Appetite, 1992, 19, 293-301
Effects of lntraruminal Infusions of Sodium Acetate and Sodium Chloride on Silage Intake by Lactating Cows J. M. FORBES, J. N. MBANYA
and M. H. ANIL
Department of Animal Physiology and Nutrition, University of Leeds
Lactating dairy cows prepared with rumen fistulas were fed on grass silage and concentrates and used in two experiments to compare the effects of sodium acetate and sodium chloride infused over 3 h into the rumen on the voluntary intake of silage. Silage intake was depressed in an approximately linear manner by increasing amounts (6-15 mol) of sodium acetate (NaOAc) and 15 mol NaCl had an effect similar to that of 12mol NaOAc. Sodium in rumen fluid was significantly correlated with intake as was osmolality. 5.5, 7.4 or 9.1 mol of NaOAc significantly depressed silage intake, while 7.4 and 9.1 mol NaCl had significant effects. There were significant negative relationships between intake and the level of NaOAc or NaCl. It is concluded that the major effect of either salt was via the elevation of osmolality of rumen fluid and the relevance to normal control of feeding is discussed.
INTRODUCTION
Ruminant animals have capacious stomachs in which food is subject to microbial fermentation, the major products being short-chain fatty acids, particularly acetic. As this is the major product of digestion and it is absorbed to provide energy in the body it has been widely studied as a potential feedback in the control of voluntary feed intake (Forbes, 1986). Most of the experimental work in this area has been carried out with mature male sheep and it is generally accepted that the results are applicable to cattle. However, there are likely to be some differences in the response between sheep and cattle, especially when the latter are lactating. The experiments reported here are part of a series carried out with lactating cows (Anil et al., 1992; Mbanya et al., 1992) in which the object was to explore the possibility that the reduction in feed previously seen when sodium acetate was infused into the rumen was due primarily to effects of osmotic pressure or sodium rather than specifically to acetate. It has been shown in many experiments that intraruminal infusions of sodium acetate depress voluntary intake and this has been used as evidence to support the concept that acetic acid plays an important part in intake control (Baile & Forbes, 1974). However, Ternouth (1967) and Bergen (1972) had already shown that
The authors are pleased to acknowledge the assistance of R. Harrison with the management of the cows, P. Pillay for volatile fatty acid analyses and the Agricultural and Food Research Council for financial support. Dr Mbanya was on leave from the Institute of Animal Research, Bambui, Cameroon. Dr Anil’s present address is Department of Meat Animal Science, University of Bristol, Langford, BS18 7DY. Address correspondence to: Professor J. M. Forbes, Department of Animal Physiology and Nutrition, University of Leeds, Leeds LS2 9JT. 01954663/92/060293+09
$OWO/O
0 1992 Academic Press Limited
294
J. M. FORBES
ET AL.
equivalent amounts of sodium chloride also depressed intake and suggested that, under some circumstances at least, elevation of the osmolality of rumen contents inhibited feeding. In the meantime, Leek (1986) and colleagues have demonstrated receptors in the rumen mucosa that were sensitive to acids but not consistently to changes in osmolality (B. F. Leek, personal communication), so that the acetate hypothesis was not rejected. In recent years the importance of osmolality in the control of feed intake has been raised again (Grovum, 1987). The voluntary intake of ensiled forage by ruminants is often lower than that of the same crop fed fresh or as hay (Gordon et al., 1961; Harris & Raymond, 1964). Infusion of the products of fermentation in silage into the rumen have resulted in reductions in voluntary intake in several studies (Thomas et al., 1961; Phillip et al., 1981; Clancy et al., 1977) but it is not known whether osmolality, specific compounds such as amines, or pH is the major factor. We had made infusions of sodium acetate and sodium propionate into the rumen of cows fed on hay or silage and shown dose-dependent depressions of intake (Anil et al., 1992). However, we used water infusion as our control and had no way of determining whether some or all of the observed effects were due to the changes in osmotic pressure or sodium induced in rumen fluid. Two experiments were therefore performed in which the effects of sodium chloride were compared with those of sodium acetate infusion into the rumen of lactating cows and rumen fluid osmolalities were monitored. The first has been briefly reported by Mbanya et al. (1988).
METHODS
The animals and their management (1992).
have been described in detail by Anil et al.
