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Splanchnic Metabolism of Dairy Cows During the Transition From Late Gestation Through Early Lactation

Centre for Dairy Research, Department of Agriculture, The University of Reading, Earley Gate, Reading RG6 6AT England

Corresponding author:
C. K. Reynolds; e-mail:
reynolds.345{at}osu.edu.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Blood flow and net nutrient fluxes for portal-drained viscera (PDV) and liver (total splanchnic tissues) were measured at 19 and 9 d prepartum and at 11, 21, 33, and 83 d in milk (DIM) in 5 multiparous Holstein-Friesian cows. Cows were fed a grass silage-based gestation ration initially and a corn silage-based lactation ration peripartum and postpartum. Meals were fed at 8-h intervals and hourly (n = 8) measures of splanchnic metabolism were started before (0730 h and 0830 h) feeding at 0830 h. Dry matter intakes (DMI) at 19 and 9 d prepartum were not different. Metabolism changes measured from 19 to 9 d prepartum were lower arterial insulin and acetate, higher arterial nonesterified fatty acids and increased net liver removal of glycerol. After calving, PDV and liver blood flow and oxygen consumption more than doubled as DMI and milk yield increased, but 85 and 93% of the respective increases in PDV and liver blood flow at 83 DIM had occurred by 11 DIM. Therefore, factors additional to DMI must also contribute to increased blood flow in early lactation. Most postpartum changes in net PDV and liver metabolism could be attributed to increases in DMI and digestion or increased milk yield and tissue energy loss. Glucose release was increasingly greater than calculated requirements as DIM increased, presumably as tissue energy balance increased. Potential contributions of lactate, alanine, and glycerol to liver glucose synthesis were greatest at 11 DIM but decreased by 83 DIM. Excluding alanine, there was no evidence of an increased contribution of amino acids to liver glucose synthesis is required in early lactation. Increased net liver removal of propionate (69%), lactate (20%), alanine (8%), and glycerol (4%) can account for increased liver glucose release in transition cows from 9 d before to 11 d after calving.

Key Words: portal-drained viscera • liver • transition

Abbreviation key: ECD = expected calving date, ME = metabolizable energy, PAH = -aminohippurate, PCV = packed-cell volume, PDV = portal-drained viscera, TAG = triacylglycerol

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The transition from late gestation to early lactation is a period of dramatic physiological and metabolic adaptation for the dairy cow. During late gestation the nutritional demands of the fetus and uterus increase exponentially, while intake is often reduced by the endocrine changes that induce parturition, parturition itself, and other factors impacting on intake (Ingvarsten and Andersen, 2000). After calving, increases in nutrient requirements for milk synthesis outpace increases in intake, while hypertrophy of visceral tissues adds to nutrient demands (Bell, 1995). For the dairy cow, the most severe nutritional imbalances typically occur during transition (Grummer, 1995). The implications of transition metabolism and management for the well-being, reproduction, longevity, and production of the cow and the profitability of the dairy enterprise, are intuitive. The last decade has seen a considerable increase in basic and applied research concerning the nutritional physiology and metabolism of the transition dairy cow, and transition metabolism and nutrition has been the subject of numerous reviews (e.g., Bell, 1995; Grummer, 1995; Drackley, 1999; Drackley et al., 2001). However, as emphasized in many of these reviews, there is a paucity of information on the quantitative changes in nutrient absorption and the metabolism of splanchnic tissues (the portal-drained viscera [PDV; the gastrointestinal tract, pancreas, spleen, and associated mesenteric and omental fat] and liver) that occur in vivo during transition.

