Changes in metabolites, metabolic hormones, and luteinizing hormone before puberty in Angus, Braford, Charolais, and Simmental heifers

We determined changes in insulin, glucose, free fatty acids (FFA), growth hormone (GH), insulin-like growth factor I (IGF-I) and LH before puberty in Angus, Braford, Charolais, and Simmental heifers. Our primary objective was to identify metabolites and metabolic hormones that serve as metabolic cues for onset of puberty. Angus (n = 12). Braford (n = 7), Charolais (n = 9), and Simmental (n = 7) heifers were assigned at weaning (289 +/- 25 d of age; 264 +/- 23 kg) to open-sided pens with slotted floors, and they were fed a corn silage-concentrate diet formulated to provide gains of .91 kg/d. Puberty was defined as the 1st d (d 0) that serum progesterone (determined in blood samples collected at weekly intervals) exceeded 1 ng/ml. Blood samples were collected before and after feeding at 15-min intervals for 8 h at 21-d intervals before puberty in a subsample of heifers (at least five per breed). Angus and Simmental heifers weighed less and were younger (P less than .05) at puberty than Charolais and Braford heifers. Serum FFA before feeding and frequency of LH release increased (P less than .05) from d-40 +/- 3 to d-17 +/- 3 in all breeds. Conversely, concentrations of insulin were greater (P less than .05) at -40 than at -17 d from puberty in Angus, but not in Braford, Charolais, or Simmental heifers. Frequency of GH release was greater at d -40 than at d -17 in Angus heifers; however, in Braford and Charolais heifers frequency of GH release was greater at d -17 than at d -40. Concentrations of IGF-I (measured every 2 wk) increased linearly (P less than .07) from d -56 to 0 from puberty in Angus but not in other breeds. In conclusion, frequency of LH release and concentrations of FFA increased before puberty in all breeds; however, consistent changes in other metabolites and hormones were observed only in Angus heifers.     

Influence of glucose infusions on luteinizing hormone secretion in the energy-restricted, primiparous, lactating sow.

The objective of this experiment was to evaluate the influence of glucose infusion during lactation on LH secretion in the energy-restricted sow. Ten primiparous Landrace x Yorkshire sows (152 kg postfarrowing) were fed a low-energy (6.5 Mcal of ME/d), high-lysine (45 g/d), corn-soybean meal diet throughout lactation. On d 18 of lactation, sows received a continuous infusion (1 L/12 h) of glucose (50% dextrose solution) or .9% saline from 1200 to 2400. Blood samples were drawn every 15 min for an 18-h period on d 18 to evaluate concentrations of plasma glucose, serum insulin, and serum LH before (600 to 1200) and during (1215 to 2400) the infusions. The glucose infusion immediately increased (P less than .001) plasma glucose and serum insulin relative to preinfusion levels. Glucose and insulin concentrations in sows receiving the glucose infusion were higher (P less than .001) throughout the infusion period relative to concentrations in sows receiving a saline infusion. Glucose infusions had no influence on LH pulsatility during the 12-h infusion period. In contrast to the response observed in the nutrient-restricted gilt, these results indicate that glucose infusions do not result in an immediate increase in pulsatile LH secretion in the energy-restricted, lactating sow.     

Effect of dietary energy restriction on metabolic and endocrine responses during the estrous cycle of the suckled beef cow

A replicated trial was conducted with suckled Angus and Polled Hereford cows (110 d postcalving) to determine metabolic and endocrine responses to an energy-restricted diet after cows had re-established postpartum estrous cyclicity. Cows were individually fed 26.5 Mcal ME (H) or 15.2 Mcal ME (L) for a 30-d preliminary period and fitted with an indwelling jugular cannula at synchronized estrus. Average daily weight change during the estrous cycle was .60 +/- .25 and -1.37 +/- .30 kg/d for H and L, respectively (P less than .05). Blood concentrations of cortisol, progesterone and LH during the estrous cycle were not affected by diet, nor did diet affect frequency or amplitude of LH pulses (P greater than .05). No dietary differences were observed for daily concentrations of total protein, glucose, nonesterified fatty acids or acetate. Mean blood concentrations of propionate and butyrate were not different between diets; however, L cows had lower concentrations of propionate and butyrate on d 11 of the cycle (P less than .05). Cows fed L had higher concentrations of blood urea nitrogen (P less than .05), but they had lower concentrations of cholesterol (P less than .05) on d 4, 11, 18 and subsequent estrus (E). Insulin was not different on d 4 and 11; however, cows fed L had lower insulin concentrations on d 18 and d E (P less than .05). Dietary energy restriction in these cyclic cows caused no change in endocrine responses. Of metabolic responses measured, only blood urea nitrogen, cholesterol and insulin showed consistent changes.     

