Review and an experimental study on the physiological and quantitative aspects of gluconeogenesis in lactating ruminants

Glucose Metabolism in Lactating Cows in Response to Isoenergetic Infusions of Propionic Acid or Duodenal Glucose

A bibliographical study showed that increasing supplies of glucogenic nutrients lead to a curvilinear increase in milk and protein yield. Increased post-hepatic glucose availability may be involved in the increase in milk yield. In the present experiment, 5 dairy cows were arranged in a 5 x 5 Latin square design to compare the respective effects of 2 amounts of either duodenal glucose or ruminal propionic acid (C3) on glucose metabolism. Treatment consisted of a grass silage-based diet supplemented with glucogenic nutrients infused into the rumen as a mixture of volatile fatty acids (control) or C3 (6.5 and 13 mol/d) or as glucose (3.4 and 6.9 mol/d) infused into the duodenum. Treatments were isoenergetic and isonitrogenous and contained 100 and 115% of energy and protein requirements, respectively, according to the Institut National de la Recherche Agronomique. Glucose appearance rate (Ra) tended to increase with the level of infusions of both glucogenic materials and with the high dose of duodenal glucose. Plasma insulin-like growth factor-I (IGF-I) concentration increased with the infusion of glucogenic materials compared with the control and was significantly higher with glucose than with C3 treatments. This experiment did not indicate whether the increased Ra was the key mechanism to increased milk yield because milk yield only tended to increase and the standard error for Ra was high. With the high dose of glucose infused into the duodenum, the Ra increase was greater than the increased lactose production in milk. Because of that connection, IGF-I may also be involved by favoring the glucose utilization by the mammary gland.

Dietary protein concentration correlates in a complex way with glucose metabolism and growth performance in pregnant rats

We studied the influence of dietary protein concentration (PC) on the adaptation mechanism of maternal glucose metabolism to gestation, and on maternal body weight (BW) gain using pregnant rats fed 4% (low protein, LP), 10% (medium protein, MP) or 20% (high protein, HP) dietary protein concentration. Feed (FI), energy (EI) and protein (PI) intakes were recorded. Plasma glucose, insulin and GH were determined, and insulin:glucose (insulin resistance) and insulin:GH ratios computed. Correlation and regression analyses were used to determine the physiological relationships between metabolic and ponderal variables. MP dams with greater FI and EI (12%), and lower PI (44%) than HP dams showed similar maternal and fetal growth performance because of the maintained energy efficiency (EE). LP dams, with similar FI and EI, and lower PI (81%) than HP dams, failed to catch up due to the low EE. PC exerted a non-linear influence on BW gain, insulin and GH, through EI and EE modifications. The EI linearly influenced hormone concentrations with the maximum and minimum hormone response in MP and LP dams, respectively. Insulin resistance increased as PC decreased, whereas the GH lipolytic effect appeared to predominate over insulin action. Circulating hormones and metabolites affected nutrient partitioning between dam and fetus, with the result that the competing demands for materno-fetal growth of HP and MP dams, but not the LP dams, were satisfied. A deeper knowledge of the nature of the materno-fetal energy relationships will enhance the manipulation of the growth performance of the fetus in several animal species.

Feeding 2-Hydroxy-4-(Methylthio)-Butanoic Acid to Periparturient Dairy Cows Improves Milk Production but not Hepatic Metabolism

Forty-eight Holstein cows, entering second or later lactation, were utilized to determine the effects of 2-hydroxy-4-(methylthio)-butanoic acid (HMB) on milk production, hepatic lipid metabolism, and gluconeogenesis during the periparturient period. Cows were fed one of 3 diets as TMR starting 21 d before expected calving. These diets contained 0 (the basal diet), 0.09 (+HMB), or 0.18 (++HMB)% HMB. From parturition to 84 DIM, cows were fed diets that contained 0, 0.13, or 0.20% HMB. Prepartum and postpartum dry matter intakes were similar among cows fed the basal diet, +HMB and ++HMB. There was a quadratic effect on milk yield such that cows fed +HMB had the greatest milk yield; yields of milk by cows fed the basal diet and ++HMB were similar. This led to trends for increased yields of 3.5% fat-corrected milk and total solids when cows were fed +HMB. Percentages of fat, protein, and total solids in milk were not affected by treatment. Despite differences in milk yield, calculated energy balance was not affected by treatment. Plasma concentrations of NEFA, beta-hydroxybutyrate, and glucose were not different among treatments. Liver triglyceride content was similar among treatments on d 1 postpartum and was increased for cows consuming +HMB on d 21 postpartum compared with the other dietary treatments. Capacities for metabolism of [1-14C]palmitate by liver slices in vitro were not affected by treatment; however, conversion of [1-14C]propionate to CO2 and glucose decreased as the amount of HMB consumed by cows increased on d 21 postpartum. Cows consuming +HMB had greater days to first ovulation compared with cows consuming the basal diet and ++HMB as measured by plasma progesterone concentrations. These data suggest that adding HMB to low Met diets to achieve a predicted Met supply of approximately 2.3% of metabolizable protein supply is beneficial for increasing milk production but does not appear to benefit hepatic energy metabolism during early lactation.

