Abstract
Context Determination of the neutral detergent fibre digestibility is one of the important parameters to consider when formulating diets. However, the in vitro determination shows low repeatability because of the source of rumen-fluid inoculum. Priming of the rumen fluid inocula, obtained through an oesophageal probe, has been proposed to overcome this issue. Aim The objective of the study was to investigate the evolution of the microbial enzymatic activities of different rumen fluids during a priming procedure, to establish the fermentation interval that minimises the differences among rumen-fluid degradative potentials. Methods Three farms for each type of diet were involved in the study. Rumen fluids were obtained from dry and lactating cows fed the following four diet types: 100% hay or a diet with 80:20 forage:concentrate ratio (F:C) as dry-cow diets, and ad libitum hay and concentrate, or a total mixed ration (both at 60:40 F:C) as lactating-cow diets. On each farm, rumen fluid was collected from three Holstein cows by using an oesophageal probe, and mixed. Two aliquots of each rumen fluid mix were added to the medium containing the same priming substrate in an in vitro batch-fermentation system. During the incubation, the fermentation fluids were sampled in duplicate at 0-, 1-, 2-, 4-, 8-, 24- and 48-h intervals. Enzymatic activities of amylase, cellulase and xylanase were determined by radial enzyme diffusion method. Key results Initial enzymatic activities were quite variable and increased with an increasing incubation time. By 24 h, amylase showed similar values among high-concentrate diet fermentation fluids, and a lower data dispersion in comparison to the other intervals; cellulase was characterised by similar values in all the fermentation fluids derived from diets including concentrates, and xylanase showed similar activity in the fermentation fluids derived from high-concentrate diets. Development of the enzymatic activity of the fermentation fluids derived from the 100% hay diet differed from the others. Conclusions A 24-h priming procedure was needed to stabilise and equalise the enzymatic activity of the rumen fluid from cows fed high-concentrate diets. This was not observed in rumen fluid from cows fed hay-based diets. Implications The 24-h-primed rumen fluid can be used to increase the repeatability of neutral detergent fibre digestibility determination.
Highlights
Determination of the neutral detergent fibre digestibility (NDFD) is one of the important parameters for the evaluation of both feed and diet nutritional values
The 24-h-primed rumen fluid can be used to increase the repeatability of neutral detergent fibre digestibility determination
The LFC and LTMR fermentation fluids showed the peak of Amy activity between 2 and 4 h of incubation, while diet comprising only hay (DTH) and DFC fermentation fluids expressed the maximal Amy enzymatic activity (EA) after 8 h from the starting point
Summary
Determination of the neutral detergent fibre digestibility (NDFD) is one of the important parameters for the evaluation of both feed and diet nutritional values This technique has no standardised procedure, showing low repeatability (17% of variability of the NDFD at 30 h of fermentation) partially dependent on rumen-fluid consistency (Hall and Mertens 2012). A further experiment was conducted by Goeser et al (2009), where rumen fluid was primed with a carbohydrate–urea mixture and fermented until a specific amount of gas was produced They found a higher repeatability of the analysis, avoiding, in this case, the NDFD depression, probably as a result of a more complete priming diet. The parameter tested was the cumulative gas production, which is, by definition, timedependent and it is not a direct expression of the degradative potential of the rumen fluid. The latter is better described by the enzymatic activity (EA) of the rumen fluid, which, in turn, can be considered as a quantitative–qualitative reflection of rumen microbiota (Raghuvansi et al 2007) and, on the basis of data from Dadvar et al (2018) and Morgavi et al (2000), a potential measure of its fermentative capacity
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