Whole-body fuel selection in ruminants: nutrient supply and utilization by major tissues

Effect of dietary forage: concentrate ratio on the behaviour, rumen fermentation and circulating concentrations of IGF-1, insulin, glucagon and metabolites of beef steers and their potential effects on carcass composition

In an investigation of factors responsible for the lower efficiency of carcass lean gain seen previously in steers offered grass silage diets 18 Simmental × British Friesian steers (361 (s.e. 5-35) kg) were offered one of three diets: a perennial ryegrass silage ad libitum and alone (S) or supplemented with rolled barley at 300 g/kg of total dry matter (SC) or supplemented as described but restricted (SCr) in quantity so as to provide the same dry matter (DM) and digestible energy (DE) intakes as for S. Eating (Eb), ruminating (Rb), standing (Sb) or lying (Lb) behaviour was quantified during four 24-h periods of manual observation. Eb was noted in more detail in a second experiment using computerized Calan-Broadbent gates and load cells to monitor times and rates of eating. Blood was taken via temporary indwelling jugular catheters at 30 to 60 min intervals on each of 4 days 1 month apart. Rumen fluid was sampled hourly for three 24-h periods from three rumen-cannulated steers given the same three diets in a separate 3 x 3 change-over design experiment.

Steers offered the restricted diet SCr ate most of their food in one extended meal within 6 h of feeding while two peak eating periods (morning and evening) were observed in steers offered the other two diets. Steers offered SCr spent more time in Sb (P < 0.05), and less time in Eb (P < 0·001) and Rb (P < 0·05) activities than did animals offered the two diets ad libitum (SC and S). Mean 24 h insulin-like growth factor-1 (IGF-1) concentrations and postprandial insulin concentrations were significantly higher with diet SCr than with diet S (P < 0·001) despite equal daily DM and DE intakes from each. Insulin appearance in the jugular vein reflected the pattern of food intake on all treatments. Rumen fermentation characteristics were largely unaffected by diet. Mean 24 h rumen volatile fatty acid, pH and ammonia concentrations did not differ between diets but post-prandial rumen pH tended to be lower in animals offered the SC and SCr diets.

Differences in patterns of food intake between animals offered food ad libitum and at a restricted level are likely to determine patterns of nutrient absorption and the secretion of nutritionally regulated splanchnic hormones. The higher proportions of Sb and Rb activities in steers offered the restricted diet represent an energy cost to these animals while the higher plasma IGF-1 and insulin concentrations also seen in these animals may collectively influence the partitioning of nutrients to the peripheral tissues and contribute to the increased efficiency of carcass lean deposition previously shown in animals offered such diets.

Responses to nutrients in farm animals: implications for production and quality

It is well known that any quantitative (energy and protein levels) and qualitative (nature of the diet, nutrient dynamic) changes in the feeding of animals affect metabolism. Energy expenditure and feed efficiency at the whole-body level, nutrient partitioning between and within tissues and organs and, ultimately, tissue and organ characteristics are the major regulated traits with consequences on the quality of the meat and milk produced. Recent progress in biology has brought to light important biological mechanisms which explain these observations: for instance, regulation by the nutrients of gene expression or of key metabolic enzyme activity, interaction and sometimes cross-regulation or competition between nutrients to provide free energy (ATP) to living cells, indirect action of nutrients through a complex hormonal action, and, particularly in herbivores, interactions between trans-fatty acids produced in the rumen and tissue metabolism. One of the main targets of this nutritional regulation is a modification of tissue insulin sensitivity and hence of insulin action. In addition, the nutritional control of mitochondrial activity (and hence of nutrient catabolism) is another major mechanism by which nutrients may affect body composition and tissue characteristics. These regulations are of great importance in the most metabolically active tissues (the digestive tract and the liver) and may have undesirable (i.e. diabetes and obesity in humans) or desirable consequences (such as the production of fatty liver by ducks and geese, and the production of fatty and hence tasty meat or milk with an adapted fatty acid profile).