Pulsatile or continuous infusion of luteinizing hormone-releasing hormone and hormonal concentrations in prepubertal beef heifers

An experiment was conducted to determine if exogenous luteinizing hormone-releasing hormone (LHRH) administered iv intermittently as pulses (P) or by continuous sc infusion (I) using osmotic minipumps could sustain pulsatile LH release and induce estrous cyclicity in prepubertal heifers. Prepubertal heifers were assigned randomly to: 1) receive pulses of LHRH (n = 6; 2.5 micrograms LHRH/2 h for 72 h), 2) be infused with LHRH (n = 11; 1.25 micrograms LHRH/h for 72 h), or 3) serve as controls (n = 16). Blood was collected at 20-min intervals for 8 h (0900 to 1700 h) from six heifers in each group on d 1, 2, 3 (during treatment), and on d 4 (during 8 h after terminating LHRH treatments). Heifers given LHRH had higher (P less than .01) LH concentrations than controls. Preovulatory-like LH surges occurred in three I, two P and no control heifers during treatment. Pulse frequencies of LH (no. LH pulses/8 h) were greater (P less than .001) for P heifers than for I and control heifers due to pulsatile LHRH treatment. Serum estradiol was higher (P less than .01) during treatment for LHRH-treated heifers than for controls. Serum follicle-stimulating hormone, cortisol, and progesterone were unchanged during treatment. High levels of cortisol on d 1 declined (P less than .001) to baseline by d 2. Characteristic progesterone rises or short luteal phases occurred within 10 d of treatment initiation in more (P less than .05) LHRH-treated heifers (I = 45%, P = 33%) than controls (6%), although days to first observed estrus and first ovulation were unaffected by treatments. Although both continuous and pulsatile administration of LHRH successfully induced LH and estradiol release as well as preovulatory-like LH surges in some heifers, earlier initiation of estrous cycles was not achieved. Estrous cycles appeared to be delayed by exposure to continuous LHRH infusions during the peripubertal period.     

Rumen protected fat in kline barley or corn diets for beef cattle: digestibility, physiological, and feedlot responses.

A rumen protected fat product composed of Ca salts of fatty acids (CAF) was evaluated in beef cattle diets. Dietary treatments in a metabolism trial (six steers, 251 +/- 15 kg) and Feedlot Trial 1 (15 individually fed heifers, 355 +/- 11 kg) consisted of 62.1% corn and 3.2% soybean meal (C), 32.6% corn and 33.1% Kline barley (CB), and 28.2% corn, 33.1% barley, and 4.5% CAF (CBF) on a DM basis. Apparent DM and OM digestibilities were higher (P less than .05) and NDF digestibility was lower (P less than .05) for C than for CB and CBF. Apparent ether extract digestibility was higher (P less than .01) for CBF than for CB. Retention of N was higher (P less than .05) for C than for CB and CBF. On d 42, Feedlot Trial 1 ruminal fluid acetate:propionate ratio was lower (P less than .01) for C than for CB and CBF, and the ratio was lower (P less than .01) for CB than for CBF. Treatments did not affect (P less than .10) ADG and DMI of heifers, but feed/gain was lower (P less than .05) for C than for CB and CBF, and feed/gain was higher (P less than .05) for CBF than for CB. In Feedlot Trial 2, steers and heifers were fed the C diet or a 57% corn, 21.4% peanut hull, 4.1% soybean meal, and 4.5% CAF diet (CF) on an ad libitum basis for 50 d. Ruminal fluid acetate and propionate were unaffected (P greater than .10) by dietary treatments. Cattle ADG tended to be lower (P greater than .10), DMI was lower (P less than .05), and feed/gain tended to be improved (P greater than .10) for CF compared with C. Kline barley has potential as a feed grain for cattle, but CAF may not improve beef cattle performance, especially when diets contain barley.     

Effects of charcoal-extracted, bovine follicular fluid on gonadotropin concentrations, the onset of estrus and luteal function in heifers

