J. Dairy Sci. 2008. 91:2011-2020. doi:10.3168/jds.2007-0696
© 2008 American Dairy Science Association ®
Metabolism of Dairy Cows as Affected by Prepartum Dietary Carbohydrate Source and Supplementation with Chromium Throughout the Periparturient Period1
K. L. Smith*,
M. R. Waldron*,
L. C. Ruzzi*,
J. K. Drackley
,
M. T. Socha
and
T. R. Overton*,2
* Department of Animal Science, Cornell University, Ithaca, NY 14853
Department of Animal Sciences, University of Illinois, Urbana 61801
Zinpro Corporation, Eden Prairie, MN 55344
2 Corresponding author: tro2{at}cornell.edu
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ABSTRACT
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Holstein cows (n = 72) entering second or later lactation were used to determine whether metabolic indices and hepatic capacities for oxidation and gluconeogenesis from propionate are affected by source of carbohydrate in the prepartum diet and chromium-L-methionine (Cr-Met) supplementation throughout the periparturient period. Cows were fed prepartum diets as total mixed rations with the concentrate portion based either on starch-based cereals [high nonfiber carbohydrate (NFC); 1.59 Mcal/kg of net energy for lactation (NEL), 14.4% crude protein (CP), 40.3% NFC] or nonforage fiber sources (low NFC; 1.54 Mcal/kg of NEL, 14.5% CP, 33.6% NFC) from 21 d before expected parturition until parturition. After parturition all cows were fed a common lactation total mixed ration (1.74 Mcal/kg of NEL, 16.5% CP, 40.0% NFC). The Cr-Met was supplemented once daily via gelatin capsule at dosages of 0, 0.03, or 0.06 mg of Cr/kg of BW0.75. Thus, treatments were in a 2 (carbohydrate source) x 3 (Cr-Met) factorial arrangement. There was no effect of prepartum carbohydrate source on pre- and postpartum plasma concentrations of glucose, nonesterified fatty acids (NEFA), β-hydroxybutyrate (BHBA), insulin, glucagon, or insulin to glucagon ratio. However, cows fed the low NFC diet during the prepartum period tended to have greater plasma NEFA and lower BHBA concentrations postpartum. Liver glycogen concentrations tended to be greater on d 1 postpartum for cows fed low NFC prepartum. Supplementing 0.03 mg/kg of BW0.75 of Cr as Cr-Met increased prepartum plasma glucose and glucagon concentrations and tended to decrease prepartum plasma NEFA concentrations compared with either 0 or 0.06 mg of Cr/kg of BW0.75. Postpartum plasma glucose concentrations decreased linearly and glucagon concentrations were increased quadratically by administering increasing amounts of Cr-Met. Supplementing Cr-Met did not affect prepartum plasma concentrations of insulin or BHBA, postpartum NEFA or BHBA, or liver composition. There was an interaction of prepartum carbohydrate source and Cr-Met supplementation such that in vitro hepatic conversion of [1-14C]propionate to both CO2 and glucose was similar or increased when Cr-Met was supplemented to cows fed the low NFC diet but decreased when Cr-Met was supplemented to cows fed the high NFC diet. Insulin addition in vitro did not affect hepatic metabolism of propionate on d 1 postpartum. Overall, both the NFC content of the prepartum diet and Cr-Met had only modest effects on metabolic indices in this experiment.
Key Words: periparturient cow • carbohydrate • chromium
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INTRODUCTION
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During the transition period from late gestation through early lactation, the dairy cow undergoes tremendous metabolic adaptations (Bell, 1995) and the increased energetic demands of lactation result in mobilization of fat reserves into the circulation in the form of NEFA. Common dietary recommendations for closeup cows following publication of the NRC (2001) included feeding diets containing relatively high (36 to 44%) concentrations of NFC (NRC, 2001) to promote DMI during the peripartum period in an attempt to increase dietary energy intake and thus decrease the cow’s reliance on NEFA. However, compared with diets lower in NFC content, excessive concentrations of starch-based NFC (>40%) in the prepartum diet may result in a greater decrease in DMI immediately before calving (Minor et al., 1998; Rabelo et al., 2003) and could potentially be detrimental to postpartum health and performance. Increasing the NFC fraction of the diet, which in turn increases propionate production, may promote insulin secretion (Harmon, 1992). Hence, circulating NEFA and BHBA and liver triglyceride (TG) content in cows fed a high NFC diet might be lower due to insulin’s negative effects on the net release of NEFA from adipose tissue. However, greater serum insulin concentrations in animals fed high NFC diets also may account for a more pronounced decrease in DMI compared with animals fed lower NFC diets (Ingvartsen and Andersen, 2000; Allen et al., 2005).
