Assessment of ruminal and fecal microbial biomass and protein content and their relationship with the digestion indicators

The aim of the present trial was to measure the proportion of microbial biomass (MB) and microbial protein (MCP) in rumen fluid (RF) and faecal samples (FS) of dairy cattle and to study their relationship with the main digestion indicators. Samples were collected from 30 high-producing fresh Holstein dairy cows raised in 6 farms and fed a hay-based total mixed ration (TMR). Diet was sampled in each farm where RF and FS were collected from 5 cows 3h after the morning feeding. The RFs were collected through oesophageal probe and analysed for their volatile fatty acids, protozoa and enzyme content. The diets and FS were chemically analysed to determine their composition and nutrient digestibility. Both RF and FS were processed in order to determine their MB and MCP content. An aliquot of 20 g of FS was diluted at 1:20 ratio in a 0.9% sodium chloride solution and blended for 10min. The RF samples and the diluted FS solution were divided each in two 200mL bottles and frozen. The thawed bottles containing RF were sequentially centrifuged at 500g×20’, 18,000g×20’ and 18,000g×25’. After the first centrifugation, samples were filtered through a 59 µm and then through a 10 µm mesh nylon filter. The diluted FS followed a similar process but were subjected to an intermediate centrifugation at 500 g x 10’ between the 2 filtrations and the last two centrifugations were limited at 15’. The supernatants were discharged obtaining 2 pellets. One was dried to obtain the weight for MB proportion calculation (%DM). The other was analysed for the determination of the MCP (Kjieldhal method). The MB were 8.02±2.74 % and 9.49±3.52 %DM respectively in RF and FS. In the same samples, the MCP content was 45.2±7.54 and 30.4±11.4 %MB. Thus, MCP was 3.68±1.51 and 3.03±1.16 %DM in the two type of samples. A negative correlation was found between the proportions of MB in RF and FS. The RF MB was negatively correlated with the dietary undigestible NDFom (uNDFom) and ruminal pH while it was positively correlated with potentially digestible NDF (pdNDF), estimated total-tract apparent NDF digestibility, propionic acid and dietary CP content. The FS MB showed a negative correlation with dietary pdNDF and fat content but it was positively related with dietary uNDFom, faecal amylase content, total protozoa and olotrichs counts. The MB and MCP determination in RF and FS are important to evaluate the digestion efficiency and appears to be related to several digestion indicators.