Carbohydrate quantitative digestion and absorption in ruminants: from feed starch and fibre to nutrients available for tissues

Abstract

Carbohydrates are the main source of energy in ruminants. Their site, extent and kinetics of digestion highly impact the amount and profile of nutrients delivered to peripheral tissues, and the responses of the animal, i.e. ingestion, efficiency of production, N and methane excretion, quality of products and welfare. Development of multi-objective feed evaluation systems thus requires a more integrated quantitative knowledge on carbohydrate digestion and yield of terminal products, as well as on their metabolism by splanchnic tissues. The objective of this paper is to review (i) quantitative knowledge on fibre, starch and sugar digestion, volatile fatty acids (VFA) and glucose production and splanchnic metabolism and (ii) modelling approaches which aim at representing and/or predicting nutrient fluxes in the digestive tract, portal and hepatic drainage. It shows that the representation of carbohydrate digestion and VFA yield is relatively homogeneous among models. Although published quantitative comparisons of these models are scarce, they stress that prediction of fibre digestion and VFA yield and composition is still not good enough for use in feed formulation, whereas prediction of microbial N yield and ruminal starch digestion seems to be more satisfactory. Uncertainties on VFA stoichiometric coefficients and absorption rates may partly explain the poor predictions of VFA. Hardly any mechanistic models have been developed on portal-drained viscera (PDV) metabolism whereas a few exist for liver metabolism. A qualitative comparison of these models is presented. Most are focused on dairy cows and their level of aggregation in the representation of nutrient fluxes and metabolism highly differs depending on their objectives. Quantitative comparison of these models is still lacking. However, recent advances have been achieved with the empirical prediction of VFA and glucose production and fluxes through PDV and liver based on the current INRA feed evaluation system. These advances are presented. They illustrate that empirical prediction of ruminal VFA and intestinal glucose production can be evaluated by comparison with measured net portal net fluxes. We also illustrate the potential synergy between empirical and mechanistic modelling. It is concluded that concomitant empirical and mechanistic approach may likely help to progress towards development of multi-objective feed evaluation systems based on nutrient fluxes.

Methane production and diurnal variation measured in dairy cows and predicted from fermentation pattern and nutrient or carbon flow

“…With decreased retention time, a lower proportion of feed is fermented in the rumen and with less complete digestion, relatively more easily fermentable parts of the CHO are digested (Huhtanen et al, 2006), which are normally considered to be least methanogenic (Beauchemin et al, 2008). It is well documented that CHO composition affects fermentation pattern (Murphy et al, 1982;Bannink et al, 2006;Nozière et al, 2010). Measured data for rumen digestibility and rumen VFA profile were used in the present study and the aforementioned effects of CHO composition on fermentation pattern should therefore already have been accounted for in the model calculations.…”

“…As butyrate is absorbed faster than other VFA, the assumption of an equal proportion of butyrate in rumen VFA to the proportion of butyrate in rumen VFA production can cause bias (Nozière et al, 2010). It was unclear why this was only revealed for DCARB and not with DOM, as it should have been a source of bias then with both approaches.…”

“…Such effects are probably closely related to those of CHO digestion, which appeared more explanatory for bias with DOM. Dietary CHO composition such as the proportion of maize silage will affect rumen digestion characteristics and therefore the VFA composition and rumen pH (Nozière et al, 2010).…”

“…Feed entering the rumen can follow three pathways: (1) outflow from the rumen as undegraded feed nutrients, (2) microbial utilization to generate metabolic energy for growth and maintenance with volatile fatty acids (VFA) and hydrogen (H 2 ) as end products or (3) microbial utilization for microbial biosynthesis. The distribution between the different pathways depends not only on the chemical composition of the feed but also on other factors such as physical structure of the diet and feed intake level (Hoover, 1986;Nozière et al, 2010). The most common approach to estimate the amount of VFA produced in the rumen is to estimate the amount of individual nutrients (protein, carbohydrates (CHO) and glycerol of fat origin) that is truly digested in the rumen, and subsequently calculate glucose units available from these nutrients for fermentation to VFA.…”

In vivo production and molar percentages of volatile fatty acids in the rumen: a quantitative review by an empirical approach

“…The stoichiometry of fermentations is governed by enzymatic laws, microbial species and thermodynamical principles. Several attempts have been made to represent stoichiometry of fermentations using mechanistic or empirical modelling approaches, as reviewed by Noziè re et al (2010b). However, such models are still unsatisfying in predicting VFA profiles in the rumen (Bannink et al, 1997;Offner and Sauvant, 2004a).…”