Several studies (summarized by Overton and Waldron, 2004) have been conducted during the past 10 yr that focused on increasing prepartum energy supply by increasing the NFC content of the prepartum diet. Most studies reported one or more positive outcomes when greater concentrations of NFC were fed during the prepartum period; however, in all cases NFC content was confounded with energy content, and experiments have not been conducted to evaluate the effect of varying NFC levels of the prepartum diet independent of energy content of the diet. Nonforage fiber sources (NFFS) such as soybean hulls and beet pulp can be substituted for high-starch ingredients in dairy cow diets to decrease NFC content while keeping the energy content similar to a diet containing larger amounts of NFC. Pickett et al. (2003) demonstrated that replacing forage with NFFS in the diet fed prepartum may be advantageous to the cow by increasing prepartum DMI, thereby decreasing her reliance on body fat stores and decreasing circulating NEFA concentrations.
Other recent information indicated that supplementation with chromium-L-methionine (Cr-Met) during the periparturient period increased insulin concentrations prepartum and decreased NEFA concentrations in plasma (Hayirli et al., 2001). Subiyatno et al. (1996) demonstrated that cows supplemented with Cr had improved glucose tolerance and insulin responsiveness postpartum. The hypolipidemic effects may be attributed to increased insulin response and increased lipogenesis. Modulating the effectiveness of insulin’s effects on peripheral tissues may have significant implications for health and performance of early lactation dairy cows. Although the exact mechanism of action remains to be elucidated, Vincent (2004) proposed that chromodulin binds to and carries chromium and this complex activates the tyrosine kinase activity of the insulin receptor to activate phosphotyrosine phosphatase, resulting in an amplification of receptor kinase activity and the insulin signal, all of which could potentially decrease NEFA concentrations in the blood and liver TG content.
Results reported from performance variables measured as part of this experiment (Smith et al., 2005) indicated that varying the concentration of NFC in the prepartum diet independent of energy or fat content of the diet did not affect periparturient performance indices; however, administering increasing amounts of Cr-Met linearly increased milk yield and DMI during the postpartum period. In addition to our interest in determining whether varying the concentration of NFC in the prepartum diet would affect metabolic variables independent of effects on overall performance, we hypothesized that the positive effects on performance from Cr-Met administration were related to differences in energy metabolism. Therefore, the objective of this part of the experiment was to evaluate the effects of varying NFC content of the prepartum diet and administration of Cr-Met on important metabolic indices related to energy metabolism, with the specific objective of determining whether the improved performance of cows administered Cr-Met is related to changes in energy metabolism.
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MATERIALS AND METHODS
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Animals, Treatments, and Sampling
The Cornell University Institutional Animal Care and Use Committee approved all procedures involving animals in this study; the experiment commenced in July 2001, and ended in February 2002. The experimental design and treatments were described more completely in a previous publication (Smith et al., 2005). Briefly, 72 cows entering second or greater lactation were assigned to 1 of 6 treatments in a completely randomized design. Treatments in a 2 x 3 factorial arrangement were 2 levels of NFC in the prepartum diet (low vs. high) and 3 amounts of Cr (0, 0.03, or 0.06 mg of Cr/kg of BW0.75) administered as Cr-Met (Micro-Plex 1000, a source that supplied 1,000 ppm of Cr from a compound containing 1 atom of Cr and 3 molecules of Met; Zinpro Corp., Eden Prairie, MN). Prepartum diets were fed from 21 d before expected parturition until parturition and then all cows were fed a common lactation TMR. Ingredient and nutrient composition of the diets fed during the prepartum and postpartum periods are provided in Table 1
. The Cr-Met was administered orally once daily (1330 h) via gelatin capsule (Torpac Inc., Fairfield, NJ) from 21 d before expected parturition through 28 d postpartum.
