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Extent of Hypophagia Caused by Propionate Infusion Is Related to Plasma Glucose Concentration in Lactating Dairy Cows

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

 
Two experiments were conducted to evaluate how dose-response effects of intraruminal infusion of propionate on feeding behavior and plasma metabolites are altered by diets differing in fermentability. Twelve ruminally cannulated Holstein cows were used in each experiment. Cows were fed diets containing either steam flaked corn or dry cracked corn (30% of dietary dry matter) in expt. 1, and diets differing in forage-to-concentrate ratio (66:34 vs. 36:64) in expt. 2. For both experiments, the experimental design was a crossover for dietary treatment, and a 6 x 6 Latin square for infusion treatment within a diet for each period. Infusion treatments were mixtures of sodium propionate and sodium acetate, containing propionate at 0, 0.2, 0.4, 0.6, 0.8 and 1.0 as a fraction of total volatile fatty acids infused. Treatment solutions were infused into the rumen continuously for 18 h starting 6 h before feeding at a rate of 23.1 mmol/min. Although propionate production from ruminal fermentation was expected to be different, dietary treatments did not affect dry matter intake (DMI) responses to propionate infusion for either experiment. However, propionate infusion decreased DMI linearly in expt. 1, but did not decrease DMI at lower rates of propionate infusion, which were much more effective at increasing plasma glucose concentration in expt. 2. Propionate had a smaller hypophagic effect at low concentrations of plasma glucose and had a greater hypophagic effect at elevated concentrations of plasma glucose, which could be explained by changes in the metabolism of propionate in the liver.

KEY WORDS: • propionate infusion • threshold response • plasma glucose concentration • hepatic oxidation

Feeding livestock diets that contain highly fermentable grains often results in greater propionate production in the rumen (1 ) and can decrease dry matter intake (DMI) in lactating dairy cows (2 ). Although hypophagic effects of propionate have been well documented (3 –6 ), some experiments in the literature have reported no effects of propionate infusion on feed intake (7 –10 ). Inconsistent hypophagic effects of propionate might be explained by a threshold response of propionate in feed intake regulation. Dose-response effects of propionate on feed intake were previously investigated for lactating dairy cows (6 ,10 ) and sheep (11 ), and infusion of propionate linearly decreased feed intake as the infusion rate of propionate increased. A threshold response in DMI was not observed in those experiments.

Fermentability of experimental diets may affect the threshold for infused propionate to decrease DMI. In a review of the literature (12 ), the amount of ruminally fermented organic matter and total volatile fatty acid (VFA) production were reported to be between 5.7 and 15.4 kg/d and 42 and 115 mol/d for lactating dairy cows, respectively. Because propionate concentration in the rumen can increase from 15 to 45% of total fermentation acids as the amount of ruminally fermented organic matter increases (1 ), propionate production can range between 6 and 52 mol/d. Lack of a threshold response for infused propionate on DMI in the experiment reported by Farningham and Whyte (11 ) might be because sheep were fed ad libitum a very fermentable diet containing 50% hay, 30% barley and 10% molasses, and propionate production from diets might have already exceeded the threshold. However, Leuvenink et al. (13 ) fed sheep a pelleted grass, and reported that propionate infusion into the mesenteric vein of mature sheep at a rate of 2 mmol/min decreased intake but the infusion at a rate of 1 mmol/min had no effect. Fermentability of diets can be altered by feeding grains differing in fermentability in the rumen or by feeding diets differing in forage-to-concentrate ratio, and is expected to affect animal responses to intraruminal infusion of propionate for feeding behavior and DMI. The objective of this experiment was to evaluate how dose-response effects of intraruminal infusion of propionate on feeding behavior and plasma metabolites are altered by diets differing in fermentability. We hypothesized that cows fed more fermentable diets decrease their DMI at lower rates of propionate infusion compared to those fed less fermentable diets.

