The utilization of acetic, propionic and butyric acids by growing heifers

Metabolic Hydrogen Flows in Rumen Fermentation: Principles and Possibilities of Interventions

Rumen fermentation affects ruminants productivity and the environmental impact of ruminant production. The release to the atmosphere of methane produced in the rumen is a loss of energy and a cause of climate change, and the profile of volatile fatty acids produced in the rumen affects the post-absorptive metabolism of the host animal. Rumen fermentation is shaped by intracellular and intercellular flows of metabolic hydrogen centered on the production, interspecies transfer, and incorporation of dihydrogen into competing pathways. Factors that affect the growth of methanogens and the rate of feed fermentation impact dihydrogen concentration in the rumen, which in turn controls the balance between pathways that produce and incorporate metabolic hydrogen, determining methane production and the profile of volatile fatty acids. A basic kinetic model of competition for dihydrogen is presented, and possibilities for intervention to redirect metabolic hydrogen from methanogenesis toward alternative useful electron sinks are discussed. The flows of metabolic hydrogen toward nutritionally beneficial sinks could be enhanced by adding to the rumen fermentation electron acceptors or direct fed microbials. It is proposed to screen hydrogenotrophs for dihydrogen thresholds and affinities, as well as identifying and studying microorganisms that produce and utilize intercellular electron carriers other than dihydrogen. These approaches can allow identifying potential microbial additives to compete with methanogens for metabolic hydrogen. The combination of adequate microbial additives or electron acceptors with inhibitors of methanogenesis can be effective approaches to decrease methane production and simultaneously redirect metabolic hydrogen toward end products of fermentation with a nutritional value for the host animal. The design of strategies to redirect metabolic hydrogen from methane to other sinks should be based on knowledge of the physicochemical control of rumen fermentation pathways. The application of new –omics techniques together with classical biochemistry methods and mechanistic modeling can lead to exciting developments in the understanding and manipulation of the flows of metabolic hydrogen in rumen fermentation.

The efficiency of utilization of metabolizable energy and apparent absorption of amino acids in sheep given spring- and autumn-harvested dried grass

Three experiments were conducted with sheep given spring-harvested dried grass (SHG) and autumn-harvested dried grass (AHG). The first was a calorimetric trial to determine the metabolizable energy (ME) content of each grass and the efficiency with which sheep utilize their extra ME intakes above the maintenance level of intake. The second examined the relative amounts of extra non-ammonia-nitrogen (NAN) and individual amino acids absorbed from the small intestine per unit extra ME intake as the level of feeding was raised from energy equilibrium (M) to approximately 1.5 M. The third was a further calorimetric trial to investigate the effect of an abomasal infusion of 30 g casein/d on the efficiency of utilization of AHG. The ME content of the SHG (11.8 MJ/kg dry matter (DM] was higher than that of AHG (10.0 MJ/kg DM). The efficiency of utilization of ME for productive purposes (i.e. above the M level of intake; kf) was higher when given SHG (kf 0.54 between M and 2 M) than when given AHG (kf 0.43 between M and 2 M). As the level of intake of each grass was raised from M to 1.5 M there was a greater increment in the amounts of NAN (P less than 0.001) and the total amino acid (P less than 0.05) absorbed from the small intestines when sheep were given the SHG (NAN absorption, SHG 5.4 g/d, AHG 1.5 g/d, SED 0.54; total amino acid absorption SHG 31.5 g/d, AHG 14.3 g/d, SED 5.24). Infusion of 30 g casein/d per abomasum of sheep given AHG at M and 1.5 M levels of intake increased (P less than 0.05) the efficiency of utilization of the herbage from kf 0.45 to kf 0.57. Consideration is given to the possibility that the higher efficiency of utilization of ME in sheep given SHG may be related to the amounts of extra glucogenic amino acids absorbed from the small intestine which provide extra reducing equivalents (NADPH) and glycerol phosphate necessary for the conversion of acetate into fatty acids.