Plasma and Tissue Sampling and Analyses
Blood samples were collected by venipuncture of the coccygeal vein/artery using heparinized Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ) at 1300 h every other day beginning 1 wk before treatment (28 d before expected parturition) and continuing through 28 d postpartum. Plasma was harvested following centrifugation (2,800 x g, 15 min at 4°C), snap-frozen in liquid N2, and stored at –20°C until analyses for metabolites and hormones. Plasma aliquots (1 mL) for glucagon analysis were added to tubes containing kallikrein protease inhibitor (aprotinin at 500 kIU/mL).
Liver samples were obtained from each cow via percutaneous trochar biopsy (Veenhuizen et al., 1991) on d 1 and 21 postpartum. Liver was blotted to remove excess blood and connective tissue and then was either snap frozen in liquid N2 and stored at –80°C until analyzed for TG and glycogen content, or was prepared for in vitro studies as described below.
Plasma concentrations of glucose were determined by enzymatic analysis (glucose oxidase) using a commercial kit (kit 510-A; Sigma Chemical, St. Louis, MO). Plasma concentrations of NEFA were analyzed by enzymatic analysis (NEFA-C; Wako Pure Chemical Industries, Osaka, Japan) using modifications described by McCutcheon and Bauman (1986) and Sechen et al. (1990). Plasma concentrations of BHBA were quantified (BHBA dehydrogenase) using a commercial kit (kit 310-UV; Sigma Chemical). All spectrophotometric measurements were conducted using a Versamax tunable microplate reader (Molecular Devices, Sunnyvale, CA). In addition, plasma was analyzed for concentrations of insulin by RIA (Ehrhardt et al., 2001) using bovine insulin (Elanco Animal Health, Greenfield, IN) and glucagon (Linco RIA kit, Linco Research, St. Charles, MO). Intra- and interassay coefficients of variation were 9.6 and 6.4%, and 9.5 and 9.9% for insulin and glucagon, respectively.
Glycogen content of liver was determined according to the procedures described in Hawk and Bergeim (1926) with modifications described by Bernal-Santos et al. (2003). Liver TG content was determined using the Folch extraction method (Folch et al., 1957) followed by a colorimetric method based upon the Hantzsch condensation for estimating serum TG (Fletcher, 1968) with modifications described by Foster and Dunn (1973).
In Vitro Studies of Liver Metabolism
Samples of liver collected from a subset of cows (n = 24) on d 1 postpartum were utilized to examine the effects of prepartum carbohydrate source and Cr-Met administration on hepatic capacity for propionate metabolism in vitro. After blotting the liver to remove excess blood and connective tissue, a portion of the liver was immersed in ice-cold PBS (0.015 M; 0.9% NaCl, pH 7.4) and transported to the laboratory within 45 min. Hepatic capacities for conversion of [1-14C]propionate (1 µCi per flask) in Krebs-Ringer bicarbonate media (final substrate concentration of 10 mM) to glucose and CO2 were measured in triplicate flasks using tissue slices (30 to 60 mg) according to procedures described by Piepenbrink and Overton (2003). Tissue metabolism was terminated using 0.5 mL of 0.75 M H2SO4 injected into the media at either 0 (blanks) or 120 min of incubation. An additional triplicate set of flasks incubated for 120 min also contained bovine insulin (10 nM; gift provided by L. Richardson, Elanco Animal Health, Greenfield, IN).
After termination of tissue metabolism, evolved CO2 was collected on NaOH-soaked (30% wt/vol) filter paper in a hanging center well for 1 h in a shaking ice-water bath. After 1 h, flasks were uncapped and the filter paper was removed to a scintillation vial and dried overnight under moving air at 39°C. Ten milliliters of scintillation cocktail (Scintisafe Econo 2; Fisher Scientific, Pittsburgh, PA) was added to each vial and radioactivity was measured using liquid scintillation spectroscopy.
