Chromium propionate enhances insulin sensitivity in growing cattle.

Thirty-six Angus and Angus×Simmental heifers, averaging 291 kg, were used to determine the effects of dietary Cr, in the form of Cr propionate (Cr Prop), on glucose metabolism and serum insulin concentrations following glucose administration. Heifers were stratified by body weight (BW) within a breed and randomly assigned to treatments. Treatments consisted of 0, 3, 6, or 9 mg of supplemental Cr/d from Cr Prop. Based on dry matter (DM) intakes, the daily doses of Cr were equivalent to 0.47, 0.94, and 1.42 mg of supplemental Cr/kg of DM. Heifers were individually fed a corn silage-based diet at a level of 2% of BW. Each heifer was also fed 0.45 kg of a ground corn supplement daily that served as a carrier for supplemental Cr. Glucose tolerance tests were performed on d 44 of the study. Glucose was infused via jugular catheters at a level of 0.45 g/kg of BW(0.75) over a course of 1 to 2 min. Blood samples were collected at -10, 0, 5, 10, 15, 30, 45, 60, 90, 120, 150, and 180 min relative to glucose dosing for glucose and insulin determination. Area under the glucose response curve was lower (1,603 vs. 1,964 mg/dL per minute) in heifers supplemented with Cr from 0 to 45 min following glucose challenge. Serum insulin concentrations were lower in Cr-supplemented heifers than in controls following glucose infusion. The molar ratio of insulin to glucose was also lower in Cr-supplemented heifers relative to controls. Serum insulin and serum insulin to glucose ratios did not differ among heifers supplemented with 3, 6, or 9 mg of Cr/d. Results indicate that Cr Prop supplementation increased tissue sensitivity to insulin in growing heifers. Based on insulin sensitivity, Cr requirements (as Cr Prop) of growing heifers can be met by supplementing with 3 mg of Cr/d or 0.47 mg of Cr/kg of DM.

The addition of cottonseed hulls to the starter and supplementation of live yeast or mannanoligosaccharide in the milk for young calves.

The objectives of this study were to investigate the effects of the addition of cottonseed hulls (CSH) to the starter and the supplementation of live yeast product (YST) or mannanoligosaccharide product (MOS) to milk, on growth, intake, rumen development, and health parameters in young calves. Holstein (n = 116) and Jersey (n = 46) bull (n = 74) and heifer (n = 88) calves were assigned randomly within sex at birth to treatments. All calves were fed 3.8 L of colostrum daily for the first 2 d. Holstein calves were fed 3.8 L of whole milk, and Jersey calves were fed 2.8 L of whole milk through weaning at 42 d. Calves continued on trial through 63 d. Six treatments were arranged as a 2 x 3 factorial. Calves received either a corn-soybean meal-based starter (21% crude protein and 6% acid detergent fiber; -CSH) or a blend of 85% corn-soybean meal-based starter and 15% CSH (18% crude protein and 14% acid detergent fiber; +CSH) ad libitum. In addition, calves received whole milk with either no supplement (NONE) or supplemented with 3 g/d of mannanoligosaccharide product (MOS) or 4 g/d of live yeast product (YST) through weaning at 42 d. Twelve Holstein steers [n = 6 (per starter type); n = 4 (per supplement type)] were euthanized for collection and examination of rumen tissue samples. Dry matter intake (DMI) was greater for Holstein calves fed +CSH (0.90 kg/d) than -CSH (0.76 kg/d). Final body weight at 63 d of Holstein calves fed +CSH (75.8 kg) was greater than that of those fed -CSH (71.0 kg). Average daily gain (ADG) was greater for Holstein calves fed +CSH (0.58 kg/d) than -CSH (0.52 kg/d). However, Holstein calves fed -CSH had a greater feed efficiency (FE; 0.71 kg of ADG/kg of DMI) than those fed +CSH (0.65 kg of ADG/kg of DMI). Also, Holstein calves fed +CSH had narrower rumen papillae (0.32 mm) compared with those fed -CSH (0.41 mm). There were no significant effects of CSH on DMI, ADG, or FE in Jersey calves. There were no significant effects of YST or MOS on DMI, ADG, FE, or rumen papillae measures in Holstein calves. Jersey calves fed YST or MOS had greater final body weight at 63 d (51.2 kg and 51.0 kg, respectively) than calves fed NONE (47.5 kg). However, there were no significant effects of YST or MOS on DMI, ADG, or FE in Jersey calves.

Starch utilization by ruminants: from basics to the bunk.