	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. Twenty-four multiparous lactating Holstein cows cannulated ruminally for previous experiments were selected from the Michigan State University Dairy Cattle Teaching and Research Center. Two experiments were conducted: 1) 12 cows were fed diets containing either steam flaked corn (SF) or dry cracked corn (DC) in expt. 1; 2) the other 12 cows were fed high forage (HF) or low forage (LF) diets in expt. 2. Infusion treatments were continuous intraruminal infusion of mixtures of sodium propionate and sodium acetate at six different ratios. Experimental designs were a crossover for dietary treatments and 6 x 6 Latin squares for infusion treatments within dietary treatments for both experiments. Throughout the experiments, cows were housed in tie stalls, and fed once daily at 110% of expected intake.

Experiment 1.

Twelve cows (99 ± 25 d in milk; 698 ± 76 kg body weight; mean ±SD) were fed diets containing either SF or DC at 297 g/kg of dietary DM. Both corn grains were obtained from Pennfield Feeds (Lancaster, PA). Flake density of SF was 0.36 kg/L, and the mean particle size of DC was 3.7 mm. Both experimental diets contained corn silage, alfalfa silage, cottonseeds, a premix of protein supplements (soybean meal, distillers grains and blood meal) and a premix of minerals and vitamins (Table 1 ). Dietary neutral detergent fiber and crude protein concentrations were 278 and 167 g/kg of dietary DM, respectively, for both diets, and fed as total mixed rations. Periods were 34 d, and each period consisted of 20 d for diet adaptation, 3 d for data and sample collection to determine the effects of dietary treatments and 11 d for data and sample collection to determine the effects of infusion treatments.

Twelve cows (53 ± 21 d in milk; 620 ± 64 kg body weight; mean ± SD) were fed diets differing in forage-to-concentrate ratio: 66:34 for HF and 36:64 for LF (Table 2 ). Both diets contained corn silage, alfalfa silage, cottonseeds, a premix of protein supplements (soybean meal, distillers grains and blood meal) and a premix of minerals and vitamins. The primary difference in diets was substitution of corn silage and distillers grains in the HF diet for dry ground corn in the LF diet. Distillers grains were used to compensate for the lower crude protein concentration of corn silage compared to dry ground corn so that diets contained similar crude protein concentrations with similar amino acid profiles. Dietary neutral detergent fiber and starch concentrations were 340 and 213 g/kg of dietary DM for HF and 252 and 351 g/kg of dietary DM for LF, respectively. Periods were 35 d, and each period consisted of 21 d for diet adaptation, 3 d for data and sample collection to determine the effects of dietary treatments and 11 d for data and sample collection to determine the effects of infusion treatments.

Cows were assigned to 6 x 6 Latin squares, balanced for carryover effects for infusion treatments. Treatment solutions were prepared by diluting 28.1 mol of sodium VFA, containing propionate at 0, 0.2, 0.4, 0.6, 0.8 and 1.0 as a molar fraction of total VFA to 18 L with deionized water. Sodium acetate was added to keep the osmolarity and pH of infusates constant across the treatments, which allowed isolation of the specific effects of propionate relative to acetate. Concentration of total VFA was 1.56 mol/L across treatments, and 16 L of each solution was infused over 18 h beginning 6 h before feeding. The infusion rate was 14.8 mL/min, which provides 23.1 mmol of VFA/min. The solutions were infused using four-channel peristaltic pumps (#78016-30; Cole-Parmer Instrument Co., Vernon Hills, IL) and Tygon tubing (7.5 m x 1.6 mm I.D.). Infusion started 6 h before feeding so that VFA concentrations in the rumen reached steady state (assuming absorption rate and passage rate of 20 and 15%/h, respectively) by feeding time when monitoring of feeding behavior began. Subperiods for infusion treatment were 2 d with 18 h of infusion followed by 30 h of recovery.

Data and sample collection.