Available data describing the metabolism of splanchnic tissues during transition in dairy cows is largely based on measurements obtained in vitro, using tissue biopsy, or in vivo measurements of blood-borne metabolite concentration or turnover (e.g., Bell, 1995; Drackley et al., 2001). The hormonal changes initiating parturition and lactation are known to induce changes in fatty acid metabolism as early as 10 d before calving (Grummer, 1995). Increases in liver protein synthesis have also been reported 10 d before parturition in dairy cows, suggesting adaptations of protein metabolism required for lactation and perhaps liver hypertrophy, may precede parturition (Bell, 1995). In sheep, liver blood flow, glucose production, and lactate and NEFA removal increased markedly in late gestation (Freetly and Ferrell, 1997, 1998, 2000). In dairy cows, glucose turnover increases after calving (Bell, 1995), but it is assumed that propionate availability limits glucose production, increasing glucogenic demands on amino acids (Drackley et al., 2001). However, quantitative effects of transition on liver glucose production and glucose precursor utilization have not been described in dairy cows in vivo. The objective of our study was to measure the effects of transition on the quantitative metabolism of the PDV and liver of dairy cows.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Cows, Diets, and Treatments
All procedures used were licensed and regulated by the UK Home Office under the Animals (Scientific Procedures) Act of 1986. Five Holstein-Friesian cows fitted with ruminal cannulas and catheters enabling measurements of splanchnic metabolism (Huntington et al., 1989) in the portal vein, a hepatic vein, two mesenteric veins, and the caudal aorta via the mesenteric artery were used. In addition, each cow’s right carotid artery was elevated to a subcutaneous position for temporary catheterization if the aortal catheter lost patency. The cows were approaching their third (three cows), fourth, or seventh lactation at the start of the study. Three cows were cannulated and catheterized for previous studies and had completed from one to four full lactations since catheterization surgery (402 to 1402 d before sampling for the present experiment). Two of these cows had duodenal and ileal cannulas that were established during abdominal surgery for catheterization of splanchnic blood vessels. The other two cows were surgically prepared for the present study. They had rumen and catheterization surgery during and at the end of, respectively, their previous lactation (at least 108 d before sampling for the present experiment). Cows were housed in an unheated tie-stall barn with forced air ventilation and translucent roofing. Yokes or head halters were used for restraint and wood-shavings on rubber mats were used for bedding. Cows had constant access to water, and the barn was artificially lit from approximately 0600 to 1800 h. Cows were moved to straw-bedded box stalls for calving and returned to tie stalls within 48 h of parturition.

Beginning at least 6 wk before expected calving date (ECD), cows were offered a grass silage-based gestation TMR (Table 1) to meet estimated requirements for metabolizable energy (ME) and CP (NRC, 1989). The amount offered was then held constant until calving. A corn silage-based TMR (Table 1) was fed at 2 kg DM/d beginning 10 d before ECD and for ad libitum intake after calving. Cows were milked at approximately 0600 and 1700 h, and daily rations were fed as 3 equal meals provided at 0830, 1630, and 2230 h. Orts were removed at 0815 h. Ration composition was adjusted weekly for the average DM content of components during the preceding week.

Sample analyses.
Blood samples were kept on crushed ice until processed. Blood for metabolite analysis was added to a composite sample for each sampling site that was immediately flash-frozen using liquid N, then mixed when thawed for analysis. Plasma was harvested from a subsample of blood after centrifugation at 1800 x g for 20 min at 2°C. Plasma was flash-frozen after the addition to a composite sample or analyzed immediately for PAH, glucose and L-lactate as described by Benson et al. (2002). Blood pH, hemoglobin, and PCV and partial pressures of CO₂ and O₂ were measured as described by Benson et al. (2002). Frozen blood and plasma were stored at -85°C until analyzed. After thawing, composite samples for each cow sampling and sampling site were analyzed for blood concentrations of ammonia, urea, BHBA, and VFA and plasma concentrations of NEFA and insulin as described by Benson et al. (2002) and Benson and Reynolds (2001). In addition, composite blood samples were deproteinized and neutralized as described by Benson et al. (2002) and analyzed for alanine as described by Reynolds and Tyrrell (1991). Composite plasma samples were analyzed for triacylglycerol (TAG) and glycerol using an enzymatic assay (Assay 337-B, Sigma-Aldrich Ltd., Poole, UK) adapted for use on a Cobas Mira analyzer (Roche Diagnostic Products, Welwyn Garden City, UK).

Calculations.
Calculation of blood O₂ and CO₂ concentrations, blood and plasma flows, and net flux rates of metabolites for the PDV, liver, and total splanchnic tissues were as described previously (Benson et al., 2002). With venous-arterial differences, positive rates denote net release of a metabolite into venous blood, while negative rates denote net removal from blood supply. For metabolites removed by the liver, extraction rate was calculated as net removal divided by total blood supply times 100. For metabolites released by the PDV and removed by the liver, the percentage of net PDV release removed by the liver on a net basis was also calculated as net liver removal divided by net PDV release times 100. In addition, the maximal possible net contribution of glucose precursors removed by the liver to glucose released by the liver, on a net basis, was calculated as 0.5 times net precursor removal divided by net glucose release times 100. A similar calculation was made for the maximal contribution of NEFA and n-butyrate removed by the liver to net BHBA release by the liver, assuming an average of 16 carbons per mole of NEFA removed. This is a conservative estimate of the carbon content of plasma NEFA and could underestimate NEFA carbon removal by as much as 9% (Drackley et al., 2001).