Starch is the major energy component of grains. Wheat contains 77% of DM as starch, corn and sorghum contain 72%, and barley and oats contain 57 to 58%. In vitro systems have provided valuable data on kinetic aspects of starch digestion. Molecular biological techniques have provided a clearer picture of the ruminal microbial milieu. Proportions of starch fermented in the rumen can be predicted satisfactorily for a variety of grains and processing methods. Compared with dry rolling, steam processing (flaking or rolling) increases ruminal digestibility of starch (percentage of intake) from 52 to 78% for sorghum, from 75 to 85% for corn, and six percentage units or less for other grains. Recent research provides new insight into pancreatic function and intestinal glucose transport systems. The capacity to digest starch in the intestine ranges from 45 to 85% of starch entering the duodenum, with that capacity apparently limited by the supply of pancreatic amylase. There is evidence that amylase secretion may be enhanced by increasing duodenal entry of protein. Capacity for active transport of glucose across of gut wall does not seem to limit the amount of starch digested that is absorbed as glucose. For ruminants eating medium- to high-concentrate diets, about 30% of their total glucose need comes from glucose absorption, 50% from organic acid absorption (substrates for hepatic gluconeogenesis), and 20% from other sources. When glucose absorption from the gut increases, ruminants generally adjust (decrease) gluconeogenesis to meet their need; that need is directly linked to DE intake. In terms of overall ME yield, grain starch is best used when it is fermented in the rumen. However, close coordination of protein and starch supply to the duodenum may improve capture of starch in the form of glucose.

Sources of variation in splanchnic blood flow in steers.

Data from six experiments were used to describe sources of variation in blood flow through portal-drained viscera and liver of 33 steers that were fed equal-sized meals every 2 h. The experiments were designed to create "steady-state" conditions under which response to various dietary or physiological treatments was assessed. Sums of squares for blood concentration of blood flow marker, venoarterial differences in blood flow marker, and blood flow were divided into variation attributable to steer; period (or time); the steer x period interaction; sampling days within steer and period; and replications (or samplings) within day, steer, and period. Steer was the largest single source of variation in arterial concentration of blood flow marker, accounting for from 42 to 80% of sums of squares among the experiments. However, replication within day, steer, and period accounted for more variation than steer in portal or hepatic blood flow in four of the six experiments. When balanced for treatment effects, steer accounted for 59%, period and the steer x period interaction accounted for 14%, and replication accounted for 27% of variation in portal blood flow. Corresponding percentages for hepatic blood flow were 45, 20, and 35%. I conclude that steer and replication within steer x period cells in a matrix of treatments are the two largest sources of variation and that there is more variation among samples on a given day within steer x period cells than among days in the same cells.

Feedlot performance, blood metabolic profile and calcium status of steers fed high concentrate diets containing several levels of calcium.

Two feeding trials with Hereford X Angus steers were conducted to evaluate effects of dietary Ca level in high concentrate diets on feedlot performance, carcass traits, blood metabolic profile and Ca status. Dietary treatments of .3, .6, .9 and 1.2% Ca (10 steers treatment-1 X trial-1) were fed for 90 (Trial 1) and 114 d (Trial 2). Ground limestone was the source of supplemental Ca. Blood samples were collected weekly throughout Trial 1 and for the first 7 wk of Trial 2. In both trials .3% Ca resulted in lighter weight (P less than .10) carcasses and lower (P less than .05) blood pH and HCO-3 than the rest of the Ca levels during the first 7 wk of feeding the high concentrate diets. Average daily gain increased linearly (P less than .10) in response to increased dietary Ca level in Trial 1, but not in Trial 2. There were no significant differences among treatments in feed intake, carcass quality traits, bone ash, bone Ca, or overall characteristics in the blood metabolic profile including total serum or plasma Ca and plasma ionizable Ca. Increasing dietary Ca to .6% or more resulted in improved blood acid-base status during the initial weeks of feeding high concentrate diets, which is associated with heavier carcasses and a trend towards more rapid weight gain. I interpret these data to suggest that the Ca requirement for steers fed high concentrate diets is greater than .3%, but no greater than .6% for maximal feedlot performance.

Metabolite flux across portal-drained viscera, liver, and hindquarters of hyperinsulinemic, euglycemic beef steers.