Effects of casein and glucose on responses of cows fed diets based on restrictively fermented grass silage

This study was conducted to investigate whether well fed dairy cows given restrictively fermented grass silage diet will respond to incremental glucose and amino acid supply at early stage of lactation. Four rumen-cannulated Finnish Ayrshire cows were used in a 4 x 4 Latin square experiment with 14-d periods. The cows were fed good quality restrictively fermented grass silage ensiled with a formic acid additive for ad libitum intake. A concentrate mixture consisting of barley (85%) and solvent extracted rapeseed meal (11.4%) was given at a rate of 9 kg/d. The four treatments were continuous abomasal infusions of water (control), casein 300 g/d, glucose 300 g/d, and casein 300 g/d + glucose 300 g/d. The infusions had only minor effects on feed intake, diet digestibility, or rumen fermentation pattern. Both casein and glucose infusions increased milk, protein and lactose yields the effects being partly additive on the combined infusion. Infused casein increased milk protein and urea as well as plasma urea concentrations. Both casein and glucose tended to increase plasma glucose concentration. Casein increased arterial plasma concentrations of essential amino acids (EAA), branched-chain AA (BCAA), and total AA (TAA). Both casein and glucose, although glucose usually less than casein, increased arteriovenous differences of EAA, nonessential AA, BCAA, and TAA. Extraction efficiencies of AA were higher for glucose than for casein. Mammary plasma flow was at highest on the control diet, but reduced owing to infused nutrients, the reduction being less with combined rather than separate infusions of casein and glucose. Based on the partly additive increases in milk production parameters and changes in plasma metabolites, it is suggested that glucose alone increased milk protein yield by sparing AA from hepatic utilization, while casein increased both supply of AA and glucose. It was concluded that cows at early stage of lactation fed diets comprising of restrictively fermented grass silage and a cereal-based concentrate suffer from both limited AA and glucose supplies.

Effects of Various Glucogenic Sources on Production and Metabolic Responses of Dairy Cows Fed Grass Silage-Based Diets

Four rumen cannulated Finnish Ayrshire cows in midlactation were used in an experiment designed as a 4 x 5 incomplete Latin square with 2-wk periods to compare effects of glucogenic substrates on grass silage-based diets. The five treatments were continuous infusions of 1) water (control), 2) casein 300 g/d, 3) glucose 300 g/d, 4) propionic acid 247 g/d, and 5) barley starch 270 g/d. Substrates were infused either into the rumen (propionic acid) or into the abomasum (other substrates). As a basal diet, cows were fed a formic acid treated grass silage ad libitum (digestible organic matter 690 g/kg dry matter [DM], crude protein [CP] 131 g/kg DM) and a barley-rapeseed concentrate (CP 141g/kg DM) at a rate of 7 kg/d. Production responses to glucogenic substrates other than casein were negligible, suggesting that glucose supply of the cows did not primarily limit milk production. However, with casein cows produced significantly more milk, milk protein, and lactose than with other glucogenic substrates. Casein increased urea and essential amino acid (EAA), and decreased nonessential AA (NEAA) in arterial plasma compared with other substrates, suggesting that casein provided precursors both in terms of NEAA for gluconeogenesis and EAA for milk protein synthesis. This puts forward that providing the AA needs of the mammary gland for milk protein synthesis are met, glucose supply may become the next limiting factor for milk protein synthesis in cows fed diets based on restrictively fermented grass silage. The limited supply of AA from the basal diet, and possibly the low production levels of cows partly invalidated the hypothesis of monitoring differing glucogenic substrates for grass silage-based diets.