Simulation of the metabolism of absorbed energy-yielding nutrients in young sheep

A mathematical model is described, which simulates the metabolism of absorbed nutrients (amino acids, acetic acid, butyric acid, glucose, lipid and propionic acid) in growing sheep. The basic assumption of the model is that each nutrient is partitioned between synthetic, oxidative or intermediate reactions with rates of reaction which are described using enzyme kinetics. These rates depend on the relationship between maximum reaction rates, constants of affinity and inhibition and the concentrations of metabolites as determined by the model. Synthetic reactions calculate fat and protein deposition while intermediate reactions involve the production of ATP and NADPH. There is a total of twelve state variables and the model, programmed in CSMP and ACSL, is solved by integration of twelve differential equations. The model calculates the efficiency of utilization of metabolizable energy for different nutrient inputs and the results may be interpreted in terms of fluxes through the metabolite pools. Simulations using inputs representing forage- and concentrate-based diets indicated decreased efficiency for the forage at high levels of intake and possible reasons for this were further studied in simulations where the inputs of protein and glucose were varied.

Net absorption and ruminal concentrations of metabolites in nonpregnant dry Holstein cows before and after intraruminal acetic acid infusion

Objectives were to define daily patterns of net absorption of various nutrients and to assess effects of intraruminal infusion of acetic acid on concentrations of ruminal fluid and net absorption of various metabolites. These characteristics were measured in three nonpregnant, dry Holstein cows (491 kg) at hourly intervals for 24 h before and after 5 days of intraruminal infusion of acetic acid to provide energy equal to 10% of daily intake of metabolizable energy. Cows were fed a completely mixed, 60% corn silage, 40% grain supplement diet at maintenance intake; daily rations were split into two feedings. Net rates of absorption were greatest after feeding and least during early morning. Net absorption of all metabolites measured was similar for the two daily feeding intervals, indicating daily net absorption could be calculated from either feeding interval. Intraruminal infusion of acetic acid caused increased ruminal and plasma concentrations of acetate, increased net absorption of acetate, and almost a twofold increased loss of urea-nitrogen from plasma to the gut. Sixty-nine percent of acetic acid infused was accounted for by increased net absorption of acetate. Ruminal fluid concentrations and net absorption of other volatile fatty acids were not affected by infusion of acetic acid. Net absorptions of ammonia-nitrogen, L-lactate, and glucose likewise were not affected. Net absorption of volatile fatty acid and L-lactate was 43% of daily intake of metabolizable energy.

[Nitrogen balance and nitrogen utilization of non-lactating and lactating cows under various feeding frequency conditions]

A cross-over trial with nine nonlactating nonpregnant cows (experiment I) and a single-reversal trial with eight lactating cows (experiment II) were conducted to study the effect of feeding frequency upon nitrogen balance. The three treatments in experiment I were twice daily forage and concentrate, twice forage and eight times concentrate, and ten times forage and concentrate per day. In experiment II the animals received their forage and concentrate in two equal feedings daily or the forage in two and the concentrate in six meals per day. The diets had a severely restricted crude fiber content of only 16% in the dry matter of the total ration. In experiment I feeding frequency had no effect on fecal or urinary nitrogen excretion, nitrogen retention and nitrogen utilization. In experiment II, the urinary nitrogen excretion was significantly decreased by 9% in response to feeding concentrate six times daily. Fecal and milk nitrogen excretion as well as nitrogen retention and utilization criteria, however, were not significantly different for the two feeding frequencies.

The utilization of diets containing acetate salts by growing lambs as measured by comparative slaughter and respiration calorimetry, together with rumen fermentation

1. In a comparative slaughter experiment, growing lambs were given concentrate diets in which 14 or 19% metabolizable energy (ME) provided by barley was replaced by sodium, calcium and potassium salts of acetic acid. As the proportion of ME as acetate was increased, energy retention decreased. ME intake was 9271, 9430, and 9217 +/- 67 kJ/d and energy retention was 2698, 2422 and 2280 +/- 71 kJ/d for the diets containong 0, 14 or 19% ME as acetate respectively. There were no differences in protein deposition. The efficiency of utilization of acetate for energy retention (kf) was calculated by difference to be 3 and 10 +/- 13% respectively for the diets containing 14 and 19% ME as acetate.

Intravenous energy infusion in lambs: effects on nitrogen retention, plasma free amino acids and plasma urea nitrogen

1. Acetate, propionate, butyrate and glucose were each administered intravenously (2.09 MJ (0.5 Mcal)/d) to lambs on either high-concentrate (HC) or high-roughage (HR) diets in two experiments with 4 × 4 Latin square design. During a control period all animals received a saline infusion.