A study was conducted to determine the effect of charcoal-extracted, bovine follicular fluid (CFF) on plasma follicle stimulating hormone (FSH) and luteinizing hormone (LH) concentrations, the interval from luteolysis to estrus, and subsequent luteal function in heifers. Fifteen Angus, Simmental and Hereford heifers were allotted by age, weight and breed to a control (C, n = 8) or a CFF (n = 7) group. Heifers received injections of saline or CFF (iv, 8 ml/injection) every 12 h from d 1 (d 0 estrus) through d 5 of the estrous cycle. On d 6, each heifer was injected (im) with 25 mg of prostaglandin F2 alpha (PGF2 alpha). Blood samples were collected every 12 h by venipuncture starting just before the first saline or CFF injection and continuing until estrus. Thereafter, blood samples were collected every other day during the subsequent estrous cycle and assayed for FSH, LH, estradiol-17 beta and progesterone by radioimmunoassay. Injections of CFF had no effect (P greater than .05) on circulating FSH or LH concentrations from d 1 to 5 relative to the C group; however, there was a transient rise (P less than .05) in FSH concentrations 24 h following cessation of CFF injections. This transient rise in FSH was not immediately followed by an increase in plasma estradiol-17 beta concentrations. Although CFF injections did not interfere with PGF2 alpha-induced luteolysis, the interval from PGF2 alpha injection to estrus was delayed (P less than .05) by 5 d in the CFF group compared with the C group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma progesterone concentration in beef heifers receiving exogenous glucose, insulin, or bovine somatotropin

Three experiments were conducted to evaluate plasma concentrations of glucose, insulin, IGF-I, and progesterone (P4) in pubertal beef heifers receiving exogenous glucose, insulin, or sometribove zinc. All heifers used had no luteal P4 synthesis but received a controlled internal drug-releasing device containing 1.38 g of P4 to estimate treatment effects on hepatic P4 degradation. In Exp. 1, 8 pubertal, nulliparous Angus × Hereford heifers (initial BW = 442 ± 14 kg; initial age = 656 ± 7 d) were randomly assigned to receive, in a crossover design containing 2 periods of 10 h, intravenous (i.v.) infusions (10 mL) of insulin (1 μg/kg of BW; INS) or saline (0.9%; SAL). Treatments were administered via jugular venipuncture in 7 applications (0.15 μg insulin/kg BW per application) 45 min apart (from 0 to 270 min). Blood samples were collected immediately before each infusion as well as at -120, -60, 330, 390, and 450 min relative to the first infusion. Heifers receiving INS had greater (P < 0.01) plasma insulin, reduced (P ≤ 0.04) plasma glucose and IGF-I, and similar (P = 0.62) plasma P4 concentrations compared with SAL heifers. In Exp. 2, the same heifers were assigned to receive, in a similar experimental design as Exp. 1, i.v. infusions (10 mL) of 1) insulin (1 μg/kg BW) and glucose (0.5 g/kg BW; INS+G) or 2) SAL. Heifers receiving INS+G had greater (P ≤ 0.02) plasma insulin, glucose, and P4 but reduced (P = 0.01) plasma IGF-I concentrations compared with SAL heifers. In Exp. 3, the same heifers were assigned to receive, in a crossover design containing 2 periods of 14 d, subcutaneous (s.c.) injections of 1) 250 mg of sometribove zinc (BST) or 2) SAL. Blood samples were collected 3 h apart (0900, 1200, 1500, and 1800 h) from heifers on d 6, 8, and 10 relative to treatment administration (d 1). Heifers receiving BST had greater (P < 0.01) plasma glucose and IGF-I and similar (P ≥ 0.67) plasma insulin and P4 concentrations compared with SAL heifers. Results from this series of experiments suggested that concurrent increases in glucose and insulin are required to reduce hepatic catabolism and increase plasma concentrations of P4 in bovine females.     

The use of a progesterone-releasing device (CIDR-B) or melengestrol acetate with GnRH,LH, or estradiol benzoate for fixed-time AI in beef heifers

The objective of this experiment was to compare two progestins and three treatments for synchronizing follicular wave emergence and ovulation in protocols for fixed-time AI in beef heifers. On d 0 (beginning of the experiment), Angus and Angus-Simmental cross beef heifers at random stages of the estrous cycle either received a CIDR-B device (n = 257) or were started on 0.5 mg x anima(-1) x d(-1) melengestrol acetate (MGA; n = 246) and were randomly assigned to receive i.m. injections of 100 microg GnRH, 12.5 mg porcine LH (pLH), or 2 mg estradiol benzoate (EB) and 50 mg progesterone (P4). The last feeding of MGA was given on d 6 and on d 7, CIDR-B devices were removed and all heifers received 500 microg cloprostenol (PG). Consistent with their treatment groups on d 0, heifers were given either 100 microg GnRH or 12.5 mg pLH 48 h after PG (and were concurrently inseminated) or 1 mg EB 24 h after PG and were inseminated 28 h later (52 h after PGF). Estrus rate (combined for both progestins) in heifers receiving EB (92.0%) was greater (P < 0.05) than that in heifers receiving GnRH and pLH (combined) and a CIDR-B device (62.9%) or MGA (34.3%). Although the mean interval from PG treatment to estrus did not differ among groups (overall, 47.8 h; P = 0.85), it was less variable (P < 0.01) in MGA-fed heifers (SD = 2.5 h) than in CIDR-B-treated heifers (SD = 8.1 h). Pregnancy rates (determined by ultrasonography approximately 30 d after AI) did not differ (P = 0.30) among the six treatment groups (average, 58.0%; range, 52.5 to 65.0%). Although fixed-time AI was done, pregnancy rates were greater in heifers detected in estrus than in those not detected in estrus (62.6 vs 51.9%; P < 0.05). In conclusion, GnRH, pLH, or EB treatment in combination with a CIDR-B device or MGA effectively synchronized ovulation-for fixed-time AI, resulting in acceptable pregnancy rates in beef heifers.     