Potential environmental benefits of ionophores in ruminant diets

A concern of the USEPA is the volatilization of NH3 from animal manure and CH4 produced from ruminal fermentation. Excess N in the environment has been associated with adverse effects on human health, and CH4 and N2O emissions are sources of greenhouse gases. The objectives of this paper are to summarize and quantify the benefits of ionophores, principally monensin, in decreasing NH3 and CH4 emissions to the environment and reducing resource utilization in cattle (Bos spp.) production. The data indicate that monensin in the diets of ruminants may decrease protein degradation in the rumen and may increase feed protein utilization by an average of 3.5 percentage units. These changes would have an effect in reducing N losses and decreasing fecal N and the amount of protein that must be fed to meet animal requirements. Additionally, CH4 is produced by enteric fermentation in ruminants, which is responsible for about 33 to 39% of CH4 emissions from agriculture. Ionophores can reduce CH4 production by 25% and decrease feed intake by 4% without affecting animal performance. The inclusion of monensin in beef and dairy cattle diets may benefit air quality by reducing CH4 and N emissions and water quality by reducing N in manure, which can potentially leave the farm through leaching into ground water and through runoff into surface water.

Extent of hypophagia caused by propionate infusion is related to plasma glucose concentration in lactating dairy cows

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.

Effect of thermal injury on relative anaplerosis and gluconeogenesis in the rat during infusion of [U-13C] propionate

A new approach for the analysis of hepatic metabolism after burn injury is introduced. Relative anaplerotic, pyruvate recycling and gluconeogenic fluxes were measured by 13C NMR isotopomer analysis of blood glucose from rats with 40% body surface area injury, and from rats exposed to sham injury. A short chain fatty acid, [U-13C] propionate which is avidly extracted by the liver, was infused intravenously to deliver 13C into the citric acid cycle. There was no difference in the multiplets detected in the glucose carbon-2 (C-2) anomer from blood or liver after 45 or 60 min of infusion of propionate, indicating that steady-state isotopic conditions were achieved. Gluconeogenesis relative to citric acid cycle flux was not altered by burn injury; in both sham and burn groups the rate of glucose production was about equal to flux through citrate synthase. In the sham group of animals the rate of entry of carbon skeletons into the citric acid cycle was about four times citric acid cycle flux in animals after thermal injury. Similarly, flux through pyruvate kinase (again relative to citrate synthase) was significantly increased in burn injury.

Duodenal glucose increases glucose fluxes and lactose synthesis in grass silage-fed dairy cows

The effect of intestinal glucose supply on whole body rate of glucose appearance (WBGRa) and mammary utilization of glucose was studied in four lactating dairy cows. Glucose (0, 443, 963 and 2398 g/d) was continuously infused in the duodenum over 14-d periods using a Latin square design. A grass silage-based diet was formulated so that treatments were isoenergetic and isonitrogenous and contained 100 and 110% of energy and protein requirements according to INRA (1989). The WBGRa was measured by the [6,6-(2)H2]glucose dilution technique, and mammary glucose balance by arteriovenous differences and blood flow measurements. Duodenal glucose infusion increased arterial glucose concentrations linearly, whereas arterial concentrations of insulin, growth hormone, and glucagon were not changed. The WBGRa increased linearly with increasing glucose loads. The increase represented 42% of the intestinal glucose supplement. Mammary blood flow dramatically increased (up to 45%) and was associated with a significant increase of arterial insulin-like growth factor-1 concentrations. Mammary gland rate of glucose disappearance ([6,6-(2)H2]glucose measurement) increased linearly, whereas net mammary balance of glucose, lactose, and milk yields increased quadratically. Net mammary balance of glucose accounted for 60% of WBGRa, except for the greatest dose (47.6%). The decrease in milk yield with 2398 g/d of glucose may be explained by an imbalance in intracellular intermediate concentrations. The milk ratio of glucose-1-phosphate to glucose-6-phosphate decreased significantly at the greatest infusion of glucose. In conclusion, exogenous glucose supply to a grass silage-based diet increased WBGRa, mammary utilization of glucose and milk synthesis.

