterial growth is greatly stimulated by amino acids and peptides, with low affinity constant values, allowing good growth in the concentrations of amino acids and peptides found in vivo

Changes of bacterial numbers and carbohydrate fermenting groups during in vitro rumen incubations with feedstuff materials

Short in vitro incubations of mixed ruminal bacteria with feedstuff materials were conducted. In each series of experiments, the rumen bacterial population was divided into equal portions. One portion was washed anaerobically by centrifugation in a mineral buffer prior to initiation of incubation, whereas the other portion was not washed. Changes of carbohydrate group composition of the populations, of bacterial cell composition, and of incubation fluid characteristics were monitored over 24 h. Washing the bacterial inoculum did not affect adversely the ensuing feed fermentation and resulted in lower background carbohydrates and fermentation acids. Unwashed bacterial incubations had a greater tendency to produce more lactate. When the feedstuff substrate contained more soluble nutrients, the microbial population exhibited batch culture type growth, and bacteria fermenting soluble carbohydrate predominated. When alfalfa hay was the sole substrate, a more balanced fermentation resulted with respect to both carbohydrate group composition and fermentation acid production. Implications of these findings in relation to use of in vitro methods to estimate digestibility of feedstuff are addressed.