Nutrient-specific preferences by lambs conditioned with intraruminal infusions of starch, casein, and water

We hypothesized that lambs discriminate between postingestive effects of energy and protein and associate those effects with a food's flavor to modify food choices. Based on this hypothesis, we predicted that 1) lambs would acquire a preference for a poorly nutritious food (grape pomace) eaten during intraruminal infusions of energy (starch) or protein (casein) and that 2) shortly after an intraruminal infusion of energy or protein (preload), lambs would decrease their preferences for foods previously conditioned with starch or casein, respectively. Thirty lambs were allotted to three groups and conditioned as follows. On d 1, lambs in each group received grape pomace containing a different flavor and water was infused into their rumens as they ate the pomace. On d 2, the flavors were switched so each group received a new flavor and a suspension of starch (10% of the DE required per day) replaced the water infusion. On d 3, the flavors were switched again, and a suspension of casein (2.7 to 5.4% of the CP required per day) replaced the starch infusion. Conditioning was repeated during four consecutive trials. Lambs in Trial 1 had a basal diet of alfalfa pellets (e.g., free access from 1200 to 1700) and 400 g of rolled barley. Lambs in Trials 2, 3, and 4 received a restricted amount of alfalfa pellets (990 g/d) as their basal diet. After conditioning, all animals received an infusion of water, and, 30 min later, they were offered a choice of the three flavors previously paired with water, starch, or casein. On the ensuing days, the choice was repeated, but starch, casein, and barley replaced the water preload. The nutrient density of the infused preloads was increased during consecutive trials. Lambs preferred the flavors paired with starch > water > casein during Trial 1 (P < .05) and the flavors paired with starch > casein > water during Trials 2 (P < .05), 3 (P < .001), and 4 (P < .001). Preloads of casein decreased preferences for flavors previously paired with casein (P < .10 [Trial 2]; P < .001 [Trial 3], and increased preferences for flavors paired with starch (P < .05 [Trial 2]; P < .001 [Trial 3]). Preloads of energy (barley) had the opposite effect (P < .05 [Trial 3]). These results indicate that lambs discriminated between the postingestive effects of starch and casein and associated the effects with specific external cues (i.e., added flavors) to regulate macronutrient ingestion.     

Nutritionally induced anovulation in beef heifers: ovarian and endocrine function preceding cessation of ovulation.

Angus x Hereford heifers were used to determine endocrine and ovarian function preceding nutritionally induced anovulation. Six heifers were fed to maintain body condition score (M), and 12 heifers were fed a restricted diet (R) until they became anovulatory. Starting on d 13 of an estrous cycle, heifers were given PGF2alpha every 16 d thereafter to synchronize and maintain 16 d estrous cycles. Ovarian structures of M and R heifers were monitored by ultrasonography daily from d 8 to ovulation (d 1 of the subsequent cycle) until R heifers became anovulatory. Concentrations of LH and FSH were quantified in serum samples collected every 10 min for 8 h on d 2 and 15 (48 h after PGF2alpha), and estradiol and IGF-I were quantified in daily plasma samples from d 8 to 16 during the last ovulatory cycle (Cycle -2) and the subsequent anovulatory cycle (Cycle -1). During the last two cycles before anovulation, M heifers had 50% larger (P < .0001) ovulatory follicles than R heifers and 61% greater (P < .0001) growth rate of the ovulatory follicles. There was a treatment x cycle x day effect (P < .001) for concentrations of estradiol. The preovulatory increase in estradiol occurred in the R and M heifers during Cycle -2 but only in M heifers during Cycle -1. A treatment x cycle x day effect (P < .05) influenced LH concentrations. During Cycle -2, LH concentrations were similar for M and R heifers, but during Cycle -1, M heifers had greater LH concentrations than did R heifers. Concentrations of FSH were greater (P < .05) in R than M heifers after induced luteolysis when R heifers failed to ovulate. There was a treatment x cycle interaction (P < .05) for IGF-I concentrations, and M heifers had 4.7- and 8.6-fold greater IGF-I concentrations than did R heifers during Cycle -2 and -1, respectively. We conclude that growth rate and diameter of the ovulatory follicle, and concentrations of LH, estradiol, and IGF-I are reduced before the onset of nutritionally induced anovulation in beef heifers.     