Mesenteric infusion of a volatile fatty acid prevents body weight loss and transiently restores luteinising hormone pulse frequency in ovariectomised, food-restricted ewes

Pulsatile luteinising hormone (LH) secretion is suppressed by food restriction and rapidly restored by return to ad lib. feeding concomitant with an increase in the oxidation of free fatty acids, although there is no increase in plasma leptin concentrations or body fat content in ovariectomised ewes. The ingestion of food may stimulate LH secretion by increasing availability of oxidisable metabolic substrates. Ruminal digestion is characterised by the production of volatile fatty acids and, of these, propionate is the major gluconeogenic substrate, and both glucose and propionate are oxidisable in a variety of tissues. To examine whether increases in mesenteric propionate concentrations are sufficient for restoration of pulsatile LH secretion during a period of food restriction, adult, food-restricted, hypogonadotrophic, ovariectomised ewes received mesenteric vein infusions of 5 μmol/min/kg body weight (BW) propionate or saline, whereas normal weight, ad lib.-fed ewes received mesenteric infusions of saline for 10 days. Blood samples were taken every 10 min for 5 h before the start of the 10-day infusion period, and continued throughout the first 5 h of infusion on the afternoon of day 1, and in the morning on days 2, 7 and 10. Propionate-infused, food-restricted and ad lib.-fed, saline-infused ewes showed a significantly higher LH pulse frequency compared to that of food-restricted-saline-infused ewes on postinfusion days 1 and 2 but not on days 7 and 10, and only the saline-infused, food-restricted group lost a significant amount of body weight. These results indicate that the reproductive system can respond acutely to infusion of metabolic fuels such as propionate, although a sustained recovery of pulsatile LH secretion requires more than an increase in this single metabolic substrate.

Partitioning of nutrient net fluxes across the portal-drained viscera in sheep fed twice daily: effect of dietary protein degradability

Extrusion is used to decrease leguminous seed protein degradability in the rumen in order to shift part of the dietary protein digestion towards the small intestine. The effect of such displacement of digestion site on the partitioning of nutrient net fluxes across the gastrointestinal tract was studied using four sheep fitted with catheters and blood-flow probes, allowing measurements across the rumen, the mesenteric-drained viscera (MDV) and the portal-drained viscera (PDV). Two diets containing 34 % of pea seeds were tested in a crossover design. They differed only according to pea treatment: raw pea (RP) or extruded pea (EP) diet. Rumen undegradable protein (RUP) accounted for 23 and 40 % of dietary crude protein for RP and EP diets, respectively. Across the rumen wall, ammonia net flux was lower with EP diet, whereas urea net flux was not different. Across the MDV, free amino acid (FAA) net flux was greater with EP diet, whereas peptide amino acid net flux was not different, accounting for 7 % of the non-protein amino acid net release. From RP to EP diet, PDV net flux of ammonia decreased by 23 %, whereas FAA net release increased by 21 %. The difference in dietary RUP did not affect the PDV net flux of SCFA, 3-hydroxybutyrate, lactate and glucose. In conclusion, the partial shift in pea protein digestion from the rumen to the small intestine did not affect the portal net balance of N, but decreased N loss from the rumen, and increased amino acid intestinal absorption and portal delivery.

Nutritional level and energetic source are determinants of elevated circulatory lipohydroperoxide concentration