Splanchnic and mammary nitrogen metabolism by dairy cows fed dry-rolled or steam-flaked sorghum grain

Objectives were to determine net release or uptake of alpha-amino N, ammonia N, and urea N across portal-drained viscera, liver, splanchnic, and mammary tissues of lactating Holstein cows (n = 8, 86 +/- 8 d in milk) fed alfalfa hay-based total mixed rations containing 40% dry-rolled or steam-flaked sorghum grain. The total mixed rations were offered at 12-h intervals in a crossover design. Blood samples were obtained from indwelling catheters in the portal, hepatic, and mammary veins and mesenteric or costoabdominal arteries, every 2 h for each cow and diet. Steam-flaking increased in vitro rate of starch hydrolysis compared with dry-rolled sorghum (66 vs. 25%). Diet did not alter dry matter intake (18.2 +/- 0.3 kg). Daily milk yield (27.6 +/- 0.8 kg), efficiency of production, and most milk components did not differ between diets, but fat yield was reduced (0.86 vs. 0.91 kg/d) by steam-flaked sorghum, and lactose concentration was increased (4.99 vs. 4.82%). Blood flows in portal and hepatic veins did not differ between diets. Steam-flaking tended to increase urea N cycling to the gut (162 vs. 95 g/d) compared with dry-rolling of sorghum, whereas net absorption of ammonia N and alpha-amino N across portal-drained viscera were decreased. Net mammary uptake of a-amino N increased more than 20% (83 vs. 67 g/d), resulting in a higher mammary extraction ratio (15 vs. 11%) for steam-flaked versus dry-rolled sorghum. Flaking of sorghum improved the efficiency of postabsorptive N metabolism by increasing urea N cycled to the gut and alpha-amino N uptake by the mammary gland.

Effects of abomasal infusions of histidine, glucose, and leucine on milk production and plasma metabolites of dairy cows fed grass silage diets

Our previous study showed that His was the first-limiting amino acid (AA) for milk protein production in cows fed grass silage and cereal-based supplement. The aim of this study was to identify the second-limiting AA and determine whether glucose was limiting responses to His. Abomasal infusion of His (6.5 g/d), glucose (250 g/d), His (6.5 g/d) + glucose (250 g/d), His 6.5 g/d) + Leu (12 g/d) and His (6.5 g/d) + Leu (12 g/d) + glucose (250 g/d) on milk production and utilization of amino acids by mammary gland was in an incomplete 5 x 6 Latin square design with 14-d periods. The diet was based on restrictively fermented grass silage fed ad libitum and 8 kg/d of concentrate comprised of barley, oats, unmolassed sugar beet pulp, urea, and minerals. The infusions did not affect feed intake, diet digestibility, or rumen fermentation pattern. The molar proportion of propionate in rumen VFA was low (15.5%), suggesting that glucose supply from the basal diet could be limiting. Milk and milk protein yields were increased by His infusion. Infusion of His increased plasma His concentration from 19 to 52 microM but decreased extraction efficiency of His. Infusion of glucose increased plasma glucose concentration, milk lactose concentration, and yield and tended to increase milk protein yield. Responses in milk protein yield to combined infusions of His and glucose were additive, suggesting that the utilization of the first-limiting AA His was limited by glucose supply. Infusion of Leu increased plasma Leu concentration but did not produce any further milk protein yield response compared with the infusions without Leu. It was concluded that the efficiency of utilization of the first-limiting AA His could be improved by increasing the supply of glucose, when the basal diet produces a rumen fermentation pattern low in propionate. Leu was not the second-limiting AA in cows fed grass silage-based diets.

Changes in mRNA expression for gluconeogenic enzymes in liver of dairy cattle during the transition to lactation