2. In lambs on the HC diet glucose was the most effective metabolite tested for improving nitrogen retention. Acetate was 51%, butyrate, 75%, propionate, 90% as effective as glucose in promoting N retention. Propionate infusion was most effective in increasing N retention in lambs on the HR diet. Acetate was 60%, butyrate, 75%, and glucose, 80% as effective as propionate. Glucose infusion did not lower plasma urea N (PUN) concentration in lambs receiving the HC diet, but the three volatile fatty acids (VFA) did lower it (P< 0.01). All energy sources significantly reduced the PUN concentrations with the HR diet, but differences between sources were not significant.

3. Plasma concentrations of valine, isoleucine, leucine and phenylalanine were depressed to 57, 71, 89 and 90% of their control values during glucose, propionate, butyrate and acetate infusion. Plasma concentrations of lysine and methionine were significantly decreased from control values (P< 0.05) but there was no significant difference between energy sources. Lysine, valine, isoleucine and leucine were significantly (P< 0.01) higher in lambs given the HR diet than in lambs given the HC diet. The concentration of alanine was greater (P< 0.05) during glucose infusion than during acetate and butyrate infusions, indicating net synthesis of alanine and possibly other non-essential amino acids during glucose infusion, since the plasma free amino acid N concentration increased despite decreased concentrations of plasma essential amino acids during glucose infusion.

4. The results indicated that glucose and propionate were superior to acetate and butyrate as energy sources for protein formation.

5. The poor relationship between PUN and N balance and plasma free essential amino acids suggested that PUN concentration may have been inappropriate to assess the protein status under some dietary conditions.

Utilization of volatile fatty acids and glucose for protein deposition in lambs

1. Glucose, and acetic, propionic and butyric acids, the major energy sources available to ruminants, have been comparatively evaluated as energy sources for growth in lambs. The energy sources were administered intravenously at 2·092 and 2·510 MJ metabolizable energy per 24 h for periods of 9–12 d. Nitrogen was given in excess of requirement so that growth rate was limited by energy, as indicated by the increased N retention after intravenous administration of energy.

2. The N-balance results from the trials showed that propionic acid promoted a significantly (P < 0·01) higher N retention than acetic acid. In some of the experiments there were no significant differences between propionic and butyric acids, and in others, no significant differences between acetic and butyric acids. Compared with isoenergetic glucose infusion, all volatile fatty acids were less efficiently utilized than glucose. The mean values for N balance (g/d) were 2·31 during control infusion, 6·53, 5·71, 5·48 and 4·59 during glucose, propionate, butyrate and acetate infusions, respectively.

3. All energy sources reduced the faecal N excretion significantly. The greatest reduction was observed during butyrate treatment. The mean values for faecal N excretion (g/d) were 4·24 for control, 4·00 for acetate, 3·89 for propionate, 3·83 for glucose, and 3·76 for butyrate infusion. The reduced faecal N excretion after butyrate infusion partly accounts for the increased N retention with that treatment.

Studies of the food intake of sheep at a single meal

1. Experiments are described in which the food intake and the water intake of sheep at a single meal were measured. The sheep were offered lucerne chaff ad lib. for 2 h only each day.

2. Following the peritoneal infusion of physiological saline the food intake increased, while the injection of a diuretic before feeding caused the food intake to be decreased.

3. In eight experiments the osmolality of the ruminal liquor was increased by the addition to the rumen of NaCl, KCl or the salts of volatile fatty acids in 250 ml water. The decrease in food intake was related to the osmolality, but not to the energy content, of the added electrolyte solution. In a further experiment, sheep receiving a highly digestible lucerne chaff, containing 1% (w/w) NaCl, increased their food intake when water was added to the rumen. Other workers have concluded that gastric osmolality is an important variable in the control of food intake in monogastric animals. The results of the eight experiments now described suggest that ruminal osmolality is of similar importance in ruminants.

4. It is suggested that all these observations are consistent with the theory, which has been proposed for non-ruminant animals, that the food intake at a single meal may be related to the degree of tissue hydration at the beginning of the meal.