Metabolic response to i.v., exogenous insulin was characterized in three younger (355 d old, 340 kg BW) and four older (480 d old, 456 kg BW) beef steers. The steers had chronic indwelling catheters to allow measurement of blood flow and net flux of metabolites across portal-drained viscera (PDV), liver, and hindquarters (HQ). Daily N and ME intakes provided at least 1.8 times maintenance requirements. Net flux was measured before and after each steer received intramesenteric infusions of insulin, which ranged from 5 to 80 mU.h-1.kg BW-1, and intrajugular infusion of glucose to maintain euglycemia. Nonlinear fits of data provided predictions of maximal glucose entry (Rmax) for glucose infused, liver glucose release, HQ glucose uptake, and total glucose entry (TGE, sum of glucose infused plus liver release). Differences in BW could account for differences (P < .05) in Rmax for TGE between older steers (688 mmol/h) and younger steers (493 mmol/h). Plasma insulin needed to elicit half-maximal response (ED50) for TGE tended (P < .17) to be greater for older (82 mU/L) than for younger steers (49 mU/L). The decrease in liver glucose release in response to infusion of insulin was greater (P < .01) for older (-170 mmol/h) than younger (-106 mmol/h) steers. The ED50 for liver production of glucose tended (P = .13) to be greater for older (45 mU/L) than for younger (5 mU/L) steers. At 80 mU.h-1.kg BW-1, liver extraction of insulin decreased to approximately 50% of control extraction, and arterial insulin concentration was at least 9.4 times control concentrations. The PDV release and liver removal of L-lactate and propionate were not reduced by insulin infusion, although liver glucose release was 51 and 76% of control for younger and older steers, respectively. We concluded that older steers tended to be less sensitive than younger steers to the effects of insulin on glucose metabolism.

Effect of dietary lactic acid on rumen lactate metabolism and blood acid-base status of lambs switched from low to high concentrate diets.

Two experiments were conducted with ruminally fistulated wether lambs to determine the effect of lactic acid addition to a hay diet on rumen lactate metabolism, blood acid-base status and subsequent adaptation to a high concentrate diet. In Exp. 1, lambs were fed mature brome hay (H), H plus 5% (w/w) D,L lactic acid (H5L) or H plus 10% lactic acid (H10L) (three lambs per treatment) for 14 days (phase I) then switched to a 90% concentrate diet for 2 days (phase II). In Exp. 2, lambs were fed alfalfa-brome hay (H) (six lambs), H plus 2.5% lactic acid (H2.5L) (six lambs) or H plus 5% lactic acid (H5L) (four lambs) during phase I, then switched to a 70% concentrate diet (3 days) followed by a 90% concentrate diet (10 days) (phase II). During both experiments rumen fluid samples were taken periodically for pH and lactate analyses and in vitro L- or D-lactate disappearance (IVLD) studies. Blood samples were taken to measure acid-base status, serum lactate, and serum calcium, magnesium and phosphorus. Dietary lactic acid enhanced IVLD during phase I of both experiments. L and D isomer IVLD rates were similar and followed zero-order kinetics. In Exp. 2, IVLD increased rapidly during phase II in response to increased concentrate level in the diet; the enhanced rates of H2.5L and H5L lambs were sustained for the first 3 days of phase II. Blood data from both experiments indicated a deleterious effect of dietary lactic acid on blood acid-base balance; however, this treatment effect was not manifested in any symptoms of acute acidosis. There was a decrease (P less than .05) in serum calcium during phase II of both experiments. In Experiment 1, serum calcium increased linearly (P less than .05) in response to dietary lactic acid level. In Exp. 1, rumen fluid total lactate and L-lactate were lower (P less than .05) for H5L vs H lambs during phase II. However, all lambs in Exp. 1 experienced acute acidosis; four of the nine lambs subsequently died. There was evidence of acidosis in Exp. 2, but there were no clear treatment effects during phase II on rumen fluid pH or lactate, or feed intake. All lambs adapted to the high concentrate diets as evidenced by rumen lactate levels and feed intakes. In both experiments, the proportion of L-lactate in rumen fluid decreased from almost 100 to about 50% of total lactate by the end of phase II.

Net absorption of amino acids by portal-drained viscera and hind half of beef cattle fed a high concentrate diet.

Two experiments were conducted to measure the effect of level of feed intake on net amino acid absorption by portal-drained viscera of six beef heifers with catheters in a mesenteric vein, portal vein and iliac artery (Exp. 1) and to evaluate intrajugular infusion of insulin or glucose on amino acid uptake by hind half of four beef steers with catheters in posterior aorta and vena cava (Exp. 2). Experiment 1 was a replicated 3 X 3 Latin square design. Treatments were calculated intakes of 84, 157 or 225 kcal metabolizable energy (ME)/kg.75 live weight. Treatments in Exp. 2 were control (no infusion), insulin infusion (1.4 IU/min for 90 min) and glucose infusion (2.5 mmol/min for 90 min) in that order. Mean live weight of animals +/- SE was 295 +/- 4 kg (Exp. 1) and 345 +/- 15 kg (Exp. 2). The diet used in both experiments was pelleted, 85% concentrate (2.9 Mcal ME/kg dry matter). Blood flow (BF) was measured by dilution of a primed, continuous infusion of para-aminohippuric acid into the mesenteric vein (Exp. 1) or the posterior aorta (Exp. 2). Net uptake or absorption was the product of BF times portal-arterial (Exp. 1) or arteriovenous (Exp. 2) differences in amino acid concentrations in blood. Increased feed intake caused linear (P less than .05) increases in net absorption of several amino acids, including lysine, methionine, leucine and valine (Exp. 1). Feed intake did not affect (P greater than .05) net absorption of glutamate or glutamine.(ABSTRACT TRUNCATED AT 250 WORDS)

The net portal and hepatic flux of metabolites and oxygen consumption in growing beef steers given postruminal casein.