The objective of this study was to profile phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC) mRNA expression in the liver of dairy cattle during the peripartum transition and determine changes in abundance of these mRNA in response to protein fed during the prepartum period. Thirty-eight multiparous Holstein cows were fed diets containing either 12% crude protein (CP) and 26% rumen undegradable protein (RUP), 16% CP and 26% RUP, 16% CP and 33% RUP, or 16% CP and 40% RUP on a dry-matter basis beginning 28 d before expected calving. After calving, all cows were fed a common diet through 56 d in milk (DIM). Northern analysis of RNA from liver biopsy samples obtained on days -28, -14, +1, +28, and +56 relative to calving indicated that PC and PEPCK mRNA expression were responsive to onset of lactation but not to prepartum protein or RUP concentration. Abundance of PEPCK mRNA was similar at -28, -14, and +1 DIM but was elevated by +28 and +56 DIM relative to precalving levels. Liver PC mRNA abundance was elevated on +1 DIM, remained elevated through 28 DIM, and declined to precalving levels by 56 DIM. The activity of PC enzyme was correlated (r2 = 0.89) with PC mRNA abundance. The data demonstrate increased abundance of PC mRNA during the early transition period followed by increased abundance of PEPCK mRNA during the postpartum period and suggest increased potential metabolism of lactate, pyruvate, and amino acids that contribute to the liver pyruvate pool.

Invited review: summary of steam-flaking corn or sorghum grain for lactating dairy cows

Nineteen lactation trials (43 grain processing comparisons) are summarized, in addition to digestibility and postabsorptive metabolism studies. The net energy for lactation (NEL) of steam-flaked corn or sorghum grain is about 20% greater than the NEL for dry-rolled corn or sorghum. Based on lactational performance, steam-flaked sorghum grain is of equal value to steam-flaked corn, and steam-flaked corn is superior to steam-rolled corn. Steam-flaking of corn or sorghum compared to steam-rolling of corn or dry-rolling of corn or sorghum consistently improves milk production and milk protein yield. This result is because of a much greater proportion of dietary starch fermented in the rumen, enhanced digestibility of the smaller fraction of dietary starch reaching the small intestine, and increased total starch digestion. Steam-flaking increases cycling of urea to the gut, microbial protein flow to the small intestine, and estimated mammary uptake of amino acids. Steam-rolling compared to dry-rolling of barley or wheat did not alter total starch digestibilities in two trials, one with each grain source. Lactation studies with these processing comparisons have not been reported. Most cited studies have been with total mixed rations (TMR) and alfalfa hay as the principal forage. Additional studies are needed with lactating cows fed steam-flaked corn or sorghum in TMR containing alfalfa or corn silage. Optimal flake density of steam-processed corn or sorghum grain appears to be about 360 g/L (approximately 28 lb/bu).

Glucose kinetic responses to protein supplementation and exogenous somatotropin in late gestation dairy cows

Glucose kinetics were measured in late gestation multiparous Holstein dairy cows fed diets with different amounts of dietary crude protein (13.3 vs. 17.8%), with and without exogenous somatotropin. The trial was conducted as a completely randomized design; 35 cows were used in the final analysis. Kinetic measurements were made using the single injection technique with uniformly labeled 13C-labeled glucose. A diet that contained 17.8% crude protein appeared to increase glucose utilization without a corresponding increase in supply. The evidence was an increased rate of glucose disposal. In contrast, exogenous bovine somatotropin (Posilac, Monsanto Co., St. Louis, MO) appeared to enhance glucose conservation, as indicated by a tendency for a decreased fractional catabolic rate and an increased glucose pool size. Somatotropin appeared to modify glucose metabolism in a pattern favorable for supporting terminal fetal development and lactogenesis and for maintaining or enhancing maternal glycemia. The latter pattern could have implications for improving the health of periparturient dairy cows.

Metabolic and production responses to dietary protein and exogenous somatotropin in late gestation dairy cows

Forty-three multiparous Holstein cows were used in a completely randomized design to evaluate the effects of protein supplementation and the use of bovine somatotropin (bST; Posilac, Monsanto Co., St. Louis, MO) in late gestation on animal metabolism and productivity in the periparturient period. Treatments were initiated 28 d prior to expected calving date and continued through parturition. Diets contained either 13.3 or 17.8% crude protein and were formulated to be similar in profile of protein fractions. Within each dietary treatment, cows were assigned to receive either 0 or 500 mg of sustained released bST once every 14 d until parturition. Following parturition, all cows were subjected to the same management and dietary treatments, and production measurements were followed until 42 d in milk. The use of bST increased plasma glucose and decreased plasma nonesterified fatty acids and beta-hydroxybutyrate prior to calving. Despite these changes in metabolism, bST did not affect concentrations of fat and triglyceride in the liver prepartum or postpartum. Feeding the 17.8% protein diet failed to stimulate glucose metabolism and tended to elevate plasma beta-hydroxybutyrate in late gestation. Cows treated with bST produced 3.3 kg/d more milk than did controls across the first 42 d of lactation; the difference was 4.6 kg/d in wk 6 of lactation. The use of bST in late gestation has considerable promise to alter cow metabolism positively. The results of these and other possible periparturient responses to somatotropin may increase milk production in early lactation.

