Abstract:
THE EFFECT OF RUMINAL HYPEROSMOLALITY ON VOLUNTARY FOOD CONSUMPTION J. H. TERNOUTH* Summary Non-pregnant Merino ewes with ruminal fistulae were offered lucerne chaff ad libitum for 2 h only each day. At the start of the feeding period they were given intra-ruminal infusions of varying quantities of NaCl, KC1 or sodium acetate in 250 ml of water. There was a decrease in feed intake and an increase in water intake with increasing levels of electrolyte infusion, i.e. with increasing ruminal osmolality. The results indicated that the osmolality of the ruminal liquor rather than the chemical properties of the infusate were important in the control of feed intake. I. INTRODUCTION Total salivary flow is highest during the period when the ruminant is feeding (Bailey 1961) and although most of the salivary glands secrete isosmolar saliva (Kay 1960), the feed boluses on entering the rumen are hyperosmolar (Bailey 1961). Fermentation of the feed further increases the osmolality of the ruminal contents and results in the inflow of plasma fluids into the rumen across the ruminal wall (Temouth 1967). Temouth (1968) reported that after feeding there was a 10% decrease in the extra-cellular space and suggested that this extra-cellular `dehydration' was a possible cause of satiety in ruminants. This paper is a report of an investigation undertaken to examine the effect of increased ruminal osmolality, on subsequent voluntary food consumption (V.F.C.). II. MATERIALS AND METHODS Six adult non-pregnant merino ewes with ruminal fistulae were used throughout these experiments. Prior to the commencement of the project they were trained to the feeding regime. The experiments were made by infusing into the rumen, in less than 1 min, known quantities of NaCl, KC1 and sodium acetate (NaOAc) dissolved in 250 ml of water immediately prior to a 2 h CFd libitum feeding period. At the start of the first and second hour of the feeding period the sheep were given 1 kg of lucerne chaff and the unconsumed residue was weighed at the conclusion of each hour period. The sheep were not fed for the remaining 22 h of the day. Water, in graduated vessels, was available at all times except when removed for experimental reasons (Experiments 3 and 4). 'Department of Animal Husbandry, Faculty of Veterinary Science, University of Queensland, St. Lucia. 369 Quantities of electrolytes (g) dissolved in 2.50 ml of water, and their calculated osmlalities, given to the ewes immediately prior to the 2 h feeding period TABLE 1 The quantities of salts given are shown in Table 1. The quantities of NaCl and KC1 were calculated to produce equal osmolar loads at the same infusion level. The NaOAc infusion levels were calculated to create twice the osmolar load, as it was desired to infuse levels of V.F.A. similar to those Wyatt (1965) found caused significant reductions of V.F.C. Each of the 5 experiments lasted 10 consecutive days and the amount of salt given to each sheep was randomized so that:1. each sheep received each level of treatment twice; 2. no two sheep had identical treatments on the same day. Individual feed and water intakes were recorded for the first and second hours of the feeding period, except in Experiments 3 and 4 where access to water was not allowed until the completion of the 2 h feeding period. In Experiments 3 and 4 water intake for the 15 min immediately following the feeding period was recorded. One hour after the start of feeding, a ruminal sample was sucked from the ventral sac of the rumen of each sheep, using a stomach pump. 50 ml of this sample was centrifuged and the liquor immediately frozen for later analysis; the remainder of the sample was returned to the rumen. The osmolality of each sample was determined using an osmometer.* The osmolalities, food and water intakes for each experiment were subjected to a balanced factorial analysis to remove between-day and between-sheep variantes. III. RESULTS The mean values for the osmolalities and food and water intakes at each level of treatment, are shown in Tables 2, 3, and 4. For each experiment the significance was determined of a single linear regression, relating the osmolality, food intake or water intake with the treatment level. *Advanced Instruments Inc., Massachusetts. 370 TABLE 2 Osmolality (mOsm) of the ruminal liquor* of 3 Merino ewes given intraruminal infusions of electrolytes when oflered lucerne chufl ad libitum The ruminal contents 1 h after feeding were all hyperosmolar with respect to normal plasma (Ternouth, 1968) and the ruminal osmolality was significantly increased with the increased quantity of electrolyte infused. When water was not available the osmolality of the ruminal liquor was higher than when water was available. Infusions of NaOAc resulted in a far greater osmolar increase, 1 h after the infusion, than did the infusions of NaCl or KCl. TABLE 3 Feed intake (g) of Merino ewes when oflered lucerne chaff ad libitum for 2 h each &y following intra-ruminal inf usim of electrolytes 371 TABLE 4 Water intake (1) of 3 Merino ewes during the feeding period, when offered lucerne chafj ad libitum for 2 h each day following intraruminal infusion of electrolytes The total V.F.C. (except in Experiment 4) was significantly reduced with increasing infusion levels. The greater quantity of the feed was eaten in the first hour, and with the exception of Experiment 4, the feed intake during this hour was significantly reduced as the infusion level rose. In the second hour the reduction of intake with treatment level was somewhat variable and was only significant in Experiment 4. In Experiment 5 the depression of V.F.C. with increasing rate of infusion was double that in Experiments l-4 although, as different sheep were used, the results are not comparable quantitatively. The total V.F.C. of Experiments l-4 showed no significant difference between trials, although there was a tendency for the sheep with water available to consume more than those without water. No water, while feeding, caused a significantly greater depression of the feed intake during the second hour (P<O.OOil) than when water was given. In all experiments linear regression methods were applied to the total feed consumption. From this it could be calculated that if a quantity of NaCl (or its equivalent of KC1 or NaOAc) representing 1% of the V.F.C. were added to the diet of sheep under the same experimental conditions, there would be reductions in voluntary feed intake of 6.4, 7.9, 