Experiment 1 Six rumen-fistulated Friesian cows in late lactation (mean milk yield = 8.2 kg/day) and 7 months pregnant were used in a 7 x 7 Latin square design with one column omitted. Cows were milked once daily in the morning. They were kept in individual stalls and given 2 h exercise each morning. Silage was offered to the cows ad libitum, plus a daily total of 3 kg concentrate supplement (Anil et al., 1992) given at milking. The cows had free access to drinking water and to trace mineralized salt licks but intake of neither was monitored. Silage refusals were collected and weighed at the end of the 3-h infusions and again 2 h after infusions. Each cow received seven treatments according to the design, each one given during a 3 h period from 1000 hrs to 1300 hrs with at least 2 days between successive treatments. The treatments were: no infusion; water; 6,9, 12 and 15 moles of sodium acetate (NaOAc); 15 moles of sodium chloride (NaCl). All solutions were made up in 4.5 litres of warm water. Rumen fluid samples were taken at hourly intervals for the analysis of sodium concentrations and osmolality. Silage was sampled on each day of the experiment for the analysis of chemical composition: dry matter (DM, g/ kg), 190; crude protein (g/kg DM) 143; modified acid detergent fibre (g/kg DM),
EFFECTS
OF SODIUM SALTS ON SILAGE
INTAKE
295
408; ether extract (g/kg DM), 43; ash (g/kg DM), 97; pH, 4.1; calculated metabolizable energy, (MJ/kg DM), 9.3. The results were analysed by the generalized linear models procedure (SAS, 1985).
Experiment 2
Seven rumen-fistulated Friesian cows in late lactation (mean milk yield= 16 kg/day) and 4-5 months pregnant were used in an experiment of 7 x 7 Latin square design and managed in the same way as in Experiment 1. They were given daily supplements of 3 kg concentrate ration and 2 kg sugar beet pulp, as described by Anil et al., (1992) in equal portions at 0700 and 1600 hrs. The treatments infused over a 3-h period from 1100 to 1400 hrs were: water (control), 5.5, 7.4 and 9.1 moles NaCl, 5.5, 7.4 and 9.1 moles NaOAc. The range of doses was chosen so as to fall closer to the “physiological” range and our interest was to compare the responses to the same range of doses of NaOAc and NaCl, rather than to seek clearly significant effects of any particular dose. Samples of rumen fluid and silage were taken as in Experiment 1 and rumen samples were additionally analysed for short chain fatty acid (SCFA) concentrations. The composition of the silage was: dry matter (DM, g/kg), 202; crude protein (g/kg DM) 148; modified acid detergent fibre (g/kg DM), 289; ether extract (g/kg DM), 42; ash (g/kg DM), 83; pH, 3.9; calculated metabolizable energy, (MJ/kg DM), 10.8. Data were analysed using analysis of variance with Duncan’s Multiple Range Test and by regression analysis (SAS, 1985).
RESULTS
Experiment
1
Voluntary feed intake
There was a significant effect of 15 mol of NaOAc compared to that of water (Table 1). The relationship between silage intake (I, kg DM/3 h) and the amount of acetate infused (AC, mo1/3 h) was: 1=2*36-0.072
(SE 0,022) AC
(r=O-62,p
Effects of lntraruminal Infusions of Sodium Acetate and Sodium Chloride on Silage Intake by Lactating Cows J. M. FORBES, J. N. MBANYA
and M. H. ANIL
Department of Animal Physiology and Nutrition, University of Leeds
Lactating dairy cows prepared with rumen fistulas were fed on grass silage and concentrates and used in two experiments to compare the effects of sodium acetate and sodium chloride infused over 3 h into the rumen on the voluntary intake of silage. Silage intake was depressed in an approximately linear manner by increasing amounts (6-15 mol) of sodium acetate (NaOAc) and 15 mol NaCl had an effect similar to that of 12mol NaOAc. Sodium in rumen fluid was significantly correlated with intake as was osmolality. 5.5, 7.4 or 9.1 mol of NaOAc significantly depressed silage intake, while 7.4 and 9.1 mol NaCl had significant effects. There were significant negative relationships between intake and the level of NaOAc or NaCl. It is concluded that the major effect of either salt was via the elevation of osmolality of rumen fluid and the relevance to normal control of feeding is discussed.
INTRODUCTION
Ruminant animals have capacious stomachs in which food is subject to microbial fermentation, the major products being short-chain fatty acids, particularly acetic. As this is the major product of digestion and it is absorbed to provide energy in the body it has been widely studied as a potential feedback in the control of voluntary feed intake (Forbes, 1986). Most of the experimental work in this area has been carried out with mature male sheep and it is generally accepted that the results are applicable to cattle. However, there are likely to be some differences in the response between sheep and cattle, especially when the latter are lactating. The experiments reported here are part of a series carried out with lactating cows (Anil et al., 1992; Mbanya et al., 1992) in which the object was to explore the possibility that the reduction in feed previously seen when sodium acetate was infused into the rumen was due primarily to effects of osmotic pressure or sodium rather than specifically to acetate. It has been shown in many experiments that intraruminal infusions of sodium acetate depress voluntary intake and this has been used as evidence to support the concept that acetic acid plays an important part in intake control (Baile & Forbes, 1974). However, Ternouth (1967) and Bergen (1972) had already shown that
The authors are pleased to acknowledge the assistance of R. Harrison with the management of the cows, P. Pillay for volatile fatty acid analyses and the Agricultural and Food Research Council for financial support. Dr Mbanya was on leave from the Institute of Animal Research, Bambui, Cameroon. Dr Anil’s present address is Department of Meat Animal Science, University of Bristol, Langford, BS18 7DY. Address correspondence to: Professor J. M. Forbes, Department of Animal Physiology and Nutrition, University of Leeds, Leeds LS2 9JT. 01954663/92/060293+09
$OWO/O
0 1992 Academic Press Limited
294
J. M. FORBES
ET AL.