After uncapping the flasks and removing the hanging center well for measurement of CO2, the contents of each flask were processed for gluconeogenesis. An internal standard ([3H] L-glucose, 0.055 µCi per flask) was added to the media, and flasks were neutralized and deproteinized by additions of saturated Ba(OH)2 solution. Radioactive glucose from media supernatants was isolated using an ion-exchange method of Azain et al. (1999) as modified by Piepenbrink and Overton (2003). Radioactivity was measured using dual-label liquid scintillation spectroscopy.
Statistical Analyses
Data for all metabolic indices from the same cows for which production data were eliminated from the data set (Smith et al., 2005) were eliminated before analysis. Data for plasma metabolites and hormones measured on the day of calving were also eliminated from the data set. Pretreatment values for blood metabolites and hormones were used as covariates during analysis of covariance applied to their corresponding measurements during the treatment period and data for the prepartum and postpartum periods were analyzed separately. An ANOVA was conducted on plasma concentrations of metabolites and hormones using the MIXED procedure of SAS (SAS Institute, 2001) for a completely randomized design with repeated measures. The model included the effects of carbohydrate source, Cr-Met supplementation, interaction of carbohydrate source and Cr-Met, day, and 2- and 3-way interactions of main effects with day; cow within the interaction of carbohydrate source and Cr-Met was considered the random variable. For each variable, cow was subjected to 6 covariance structures (first-order autoregressive, heterogeneous first-order autoregressive, compound symmetry, heterogeneous compound symmetry, first-order ante-dependence, and unstructured). The structure yielding the smallest Akaike’s information criterion was selected. The method of Kenward-Rogers was used for calculation of denominator degrees of freedom. Covariates were dropped from the model if P > 0.10 and the data reanalyzed. Orthogonal contrasts were used to assess linear and quadratic effects of increasing Cr-Met supply.
Before statistical analysis, triplicate values for blanks and live flasks from the liver metabolic incubations were averaged and blanks subtracted from values obtained from live flasks. Rates of conversion of [1-14C]propionate to CO2 and glucose were calculated based upon the known amount of radioactivity added to each flask that was converted to end products (corrected using the internal standard) during the 2-h incubation period. Data for liver composition and in vitro metabolism were subjected to ANOVA using the MIXED procedure (SAS Institute, 2001). The model included the effects of carbohydrate source, Cr-Met, the interaction of carbohydrate source and Cr-Met, the effect of insulin addition in vitro, and the 2- and 3-way interactions of insulin addition with in vivo treatments. Orthogonal contrasts were used to assess linear and quadratic effects of increasing Cr-Met supply. Significance was declared at P < 0.05, trends were declared at 0.05 < P < 0.15, and least squares means are presented throughout.
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RESULTS
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Results are reported as the main effects of carbohydrate source and Cr-Met supplementation because interactions of prepartum dietary carbohydrate source and Cr-Met supplementation were not significant (P > 0.15) for nearly all variables measured in this experiment. Variables for which significant 2- and 3-way interactions occurred involving the main effects and the main effects with time will be discussed where appropriate later in the text.
Effects of Prepartum Dietary Carbohydrate Source on Prepartum Metabolism
There was no effect of prepartum carbohydrate source on overall prepartum concentrations of glucose, NEFA, BHBA, insulin, glucagon, and the ratios of insulin to glucagon, glucose to insulin, and insulin to NEFA (Table 2). A statistically significant interaction (P = 0.05) between carbohydrate source and day occurred during the prepartum period such that plasma BHBA increased slightly (d –20 BHBA = 5.9 mg/dL; d –2 BHBA = 6.4 mg/dL) for cows fed high NFC as cows approached parturition, although differences were slight.