4.2, 5.5 and 4.2% respectively for Experiments 1-5. The quantity of salt infused into the rumen was positively correlated with total water intake in Experiments 1, 2 and 5, and with water consumption in the 15 min after feeding in Experiment 4. In Experiment 3 there was a higher vari372 ability in the results and the correlation was not significant. No significant difference was found in the amount of water drunk after eating in Experiments 3 and 4 compared with the total consumption while feeding in Experiments 1 and 2. IV. DISCUSSION In these experiments the salts were given directly into the rumen so the influence of oral factors in determining V.F.C. has been eliminated. This contrasts with the work of Wilson ( 1966) who added NaCl to the food or drinking water. The infusions were calculated to create equal osmolar loads when placed in the rumen. As the sheep had not been fed for the previous 22 h the osmolality of the ruminal liquor and the ruminal volume would be similar on successive days of the experiments (Hecker et al 1964; Warner and Stacy 1965; Ternouth 1967). Thus the effects of the infusions were expected to be the same for each experiment, and to be modified between experiments only by the presence or absence of water during the feeding period and the different rates of absorption of the two ions. In Experiments l-4 the measured osmolalities 1 h after the start of feeding indicate that the different ions were not an important influence on the osmolality but the absence of water normally resulted in a considerably higher ruminal osmolality being measured. Smith (1966) has concluded, based largely on his earlier findings (Smith et al 1959) that in the monogastric the 'colligative' rather than the chemical properties of solutions, injected into the stomach or peritoneal cavity, are important in determining appetite. Similar conclusions have been made by Harper and Spivey ( 1958) who also reported increases in the quantity of fluid in the stomach following the intragastric injection of hyperosmolar solutions. The high variability of the feed intake in the second hour was surprising and it is suggested that this may be the result of an increased rate of absorption of fluids from parts of the alimentary tract posterior to the reticulum, i.e. the commencement of a rehydration (Ternouth 1968). A comparison of the significance of the reduction of the intake of feed for the first and second hour and the total V.F.C. suggests some compensatory feed intake in the second hour, i.e. on some days feed intake in the second hour was increased or decreased when the intake for the first hour had been unusually low or high so the significance of the total feed intake with treatment level was increased. The increased osmolality of the ruminal contents was created almost instantaneously by the addition of the salts and the results show that the water intakes during the first hour of feeding (Experiments 1 and 2) increased with increasing infusion levels. However, the osmolality was still higher than that of plasma 1 h after the start of feeding and still increased with increasing levels of infusion. Ternouth (1967, 1968) has shown that the osmolality of plasma does not start to rise until one hour after feeding and these results suggest that osmolar receptors are not responsible for the control of water consumption. The close agreement of the water intake 15 min after feeding in Experiments 3 and 4 with the water intake during feeding in Experiments 1 and 2 also suggest that the 373 stimulus to drink is not directly associated with ruminal osmolality. Bott, Denton and Weller (1965) found that if a sheep with a raised plasma Na concentration, because of dehydration, was given water the plasma sodium level did not start to fall appreciably for 30-45 min afterwards. Thus a parameter of the volume of water in the body (excluding ruminal volume) is proposed for the control of water consumption and food satiation. The lower oesophagus and the entry of water into the rumen are both considered important in determining water satiation (Bott et al 1965). Wilson (1966) added 2% NaCl to the drinking water and found that the sheep which reduced their feed intake least were those who had the highest water intake. This duality of control is suggested to be the reason why the first hour V.F.C. and total V.F.C. in Experiment 4, and the water intake immediately after feeding in Experiment 3, were not significant. Weston ( 1966) found that the level of infused acids (given continuously over 16-24 h) caused a reduction of V.F.C. such that 'the decrease in M.E. intake . . . was of the same order as the energy supplied by the V.F.A.'. Using Weston's conversion value for the digestible energy (D.E.) content of lucerne chaff (2.6-2.8 kcal/g D.M.) it can be calculated that in the present experiments the infusion of 17.5 kcal D.E. (as NaOAc) resulted in a reduction of V.F.C. of 55-59 kcal D.E. (i.e. with the present experimental method the infusate and the reduction in V.F.C. were not of the same order). Ruminal hyperosmality has not previously been considered important in the control of V.F.C. in the ruminant (Balch and Campling 1962; Conrad 1966). From these experimental results it is postulated that high osmolar load may be responsible, at least in part, for controlling V.F.C., and the mechanism of action is considered to involve a reduction of non-gastrointestinal body water (Ternouth 1967, 1968). The practical conditions under which osmolar load would be important in limiting V.F.C. are considered to be:1. Following the consumption of highly digestible carbohydrates. 2. Following the consumption of feed with a high ionizable-ash content. 3. When a high proportion of the day's food is consumed in a short period of time. 4. When water intake is restricted. V. ACKNOWLEDGMENTS I wish to thank the Reserve Bank of Australia for their financial assistance, Mr. A. W. Beattie for his assistance with the statistical analysis, and Mr. P. J. Amos for his care of the animals. VI. REFERENCES BAILEY, C. B. (1961). Br. J. Nutr. 15: 443. B ALCH , C. C., and C AMPLING, R. C. (1962). Nutr. Abstr. Rev. 32: 389. B OTT, E LSPETH, DENTON, D. A., and W ELLER, SIGRID (1965). J. Physiol. Lond. 176: 323. CONRAD, H. R. (1966). J. Anim. Sci. 25: 227. HARPER, A. E., and S PIVEY, H. E. (1958). Amer. J. Physiol. 193: 483. HECKER, J. F., BUDTZ-OLSEN, 0. E., and OSTWALD, MARY ( 1964). Azlst. J. agric. 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