Endocrine changes in beef heifers superovulated with follicle-stimulating hormone (FSH-P) or human menopausal gonadotropin.

The effects of superovulatory treatment (FSH-P vs human menopausal gonadotropin, HMG) and of route of administration (i.m. vs. i.v.) of prostaglandin F2 alpha (PGF2 alpha) on hormonal profiles were determined in 32 Angus x Hereford heifers. Heifers were superstimulated with either FSH-P (total of 26 mg) or HMG (total of 1,050 IU) beginning on d 9 to 12 of an estrous cycle and PGF2 alpha (40 mg) was administered at 60 and 72 h after the beginning of superovulatory treatments. Heifers were artificially inseminated three times at 12-h intervals beginning 48 h after PGF2 alpha treatment. Blood serum samples were collected immediately before treatments began, at 12-h intervals during the first 60 h, each 4 h during the next 96 h, and each 12 h until day of embryo collection. Concentrations of LH and FSH were not affected by hormone treatments, route of PGF2 alpha injection, or interactions between them. Estradiol-17 beta (E2-17 beta) levels were higher (P less than .05) in HMG- than in FSH-P-treated heifers 60 h after gonadotropin treatment. Peak concentration of E2-17 beta occurred earlier (P less than .05) in HMG- than in FSH-P-treated heifers and earlier in heifers injected with PGF2 alpha i.m. than in those injected i.v. Progesterone concentrations were not influenced by treatment or route of PGF2 alpha administration, but were affected (P less than .01) by the interactions between treatment and route of PGF2 alpha administration. Progesterone declined to basal levels earlier in the FSH-P- than in the HMG-treated heifers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitrogen metabolism and somatotropin secretion in beef heifers receiving abomasal arginine infusions

Twelve Angus x Hereford heifer calves (233 kg) were fitted with abomasal infusion cannulas and used to study N and endocrine responses to abomasally infused arginine (Arg). Heifers were allotted randomly to three treatment groups and received continuous abomasal infusions (2 liters/d) of water (CON) or Arg solutions providing .33 g Arg.HCl/kg BW (LOW) or .50 g Arg.HCl/kg BW (HIGH) each day. A 12-d dietary adjustment period preceded a 7-d infusion and collection period. Each calf received 4,544 g DM/d of a basal diet in equal portions at 0600, 1200, 1800 and 2400. Calves were housed in individual metabolism crates and fitted with urinary bladder catheters for total excreta collection. On d 1 and 5, blood samples were collected at 15-min intervals for 8 h between 1200 and 2000. Single samples were obtained at 1400 on remaining days. The infusion of Arg increased the quantity of N retained by heifers (P less than .01) and the percentage of total N retained (P less than .10); however, no differences were observed between LOW and HIGH heifers. Increased (P less than .01) urinary N excretion by Arg heifers was associated with greater (P less than .05) quantities of urinary urea N and ammonia N. Blood urea N and serum Arg concentrations were highest (P less than .05) in Arg heifers, whereas total serum AA concentrations were lower (P less than .05) in Arg heifers than in CON heifers. Serum glucose and insulin concentrations were not affected (P greater than .10) by treatment. Characterization of somatotropin (STH) profiles revealed that amplitude and frequency of STH pulses were not affected (P greater than .10) by treatment, whereas mean (P less than .10) and basal (P less than .05) STH concentrations were elevated in HIGH compared to LOW heifers on d 1 and 5. The similar N retention responses of LOW and HIGH heifers and similar STH profiles of CON and LOW heifers suggest that the stimulatory effect of the HIGH dose on STH secretion occurred only after tissue N requirements had been satisfied.     