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Table 2. Effects of varying the NFC content of the prepartum diet on plasma metabolites and hormones in dairy cows1
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Effects of Prepartum Diet on Postpartum Metabolism
Cows fed the low NFC diet during the prepartum period tended (P = 0.13) to have greater postpartum plasma NEFA concentrations and tended (P = 0.14) to have lower plasma BHBA concentrations than cows fed the high NFC diet during the prepartum period (Table 2
). There was no effect of prepartum carbohydrate source on overall plasma concentrations of glucose, insulin, glucagon, and the ratios of insulin to glucagon, glucose to insulin, and insulin to NEFA (Table 2) during the postpartum period. There was a trend (P = 0.12) for an interaction during the postpartum period between prepartum carbohydrate source and day for plasma glucose such that concentrations for plasma glucose were slightly greater during the immediate postpartum period for cows fed the low NFC diet prepartum and then became more similar during wk 3 postpartum (data not shown). Consistent with the transient increase in plasma glucose during the postpartum period for cows fed the low NFC diet prepartum, cows fed the low NFC diet during the prepartum period had greater (P = 0.04) glycogen content in liver on d 1 after calving. There was no effect of prepartum carbohydrate source on liver TG content on d 1 postpartum or on liver TG and glycogen content on d 21 postpartum (Table 3).
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Table 3. Effects of varying the NFC content of the diet fed prepartum on liver triglyceride (TG) and glycogen content in periparturient dairy cows
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Effects of Chromium on Prepartum Metabolism
Chromium administration resulted in quadratic relationships for some of the plasma metabolites measured in this experiment such that cows administered 0.03 mg of Cr/kg of BW0.75 had greater (P < 0.01) prepartum glucose and glucagon concentrations in plasma compared with cows administered the other 2 doses of Cr-Met (Table 4). There was no effect of Cr on prepartum plasma NEFA, BHBA, insulin, or the ratios of plasma insulin to glucagon, glucose to insulin, or insulin to NEFA (Table 4).
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Table 4. Effects of Cr-Met supplementation during the periparturient period on plasma metabolites and hormones in dairy cows1
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Effects of Chromium on Postpartum Metabolism
Chromium administration resulted in quadratic relationships for some of the plasma metabolites measured in this experiment such that cows administered 0.03 mg of Cr as Cr-Met/kg of BW0.75 tended to have greater (P = 0.09) postpartum plasma glucagon concentrations and tended to have a greater (P= 0.06) glucose to insulin ratio. There was no effect of periparturient Cr supplementation on postpartum glucose, NEFA, BHBA, or insulin concentrations in plasma, or the plasma ratios of insulin to glucagon or insulin to NEFA (Table 4). Chromium supplementation during the periparturient period had no effect on liver concentrations of TG and glycogen on either d 1 or 21 postpartum (Table 5).
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Table 5. Effects of Cr-Met supplementation during the periparturient period on liver triglyceride (TG) and glycogen content in dairy cows
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Effects of Prepartum Diet and Periparturient Chromium Supplementation on In Vitro Liver Metabolism
Results for studies of [1-14C]propionate metabolism in liver slices in vitro are depicted in Figures 1, 2, and 3. There was an interaction of prepartum carbohydrate source and Cr-Met supplementation such that conversion of [1-14C]propionate to both CO2 (Figure 1) and glucose (Figure 2) decreased (P < 0.01) when Cr-Met was supplemented to cows fed the high NFC diet but was either not affected or slightly increased when Cr-Met was supplemented to cows fed the low NFC diet. Insulin addition in vitro did not affect (P > 0.15) conversion of [1-14C]propionate to CO2 or glucose (Figure 3), and there were no interactions of insulin with in vivo treatments (P > 0.15).
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Figure 1. Conversion of [1-14C]propionate to CO2 by liver slices from cows as affected by dietary carbohydrate source and Cr-Met supplementation. Values are least squares means; n = 9 for high NFC, 0 mg/kg of BW0.75 Cr-Met (white bars), n = 12 for high NFC, 0.03 mg/kg of BW0.75 Cr-Met (gray bars), n = 13 for high NFC, 0.06 mg/kg of BW0.75 Cr-Met (black bars), n = 13 for low NFC, 0 mg/kg of BW0.75 Cr-Met, n = 13 for low NFC, 0.03 mg/kg of BW0.75 Cr-Met, and n = 12 for low NFC, 0.06 mg/kg of BW0.75 Cr-Met; P-value for carbohydrate x Cr-Met interaction is 0.01.