Changes in net portal and hepatic nutrient flux and oxygen consumption in response to 3-d abomasal casein infusions were studied in seven multicatheterized beef steers. Steers were fed 4.3 kg DM/d of a high-concentrate diet in 12 equal meals. Blood flow (para-aminohippurate dilution) and net flux (venoarterial concentration difference x blood flow) across portal-drained viscera (PDV) and hepatic tissues were measured on d 3 of the abomasal infusions. In two experiments, the response to 300 (300C) and 150 (150C) g casein/d were compared, respectively, to a control water infusion. The 300C increased (P less than .05) arterial blood concentrations of alpha-amino N (AAN), urea N and ammonia; 150C increased (P less than .05) arterial urea N. Urinary urea N excretion was increased (P less than .01) by 300C and 150C. Although 300C increased net PDV release of AAN (P less than .07) and alanine (P less than .10), there was no net change in total splanchnic (TSP) flux due to an increased net hepatic uptake of AAN (P less than .01) and alanine (P less than .05). Net PDV glucose flux was decreased (P less than .05) by 300C, but net hepatic glucose flux was not affected by either level of casein. The 150C increased TSP oxygen consumption (P less than .05) and hepatic oxygen extraction (P less than .10). Approximately 26 and 30% of the casein N infused abomasally appeared in the portal blood as AAN for 150C and 300C, respectively. The sum of net PDV ammonia and AAN fluxes accounted for 47 and 88% of the N infused for 150C and 300C, respectively. These data emphasize the importance of intestinal and liver tissues in regulating the flux of nitrogenous compounds absorbed from the diet.

Response of nitrogen metabolism in preparturient dairy cows to methionine supplementation.

Three multiparous Holstein cows (607 kg of BW) were surgically prepared with an elevated carotid artery and indwelling catheters in the hepatic, portal, and two mesenteric veins to study the effects of methionine supplementation on amino acid metabolism during the last 2 wk of pregnancy. The study began 15 d before the expected calving date. Dietary treatments were Control (1.53 Mcal NE(l)/kg, 15.6% CP, and 40% ruminally undegradable protein) and Control supplemented with 60 g/d of ruminally protected methionine (MET, supplying 39 g/d of DL-methionine and approximately 18 g/d of methionine available for intestinal absorption). Each cow received both dietary treatments in a crossover design. Cows were fed once daily. After 5 d on treatment, a blood flow marker (para-aminohippurate) was infused into a mesenteric vein, and arterial, portal, and hepatic blood samples were obtained at 0, 2, 6, 12, and 18 h after feeding. Net flux of methionine was calculated as the plasma arteriovenous difference multiplied by plasma flow. Dry matter intake (10.8 kg/d) and portal (824 L/h) and hepatic (995 L/h) plasma flows were not affected (P > .10) by treatment. Arterial plasma concentration of methionine was greater (P = .10) with MET (27.67 microM) than with Control (16.42 microM). Net portal absorption of methionine increased (P = .10) with MET (26.2 g/d) compared with Control (9.5 g/d). The net portal methionine flux was negatively correlated (r = -.59; P < .001) with arterial urea concentrations. Net flux of methionine across splanchnic tissues shifted (P = .06) from a net uptake with Control (4 g/d) to a net output with MET (11 g/d). Therefore, MET increased by 15 g/d the methionine supply to the rest of the body. The net uptake of methionine by splanchnic tissues observed with Control indicated a net mobilization of methionine by peripheral tissues. Results indicate that methionine was the limiting amino acid with Control and that MET was beneficial because it increased methionine supply to peripheral tissues and reduced arterial urea concentrations.

Net nutrient flux by visceral tissues of beef steers given abomasal and ruminal infusions of casein and starch.