Influence of feed intake and source of dietary carbohydrate on the metabolic response to propionate and glucose challenges in lactating goats

The influence of type of nutrients in the diet (rapidly degraded starch plus rapidly degraded N or highly digestible fiber plus slowly degraded N) and amount of feed offered (2.40 +/- 0.10 or 2.00 +/- 0.10 kg of dry matter/d) on plasma constituents after a glucose tolerance test or a propionate challenge test were studied using 12 multiparous dairy goats. Type of diet and interaction between type of diet and amount of feed offered had no effect on the concentrations of any plasma metabolite measured during propionate challenge. During propionate challenge, goats fed at the high dry matter intake (DMI) had a higher clearance of propionate and glucose and a higher ratio of plasma insulin to glucose. During glucose challenges, goats fed diets that were rich in highly digestible fiber tended to have lower peak concentrations of glucose, and goats fed at the high DMI had a higher clearance of glucose. The plasma insulin response to the glucose test was not altered by treatment. The ratio of plasma insulin to glucose was numerically, although nonsignificantly, higher for goats fed at the high rate of DMI. Most of the differences concerning glucose metabolism, insulin response, and tissue responsiveness to insulin appeared to be primarily due to the amount of feed offered and not to the type of diets fed to dairy goats in midlactation.

Absorption and metabolism of short-chain fatty acids in ruminants

Short-chain fatty acids (SCFA), viz. acetate, propionate and butyrate are quantitatively important substrates in ruminant energy metabolism. In the reviewed literature, 16 44% of ME intake was recovered as portal appearance of SCFA. This is considerably lower than expected when related to the estimated intragastric flux of SCFA. The discrepancy is caused by portal drained viscera metabolism of arterially abundant metabolites e.g., acetate and the metabolism of acetate and butyrate to acetoacetate and D-3-hydroxybutyrate in the absorptive epithelia. Even though considerable variations between experiments on acetate and propionate appearance are found, there seems to be a great deal of evidence that the proportion of gastrointestinally produced acetate and propionate absorbed to the portal blood is 50-75%. The portal recovery of butyrate has been found to be between 10 and 36% dependent on intraruminal infusion rate. It is concluded that major parts of acetate and propionate are directly absorbed to the portal vein. The true absorption rate of acetate can only be estimated by taking the portal drained viscera metabolism of arterial acetate into account. Butyrate is generally found to have a low recovery in the portal vein, but the production of D-3-hydroxybutyrate seems to be underestimated in major parts of the literature. It is therefore necessary to measure portal appearance as well as portal drained viscera metabolism to assess the quantitative as well as the qualitative contribution of SCFA and SCFA metabolites to whole animal metabolism.

Protein density and its influence on metabolite concentration and nitrogen retention by Holstein cows in late gestation

Multiparous Holstein cows in late gestation were used in a completely randomized design to test the effects of prepartum protein supply on prepartum N balance, blood metabolite and hormone concentrations, and postpartum intake and milk production. Cows were assigned to one of three isocaloric diets that differed in amount of total dietary crude protein (CP) (10.6, 12.7, or 14.5% of dry matter) but not in CP degradability or solubility. All diets contained the following ingredients: corn silage, chopped grass hay, ground corn, soybean meal, expeller soybean meal, minerals, and vitamins. Following parturition, all cows were offered a similar diet. Nitrogen balance was measured on d 12 to 7 prior to the expected calving date. Cows were bled on d 5 prior to the expected calving date from just prior to feeding to 8 h postfeeding. As dietary CP increased, plasma glucose concentrations increased linearly, but no change was detected in plasma nonesterified fatty acids or serum insulin concentrations. Nitrogen intake, apparent and true digestibilities, fecal and urinary concentration of and N balance increased as the concentration of dietary protein increased. The efficiency of absorbed protein utilization decreased as protein intake increased. No change in postpartum intake or milk production was observed. An increase in N retention in late gestation cows that were in positive N balance did not increase postpartum milk production.