Influence of insulin and energy intake on ovulation rate, luteinizing hormone and progesterone in beef heifers

Ovarian and gonadotropin responses to insulin and energy restriction were investigated in a 2 X 2 factorial experiment using 2-yr-old Brangus heifers. Thirty heifers were paired by weight and body condition, then assigned to treatment groups receiving 75 (LE) or 180% (HE) of NRC recommendations for dietary energy for maintenance. Diets were adjusted weekly to maintain daily .25 to .5 kg weight loss or 0 to .25 kg weight gain, respectively. On d 10 of the first estrous cycle subsequent to the initial 45 d of feeding, heifers within each dietary group were allocated to receive twice daily infusions of either 40 U insulin (I) or saline (C). Infusions began at 5 and 10 h postprandial and were given in six boluses, 20 min apart. Infusions continued daily until d 20 or estrus, whichever occurred first. On d 11, blood samples were collected at 15-min intervals for 12 h to determine luteinizing hormone (LH) and insulin concentrations. On d 16 to 20, twice daily im injections of 1 mg follicle stimulating hormone (FSH) were administered. Heifers were ovariectomized on d 11 after estrus. Number of corpora lutea (CL) in LE-I heifers was greater (P less than .05) in LE-C, HE-C or HE-I. Total CL weight (g) per heifer was greater (P less than .05) in HE-C and LE-I heifers than in LE-C. Individual CL wt was heavier in HE than in LE heifers (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of isoenergetic infusions of propionate and glucose on portal-drained visceral nutrient flux and concentrations of hormones in lambs maintained by total intragastric infusion

Three lambs were used in a repeated Latin square design to determine the influence of isoenergetic infusions of propionate or glucose on portal-drained visceral flux (PDV) of nutrients and concentrations of insulin, glucagon, growth hormone and prolactin. Lambs were fitted with appropriate catheters for blood sampling and maintained on total intragastric infusion of nutrients. Basal VFA, casein, mineral and vitamin infusions (isocaloric and isonitrogenous) were supplemented with an additional 22 +/- .5 kcal/h from propionate, glucose or a combination of propionate plus glucose. Ruminal fluid proportion and arterial blood concentration and PDV flux of propionate increased (P less than .10) by 17 mol/100 mol, .02 mM and 40 mmol/h, respectively, with infusion of an additional 61 mmol/h of propionate. Regression equations predicted that, on a net basis, 67% of ruminally infused propionate and 43% of abomasally infused glucose appeared in portal blood. Arterial L-lactate, beta-hydroxybutyrate and acetate concentrations, and beta-hydroxybutyrate flux were increased (P less than .10) by .34 mM, .20 mM, .50 mM and 4.2 mmol/h, respectively, with infusion of 33 mmol/h of added glucose. Net utilization of glucose by the PDV was approximately 4.4 mmol/h when no glucose was infused. Increased infusion of propionate resulted in a 22.2-micrograms/h increase in PDV flux of insulin (P less than .08) but had no effect on arterial insulin, glucagon and prolactin concentrations (P greater than .10). Arterial growth hormone increased by 3.8 ng/ml with increasing glucose infusion (P less than .08).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of an estrogen antagonist (tamoxifen) on cloprostenol-induced luteolysis in heifers

Experiments were conducted to determine the role of estrogens on endogenous PGF2 alpha secretion and luteolysis following injection of cloprostenol in heifers. In Exp. 1, eight luteal-phase heifers were used to evaluate tamoxifen (T) as an estrogen antagonist. Heifers received T (35 mg i.v.) or ethanol:saline vehicle (ES) every 4 h for 44 h. All received cloprostenol (500 micrograms i.m.) immediately after the start of T or ES, and received estradiol-17 beta (500 micrograms i.m.) 12 h later. Each ES heifer had a surge of luteinizing hormone (LH) within 48 h of estradiol injection, whereas T-treated heifers did not. Estrus was observed in three ES-treated heifers, but not in T-treated heifers. In Exp. 2, 10 heifers received T (35 mg i.v.) or ES every 4 h for 64 h beginning on d 15 postestrus. Cloprostenol (500 micrograms i.m.) was injected 16 h after the start of treatment. Concentrations of LH were similar (P greater than .05) in both groups. In ES heifers, concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) increased; in T-treated heifers, PGFM remained at pre-cloprostenol levels. Luteolysis was induced in all heifers. Progesterone (P4) decreased to less than or equal to 1 ng/ml at similar (P greater than .05) rates in ES-treated and T-treated heifers. Mean concentration of P4 288 h post-cloprostenol was greater (P less than .05) in ES-treated than in T-treated heifers. Three ES-treated heifers, but no T-treated heifers, were in standing estrus. We conclude that T effectively antagonizes estrogen in cattle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of unilateral ovariectomy and injection of bovine follicular fluid on gonadotropin secretion and compensatory ovarian hypertrophy in prepuberal heifers