The objective of this study was to quantify changes of net nutrient metabolism by portal-drained viscera (PDV) and liver of four beef steers (253 +/- 7 kg) in response to combinations of ruminal (R) or abomasal (A) infusions of cornstarch (S) and casein (C). The four treatments in a Latin square design were SACA, SACR, SRCA, and SRCR. Steers were fed alfalfa hay (DMI = 4 kg/d) as a basal diet in 12 equal meals delivered every 2 h and they received continuous infusion of S (800 g/d) and C (200 g/d) in 11-d periods. Digestibilities of DM, N, NDF, and starch, ruminal outflow of liquids and DM, and energy and N retention were less (P < .05) for SA than for SR. Net ammonia and glucose release from PDV were greater (P < .01) for SA than for SR. Net total VFA, acetate, and beta-hydroxybutyrate release from PDV and total splanchnic acetate and beta-hydroxybutyrate release were greater (P < .05) for SR than for SA, but starch infusion site had no effect (P > .10) on net urea N transfer or alpha-amino N release by PDV. Net release of ammonia N, propionate, and total VFA by PDV and uptake of urea N by PDV were greater for CR than for CA, but net alpha-amino N release by PDV and total splanchnic tissues were greater for CA than for CR (P < .05). Summation of energy supply by PDV indicated no difference in total supply among the treatments, but relative contribution of energy sources was affected by infusion site. Energy release by PDV per unit of DE intake was .68 and .66 for SA and SR, respectively. Net release of glucose by PDV was greater for SACA than for SACR (P < .05). These results suggest that site of digestion of starch and casein varies the relative contribution of nutrients to energy supply by visceral tissues and therefore varies N use in beef steers.

Nitrogen metabolism of beef steers fed endophyte-free tall fescue hay: effects of ruminally protected methionine supplementation.

Level of nitrogen (N) intake and ruminally protected methionine supplementation were evaluated in eight Angus growing steers (initial BW 253+/-21 kg, final BW 296+/-21 kg) in a replicated 4+/-4 Latin square design. The steers were fed two endophyte-free tall fescue (Festuca arundinacea) hays that contained 2.2 (LO) or 2.8% (HI) of DM as N and were either supplemented or not with ruminally protected methionine (10 g metabolizable methionine/d). Diets were fed to provide adequate energy for 0.5 kg ADG and sufficient protein for maintenance (LO), or protein to support 0.5 kg ADG (HI). Following at least 14 d of adjustment, N balance was measured for 6 d. Isotopic urea was infused (15N15N-urea, 0.164 mmol urea N/h) via a jugular catheter for 56 h and urine was collected from 48 to 56 h to measure urea kinetics. Jugular blood was collected during the balance trial, and serum was analyzed for serum urea N (SUN). By design, daily N intake was greater (P < 0.05) for HI (112 g) than for LO (89 g). Compared with LO, steers when fed HI had greater (P < 0.05) daily DMI (4,217 vs 4,151 g), fecal N (34.4 vs 31.2 g), N digested (77.1 vs 57.7 g), urine N (48.3 vs 37.5 g), urine urea N excretion (34.6 vs 24.8 g), and N retained (29.8 vs 21.1 g). When fed HI steers also had higher (P < 0.05) urine urea N concentration (276 vs 219 mM), SUN (8.7 vs 6.7 mM), N digestibility (69.1 vs 64.9%), percentage of urinary N present as urea (71.5 vs 66.7%, P < 0.053), and rate of urea N production (59.6 vs 49.2 g/d) but lower (P < 0.05) percentage of urea N produced that was returned to the ornithine cycle (15.03 vs 19.2 1%) than when fed LO. Methionine supplementation decreased daily urine N (41.2 vs 44.6 g, P = 0.10) and increased both the amount of N retained daily (27.9 vs 23.7 g, P < 0.089) and the percentage of N digested that was retained (40.4 vs 34.6%, P < 0.094). In summary, supplemental methionine met a specific dietary limitation by increasing the amount of digested N that was retained by the steers.

Comparison of insulin infusion sites on metabolite net flux and insulin kinetics in growing euglycemic beef steers.

Four beef steers (average BW, 246 kg) were used in a split-plot design with five bovine insulin (I) infusion rates (5, 10, 20, 40, and 80 mU.kg-1.h-1) in the main plot and two infusion sites, mesenteric (M) and jugular (J) veins, in the subplot. Steers were fed a medium-energy diet at .235 Mcal of ME/kg BW.75 daily in 12 equal feedings at 2-h intervals. Catheters were placed in the mesenteric, hepatic-portal, and hepatic veins and in the abdominal aorta. Blood was sampled from the arterial, portal, and hepatic catheters at 20-min intervals for 1 h before I infusion. Glucose was infused intrajugularly to maintain euglycemia during the I infusion, with arterial glucose monitored at 10- to 15-min intervals. After at least 2.5 h, blood was again sampled at 20-min intervals for 1 h. Blood flow was determined by downstream dilution of p-aminohippurate. Arterial I concentrations (+/- SE) at the greatest I infusion rates were 183.5 +/- 10.46 (J) and 179.0 +/- 6.64 (M) microU/mL. Portal I concentration tended to be greater during M than during J infusion (e.g., J, 199.9 +/- 10.48 vs M, 225.8 +/- 8.99 microU/mL at the greatest dose). Hepatic glucose production at the larger three I doses reached a plateau near 40% of the preinfusion production rate (.57 +/- .02 mmol.kg-1.h-1 vs J, .23 +/- .029 and M, 27 +/- .037). Urea N concentration decreased, but portal uptake or hepatic release of urea N was largely unaffected by I dose or site of infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of nutrient interchange and oxygen use among portal-drained viscera, liver, and hindquarters of beef steers from 235 to 525 kg body weight.