equivalent amounts of sodium chloride also depressed intake and suggested that, under some circumstances at least, elevation of the osmolality of rumen contents inhibited feeding. In the meantime, Leek (1986) and colleagues have demonstrated receptors in the rumen mucosa that were sensitive to acids but not consistently to changes in osmolality (B. F. Leek, personal communication), so that the acetate hypothesis was not rejected. In recent years the importance of osmolality in the control of feed intake has been raised again (Grovum, 1987). The voluntary intake of ensiled forage by ruminants is often lower than that of the same crop fed fresh or as hay (Gordon et al., 1961; Harris & Raymond, 1964). Infusion of the products of fermentation in silage into the rumen have resulted in reductions in voluntary intake in several studies (Thomas et al., 1961; Phillip et al., 1981; Clancy et al., 1977) but it is not known whether osmolality, specific compounds such as amines, or pH is the major factor. We had made infusions of sodium acetate and sodium propionate into the rumen of cows fed on hay or silage and shown dose-dependent depressions of intake (Anil et al., 1992). However, we used water infusion as our control and had no way of determining whether some or all of the observed effects were due to the changes in osmotic pressure or sodium induced in rumen fluid. Two experiments were therefore performed in which the effects of sodium chloride were compared with those of sodium acetate infusion into the rumen of lactating cows and rumen fluid osmolalities were monitored. The first has been briefly reported by Mbanya et al. (1988).
METHODS
The animals and their management (1992).
have been described in detail by Anil et al.
Experiment 1 Six rumen-fistulated Friesian cows in late lactation (mean milk yield = 8.2 kg/day) and 7 months pregnant were used in a 7 x 7 Latin square design with one column omitted. Cows were milked once daily in the morning. They were kept in individual stalls and given 2 h exercise each morning. Silage was offered to the cows ad libitum, plus a daily total of 3 kg concentrate supplement (Anil et al., 1992) given at milking. The cows had free access to drinking water and to trace mineralized salt licks but intake of neither was monitored. Silage refusals were collected and weighed at the end of the 3-h infusions and again 2 h after infusions. Each cow received seven treatments according to the design, each one given during a 3 h period from 1000 hrs to 1300 hrs with at least 2 days between successive treatments. The treatments were: no infusion; water; 6,9, 12 and 15 moles of sodium acetate (NaOAc); 15 moles of sodium chloride (NaCl). All solutions were made up in 4.5 litres of warm water. Rumen fluid samples were taken at hourly intervals for the analysis of sodium concentrations and osmolality. Silage was sampled on each day of the experiment for the analysis of chemical composition: dry matter (DM, g/ kg), 190; crude protein (g/kg DM) 143; modified acid detergent fibre (g/kg DM),
EFFECTS
OF SODIUM SALTS ON SILAGE
INTAKE
295
408; ether extract (g/kg DM), 43; ash (g/kg DM), 97; pH, 4.1; calculated metabolizable energy, (MJ/kg DM), 9.3. The results were analysed by the generalized linear models procedure (SAS, 1985).
Experiment 2
Seven rumen-fistulated Friesian cows in late lactation (mean milk yield= 16 kg/day) and 4-5 months pregnant were used in an experiment of 7 x 7 Latin square design and managed in the same way as in Experiment 1. They were given daily supplements of 3 kg concentrate ration and 2 kg sugar beet pulp, as described by Anil et al., (1992) in equal portions at 0700 and 1600 hrs. The treatments infused over a 3-h period from 1100 to 1400 hrs were: water (control), 5.5, 7.4 and 9.1 moles NaCl, 5.5, 7.4 and 9.1 moles NaOAc. The range of doses was chosen so as to fall closer to the “physiological” range and our interest was to compare the responses to the same range of doses of NaOAc and NaCl, rather than to seek clearly significant effects of any particular dose. Samples of rumen fluid and silage were taken as in Experiment 1 and rumen samples were additionally analysed for short chain fatty acid (SCFA) concentrations. The composition of the silage was: dry matter (DM, g/kg), 202; crude protein (g/kg DM) 148; modified acid detergent fibre (g/kg DM), 289; ether extract (g/kg DM), 42; ash (g/kg DM), 83; pH, 3.9; calculated metabolizable energy, (MJ/kg DM), 10.8. Data were analysed using analysis of variance with Duncan’s Multiple Range Test and by regression analysis (SAS, 1985).
RESULTS
Experiment
1
Voluntary feed intake
There was a significant effect of 15 mol of NaOAc compared to that of water (Table 1). The relationship between silage intake (I, kg DM/3 h) and the amount of acetate infused (AC, mo1/3 h) was: 1=2*36-0.072
(SE 0,022) AC
(r=O-62,p