Utilization of salts of volatile fatty acids by growing sheep. IV. Effects of type of fermentation of the basal diet on the utilization of salts of volatile fatty acids for nitrogen retention and body gains

1. Two experiments are reported in which sodium and calcium salts of volatile fatty acids (VFA) were given to lambs as additions to two basal dietsof hay andconcentrate. The two diets induced distinctly different types of rumen fermentation. A low-concentrate diet gave higher acetic and lower butyric acid proportions in the rumen contents than a high-concentrate diet. The two diets were given to lambs to provide equal intake of digestible organic matter. In one change-over experiment with twelve sheep the nitrogen retention was determined, and in a second experiment involving sixty-four lambs the efficiency of the VFA salt to promote body-weight gain was estimated. 2. The estimated metabolizable energy (ME) from the high-concentrate diet was utilized significantly more efficiently than that from the low-concentrate diet to promote positive nitrogen retention and to increase empty body-weight and carcass weight. These differences in final live weight were not significant owing to the greater content of the alimentary tract of lambs given the low-concentrate diet. 3. On both diets the lambs receiving the additions of VFA salts grew faster and produced significantly greater empty body and carcass weights than the lambs receiving only the basal diets. 4. For promoting carcass gain the energy derived from the VFA salts appeared to be utilized more efficiently than the calculated ME above maintenance of the two basal diets. 5. There were no significant differences between the efficiency of utilization of the energy from acetate, propionate and butyrate to promote carcass gains. There was, however, a tendency for the response to acetate to be greater on the high-concentrate diet than on the low-concentrate diet. 6. With a low level of feeding but with positive N balance, addition of acetate resulted in a greater N retention than that of propionate with both diets. 7. It is concluded that if the reported low utilization of the ME of diets high in roughage is related to the proportion of acetic acid in the rumen contents, then the implied losses of energy may be associated with the formation of acetic acid rather than its utilization.

Utilization of salts of volatile fatty acids by growing sheep. I. Acetate, propionate and butyrate as sources of energy for young growing lambs

1. An experiment was conducted in which sodium and calcium salts of acetic, propionic and butyric acids were given to groups of eight lambs as additions to basal diets of hay and concentrate. Two control groups were included, one group which received only the basal ration and one high-level control group which received sufficient additional concentrate to achieve growth rates greater than those of the groups receiving volatile fatty acid (VFA) salts. 2. With rumen-fistulated sheep, the effect on the rumen VFA composition and the pH of the rumen liquor, of diets supplemented with VFA salts given twice daily, was also investigated and compared with the effect of the basal diet alone. 3. The lambs receiving the VFA salts grew faster and achieved significantly greater empty body and carcass weights than those receiving only the basal rations; the high-level control group had significantly greater empty body and carcass weights than groups receiving salts of VFA. 4. There were no differences approaching significance in the efficiency of the three VFA in promoting gains in live weight, empty body weight and carcass weight. There appeared to be equal efficiency of utilization of the gross energy of the VFA and of the calculatedmetabolizable energy of the concentrates. 5. The effect of twice-daily feeding on the rumen VFA composition was that the concentration of the supple- mentary acid was greatest shortly after feeding, and about 5–6 h after feeding the effect was difficult to detect. There were no differences in the pH of the rumen liquor between animals given diets containing VFA salts and those given the basal ration.

The relationship in the cow between milk-fat secretion and ruminal volatile fatty acids

1. The effect of reducing the hay and of increasing the proportions of concentrate and flaked maize in the diet of cows on the secretion of milk fat and its component fatty acids and on the proportions of volatile fatty acids in the rumen liquor has been studied. 2. The low-hay diet caused a fall in the milk fat content to about half of the values in the initial control period and the secretion of all the major fatty acids in the milk was reduced. The general pattern of change in the proportions of rumen VFA during the change to the low-hay diet was a decrease in acetic acid, an increase in propionic and n-valeric acids, relatively little change in n-butyric acid and also an increase in the concentration of lactic acid. Approximately 60% of the variation in milk fat content during the change of diet was associated with the increase in propionic acid. 3. In cows established on the low-hay diet there were marked variations in the relative proportions of acetic and propionic acids, but there was no related change in milk fat content. On return of the animals to the high-hay diet, recovery of the initial proportions of acetic, propionic and valeric acids occurred within about 4 days but the recovery in milk fat content was not complete until 2–3 weeks had elapsed. 4. Although the intraruminal infusion of acetic acid in cows on the low-hay diet caused increases and decreases respectively in the molar proportions of acetic and propionic acids in the rumen, an increase in milk fat content was observed amounting to only one-quarter of the loss associated with the transfer to the low-hay diet; therem was no characteristic pattern of increase in the yields of the individual fatty acids of milk fat. No consistent effects of intraruminal infusions of butyric acid, in cows on the low-hay diet, on the yield of milk fat or of the individual fatty acids were observed.