Our objective was to quantify changes in supply and use of nutrients and O2 by large-frame, multicatheterized beef steers as they grew from 235 to 525 kg BW. Steers consumed 5.25 to 9.87 kg DM/d of a 62% concentrate diet that provided 126 to 217 g N/d and 1 kg ADG. Steers were assigned to three groups (eight, nine, and eight steers each) that divided the BW range into thirds. Weights at first sampling for the three groups were 236, 319, and 445 kg, respectively. Each group was sampled twice. Groups were killed after the second sampling. Tissue weights and hindquarters (HQ) contents of fat, protein, and ash were measured. Blood flow, oxygen uptake, and net uptake or release of metabolites were regressed against functions of BW.75 to assess changes during growth. Blood flow in all tissues except liver and oxygen use by all tissues decreased per unit tissue weight as BW.75 and age increased. Changes with age per unit liver weight were as follows: decreased uptake of propionate and lactate, increased uptake of alpha-amino N and glutamine, decreased production of urea and glutamate, and increased production of acetate and beta-hydroxybutyrate. Glucose and urea production per unit liver weight was constant. Changes with age per unit HQ weight were as follows: increased uptake of glucose, decreased uptake of alpha-amino N and glutamate, decreased release of lactate, and increased release of glutamine. Weight of the portal-drained viscera (PDV) increased from 91 to 97 g/kg EBW as BW increased from 236 to 522 kg; PDV fat increased from 375 to 552 g/kg PDV tissues. Liver decreased from 16 to 12 g/kg EBW. Hindquarters decreased from 286 to 266 g/kg EBW; HQ protein was 200, 197, and 200 g/kg HQ tissue for Groups 1, 2, and 3, respectively. Corresponding fat was 131, 182, and 177 g/kg HQ tissue. Changes in net flux reflect changes in nutrient partitioning and tissue deposition as steers grew and aged.

Urea flux in beef steers: effects of forage species and nitrogen fertilization.

The effects of two forage species and N levels on urea kinetics and whole-body N metabolism were evaluated in eight Angus steers (initial BW 217+/-15 kg). In a replicated, 4 x 4 Latin square design, steers were fed gamagrass (Tripsacum dactyloides L.) or switchgrass (Panicum virgatum L.), each of which had 56.2 (LO) or 168.5 (HI) kg of N fertilization per hectare. Diets provided adequate energy for 0.5 kg ADG. Nitrogen balance and urea kinetics were measured from d 22 to 27 of each period. Urine samples collected during intravenous infusion of bis 15N urea were used to calculate production and recycling of urea N from relative abundance of urea isotopomers. Jugular blood serum was analyzed for serum urea N (SUN). Gamagrass differed from switchgrass (P < 0.05) in daily DMI (4,273 vs 4,185 g), N intake (72 vs 67 g), DM digestibility (61.0 vs 63.6%), fecal N (30.6 vs 28.3 g/d), urine urea N (10.5 vs 8.0 g/d), and percentage of urinary N present as urea N (53.5 vs 40.0%). After adjustment for differences in N intake, fecal N still tended to be greater (P < 0.09) for gamagrass than for switchgrass. The LO differed from the HI (P < 0.01) in daily N intake (63 vs 76 g), DM digestibility (61.3 vs 63.3%), urine N (13.6 vs 25.9 g/d), and N retained as a percentage of N digested (57.3 vs 43.5%). Compared to switchgrass, gamagrass had greater SUN, N digestibility, and N digested as N level increased (forage x N level interactions, P < 0.05). As N level increased, N retention increased from 19.5 to 23.5 g/d in gamagrass and decreased from 20.5 to 18.1 g/d in switchgrass (interaction, P < 0.07). The HI group was greater than the LO intake group (P < 0.03) in endogenous production of urea N (44.4 vs 34.0 g/d), gut entry rate of urea N (31.6 vs 28.2 g/d), and the amount of urea N that re-entered the ornithine cycle (9.4 vs 7.9 g/d). However, the percentage of urea N entering the gastrointestinal tract that was recycled was constant among treatments (29.1%), indicating that almost 70% of the urea N that entered the gastrointestinal tract was potentially available for anabolic purposes of the steers as a component of microbial products that were absorbed or excreted in the feces. In summary, N levels affected N metabolism of steers more when they were fed gamagrass than when they were fed switchgrass. Although the absolute amounts of N moving through the system changed with variations in intake, the proportions remained similar, with a greater efficiency of N use at low N intakes.