Statistical analyses.
Average rates of metabolism for each cow sampling were analyzed statistically for effects of average day relative to calving using the mixed procedure of SAS (2001). Data were analyzed as repeated measures within cow using either a compound symmetry or spatial power covariance structure, depending on goodness of fit criteria (Littell et al., 1996). In addition, the probability of linear, quadratic, cubic, or quartic effects of average day relative to calving were tested using sequential sums of squares and the mixed procedure of SAS (2001) as described by Littell et al. (1996). To aid interpretation of changes as cows progressed through transition, means for measurements at 19 d before calving (11 d after calving for milk variables) were compared to each subsequent mean using the Dunnet’s adjustment for LS means comparisons within the mixed procedure. One cow was not sampled at 12 wk postpartum, therefore least squares means are presented, and the SEM for n = 5 is given. There were no missing blood samples for any sampling day included in the statistical analysis of the data. Blood flow and net flux rates are presented as the average hourly rate for the 8-h sampling period, which may differ from average rates of metabolism for the entire day. As the number of animals used was limited, differences are discussed as significant at P < 0.10. Three additional measurements, obtained before calving in cows that calved later than, or on, their expected calving date, were not included in the statistical analysis. These samplings occurred on d -33 and -1 relative to calving in one cow, and during calving in another cow. This meant that one measurement included for 9 d before calving (11 d for that cow) was obtained from a cow fed 2 kg of lactation ration DM for 10 d.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
There were relatively few transition problems or veterinary treatments during the course of the study. One cow (6132) suffered from milk fever and was "down" for 24 h after calving but responded to treatment. Another cow had excessive uterine discharge and received a medicated uterine "flush" at 15 d postpartum. Two cows were inseminated before their measurement at 12 wk postpartum.

Dry Matter Intake, Milk Yield, and Body Weight
Dry matter intake was the same on the two sampling days before calving (Table 2), only declining, on average, on d -1, 0, 1, and 2 relative to parturition (Figure 1). This slight dip in daily DMI coincided with the period that cows were in box stalls for calving, so may in part reflect consumption of straw bedding not included in measurements of DMI. After returning to tie stalls and the change to the lactation ration, DMI increased (cubic, P < 0.01) and was highest during sampling at 12 wk postpartum (Table 2 and Figure 1). Milk (quadratic, P < 0.001) and 4% FCM (quadratic, P < 0.09) yield on sampling days was highest 33 d after calving (Table 2). Milk fat and protein yield were not affected by day postpartum. Milk protein concentration was higher during sampling at 11 d postpartum than on subsequent sampling days (quadratic, P < 0.006), while milk lactose concentration and yield increased (quadratic, P < 0.04) as lactation progressed (Table 2). The BW of cows decreased 90 kg at calving, then continued to decline gradually through wk 12 postpartum (quartic, P < 0.001; Table 2).

View larger version (18K): [in this window] [in a new window]   Figure 1. Dry matter intake () and milk yield (), and milk fat () and protein () concentration in transition dairy cows.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

Lactate metabolism.
One of the major changes in liver metabolism observed in gestating ewes was an increase in liver lactate removal and glucose release as gestation progressed (Freetly and Ferrell, 1998), presumably as a consequence of Cori cycling between the liver and uterus. In contrast, we observed no significant change in net liver flux of these compounds between 19 and 9 d before calving. In the cow sampled 4 times over the last 30 d of gestation in the present study (6698), there was little evidence of any change in liver lactate or glucose metabolism until the day before parturition (Figure 2). In the cow sampled on the day of calving (6132), no outward signs of parturition were evident as the cow approached her seventh calving. The cow was eating and ruminating as normal. Sampling was terminated after five hourly samples were obtained because excessively high levels of plasma lactate (>1.5 mM) and glucose (>6 mM) were measured on repeated samplings. A rectal palpation was performed and confirmed the calf was engaged in the pelvis. She calved within an hour of the cessation of blood sampling. The most striking observation in this cow was the increase in liver lactate removal and glucose release measured (Figure 2), which likely represented cycling of glucose and lactate carbon between the uterus and liver to provide energy for uterine contractions. In this regard, the increase in liver lactate removal measured in the cow sampled 1 d before calving (6698; Figure 2) may have been attributable to an increase in the number and magnitude of preliminary (Braxton Hicks) uterine contractions preceding parturition. Alternatively, these changes in lactate and glucose metabolism may have been associated with an increase in milk synthesis before calving. However, glucose use for prepartum milk synthesis would not give rise to lactate and thus could not explain the increase in arterial lactate concentration observed at calving.