A study was conducted to determine whether compensatory ovarian hypertrophy occurred in prepuberal heifers, and if so, could compensatory ovarian hypertrophy be prevented by injecting charcoal-extracted bovine follicular fluid (CFF). An additional objective was to determine the effect of CFF injections on follicle stimulating hormone (FSH) and luteinizing hormone (LH) concentrations following unilateral ovariectomy (ULO). Thirty-one prepuberal Angus, Hereford, and Angus X Hereford heifers were assigned by weight and breed to the following groups: Control-saline injections (C), unilateral ovariectomy-saline injections (ULO) and unilateral ovariectomy-CFF injections (ULO-CFF). In group C, a sham operation was performed on d 0 and both ovaries were removed on d 7. In groups ULO and ULO-CFF, either the right or left ovary was removed on d 0 and the remaining ovary was removed on d 7. From d 0 to 7, each heifer received an injection (iv, 7 ml/injection) of saline or CFF every 12 h. Jugular blood samples were collected immediately before each saline injection from d 0 to 7 and analyzed for FSH and LH. Unilateral ovariectomy increased ovarian (P less than .08) and follicular fluid (P less than .05) weights compared with groups C and ULO-CFF; however, lyophilized ovarian weight was similar (P greater than .05) for all groups. Injections of CFF blocked the increase in ovarian and follicular fluid weight following ULO. The total number of visible follicles per ovary was similar for all groups; however in the ULO group there was an increase (P less than .01) in those 5 to 6 mm and greater than or equal to 9 mm.(ABSTRACT TRUNCATED AT 250 WORDS)     

Early-weaning and postweaning nutritional management affect feedlot performance, carcass merit, and the relationship of 12th-rib fat, marbling score, and feed efficiency among Angus and Wagyu heifers

Twelve 3/4 Angus (Angus) and 12 Wagyu-cross (1/2 Wagyu x 1/2 Angus) (Wagyu) heifers were weaned at 180 d of age and grazed on endophyte-infected tall fescue for 16 mo before entering the feedlot as 2-yr-olds. Twelve 3/4 Angus heifer calves and 12 Wagyu-cross heifer calves from the following year's calf crop were weaned at 142 +/- 4.1 d of age, immediately adjusted to an 80% concentrate diet, and finished as calves. All heifers were fed a common finishing diet until an estimated 50% of their respective group would grade USDA low Prime or better based on ultrasound predictions. Ultrasound measurements of s.c. and i.m. fat depots were recorded at 60-d intervals throughout the finishing period. Heifers finished as calves had higher (P = 0.02) marbling scores at any given fat thickness and gained more efficiently (P < or = 0.01) at any given marbling score than heifers finished as 2-yr-olds. Gain:feed decreased quadratically (P < or = 0.05) as 12th-rib fat thickness increased for Angus and Wagyu heifers. Gain:feed decreased linearly (P < or = 0.01) for Wagyu calves and quadratically (P < or = 0.01) for Angus calves as 12th-rib fat thickness increased. However, these differences in slope were not different (P = 0.34) as a result of breed among heifers finished as calves. Marbling score increased linearly (P < or = 0.01) as 12th-rib fat thickness increased for Angus and Wagyu heifers finished as 2-yrolds or as calves. However, Wagyu heifers, regardless of age at feedlot entry, had a higher marbling score (P < or = 0.05) at any given 12th-rib fat thickness than Angus heifers. Finishing early-weaned heifers as calves as opposed to 2-yr-olds results in i.m. fat deposition during a period of more efficient growth. Additionally, including Wagyu genetics into the breeding of early-weaned heifers finished as calves or as 2-yr-olds results in higher marbling scores at any 12th-rib fat thickness.     

Effects of feeding diets containing endophyte-infected fescue seed on luteinizing hormone secretion in postpartum beef cows and in cyclic heifers and cows.

Two experiments were conducted to investigate the effect of feeding endophyte (Acremonium coenophialum)-infected fescue (Festuca arundinacea Shreb.) seed on LH secretion in postpartum beef cows and in cycling heifers and cows. In Exp. 1, spring-calving primiparous Angus cows (n = 16) were pair-fed for 75 d diets that contained endophyte-free or endophyte-infected (95%) fescue seed that contained 1.3 micrograms/g of ergovaline and 5.2 mg/g of saturated pyrrolizidines. Serial blood samples for basal and GnRH-stimulated serum LH analysis were obtained on d 7, 28, 42, and 56 of the study. The endophyte had no effect on LH secretion (basal, pulse frequency, and amplitude) or milk production. Average daily gain was decreased (P < .05) in cows that consumed infected fescue seed compared with controls (-.20 vs -.01 kg, respectively). Basal serum prolactin concentrations were reduced (P < .01) in treated compared with control cows (8.9 vs 25.4 ng/mL, respectively) on d 70. In Exp. 2, cycling Angus heifers (n = 8; age = 2 yr) and cows (n = 8; age = 4 yr) stratified by age were pair-fed for 40 d diets that contained the noninfected or the highly infected fescue seed. Estrus was synchronized by prostaglandin F2 alpha (d 18 and 28). Serial blood samples for serum LH analysis were obtained on d 28 (luteal phase) and d 30 (follicular phase). The endophyte did not affect LH (P > .28) or prolactin (P > .16) secretion, whereas ADG was decreased (P < .05) in treated compared with control animals (.32 vs .70 kg/d, respectively).     

