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© 2003 The American Society for Nutritional Sciences J. Nutr. 133:1100-1104, April 2003

Hypophagic Effects of Ammonium Are Greater When Infused with Propionate Compared with Acetate in Lactating Dairy Cows

Masahito Oba and Michael S. Allen4

Department of Animal Science, Michigan State University, East Lansing, MI

4To whom correspondence should be addressed. E-mail: allenm{at}msu.edu.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
The objective of this experiment was to determine interactions between hypophagic effects of propionate and ammonium in lactating dairy cows. Eight ruminally cannulated Holstein cows in mid-lactation were used in a duplicated 4 x 4 Latin square design experiment with a 2 x 2 factorial arrangement of treatments. Factors evaluated were type of volatile fatty acid (VFA; acetate vs. propionate) and type of salt (sodium vs. ammonium). Treatment solutions were infused continuously into the rumen at a rate of 16.7 mmol of VFA salts/min starting 2 h before feeding and ending 12 h after feeding. Hypophagic effects of ammonium were significantly greater for cows infused with propionate (4.3 vs. 12.1 kg/12 h; SEM = 1.3) compared with acetate (13.5 vs. 15.3 kg/12 h; SEM = 1.3; interaction P < 0.01). This interaction was attributed to a greater reduction in meal frequency for ammonium treatment compared with sodium treatment when infused with propionate (3.9 vs. 7.2/12 h; SEM = 0.8) compared with acetate (6.6 vs. 7.0/12 h; SEM = 0.8), indicating that infusion of ammonium propionate reduced hunger. Meal size was decreased by infusion of propionate compared with acetate, but was not affected by ammonium compared with sodium, indicating that ammonium did not affect satiety. Mechanisms to explain the interactions between ammonium and propionate on meal frequency and feed intake warrant further investigation.

KEY WORDS: • propionate infusion • ammonium • urea synthesis • feeding behavior • dairy cows

Propionate has hypophagic effects in ruminants that have been documented extensively and reviewed recently (1 ). Ammonium also has hypophagic effects in ruminants; intraruminal infusion of urea with subsequent increase in ruminal ammonium concentration decreased feed intake of lactating dairy cows (2 ,3 ). Regulation of feed intake is likely a result of the interaction and integration of a variety of signals (4 ). Allen (1 ) proposed that hypophagic effects of propionate are mediated by enhanced oxidative metabolism in the liver. Therefore, an interaction between hypophagic effects of propionate and ammonium is of interest because ammonium is also metabolized extensively in the liver, and both propionate and ammonium production vary in the rumen, depending on the fermentability of the diets and the concentration of ruminally degraded protein.

Evaluation of feeding behavior will help to elucidate mechanisms for regulation of feed intake because feed intake is a function of meal size and meal frequency, which are determined by satiety and hunger, respectively. Oba and Allen (5 ) reported that intraruminal infusion of ammonium propionate decreased dry matter intake (DMI) compared with sodium and potassium propionate by decreasing meal frequency without affecting meal size. This observation was noteworthy because hypophagia from other infusates affects feeding behavior differently. Infusion of a hyperosmotic solution into the rumen decreased meal size but increased meal frequency, resulting in no change in feed intake (6 ), and infusion of propionate decreased both meal size and meal frequency compared with infusion of acetate (6 ,7 ). However, the interactions of ammonium and propionate on feeding behavior are not known. Therefore, the objective of this experiment was to evaluate interactions between intraruminal infusion of propionate and ammonium on feeding behavior and feed intake of lactating dairy cows. We hypothesized that hypophagic effects of ammonium are exacerbated by propionate primarily by decreasing meal frequency.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Experimental procedures were approved by the All University Committee on Animal Use and Care at Michigan State University. Multiparous Holstein cows (n = 8; 151 ± 26 d in milk; mean ± SD) cannulated ruminally for previous experiments were selected from the Michigan State University Dairy Cattle Teaching and Research Center. These cows were used for other experiments (5 ,7 ), and the cows and the experimental diet were described previously (7 ).