Amounts of feed offered and orts were weighed for each cow daily during the collection period. Samples of all dietary ingredients (0.5 kg) were collected daily during each 3-d collection period and on feeding behavior-monitoring days during each infusion period (d 1, 3, 5, 7, 9 and 11) and composited to one sample per diet period. Samples of orts (12.5%) were collected daily during the 3-d collection period and composited into one sample per cow per period. Cows were milked twice daily in the milking parlor except for the evening milking for days in which feeding behavior was monitored (d 1, 3, 5, 7, 9 and 11 of each infusion period) when cows were milked in their stalls. Milk yield was measured daily during the 3-d collection period and was averaged to determine the effects of dietary treatments. Milk was sampled at every milking, and analyzed for fat, true protein, lactose and nonfat solids with infrared spectroscopy by Michigan DHIA (East Lansing).

Samples of feces, ruminal fluid and blood were collected every 9 h during the 3-d collection period. Ruminal fluid samples were collected from five different sites in the rumen and squeezed through a nylon screen (1 mm pore size), and pH was determined immediately after collection. Samples were frozen at -20°C until further analysis. Blood samples were collected from coccygeal vessels into two Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ), one with sodium heparin and the other with potassium oxalate and sodium fluoride as a glycolytic inhibitor. Both were centrifuged at 2000 x g for 15 min immediately after sample collection, and plasma was harvested and frozen at -20°C until analysis.

On feeding behavior monitoring days (d 1, 3, 5, 7, 9 and 11 of infusion periods), infusion started at 0800 h, 6 h before feeding, and continued for 18 h. Cows were not allowed access to feed between 1000 and 1400 h to minimize the confounding effects of ruminal fermentation from the previous feeding. Feeding behavior was monitored for 12 h (1400 h to 0200 h) by a computerized data acquisition system (14 ). Data on chewing activity, feed disappearance and water consumption were recorded for each cow every 5 sec, and meal bouts, interval between meals and meal size were calculated. At the end of the feeding behavior monitoring period (0200 h), ruminal fluid and blood were sampled from each cow and processed as described above. Diet ingredients, orts and fecal samples were processed and analyzed for ash, neutral detergent fiber, acid detergent fiber, crude protein and starch as described previously (6 ). Indigestible neutral detergent fiber was estimated as neutral detergent fiber residue after 120 h in vitro fermentation (15 ) and used as an internal marker to calculate apparent total tract digestibility (16 ). Metabolizable energy (ME) intake from diets was calculated according to National Research Council (17 ) based on actual digestibility of diets. Ruminal fluid and plasma samples were analyzed for VFA concentrations according to the method described previously (18 ). Commercial kits were used to determine concentrations of glucose (Glucose kit #510; Sigma Chemical, St. Louis, MO) and insulin (Coat-A-Count; Diagnostic Products, Los Angeles, CA) in plasma.

For both experiments, all data for dietary effects from the 3-d collection periods were analyzed using the fit model procedure of JMP (version 4.0, SAS Institute, Cary, NC) according to the following model:

where

µ = overall mean

S_i = fixed effect of diet sequence (i = 1 to 2)

C(S)_j(i) = random effect of cow nested within diet sequence (j = 1 to 6)

P_k = fixed effect of period (k = 1 to 2)

D_l = fixed effect of diet (l = 1 to 2)

e_ijkl = residual, assumed to be normally distributed

All data from the 11-d infusion periods were analyzed by use of the fit model procedure of JMP according to the following model:

where

µ = overall mean

S_i = fixed effect of diet sequence (i = 1 to 2)

C(S)_j(i) = random effect of cows nested within diet sequence (j = 1 to 6)

P_k = fixed effect of period (k = 1 to 2)

SP(P)_l(k) = fixed effect of subperiod nested within period (l = 1 to 6)

D_m = fixed effect of diet (m = 1 to 2)

L_n = linear effect of infusion

Q_n = quadratic effect of infusion

DL_mn = interaction between effect of diet and linear effect of infusion

DQ_mn = interaction between effect of diet and quadratic effect of infusion

e_ijklmn = residual, assumed to be normally distributed

Data from the 3-d collection periods before infusion were analyzed to characterize the animals used in expts. 1 and 2, by use of the fit model procedure of JMP according to the following model:

where

µ = overall mean

E_i = fixed effect of experiment (i = 1 to 2; expt. 1 or expt. 2)

e_ij = residual, assumed to be normally distributed

Main treatment effects were declared significant at P < 0.05, and the tendency for treatment effects was declared significant at P < 0.10. Interaction effects were declared significant at P < 0.10.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Experiment 1.