Dietary energetic impact on oxidative stress is incompletely understood. Therefore, effects of diets on oxidative stress were studied using a crossover block design. In Expt 1, intake of metabolizable energy (ME) was restricted orad libitum. In Expt 2, isoenergetic and isonitrogenic diets were fed, replacing carbohydrate energy by energy of fatty acids. Circulatory lipohydroperoxides (LOOH), markers of acute oxidative stress, were expressed absolutely and in terms of cholesterol or TAG levels. In Expt 1, plasma (jugularis vein) LOOH was assayed in combination with whole-body oxidative metabolism using gas exchange and heart rate (HR) during feeding periods and at rest. In Expt 2, LOOH was assayed in plasma from portal and a large udder vein and a mesenteric artery. In Expt 1, intake increased VO2, HR and LOOH following overnight fast with higher values (P < 0·05) when feeding MEad libitum. Intake of MEad libitum(3 weeks) increased cardiac protein of cytochrome oxidase and endothelial-type nitric oxide synthase (P < 0·05), indicating adaptation of the heart to higher activity. Transient HR responses evoked by an antidiabetic drug (levcromakalim) revealed a linear positive correlation with relative LOOH (r20·79), supporting the relationship between oxidative metabolic rate and lipoperoxidation. Evidence for exogenous lipids as LOOH source provided the vessel-specific rise in LOOH through replacing carbohydrate ME by lipid ME (Expt 2). Thus, dietary energy level and energetic source are important for circulatory LOOH with a role of vascular activity in production of oxidant.

Conception and development of a bibliographic database of blood nutrient fluxes across organs and tissues in ruminants: data gathering and management prior to meta-analysis

In the organism, nutrient exchanges among tissues and organs are subject to numerous sources of physiological or nutritional variation, and the contribution of individual factors needs to be quantified before establishing general response laws. To achieve this, meta-analysis of data from publications is a useful tool. The objective of this work was to develop a bibliographic database of nutrient fluxes across organs and tissues of ruminant animals (Flora) under Access using the Merise method. The most important criteria for Flora were the ease to relate the various information, the exhaustivity and the accuracy of the data input, a complete description of the diets, taking into account the methods of the methodological procedures of measurement and analysis of blood nutrients and the traceability of the information. The conceptual data model was built in 6 parts. The first part describes the authors and source of publication, and the person in charge of data input. It clearly separates and identifies the experiments, the groups of animals and the treatments within a publication. The second part is concerned with feeds, diets and their chemical composition and nutritional value. The third and fourth parts describe the infusion of any substrates and the methods employed, respectively. The fifth part is devoted to the results of blood flows and nutrient fluxes. The sixth part gathers miscellaneous experimental information. All these parts are inter-connected. To model this database, the Merise method was utilised and 26 entities and 32 relationships were created. At the physical level, 93 tables were created, corresponding, for the majority, to entities and relationships of the data model. They were divided into reference tables (n = 65) and data tables (n = 28). Data processing was developed in Flora and included the control of the data, generic calculations of unknown data from given data, the automation of the estimation of the missing data or the chemical composition of the diets. It also included the construction of tables for meta-analyses and the study of the variations of several factors within publications (pre-coding of meta-analyses). Overall, the system was built to facilitate the gathering, input, validation, management and retrieval of data from publications.

Digestion and nutrient net fluxes across the rumen, and the mesenteric- and portal-drained viscera in sheep fed with fresh forage twice daily: net balance and dynamic aspects

Digestion and portal net flux of nutrients were studied in sheep fed twice daily with fresh orchard-grass. Digestive flows were measured in six fistulated sheep using the double-marker technique. Three sheep were fitted with catheters and blood-flow probes, allowing nutrient net flux measurements across the portal-drained viscera (PDV), the mesenteric-drained viscera (MDV) and the rumen. Total tract apparent digestion of N was similar to portal net appearance of N, calculated as the sum of free amino acids (FAA), peptide amino acids (PAA), NH3, and urea net fluxes. PAA accounted for 25 % of non-protein amino acid net release across the PDV. With the exception of glycine and glutamate, the small intestine was the main contributor to this PAA net release. The essential amino acid (EAA) apparent disappearance between the duodenum and the ileum was lower than the net appearance of EAA (FAA + PAA) across the MDV. The value of PDV:MDV flux of free EAA was, on average, 78 %. The rumen accounted for 30 % of the net uptake of EAA by the PDV tissues not drained by the mesenteric vein. Rumen net release of acetate, propionate, butyrate, 3-hydroxybutyrate, and lactate accounted for 70, 55, 46, 77 and 52 %, respectively, of their portal net releases. Conversely, the small intestine was a net consumer of arterial acetate and 3-hydroxybutyrate. Dynamic study of nutrient net fluxes across the PDV showed that throughout a feeding cycle, the liver faced a constant flux of amino acids (AA), whereas volatile fatty acid and NH3 net fluxes varied in response to the meal. The present study specified, in forage-fed sheep, the partitioning of nutrient net fluxes across the PDV and the role of peptides in portal net release of AA.