Cows were assigned to duplicate 4 x 4 Latin squares balanced for carry-over effects with a 2 x 2 factorial arrangement of treatments. Factors evaluated were type of volatile fatty acid (VFA; acetate vs. propionate) and type of salt (sodium vs. ammonium). Treatments were continuous intraruminal infusion of sodium acetate, ammonium acetate, sodium propionate or ammonium propionate. Treatment solutions were prepared by diluting 16.8 mol of VFA salts to 18 L with deionized water. Concentration of VFA salts were 0.93 mol/L across treatments, and 15 L of each solution was infused over 14 h. The infusion rate of 17.9 mL/min provided 16.7 mmol of VFA salts/min. Solutions were infused using 4-channel peristaltic pumps (#78016–30, Cole-Parmer Instrument, Vernon Hills, IL) and Tygon tubing (7.5 m x 1.6 mm i.d.; Fisher Scientific Co., Pittsburgh, PA). Treatment periods were 2 d with 14 h of infusion followed by 34 h of recovery. Infusions started 2 h before feeding and ended 12 h after feeding. The infusion rate of ammonium was higher than for a previous experiment [11.9 mmol/min; (5 )] because we thought there would be little risk of ammonia toxicity based on milk urea nitrogen (MUN) data from that experiment. We expected a greater treatment effect on feeding behavior and DMI for this experiment compared with the previous experiment (5 ) because of the higher infusion rate of salts. Total amount of ammonium infused into the rumen in this experiment was equivalent to 1227 g of crude protein over 14 h.

Total metabolizable energy (ME) intake was calculated by adding ME from infusates to ME of the diet because the energy concentration of infusates differed. The experimental diet was assumed to contain 11.4 MJ/kg of ME on the basis of values from the NRC (8 ). Acetate and propionate were assumed to contain 0.876 and 1.536 MJ/mol of ME, respectively (9 ). Infusates were weighed before and after infusion, and actual amount of solutions infused into the rumen was calculated. The ME from infusates was calculated by multiplying ME concentration of infusates by the amount actually infused into the rumen for 12 h.

Throughout the experiment, cows were housed in tie-stalls, and fed once daily (1030 h) at 110% of expected intake. Cows were not allowed access to feed between 0830 and 1030 h. The amount of feed offered and orts were weighed for each cow daily. On every infusion day, samples of all dietary ingredients (0.5 kg) were collected, and treatment solutions were infused from 0830 to 2230 h for all cows. Cows were milked twice daily in the milking parlor except for the evening milking on infusion days, for which cows were milked in their stalls. Feeding behavior was monitored from 1030 to 2230 h on each infusion day by a computerized data acquisition system (10 ). Data of chewing activities, feed disappearance, and water consumption were recorded for each cow every 5 s, and meal bouts, interval between meals, meal size, eating time, ruminating time and total chewing time were calculated. Milk yield was recorded and milk samples were taken at both milkings on each infusion day. Milk samples were analyzed for fat, true protein, lactose, solids-nonfat and MUN concentration with infrared spectroscopy by the Michigan Dairy Herd Improvement Association (East Lansing). Diet ingredients were analyzed as previously described (7 ).

All data except for MUN were analyzed using the fit model procedure of JMP (version 4, SAS Institute, Cary, NC) according to the following model:

in which Yijklm is a dependent variable, µ is the overall mean, Si is the fixed effect of square (i = 1 to 2), C(S)j(i) is the random effect of cow nested in a square (j = 1 to 4), Pk is the fixed effect of period (k = 1 to 4), Ll is the linear effect of treatment, Ql is the quadratic effect of treatment, CovINF is the effect of actual amount of solution infused into the rumen as a covariate and eijklm is the residual error. One pump was used for each square of four cows, and the random effect of cow was nested in a square that shared the same infusion pump. Interactions of square x treatment and period x treatment were originally evaluated, but they were removed from the statistical model because interactions were not significant for response variables of interest. The volume of solution infused into the rumen was included in the statistical model as a covariate. Orthogonal contrasts were made for the effect of VFA type (acetate vs. propionate), salt type (sodium vs. ammonium) and interaction of VFA and salt. Treatment effects and their interaction were declared significant at P < 0.05 and P < 0.10, respectively.


   RESULTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
The volume of solution infused into the rumen with sodium treatment was greater than that with ammonium treatment (P < 0.001; Table 1 ). This was unexpected because a 4-channel peristaltic pump with identical tubing and fittings was used for each square, but a difference in viscosity of infusates might have affected flow rate. Therefore, the volume of solution infused into the rumen was included in the statistical model as a covariate.


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  		TABLE 1 Effects of intra-ruminal infusion of volatile fatty acids (VFA) salts on feeding behavior of lactating dairy cows

 
Interactions of main effects were observed for DMI (P < 0.01; Fig. 1 ). Infusion of ammonium propionate decreased DMI compared with sodium propionate (4.3 vs. 12.1 kg/12 h; SEM = 1.3), but ammonium acetate treatment did not decrease DMI compared with sodium acetate treatment (13.1 vs. 15.3 kg/12h; SEM = 1.3). Propionate infusates contained more energy than acetate infusates; however, total ME intake was also decreased by infusion of propionate compared with acetate. In addition, total ME intake reduction by ammonium infusion was much greater when infused with propionate than when infused with acetate. Ammonium treatment increased milk urea nitrogen concentration compared with sodium treatment similarly for acetate and propionate treatments (15.4 vs. 5.3 mmol/L; SEM = 0.9). Infusion of ammonium decreased meal frequency, and this effect was greater when infused with propionate than with acetate (Fig. 2 ). Ammonium propionate decreased the number of meal bouts compared with sodium propionate treatment (3.9 vs. 7.2/12 h; SEM = 0.8), but ammonium acetate treatment did not decrease the number of meal bouts compared with sodium acetate treatment (6.6 vs. 7.0/12 h; SEM = 0.8). Meal size was not affected by ammonium treatment. This is consistent with observations from our previous experiment (5 ), in which infusion of ammonium propionate decreased meal frequency without affecting meal size compared with sodium and potassium propionate. These observations indicate that ammonium does not increase satiety, but decreases hunger when infused with propionate. Propionate infusion decreased DMI by decreasing both meal size and meal frequency compared with acetate infusion, which is consistent with previous observations (7 ).



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  		FIGURE 1 Effect of intraruminal infusion of volatile fatty acid (VFA) salts on dry matter intake (DMI; kg/12 h) of lactating dairy cows. Values are least square means ± SEM (n = 8). Effect of VFA type: P < 0.001; effect of salt type: P < 0.001; effect of interaction between VFA type and salt type: P < 0.01.

 


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  		FIGURE 2 Effect of intraruminal infusion of volatile fatty acid (VFA) salts on number of meal bouts (/12 h) of lactating dairy cows. Values are least square means ± SEM (n = 8). Effect of VFA type: P < 0.03; effect of salt type: P < 0.01; effect of interaction between VFA type and salt type: P < 0.01.

 
Eating time was decreased by infusion of propionate (P < 0.001) and ammonium (P < 0.01) compared with acetate and sodium, respectively (Table 1) . In addition, the reduction in eating time was greater when ammonium was infused with propionate compared with acetate. Ruminating time was decreased for ammonium treatment compared with sodium treatment when infused with propionate (75 vs. 124 min; SEM = 22) probably from lower fiber intake and lower expected fiber mass in the rumen, which are positively related to ruminating time (11 ). However, sodium treatment decreased ruminating time compared with ammonium treatment when infused with acetate (137 vs. 211 min; SEM = 22), and ruminating time/kg DMI was lower for sodium treatment than for ammonium treatment (P < 0.04). This might be attributed to the expected greater tonicity of ruminal fluid for sodium treatment compared with ammonium treatment because of ammonium incorporation into microbial N. Greater tonicity of ruminal fluid is expected to result in increased blood tonicity, which could decrease rumination (12 ). Water intake (P < 0.0001) and the number of drinking bouts (P < 0.02) were greater for sodium treatments compared with ammonium treatments, which is similar to treatment effects on ruminating time/kg DMI. An interaction of main effects was significant for water intake (P < 0.01), indicating that water intake was lower for ammonium propionate compared with ammonium acetate treatment, whereas VFA type did not affect water intake for sodium treatments.