During the 3-d collection period before infusion, DMI and feeding behavior were not affected by dietary treatment (Table 3 ). Cows fed SF had higher apparent total tract digestibility of starch (P < 0.001), but lower total tract neutral detergent fiber digestibility (P < 0.001) compared to cows fed DC. Ruminal propionate concentration was greater (P < 0.001) and ruminal acetate concentration was less (P < 0.001) for SF compared to DC treatment, although ruminal pH was not affected by treatment. Plasma insulin concentration was greater for SF compared to DC treatment (P < 0.001), whereas plasma glucose concentration was not affected by dietary treatment. The SF treatment increased milk yield (P < 0.01), milk protein (P < 0.001) and milk lactose concentrations (P < 0.01), but decreased milk fat concentration (P < 0.05) compared to the DC treatment. These observations are consistent with expected greater ruminal fermentation and greater ruminal propionate production for the SF treatment compared to the DC treatment.

During the 3-d collection period before infusion, no effect of diet was observed on DMI, feeding behavior or milk yield (Table 5 ). Although cows fed LF had lower apparent total tract digestibility of neutral detergent fiber (P < 0.05) compared to cows fed HF, the apparent total tract digestibility of organic matter was not affected by dietary treatment because LF treatment, compared to HF treatment, contained more starch, which is a highly digestible fraction of the diet. The LF treatment, compared to HF treatment, increased ruminal propionate concentration (P < 0.01) but decreased acetate concentration (P < 0.001) and ruminal pH (P < 0.01). The LF treatment, compared to HF treatment, decreased milk fat concentration (P < 0.01). These observations are consistent with our expectation of greater ruminal fermentability and ruminal propionate production for the LF compared to the HF diet.

Digestibility of diets was relatively low for both experiments, and dietary ME concentration calculated from actual digestibility of nutrient might be underestimated. Underestimation of dietary ME concentration would reduce the effect of the reduction in DMI from propionate infusion on total ME intake (ME intake from diet and infusates). However, total ME intake decreased linearly (expt. 1) and quadratically (expt. 2) as the proportion of propionate in infusates increased. Therefore, the significant effects of infusion treatment on total ME intake with a possible underestimation of dietary ME concentration provide strong evidence for specific hypophagic effects of propionate relative to acetate.

During the infusion period, milk yield was greater for more fermentable diets (SF and LF) compared to less fermentable diets (DC and HF) for expts. 1 and 2, respectively. Infusion treatment did not affect milk yield despite an increase in plasma glucose concentration as the proportion of propionate in infusates increased. Therefore, it is not likely that availability of glucose for lactose synthesis limited maximum milk yield during the infusion period. It is more plausible that greater milk yield for more fermentable diets was attributable to greater microbial protein production in the rumen, and that availability of metabolizable protein was a dominant factor limiting maximum milk yield during the infusion period.

In expt. 1, a threshold for the effect of propionate on DMI did not exist and infused propionate linearly decreased DMI (Fig. 1 ). However, in expt. 2, a threshold for the effect of propionate on DMI was observed; infused propionate did not decrease DMI at lower rates of propionate infusion and linearly decreased DMI after a threshold was reached (Fig. 1) . It is unlikely that inconsistent responses observed between the experiments are a result of differences in dietary characteristics between the experiments. Both experiments were designed to evaluate how dose-response effects of intraruminal infusion of propionate are affected by fermentability of diets, and differences of diets within experiments were greater than differences of diets between experiments. Apparent total tract starch digestibility was different by 179 g/kg for SF and DC treatments in expt. 1 (952 ± 13 vs. 773 ± 13 g/kg), and dietary neutral detergent fiber concentration was different by 88 g/kg of diet dry matter for LF and HF treatments in expt. 2 (340 vs. 252 g/kg of diet dry matter). Dietary differences within each experiment should have been great enough to detect interactions between effects of diet and infusions. Therefore, the differences in threshold response observed between experiments are more likely attributable to differences in animal characteristics, given that different cows were used in each experiment.