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Figure 2. Conversion of [1-14C]propionate to glucose by liver slices from cows as affected by dietary carbohydrate source and Cr-Met supplementation. Values are least squares means; n = 9 for high NFC, 0 mg/kg of BW0.75 Cr-Met (white bars), n = 12 for high NFC, 0.03 mg/kg of BW0.75 Cr-Met (gray bars), n = 13 for high NFC, 0.06 mg/kg of BW0.75 Cr-Met (black bars), n = 13 for low NFC, 0 mg/kg of BW0.75 Cr-Met, n = 13 for low NFC, 0.03 mg/kg of BW0.75 Cr-Met, and n = 12 for low NFC, 0.06 mg/kg of BW0.75 Cr-Met; P-value for carbohydrate x Cr-Met interaction is 0.01.
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Figure 3. Conversion of [1-14C]propionate to CO2 and glucose by liver slices from cows as affected by addition of insulin [10 nM (4 ng/mL); black bars] or without insulin (0 nM; white bars) in vitro. Values are least squares means; P-values are 0.46 and 0.73 for CO2 and glucose, respectively.
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Interactions of Treatments
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Although interactions of prepartum dietary carbohydrate source and Cr-Met administration were not significant for most variables measured in this experiment, there were trends for interactions of carbohydrate, Cr-Met, and time. The biological significance of these interactions is questionable. There were prepartum carbohydrate source x Cr-Met supplementation x day interactions during the postpartum period for glucose (P = 0.02) and insulin to glucagon ratio (P = 0.06). All cows had comparable glucose concentrations during the prepartum period. In general during the postpartum period, cows not administered Cr-Met had greater plasma glucose concentrations, regardless of prepartum diet. Significant interactions of prepartum carbohydrate source and Cr-Met supplementation occurred in the postpartum period for glucagon (P < 0.01) and trends occurred in the postpartum period for NEFA (P = 0.07). Administration of increasing amounts of Cr-Met decreased concentrations of NEFA when cows were fed the low NFC diet prepartum and increased plasma concentrations when cows were fed the high NFC diet during the prepartum period. Cows fed the intermediate dose of Cr-Met and the low NFC diet prepartum had increased concentrations of glucagon during the postpartum period and cows fed the intermediate dose of Cr-Met and the high NFC diet prepartum had decreased concentrations of glucagon during the prepartum period when compared with cows fed the other 2 doses of Cr-Met. As reported above, cows fed the high NFC diet prepartum and supplemented with Cr-Met converted less [1-14C]propionate to CO2 and glucose than cows fed the low NFC diet prepartum. Collectively, these data suggest that there may be some modest, diet-dependent effects of Cr-Met administration on aspects of intermediary metabolism.
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DISCUSSION
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During this portion of our experiment, we sought to determine whether varying the NFC content of the prepartum diet would affect aspects of metabolism in periparturient dairy cows independent of changes in performance, given the overall lack of response of production-related variables to this treatment reported previously (Smith et al., 2005). Differences between treatments in metabolic variables in this experiment were minimal, but it is interesting to note that cows fed an isocaloric diet that was lower in NFC during the prepartum period actually had greater concentrations of glycogen in liver on 1 d postpartum than cows fed a diet containing a greater concentration of NFC. However, these cows tended to have greater NEFA and lower BHBA during the postpartum period. In general, these results demonstrate the potential to replace NFC in the prepartum diet with other feedstuffs without compromising performance and metabolism. Pickett et al. (2003) included NFFS in the prepartum diet to replace forage and measured decreased NEFA concentrations in plasma on d 5 and 3 prepartum, but there was no effect on plasma metabolites during the postpartum period. In contrast to our experiment, they replaced forages with NFFS, which likely increased the passage rate of the diet (Firkins, 1997) and led to the increase in prepartum DMI and the early postpartum decrease in circulating NEFA concentrations. Minor et al. (1998) reported increased glucose concentrations in plasma and decreased plasma concentrations of NEFA and BHBA when cows were fed increased concentrations of NFC in the prepartum diet. Furthermore, they reported that feeding the higher NFC diet during the prepartum period increased liver glycogen and decreased liver TG concentrations. However, carbohydrate source and energy content of the prepartum diet were confounded; therefore, the responses to feeding the prepartum diet higher in NFC content measured in their experiment may simply have been caused by increased energy intake (13.4 Mcal/d for low NFC vs. 20.9 Mcal/d for high NFC). Overall, our results suggest that varying the source of carbohydrate (starch vs. digestible NDF) has minimal effects on metabolism during the prepartum and postpartum periods.