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
One possible explanation for the interaction observed for the hypophagic effects of ammonium and propionate is discomfort from toxic effects of ammonium in the blood (not measured). Propionate can decrease hepatic capacity to detoxify ammonium by inhibiting the synthesis of N-acetyl glutamate, the activator of carbamoyl phosphate synthetase (13 ). Choung and Chamberlain (13 ) showed that intraruminal infusion of propionate and urea markedly increased plasma ammonium concentration in dairy cows compared with infusion of urea alone. Mutsvangwa et al. (14 ) evaluated the effect of propionate on urea production with isolated ovine hepatocytes, and found that urea production was halved when hepatocytes were incubated with 1.25 mmol/L of propionate and 1.25 mmol/L of NH4Cl compared with when they were incubated with 1.25 mmol/L of NH4Cl alone. However, in the current experiment, MUN concentrations for the ammonium treatments were not excessive relative to the normal range of 3.3–20.1 mmol/L [mean ± 2 SD; n = 133,057; (15 )], and ammonium propionate treatment increased MUN to the same extent as the ammonium acetate treatment. Because urea equilibrates within body fluids, MUN is highly correlated with blood urea nitrogen and is an indicator of hepatic urea production (16 ). Therefore, our data indicate that ammonium propionate infusion did not inhibit urea production in the liver over time compared with ammonium acetate infusion. Arterial ammonium concentration was not determined in this experiment, and similar MUN concentrations for the ammonium treatments alone do not eliminate the possibility that temporal discomfort from greater blood ammonium concentrations affected feeding behavior to a greater extent for cows infused with ammonium propionate than with ammonium acetate. However, toxic effects of ammonium are expected to affect meal size because of the increased ammonium flux from the rumen during a meal, and meal size was not affected by ammonium infusion in this experiment or in a previous experiment (5 ).

Ammonium propionate treatment decreased meal frequency without affecting meal size, indicating that ammonium infusion decreased feed intake by reducing hunger and that propionate exacerbated the reduction in hunger. Greater ammonium flux from the rumen might have reduced hunger by increasing propionate oxidation in the liver. The addition of NH4Cl in ruminant hepatocytes decreased propionate utilization for gluconeogenesis (17 –19 ), and increased propionate oxidation (17 ,19 ). Oxidative metabolism in the liver has been shown to affect satiety in rats (20 –22 ) and a temporal relationship between feeding behavior and hepatic ATP concentration has been demonstrated (23 ). Langhans et al. (22 ) proposed that metabolic fuels that are extensively metabolized in the liver have hypophagic effects. A stimulatory effect of ammonium on propionate oxidation in the liver is a plausible mechanism for the interaction observed for hypophagic effects of ammonium and propionate in this experiment.

Ammonium infusion might also have enhanced oxidative metabolism in the liver by stimulating urea synthesis, increasing the oxidation of amino acids. One of the two amino groups of urea is from ammonium, but the other is from amino acids via aspartate; urea production in the liver is associated with {alpha}-amino nitrogen removal in the liver (24 ,25 ). Infusion of NH4Cl into the mesenteric vein of sheep increased oxidation of leucine by splanchnic tissues (26 ). Therefore, urea synthesis might increase net ATP production in the liver by increasing hepatic amino acid oxidation despite utilization of ATP by the urea cycle. Although carbon from some amino acids can be utilized for gluconeogenesis and consume ATP in the liver, the maximum rate of gluconeogenesis at any point in time is affected by enzyme activity regulated by hormones such as insulin and glucagon. Therefore, carbon from amino acids might be oxidized to a greater extent when the glucose demand of peripheral tissues is low or when the liver has alternative substrates for gluconeogenesis. If enhanced oxidative metabolism in the liver decreases feed intake (20 –22 ), hypophagic effects of ammonium are expected to be greater when it is infused with propionate compared with acetate because propionate is the primary substrate for gluconeogenesis in ruminants, whereas acetate is not metabolized in the liver (27 ). When ammonium is infused with acetate, carbon from amino acids might be utilized for gluconeogenesis to a greater extent. We speculate that the hypophagic effects of ammonium infusion vary, depending on its stimulatory effect on oxidative metabolism in the liver, and increase when it is infused with propionate compared with acetate.