View larger version (23K): [in this window] [in a new window]   FIGURE 1 Dose-response effects of intraruminal propionate infusion on dry matter intake (DMI, kg/12 h) in lactating dairy cows. Values are pooled means ± SEM for DMI within an experiment at each infusion rate (n = 24).

View larger version (20K): [in this window] [in a new window]   FIGURE 2 Dose-response effects of intraruminal propionate infusion on plasma glucose concentration (mmol/L) in lactating dairy cows. Values are pooled means ± SEM for plasma glucose concentration within an experiment at each infusion rate (n = 24).

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

In expt. 2, lower rates of propionate infusion substantially increased plasma glucose concentration but did not affect DMI; we speculate that propionate does not cause hypophagia when propionate is extensively used for glucose synthesis. Ruminant animals rely on gluconeogenesis as a primary source of plasma glucose (22 ) and 40–70% of plasma glucose carbon is derived from propionate carbon, depending on the availability of propionate (23 ). Therefore, the increase in plasma glucose concentration, observed in this experiment, is likely attributable to extensive propionate use for gluconeogenesis. However, the marginal effect of infused propionate on plasma glucose concentration decreased at higher rates of propionate infusion, indicating that use of infused propionate for gluconeogenesis decreases as the glucose demand of body tissues is satisfied. Progressive increases in oxidative metabolism of propionate in the liver might have caused the reduction in DMI at higher rates of propionate infusion in expt. 2. In expt. 1, cows had greater plasma glucose concentrations before the infusion period compared to those used in expt. 2. Although propionate infusion linearly increased plasma glucose concentration in expt. 1, the response in glucose concentration to propionate infusion was far less than that observed at lower rates of propionate infusion in expt. 2. Therefore, infused propionate might have been more extensively oxidized in the liver even at lower rates of propionate infusion in expt. 1 compared to expt. 2, resulting in a linear decrease in DMI.

Within the data set of the current experiments, the extent of hypophagia caused by propionate infusion was positively related to plasma glucose concentration. Treatment means were plotted to determine the relationship between plasma glucose concentration and marginal response in DMI (g/12 h) to infused propionate (mmol/min) (Fig. 3 ). As plasma glucose concentration increased, DMI decreased to a greater extent by propionate infusion (P < 0.01). It is likely that a greater concentration of plasma glucose is an indicator that glucose demand of body tissues is more nearly satisfied and that infused propionate is less used for gluconeogenesis but increasingly oxidized in the liver, resulting in greater hypophagia. Our observations imply that the threshold for propionate to affect DMI becomes greater for cows with lower plasma glucose concentration and that propionate has a greater hypophagic effect for cows with high plasma glucose concentration.

View larger version (19K): [in this window] [in a new window]   FIGURE 3 Relationship between plasma glucose concentration (mmol/L) and marginal response in dry matter intake (DMI, g/12 h) to infused propionate (mmol/min). Marginal response in DMI = 3842 - 1166 x plasma glucose concentration (r2 = 0.45; P < 0.01).