View larger version (22K): [in this window] [in a new window]   Figure 2. Net portal-drained viscera release (open bars) and liver removal (solid bars) of L-lactate and net liver release of glucose (gray bars) in 2 cows during transition.

In pregnant and nonpregnant sheep a net release of TAG was measured across the liver using multicatheterization procedures (Freetly and Ferrell, 2000), whereas a gross removal of labeled TAG by the liver of catheterized sheep was measured by Bergman et al. (1971). Reid et al. (1979) measured a net release of TAG by the liver of two dry and two lactating cows fed to requirements and a net removal of TAG when the same cows were fasted for 6 d. In the present study, net liver removal of TAG was near 0, suggesting equal rates of gross removal and release, except 9 d before calving, when a significant net removal was measured ( Table 6). Reasons for this increase in liver removal of TAG by the liver are not certain. Reid et al. (1979) suggested that the increase in net liver removal of TAG by the liver of dairy cows during fasting was a consequence of decreased gross release, due to an increase in NEFA oxidation, rather than an increase in gross removal. If the simultaneous increase in arterial NEFA concentration observed 9 d before calving reflects a decrease in lipogenesis within adipose tissue and associated reductions in adipose lipoprotein lipase activity, then the increase in net liver removal of TAG may represent an alternative disposal of absorbed TAG. The potential role of liver lipoprotein lipase in the uptake of plasma TAG by the bovine liver was considered previously (Drackley, 1999).

Changes in lactate and NEFA removal by the liver between 19 and 9 d before calving were significant when considered relative to their net PDV release (Table 7) and liver release of glucose and BHBA, respectively (Table 8). In support of this observation, isotopic measurements of glucose and lactate turnover, and interconversion, obtained in one cow at 28, 21, and 5 d prepartum, showed an increased proportional conversion of lactate to glucose at 5 d prepartum (Baird et al., 1983). Liver removal of NEFA is dictated by the concentration of NEFA in plasma reaching the liver, as liver extraction of NEFA supply is constant across a range of plasma NEFA levels (Bell, 1995). In support of this generality, liver extraction of total NEFA supply was not significantly different across sampling times in the present study. Although liver NEFA extraction was slightly lower in early lactation (Table 7), when arterial concentration was highest (Table 4), overall changes in liver extraction of NEFA supply were relatively small. As the liver receives as much as 38% of cardiac output (Huntington et al., 1990), it is particularly susceptible to an oversupply of NEFA when arterial concentrations increase during transition.

Splanchnic Blood Flow and Oxygen Consumption
Portal vein and liver blood flow increased during lactation to more than double the average rates before calving by d 83 postpartum (213 and 215%, respectively), but the majority of this increase (85 and 93%, respectively) occurred in the first 11 d after calving (Figure 3). Previous studies have suggested that PDV and liver blood flow are largely determined by ME intake, but that other factors, such as total DMI and the fiber content of the diet and resulting effects on gut fill, mass, and work load, modulate this overriding relationship between splanchnic blood flow and ME (Reynolds, 1995). In the present study, increases in ME intake after calving were greater than for DMI, as the ME concentration of the lactation ration was greater (Figure 3). While the relative increase in ME intake was similar to the relative increase in portal vein blood flow at 11 d postpartum (Figure 3), ME intake continued to increase with successive samplings and at a greater rate than blood flow. These comparisons illustrate that the relationship between ME intake and blood flow changed as cows progressed through transition. This may in part reflect the influence of the fiber content of the diet on PDV blood flow during gestation, as well as additive effects of liver O₂ requirements, and PDV and liver growth, on splanchnic blood flow. Reynolds (1995) noted that liver blood flow is not determined solely by ME intake but also influenced by oxidative metabolism, which is in part reflected by variations in hepatic artery blood flow to meet liver O₂ demands.

View larger version (15K): [in this window] [in a new window]   Figure 3. Postpartum increase in DM (open triangle) and metabolizable energy intake (solid triangle), PDV (open square) or liver (solid square) blood flow, and PDV (open circle) or liver (solid circle) oxygen consumption relative to average rates prepartum in transition dairy cows.