The effect of abomasal infusion of casein and recombinant somatotropin hormone injection on nitrogen balance and amino acid fluxes in portal-drained viscera and net hepatic and total splanchnic blood in Holstein steers.

Six Holstein steers, 6 mo of age, with a BW range of 180 to 200 kg were used to determine effects of abomasal casein infusion and recombinant bovine somatotropin on amino acid absorption and metabolism in the gut and liver. Catheters were positioned in the hepatic vein, the hepatic portal vein, two mesenteric veins, and a mesenteric artery. Using the same basal diet, treatments consisted of 1) basal diet only (Control), 2) abomasal casein infusion of 300 g/d (Casein), and 3) abomasal casein infusion plus daily injection of 20 mg of bovine somatotropin (ST). All steers were fed the basal diet at a rate of 24 g/kg of BW on a DM basis, in 12 equal meals at 2-h intervals. Initially, Casein and ST treatments were in a balanced cross-over design, and then all steers received the Control treatment. Casein infusion tended to increase BW gain but did not affect BW gain per unit of N intake. The ST treatment increased weight gain (P < .01) and efficiency of dietary DM (P < .01) and N (P < .01) utilization by more than 40% and increased the percentage of absorbed N retained. Nonessential, essential, and total amino acid fluxes in portal-drained viscera (PDV) and their net removal by the liver were not affected by casein infusion. However, ST tended to reduce PDV flux of essential, nonessential, and total amino acids by approximately 30%, and their removal by the liver (P < .08). As a result, ST increased, compared with casein, release of essential (P < .06) and total (P < .10) amino acids from the total splanchnic pool (TSP) into peripheral blood. The ST treatment increased the availability of absorbed amino acids to peripheral tissues, but it did not affect the amino acid profile delivered from splanchnic tissues, especially that of essential amino acids.

Effects of dietary concentrate level on nutrient absorption, liver metabolism, and urea kinetics of beef steers fed isonitrogenous and isoenergetic diets.

Six multicatheterized beef steers (421 +/- 21 kg BW) were used to predict the effect of dietary concentrate level on blood flow and net flux of urea and other metabolites across splanchnic tissues. Diets ranged from 0% (switchgrass hay) to 90% concentrate (10% switchgrass hay, 89% cracked corn, 1% urea). Daily DMI varied from 8.01 to 5.34 kg/d. Nitrogen intake (99 g/d) and calculated ME intake (16.8 Mcal/d) were equal among diets. As dietary concentrate increased from 0 to 90%, liver blood flow decreased from 850 to 795 L/h, portal-drained visceral (PDV) blood flow decreased from 750 to 620 L/h, and mesenteric-drained visceral (MDV) blood flow decreased from 270 to 250 L/h. Liver release of urea N was 94 mmol/h when dietary concentrate was less than 20%, then increased to 146 mmol/h at 55% concentrate. Urinary excretion of urea N was 13 mmol/h or less when dietary concentrate was 20% or less, increased to 53 mmol/h at 55% concentrate, then continued to increase to 76 mmol/h at 90% concentrate. Transfer of urea N to PDV ranged from 71 to 91 mmol/h and transfer to MDV ranged from 0 to 10 mmol/h among diets. As dietary concentrate increased from 27 to 63%, VFA release by PDV decreased, net MDV and splanchnic release of glucose increased, and splanchnic tissues switched from net uptake to net release of L-lactate. Net PDV release or liver removal of ammonia or alpha-amino N and net liver release of glucose were not affected. We conclude that the liver responded to changes in the percentage of dietary concentrate by altering urea production and by altering the role of lactate in intermediary metabolism.

Insulin sensitivity and responsiveness of portal-drained viscera, liver, hindquarters, and whole body of beef steers weighing 275 or 490 kilograms.