If propionate decreases the rate of urea production in the liver as previously demonstrated (13 ), urea synthesis after meals probably was extended over a longer period of time for propionate compared with acetate treatment. The liver is a heterogeneous organ, varying in enzyme activity between periportal and perivenous regions. Urea synthesis occurs in periportal hepatocytes; at physiologic portal concentrations of ammonium, about two thirds of ammonium is incorporated into urea, whereas glutamine synthesis in perivenous hepatocytes scavenges the remaining ammonium (28 ). Glutamine synthesis minimizes the amount of ammonium escaping hepatic detoxification when ammonium flux to the liver exceeds the rate of urea synthesis; Rodriguez et al. (29 ) showed that diurnal variation in ruminal ammonium concentration is 10 times greater than that for plasma urea nitrogen concentration. Ammonium incorporated into glutamine is available for later urea synthesis because of significant activity of glutaminase in periportal hepatocytes (28 ). The generation of ammonium from glutamine in periportal hepatocytes allows urea synthesis to continue over time after meals. Propionate infusion might have decreased the rate of urea synthesis and extended urea synthesis over time after meals, providing carbon from amino acids for oxidative metabolism in the liver for an extended period, and reducing hunger and meal frequency.

Evidence supporting the idea that propionate exacerbates hypophagic effects of ammonium by stimulating oxidative metabolism in the liver is not conclusive. Therefore, other factors that have not been discussed, such as hormonal control, cannot be eliminated as possible mechanisms explaining the treatment effects observed in this experiment. However, interactions observed between treatments in this experiment were not likely mediated by insulin because Choung and Chamberlain (13 ) reported that intraruminal infusion of urea and propionate decreased insulin concentration compared with infusion of propionate alone.

The results of this experiment should be considered with caution for practical application because the alteration of dietary crude protein concentration has many confounding effects on feed intake. Increasing dietary crude protein concentration often enhances DMI by dairy cows, possibly because of a reduction in the filling effects of a diet from enhanced rumen microbial population and increased ruminal digestibility or because an increased amino acid supply stimulates clearance of metabolic fuels, increasing hunger (1 ). In addition, synergistic hypophagia from propionate and ammonium might not be observed under normal feeding conditions because highly fermentable diets usually increase propionate production but decrease ruminal ammonium concentration from enhanced production of microbial protein. However, an unbalanced amino acid profile of metabolizable protein, excess dietary protein intake relative to requirement or excessive body protein degradation contribute to enhanced urea production in the liver. Mechanisms by which ammonium propionate decreases feed intake warrant further investigation.

In conclusion, the hypophagic effects of ammonium were exacerbated when infused with propionate compared with acetate. Infusion of ammonium decreased meal frequency and this effect was greater when infused with propionate compared with acetate. Meal size was decreased by infusion of propionate compared with acetate, but not by infusion of ammonium compared with sodium. These observations indicate that ammonium decreases feed intake by affecting hunger, not satiety, and that propionate exacerbated the reduction in hunger. The potential role of hepatic nitrogen metabolism in regulation of feed intake for ruminants warrants further investigation.


   ACKNOWLEDGMENTS
 
We thank R. E. Kreft and D. G. Main for technical assistance.


   FOOTNOTES
 
1 Presented in part at the American Dairy Science Association annual meeting, July 2001, Indianapolis, IN [Oba, M. & Allen, M. S. (2001) Effects of intra-ruminal infusion of propionate salts on feeding behavior of lactating dairy cows. J. Dairy Sci. 84(Suppl. 1): 122 (abs.)]. Back

2 Supported by the Michigan Agricultural Experiment Station, East Lansing, Michigan. Back

3 Present address: Department of Animal and Avian Sciences, University of Maryland, College Park, MD. Back

5 Abbreviations used: DMI, dry matter intake; ME, metabolizable energy; VFA, volatile fatty acids; MUN, milk urea nitrogen. Back

Manuscript received 6 August 2002. Initial review completed 20 October 2002. Revision accepted 23 December 2002.