Nutrient Metabolism by the Portal-Drained Viscera
The transitional response of net PDV nutrient release was generally characterized by two patterns of response, reflecting the origin of the compounds released. For compounds whose supply was determined primarily by the absorption of products of diet digestion and fermentation, increases in their net PDV release appeared to be driven by relative increases in diet intake (e.g., VFA, lactate, BHBA, and ammonia). A linear relationship between diet or diet component intake and net PDV release of these nutrients has been reported previously in a number of studies (e.g., Nozière et al., 2000). For compounds released from PDV tissues as a consequence of adipose turnover (NEFA and glycerol), maximum rates of release were measured in early lactation, presumably as a consequence of negative energy balance and the mobilization of portal-drained adipose tissue. Glycerol and NEFA released into the portal vein predominantly reflect release from omental and mesenteric fat, which can account for up to 28% of body adipose tissue in postpartum dairy cows (Gibb et al., 1992). For alanine, which is both absorbed from the small intestine and synthesized from degraded amino acids and pyruvate, increases in early lactation reflect both increased protein digestion in the small intestine and protein turnover within PDV tissues. In addition, interpretation of net PDV flux rates for amino acids and other absorbed nutrients not subject to immediate clearance by the liver is complicated by simultaneous absorption into portal blood and extraction from arterial blood by PDV tissues. In this regard, net PDV flux of glucose, urea, and CO₂ showed considerable variation in the present study, reflecting the small venous-arterial concentration differences for these compounds and their potential bi-directional transfer across PDV tissues. Although insulin concentration was reduced during transition, this reduction was not attributable to significant differences in net PDV or liver flux. However, circulating insulin concentrations are also determined by kidney clearance, and measurement of secretion ideally employs more frequent sampling than possible in the present study (Benson and Reynolds, 2001).

Liver Metabolism
Glucose metabolism.
Whole body glucose and NEFA turnover are greater in lactating than in gestating cattle and sheep (Baird et al., 1983; Bell, 1995; Drackley, 1999). In sheep, glucose turnover is greater during lactation than late gestation (Baird et al., 1983) as a result of greater hepatic production (van der Walt et al., 1983), but the contribution of lactate to glucose synthesis was greater during gestation. In the present study, rates of liver glucose production in late gestation were similar to rates of glucose turnover measured by isotope dilution in two Holstein cows at the same stage of gestation (Baird et al., 1983). After calving, liver glucose production increased as milk yield and ME intake increased, nearly doubling within 11 d of lactation (Table 6). Liver glucose production is highly regulated and coordinated with body requirements, which in a majority of physiological circumstances are determined by, or related to, ME intake (Reynolds, 1995). However, this relationship is challenged in states of undernutrition, such as feed restriction, early lactation in the dairy cow, or late gestation in the ewe, when body requirements are greater than nutrient inputs.

Predicted glucose supply and requirement.
In the present study, measured glucose output by the liver, which excludes the contributions of the kidneys, was more than adequate to meet body glucose requirements calculated using estimated requirements for maintenance, gestation, and measured milk lactose synthesis (Elliot, 1976; Drackley et al., 2001; Figure 4). The calculated surplus, relative to estimated requirement, was least in gestation and increased as lactation progressed, perhaps reflecting a shift in ME utilization towards body energy deposition. The estimate of glucose requirement of Elliot (1976) assumed a minimal body glucose requirement for maintenance. At 83 d postpartum, predicted glucose requirement is much lower than measured glucose production, reflecting an underestimate of glucose use, and thus requirement for tissue energy deposition.

View larger version (15K): [in this window] [in a new window]   Figure 4. Measured liver glucose release (open bar) and predicted glucose requirement (solid bar) in transition dairy cows.