For expt. 2, propionate production from experimental diets is expected to be lower for HF than for LF, and similar glucose concentration between HF and LF might indicate that a greater proportion of propionate flux from the rumen is used for gluconeogenesis for HF than for LF treatment. If greater use of propionate for gluconeogenesis in the liver decreases the hypophagic effects of propionate, infused propionate is expected to cause less hypophagia for HF than for LF treatment. Contrary to our hypothesis, propionate infusion decreased DMI similarly regardless of dietary treatment. However, it is difficult to determine which dietary treatment increased use of propionate for gluconeogenesis compared to the other because of the difference in milk yield observed during the infusion period. Greater milk yield for LF (+2.6 kg/d) compared to HF treatment increased glucose demand and likely enhanced gluconeogenesis in the liver, which might have resulted in a similar proportion of propionate used for gluconeogenesis compared to HF and resulted in the failure to detect an interaction of treatments. Another possible reason for the absence of an interaction between diets and infusion treatments is that a physical fill was likely a more dominant mechanism regulating feed intake for cows fed HF, whereas satiety related to propionate metabolism was likely a more dominant factor regulating feed intake for cows fed LF. The hypophagic effects of fill for HF treatment and greater basal propionate production for LF treatment might have had similar effects on DMI because integration of both physical fill and metabolic satiety signals contributes to the regulation of voluntary feed intake (26 ). Mbanya et al. (27 ) infused acetate, propionate or both, with or without distention of the rumen by a balloon. Combination of VFA infusion and distention significantly depressed DMI, whereas VFA infusion or distention alone did not. Thus, the threshold for infused propionate to decrease DMI can be altered by dietary fill.

The threshold response for effects of propionate on DMI observed in this study has important practical implications because greater ruminal fermentation is more desirable to increase productivity of animals, unless energy intake is decreased. In this experiment, the extent of hypophagia caused by propionate was positively related to plasma glucose concentration. The hypophagic effects of propionate might be lessened for cows with low plasma glucose concentration; thus, increasing the fermentability of diets likely enhances productivity of these animals. However, cows with high plasma glucose concentration might decrease productivity by a similar diet change because of greater risk of reducing energy intake. Nonetheless, the relationship between the extent of hypophagia caused by propionate and plasma glucose concentration was indirectly inferred from the observations made in the current experiments, and future research should evaluate directly the effect of glucose demand of animals on hypophagia from propionate.

In conclusion, the hypophagic effects of propionate were not affected by fermentability of dietary starch sources in expt. 1 or by forage-to-concentrate ratio in expt. 2. Our results indicate that propionate flux from the rumen per se did not generate satiety signals. A quadratic effect of propionate infusion on DMI was observed in expt. 2 but not in expt. 1, regardless of dietary treatments. Lower rates of propionate infusion in expt. 2 greatly increased plasma glucose concentration but did not decrease DMI. However, DMI was linearly decreased by propionate infusion in expt. 1 and by higher rates of propionate infusion in expt. 2, in which the marginal effect of propionate infusion on plasma glucose concentration was much lower. Propionate may exert less hypophagic effects while infused propionate is extensively used for glucose synthesis. The extent of hypophagia caused by propionate infusion was positively related to plasma glucose concentration in these experiments. These observations were consistent with the hypothesis that propionate decreases feed intake by stimulating oxidative metabolism in the liver.

	ACKNOWLEDGMENTS

 
We thank R. E. Kreft, R. A. Longuski, D. G. Main, C. S. Mooney, J. A. Voelker and Y. Ying for technical assistance and Pennfield Feeds (Lancaster, PA) for donation of corn grain used in expt. 1.

	FOOTNOTES

 
¹ Presented in part at the American Dairy Science Association annual meeting, July 2002, Quebec City, Canada [Oba, M. & Allen, M. S. (2002) Dose-response effects of propionate infusion on feeding behavior and plasma metabolites in lactating dairy cows. J. Dairy Sci. 85(Suppl. 1): 399].

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

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

⁵ Abbreviations used: DC, dry cracked corn; DMI, dry matter intake; HF, high forage diet; LF, low forage diet; ME, metabolizable energy; SF, steam flaked corn; VFA, volatile fatty acids.

Manuscript received 5 September 2002. Initial review completed 20 October 2002. Revision accepted 2 January 2003.

	LITERATURE CITED

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

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				M. Oba and M. S. Allen Dose-Response Effects of Intrauminal Infusion of Propionate on Feeding Behavior of Lactating Cows in Early or Midlactation J Dairy Sci, September 1, 2003; 86(9): 2922 - 2931. [Abstract] [Full Text] [PDF]

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