   LITERATURE CITED
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 

1. Allen, M. S. (2000) Effects of diet on short-term regulation of feed intake by lactating dairy cattle. J. Dairy Sci. 83:1598-1624.[Abstract]

2. Choung, J. J., Chamberlain, D. G., Thomas, P. C. & Bradbury, I. (1990) The effects of intraruminal infusions of urea on the voluntary intake and milk production of cows receiving grass silage diets. J. Dairy Res. 57:455-464.[Medline]

3. Kertz, A. F., Koepke, M. K., Davidson, L. E., Betz, N. L., Norris, J. R., Skoch, L. V., Cords, B. R. & Hopkins, D. T. (1982) Factors influencing intake of high urea-containing rations by lactating dairy cows. J. Dairy Sci. 65:587-604.

4. Forbes, J. M. (1995) Voluntary Food Intake and Diet Selection in Farm Animals 1995 CAB International Oxon, UK.

5. Oba, M. & Allen, M. S. (2003) Effects of intra-ruminal infusion of sodium, potassium, and ammonium on hypophagic effect of propionate in lactating dairy cows. J. Dairy Sci. 86in press.

6. Choi, B. R. & Allen, M. S. (1999) Intake regulation by volatile fatty acids and physical fill. S. Afr. J. Anim. Sci. 29:40-41(abs.).

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13. Choung, J. J. & Chamberlain, D. G. (1995) Effects of intraruminal infusion of propionate on the concentrations of ammonia and insulin in peripheral blood of cows receiving an intraruminal infusion of urea. J. Dairy Res. 62:549-557.[Medline]

14. Mutsvangwa, T., Buchanan-Smith, J. G. & McBride, B. W. (1997) Effects of ruminally degradable nitrogen intake and in vitro addition of ammonia and propionate on the metabolic fate of L-[1-14C] alanine and L-[1-15N] alanine in isolated sheep hepatocytes. J. Anim. Sci. 75:1149-1159.[Abstract/Free Full Text]

15. Jonker, J. S., Kohn, R. A. & Erdman, R. A. (1999) Milk urea nitrogen target concentrations for lactating dairy cows fed according to National Research Council recommendations. J. Dairy Sci. 82:1261-1273.[Abstract]

16. Butler, W. R., Calaman, J. J. & Beam, S. W. (1996) Plasma and milk urea nitrogen in relation to pregnancy rate in lactating dairy cattle. J. Anim. Sci. 74:858-865.[Abstract]

17. Aiello, R. J. & Armentano, L. E. (1987) Gluconeogenesis in goat hepatocytes is affected by calcium, ammonia and other key metabolites but not primarily through cytosolic redox state. J. Comp. Biochem. Physiol. 88B:193-201.

18. Demigné, C., Yacoub, C., Morand, C. & Rémésy, C. (1991) Interactions between propionate and amino acid metabolism in isolated sheep hepatocytes. Br. J. Nutr. 65:301-317.[Medline]

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20. Langhans, W., Wiesenreitter, F. & Scharrer, E. (1983) Different effects of subcutaneous D, L-3-hydroxybutyrate and acetoacetate injections on food intake in rats. Physiol. Behav. 31:483-486.[Medline]

21. Langhans, W., Damaske, U. & Scharrer, E. (1984) Subcutaneous glycerol injection fails to reduce food intake in rats fed a high protein diet. Physiol. Behav. 32:785-789.[Medline]

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23. Koch, J. E., Ji, H., Osbakken, M. D. & Friedman, M. D. (1998) Temporal relationships between eating behavior and liver adenine nucleotides in rats treated with 2, 5-anhydro-D mannitol. Am. J. Physiol. 274:R610-R617.[Abstract/Free Full Text]

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26. Lobley, G. E., Connell, A., Lomax, M. A., Brown, D. S., Milne, E., Calder, A. G. & Farningham, D.A.H.F. (1995) Hepatic detoxification of ammonia in the ovine liver: possible consequences for amino acid catabolism. Br. J. Nutr. 73:667-685.[Medline]

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