Ryegrass-based diet and barley supplementation: partition of energy-yielding nutrients among splanchnic tissues and hind limbs in finishing lambs

Splanchnic metabolism of energy-yielding nutrients and their uptake by the hind limb were studied in finishing lambs receiving ryegrass harvested at grazing stage (ear at 10 cm) with or without barley supplementation. Six ruminally cannulated and multicatherized lambs (40.2 +/- 1.5 kg) were fed with frozen ryegrass (RG) at 690 kJ of metabolizable energy intake (MEI) x d(-1) x BW(-0.75) successively with and without barley supplementation (RG + B), according to a triplicated Latin square design. Barley supplementation represented 21% of DM intake and increased the MEI by 32% (P < 0.002). In ruminal fluid, barley supplementation increased the acetate and butyrate concentrations by 21.2 and 49.6%, respectively (P < 0.04), without modifying those of propionate. Thus, molar proportions of acetate and butyrate were not modified, and those of propionate tended (P < 0.06) to decrease from 26 to 23%. As a result, the net portal appearance of propionate was not modified. Net portal appearance of butyrate and beta-hydroxybutyrate increased (P < 0.03), and that of acetate was not modified. Consequently, hepatic uptake of butyrate increased and probably spared acetate from hepatic metabolism. The hepatic fractional extraction of propionate decreased (P < 0.03), whereas the net flux of lactate switched from a net release to a net uptake, suggesting an alteration in the contribution of gluconeogenic substrates to glucose synthesis without modification in net hepatic glucose release. As a consequence, barley supplementation increased net splanchnic release of acetate (P < 0.02), propionate (P < 0.001), and beta-hydroxybutyrate (P < 0.01) by 60, 157, and 78%, respectively. In addition, the net splanchnic release of insulin increased (P < 0.03) because of a decrease (P < 0.02) in its hepatic extraction. Despite those changes, the net uptake of nutrients by the hind limb was not modified and even decreased in the case of glucose (P < 0.02), suggesting a stimulation of lipogenesis in adipose tissues. Results from the present study suggested that supplementation of a ryegrass-based diet would likely have little effect on the orientation of muscle energy metabolism and on meat quality because the net uptake of nutrients by the hind limb was unchanged.

Absorption and metabolism of short-chain fatty acids in ruminants

Short-chain fatty acids (SCFA), viz. acetate, propionate and butyrate are quantitatively important substrates in ruminant energy metabolism. In the reviewed literature, 16 44% of ME intake was recovered as portal appearance of SCFA. This is considerably lower than expected when related to the estimated intragastric flux of SCFA. The discrepancy is caused by portal drained viscera metabolism of arterially abundant metabolites e.g., acetate and the metabolism of acetate and butyrate to acetoacetate and D-3-hydroxybutyrate in the absorptive epithelia. Even though considerable variations between experiments on acetate and propionate appearance are found, there seems to be a great deal of evidence that the proportion of gastrointestinally produced acetate and propionate absorbed to the portal blood is 50-75%. The portal recovery of butyrate has been found to be between 10 and 36% dependent on intraruminal infusion rate. It is concluded that major parts of acetate and propionate are directly absorbed to the portal vein. The true absorption rate of acetate can only be estimated by taking the portal drained viscera metabolism of arterial acetate into account. Butyrate is generally found to have a low recovery in the portal vein, but the production of D-3-hydroxybutyrate seems to be underestimated in major parts of the literature. It is therefore necessary to measure portal appearance as well as portal drained viscera metabolism to assess the quantitative as well as the qualitative contribution of SCFA and SCFA metabolites to whole animal metabolism.