In general, our experiment showed no significant biological changes in metabolites or hormones in response to chromium supplementation during the periparturient period, showing no direct reason for enhanced milk yield and DMI postpartum (Smith et al., 2005). Most likely the production response was due to variables not measured in our experiment because we expected Cr-Met to decrease plasma NEFA and improve metabolic status. Hayirli et al. (2001) reported effects of administering Cr-Met on plasma glucose concentrations during the prepartum and postpartum periods. However, cows administered Cr-Met in their experiment had greater prepartum serum insulin and lower NEFA than cows not receiving Cr-Met, perhaps caused by greater DMI in supplemented cows compared with the controls or because cows on their study were sampled only once during the prepartum period whereas our cows were sampled frequently during the last 21 d prepartum. They reported that postpartum concentrations of BHBA, glucose, NEFA, and glucagon were not affected by administration of Cr-Met; however, administering Cr-Met decreased concentrations of insulin during the postpartum period (Hayirli et al., 2001). Similar to the lack of response of liver TG to Cr-Met administration reported in our study, Hayirli et al. (2001) reported that Cr-Met administration did not affect liver TG concentrations. Contrary to other published results, Yang et al. (1996) reported that cows receiving Cr had increased plasma NEFA during the first 8 wk postpartum, probably due to increased milk yield. Besong et al. (1996) reported that Cr did not affect circulating concentrations of glucose, NEFA, or insulin, but cows fed supplemental Cr had lower plasma BHBA and liver TG concentrations than control cows.
Recently, McNamara and Valdez (2005) reported that cows supplemented with 10 mg/d of chromium as chromium propionate during the transition period had increased milk yield and DMI during the first 90 DIM as compared with controls. Similar to our results, they reported no significant change in serum glucose or NEFA concentrations in cows supplemented with chromium. In addition, they showed that supplemented cows had increased rates of net synthesis of fat in adipose and decreased net release. They hypothesized that chromium may be working through a linkage of chromodulin, the low molecular weight chromium binding protein (Vincent, 2004), with the insulin receptor and glucose transporters, thus facilitating reduced lipolysis, increased milk production, and increased DMI. This exact molecular mechanism remains to be elucidated in dairy cows; however, it is possible that chromium supplementation was working through similar mechanisms in our study to increase production and intake. In addition, other researchers (Yang et al., 1996; Al-Saiady et al., 2004) have hypothesized that chromium supplementation might promote the activity of the IGF receptor, mimicking the action of bST, and thus increasing milk yield. Subiyatno et al. (1996) reported an increase in the level of circulating IGF-1 in chromium-supplemented cows following propionate challenge. It is possible that chromium may have been working through this pathway in our experiment to increase milk production as well.