Ovarian and endocrine characteristics during an estrous cycle in Angus, Brahman, and Senepol cows in a subtropical environment

To determine breed differences in ovarian function and endocrine secretion, daily rectal ultrasonography was conducted on multiparous lactating Angus (temperate Bos taurus; n = 12), Brahman (tropical Bos indicus; n = 12), and Senepol (tropical Bos taurus; n = 12) cows during an estrous cycle in summer. Blood was collected daily to quantify plasma concentrations of FSH, LH, progesterone, estradiol, GH, insulin-like growth factor (IGF)-I, IGF-II, IGF binding proteins (IGFBP), insulin, glucose, and plasma urea nitrogen (PUN). Numbers of small (2 to 5 mm), medium (6 to 8 mm), and large follicles (> or = 9 mm) were greater (P < .05) in Brahman than in Angus and(or) Senepol cows. Length of the estrous cycle (SEM = .6 d) was similar (P > .10) among Senepol (20.4 d), Angus (19.5 d), and Brahman (19.7 d) cows. Senepol cows had greater (P < .05) diameters of the corpus luteum (CL) and a delayed regression of the CL as compared with Angus cows. The secondary surge of FSH (between d 1 and 2; d 0 = estrus) was greater in Angus than Brahman or Senepol cows (breed x day, P < .05). Between d 2 and 14 of the estrous cycle, concentrations of progesterone, LH, IGF-II, and binding activities of IGFBP-3, IGFBP-2, and the 27- to 29-kDa IGFBP in plasma did not differ (P > .10) among breeds. Concentrations of GH, IGF-I, insulin, and PUN were greater (P < .001) and binding activities of the 22-kDa and 20-kDa IGFBP tended (P < .10) to be greater in plasma of Brahman than in Angus or Senepol cows. Plasma glucose concentrations were greater (P < .05) in Senepol than in Brahman or Angus cows. In conclusion, Brahman (Bos indicus) and Senepol cows (tropical Bos taurus) had greater numbers of follicles in all size categories and greater diameter of CL than Angus (temperate Bos taurus) cows. These ovarian differences may be due to changes in the pattern of secretion of FSH, insulin, IGF-I, and GH but not LH, IGF-II, or IGFBP-2 or -3.     

The effect of estradiol, trenbolone acetate, or zeranol on growth rate, mammary development, carcass traits, and plasma estradiol concentrations of beef heifers.

The effect of a single implantation (on d 1) with one or two long-acting, biodegradable estradiol implants (1E or 2E) on plasma estradiol concentrations in beef heifers was determined. The growth rates of these (2E) heifers, and of heifers repeatedly implanted with trenbolone acetate (TBA) or zeranol (Z) on d 1, 84, 168, and 252 of the trial, were compared to growth rates of controls. Trenbolone acetate alone was compared to TBA + 2E, and 2E was compared to 1E. At a mean age of 84 d (d 1 of experiment), 81 Hereford x Friesian heifers were allocated at random to the following treatments: Control (n = 15); TBA (n = 15); 1E (n = 12); 2E (n = 15); Z (n = 13); or TBA + 2E (n = 11). Mean live weight (kg) prior to slaughter on d 368 and hot carcass weight (kg) for heifers assigned to treatment Groups 1 to 6, respectively, were 366 and 200, 391 and 212, 374 and 201, 386 and 207, 387 and 210, and 391 and 208 (residual SD = 30.3 and 20.2). Heifers assigned to both the 2E and Z treatments were heavier on d 368 (P less than .05) and had longer teats on d 279 (P less than .05), less pelvic fat (P less than .05), and heavier kidneys (P less than .005) than control heifers. Heifers assigned to the TBA treatment had shorter teats on d 279 (P less than .001) but greater final live weight (P less than .05) and carcass weight than control heifers. Heifers given TBA alone had more pelvic fat (P less than .05) and lighter kidneys (P less than .05) than those given TBA + 2E. Mean estradiol concentrations in both the ipsilateral and contralateral jugular veins of heifers assigned to the 2E and TBA + 2E treatments, and in the ipsilateral jugular veins of heifers given 1E, were greater (P less than .05) than those in control heifers; concentrations did not decline during the experiment.