With the exception of alanine, data from the present study do not support the concept that a greater relative contribution of amino acids to glucose synthesis is required in early lactation. The removal of other amino acids by the liver was not measured and surely increased after calving. However, based on relative rates of net liver glucose release and measured precursor removal (Table 9), the minimum required amino acid contribution in addition to alanine (100 minus total), was not different across sampling times but numerically lowest (15.7%) at 11 DIM. While there was a doubling of the potential contribution of alanine to liver glucose synthesis, in the present study there were no indications of a shortage of glucose precursors at 11 d postpartum. On an incremental basis, increases in net propionate, lactate, alanine, and glycerol removal by the liver between 9 d before and 11 d after calving can account for 100% (69.2, 19.5, 7.5, and 4.2%, respectively), of the concurrent increase in net liver glucose production. A greater contribution of amino acids to glucose synthesis may have occurred in the first days of lactation, before intake increased (Figure 1). However, considering the importance of essential AA for milk and visceral tissue protein synthesis during this period, the use of their carbon for glucose synthesis may reflect obligatory catabolic processes and ureagenesis rather than a metabolic requirement for lactation.

A greater potential contribution of propionate to glucose synthesis at 83 d postpartum was balanced by a reduction in the potential contribution of lactate, which is the precursor most heavily recruited for glucose synthesis when propionate is in short supply (Lomax and Baird, 1983). However, if lactate is derived from Cori cycling of glucose with other body tissues, less ATP will be available for anabolism of peripheral tissues. Similarly, the alanine removed by the liver may be synthesized using pyruvate derived from glycolysis; thus, an increased contribution of lactate and alanine to glucose synthesis in early lactation reflects a state of energy movement from body tissues to milk. Results of the present study do not refute the concept that propionate availability limits glucose production in early lactation. However, based on other studies, the provision of additional propionate may have simply reduced lactate removal by the liver, without increasing glucose production (Reynolds, 1995).

Metabolism of other nutrients.
Aspects of liver lipid and glucose metabolism during transition were discussed previously. While liver metabolism of these nutrients appeared to be driven by increases in milk yield, changes in net liver metabolism of many metabolites after calving were generally associated with, or dictated by, changes in their net PDV release as DMI increased. In the case of the VFA and ammonia removed by the liver, increases in net PDV release were largely matched by increases in net liver removal of these compounds. One exception in this regard was the reduced extraction of propionate, i-butyrate, n-butyrate, and i-valerate as a percentage of total supply at 11 d postpartum (Table 8), when arterial concentrations were increased (Table 4), which may reflect a limitation of liver metabolic capacity during transition. For BHBA, changes in net liver release after calving could be attributed both to changes in PDV release of n-butyrate and thus DMI, as well as changes in liver removal of NEFA, and thus body energy balance. On a net basis, estimates of liver removal of NEFA and n-butyrate carbon were nearly equal to, or exceeded, net BHBA carbon release (Table 9). These measurements excluded acetoacetate released by the PDV, which is normally removed by the liver and converted to BHBA (Reynolds, 1995). Other fates of NEFA carbon include oxidation, TAG synthesis, and release as acetate.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Apart from an increase in net liver removal of glycerol at 9 d before calving and numerical increases in liver NEFA and lactate removal, there was little evidence in the present study of substantial changes in net splanchnic metabolism as dairy cows approached parturition. This suggests that even in late gestation, diet intake is the primary determinant of splanchnic tissue metabolism. After calving, substantial increases in PDV and liver blood flow were measured by 11 d postpartum, which in part reflect increases in DM and ME intake, as well as liver oxidative metabolism, and perhaps splanchnic tissue growth to support lactation. Changes in net splanchnic metabolism after calving generally reflected changes in DMI (e.g., PDV release of VFA and ammonia), milk yield (e.g., liver glucose release) or body energy status (e.g., liver removal of NEFA, glycerol, lactate, and alanine), depending on the origins of the nutrients measured. Measured net glucose release by the liver was greater than calculated requirements, with an increasing surplus evident as lactation progressed. In the present study, the maximal potential contribution of lactate, alanine, and glycerol to liver glucose synthesis was greatest 11 d after calving, but there was no evidence of an increased contribution of other amino acids to glucose synthesis as milk yield and liver glucose production increased. These data emphasize the importance of lactate and alanine conversion to glucose via pyruvate carboxylase in early lactation, but suggest that the gluconeogenic contribution of other amino acids may reflect obligatory catabolic processes and ureagenesis, rather than a metabolic requirement in early lactation.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Funded by the Milk Development Council of England and Wales. The dedicated support of technical staff of the CEDAR Metabolism Unit in the daily management and care of cows and the contributions of J. A. Benson are gratefully acknowledged.

	FOOTNOTES

 
¹ Present address: Department of Animal Sciences, The Ohio State University, OARDC, 1680 Madison Ave., Wooster, OH 44696-4076.

Received for publication June 21, 2002. Accepted for publication September 27, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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