Reasons are unclear for the interaction of prepartum carbohydrate source and Cr-Met supplementation on the conversion of [1-14C]propionate to both CO2 (Figure 1) and glucose (Figure 2). Conversion to both decreased when Cr-Met was supplemented to cows fed the high NFC diet but was either not affected or slightly increased when Cr-Met was supplemented to cows fed the low NFC diet. However, we speculate that Cr-Met supplementation may have altered metabolism of the liver and tissues of the portal-drained viscera (gastrointestinal tract, pancreas, spleen, and associated mesenteric and omental fat) such that glucose supply from the portal-drained viscera was increased and the demand for the liver to produce glucose may have been lower in cows fed the high NFC diet prepartum. This speculation is consistent with the observations of Drackley et al. (2001), who reported that hepatic capacity for propionate metabolism was correlated with fat-free NEL intake (a proxy for propionate supply) only during the immediate postpartum period. We also speculate that intestinal glucose supply from cows fed the low NFC diet prepartum would be low; therefore, hepatic gluconeogenic capacity could be more unresponsive to inhibition by dietary or other factors. In addition, Subiyatno et al. (1996) reported an increase in the conversion of propionate to glucose following propionate challenge in early-lactation heifers supplemented with chromium. These results are consistent with the increase in conversion of propionate to glucose in cows supplemented with chromium and fed the low NFC diet prepartum.
We hypothesized that insulin would decrease conversion of propionate to glucose by liver slices, and that liver from Cr-Met cows would be more sensitive to the inhibitory effects of insulin on gluconeogenesis. Insulin addition in vitro did not affect (P > 0.15) conversion of [1-14C]propionate to CO2 or glucose (Figure 3), and interactions of insulin with in vivo treatments were not significant (P > 0.15). Perhaps this is due to the possibility that the liver is unresponsive to insulin at 1 d postcalving as a component of the homeorhetic adaptations that occur with the onset of lactation. A positive control including liver tissue from midlactation cows was not included in this experiment; doing so would have provided comparative information to perhaps strengthen this contention. In addition, it is possible that the 2-h incubation time was not long enough for insulin to affect enzyme transcription.
Primiparous cows fed chromium during the periparturient period were reported to have increased insulin sensitivity during the prepartum period and decreased insulin sensitivity postpartum (Subiyatno et al., 1996). In general, some degree of insulin resistance is beneficial during early lactation to direct nutrients toward the mammary gland and away from peripheral tissues (Bell, 1995). Isolated hepatocytes from nonlactating or lactating sheep converted propionate to glucose at similar rates and addition of insulin in vitro tended to increase the rate of gluconeogenesis from propionate, but only when insulin was added at very high, nonphysiological concentrations (100 nM; Emmison et al., 1991); there were no differences when 1 nM or 10 nM insulin were added. Donkin and Armentano (1995) isolated hepatocytes from ruminating calves and demonstrated that insulin addition did not alter conversion of propionate to glucose. Our results indicate the liver was not responsive to insulin at 1 d postpartum under our experimental conditions.
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CONCLUSIONS
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Feeding a high-energy diet containing a lower content of NFC during the prepartum period resulted in modest increases in liver glycogen content on d 1 postpartum compared with feeding a high-energy diet containing a greater content of NFC during the prepartum period; however, varying NFC content of the prepartum diet results in few other changes in metabolic variables measured in this experiment. Although changes in concentrations of metabolites and hormones in response to Cr-Met administration were statistically significant in some cases, the collective response of these metabolic indices was biologically modest. Results from metabolic incubations indicate that effects of Cr-Met on liver propionate metabolism may be diet dependent and that hepatic capacity for propionate metabolism on d 1 postpartum is refractory to insulin addition in vitro. The increased milk yield and DMI in response to Cr-Met administration in this experiment reported previously (Smith et al., 2005) possibly was a result of improved energy metabolism, but unlikely to be caused by changes in circulating metabolites measured in this experiment.
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ACKNOWLEDGEMENTS
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The assistance of the following students and colleagues at Cornell University in implementing the study is gratefully acknowledged: M. Partridge, T. Muscato, R. Ehrhardt, M. Piepenbrink, A. Kulick, G. Johnson, A. Rauf, K. James, and the staff at the Cornell University Dairy Teaching and Research Center.
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FOOTNOTES
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1 Supported in part by Zinpro Corporation (Eden Prairie, MN) and in part by the Cornell University Agricultural Experiment Station federal formula funds, Project No. 127453 received from Cooperative State Research, Education, and Extension Service, US Department of Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the US Department of Agriculture. 
Received for publication September 17, 2007.
Accepted for publication December 29, 2007.
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ABSTRACT
INTRODUCTION