Our objective was to quantify effects of age, weight, and body composition on responsiveness (Rmax or Rmin) and sensitivity (ED50) of several parameters of glucose metabolism to insulin in growing beef steers. Steers ate equal-sized meals every 2 h; the diet contained 62% concentrate and sustained 1 kg ADG. Treatments were euglycemic clamps at 10, 20, 40, 80, 160, and 320 mU.h-1.kg BW-1 of insulin infused into a mesenteric vein of seven younger (275 kg BW) and seven older (490 kg BW) steers. Most steers received three of the six treatments; two extra steers were added to compensate for missing data. Steers had blood vessel catheters and ultrasound flow probes that allowed measurement of net uptake or release of glucose and insulin by portal-drained viscera (PDV), liver, and hindquarters (HQ). Steady-state glucose infusion rate (SSGIR) was intrajugular glucose infused during treatments to maintain euglycemia. Within age groups, Rmax or Rmin and ED50 were estimated by nonlinear regression of glucose flux on arterial plasma insulin concentrations. Steers were killed after sampling, tissues were weighted, and HQ content of fat and protein was determined. Those data were used to predict tissue weights and HQ composition at the time of the euglycemic clamps. Predicted EBW (243 vs 444 kg), liver (4.24 vs 6.19 kg), and HQ (73 vs 122 kg) were heavier for older than for younger steers. Fat in HQ was higher for older than for younger steers (173 vs 134 g/kg), but protein was similar (198 g/kg). The ED50 (mU of insulin/L of plasma) for SSGIR (237 +/- 65 vs 113 +/- 22), liver glucose release (89 +/- 22 vs 44 +/- 11), total glucose entry (418 +/- 184 vs 125 +/- 20), and HQ glucose uptake (488 +/- 151 vs 243 +/- 78) was higher for older than for younger steers. The Rmax (mmol glucose.h-1.kg tissue-1) for SSGIR (2.68 +/- .22 vs 2.09 +/- .23) and HQ (3.08 +/- .33 vs 2.46 +/- .30) was higher for younger than for older steers. Liver glucose release decreased in response to insulin; Rmin (mmol glucose.h-1.kg liver-1) was higher for younger (36.0 +/- 6.9) than for older (24.7 +/- 3.2) steers. We conclude that as steers grew older, heavier, and fatter, their peripheral tissues and liver became less sensitive and less responsive to insulin.

Portal blood flow in beef steers: comparison of techniques and relation to hepatic blood flow, cardiac output and oxygen uptake.

We compared two techniques for measuring blood flow through portal-drained viscera (PDV) of beef steers and measured portions of cardiac output and total oxygen uptake attributable to PDV and hepatic tissues. Four steers (198 +/- 2 kg), equipped with chronic catheters in appropriate vessels, a transit-time ultrasound probe around the hepatic portal vein and a temporary cardiac output thermodilution catheter, were fed a 60:40 hay: concentrate diet. Treatments, designed to alter blood flow, were: 12 equal meals every 2 h (CNTL); CNTL plus 2 mg clenbuterol in one meal (CLEN); and a 65-h fast (FAST). Blood flow through PDV was measured by dilution of p-aminohippurate (PAH) and transit-time ultrasound. Hepatic blood flow was measured by PAH dilution and cardiac output was measured by thermodilution. Blood flow measured by transit-time ultrasound was consistently slower (45%, P less than .01) than blood flow measured by PAH dilution. Necropsy revealed anatomical constraints that precluded proper placement and function of the flow probes. Cardiac output (liters/h) was greater (P less than .05) for CLEN (3,082) than for CNTL (1,655) or FAST (1,047). Percentage of cardiac output flowing through PDV and hepatic tissues was less (P less than .05) for CLEN (23 and 24%) than for CNTL (31 and 38%) or FAST (32 and 38%). Whole body oxygen uptake (mmol/h) was greatest (P less than .05) for CLEN (4,220), intermediate for CNTL (2,999) and least for FAST (1,965). Percentage of oxygen uptake attributable to hepatic tissues was greater (P less than .05) for FAST (31%) than for CLEN (18%), with CNTL intermediate (24%). Percentage of oxygen uptake attributable to PDV (22%) was not affected (P greater than .05) by treatments.

Effects of dietary clenbuterol on metabolism of the hindquarters in steers.

The objective of this study was to measure acute (d 1) and chronic (d 9) effects of dietary clenbuterol on heart rate, blood flow, oxygen uptake, and net uptake/release of metabolites in the hindquarters of growing steers. The design was a single reversal with two 9-d periods of control or 8 mg clenbuterol/d with 5 d between periods. Within 2 h of initial consumption of 2 mg clenbuterol (d 1), heart rate and blood flow doubled and arterial plasma concentrations of glucose, L-lactate and nonesterified fatty acid (NEFA) increased, whereas alpha-NH2 N and NH3 concentrations decreased, demonstrating an acute response. Uptake of oxygen increased and net uptake of alpha-NH2 N decreased. Net release of both L-lactate and NEFA increased. On d 9, there were no acute responses to clenbuterol consumption; however, heart rate, blood flow, and NEFA concentration remained chronically elevated, and plasma concentrations of acetate and propionate decreased compared with control feeding. Net uptake of alpha-NH2 N, oxygen and release of L-lactate by the hindquarters chronically increased on d 9 of clenbuterol feeding. Changes in both blood flow and arteriovenous (AV) concentration difference contributed to changes in uptake/release. The chronic metabolic changes and oxygen uptake were consistent with increased N retention in the hindquarters through increased protein synthesis, decreased use of acetate and increased reliance on NEFA for cellular energy. In conclusion, the data show that the perturbation of homeostatic regulation by dietary clenbuterol on d 1 evolved to establishment of homeorhetic regulation by d 9 that is consistent with increased skeletal protein accretion in growing steers.