Influence of chlortetracycline and dietary protein level on visceral organ mass of growing beef steers

Thirty-two beef steers (285 +/- 3 kg BW) were used to determine the effects of chlortetracycline and dietary protein level on visceral tissue mass, chemical composition, intestinal morphology, and proliferation rate indices. Steers were allotted randomly by weight to a factorial arrangement of dietary treatments consisting of either 10 or 13% CP diets top-dressed with a corn meal carrier (500 g/d) containing either 0 or 350 mg of chlortetracycline. After 84 d, steers were slaughtered and visceral organs removed and separated. Rinsed wet tissue mass was recorded; total RNA, total DNA, tissue DM, and tissue N content were determined; and tissue sections were prepared for immunohistochemical analysis. Thin tissue sections were evaluated to determine crypt depth and villus height as well as proliferation rate by immunohistochemical detection of the nuclear antigen Ki67. Rumen and abomasum weights and small intestinal length were greater (P < 0.04) in steers fed the 13% CP diet than in those fed the 10% CP diet on both an absolute weight basis and a percentage of empty BW. Chemical composition of the small intestinal and ruminal segments were largely unaffected by increased dietary protein. Increasing the dietary CP also increased the villus height in duodenal (P = 0.02) and the crypt depth of jejunal (P = 0.03) sections. Dietary administration of chlortetracycline decreased (P < 0.01) small intestinal weight both on absolute and empty BW bases. Nitrogen and RNA concentrations of the small intestinal segments were unaffected (P > 0.1) by dietary administration of subtherapeutic levels of chlortetracycline; however, because of increases (P < 0.05), or tendencies for an increase (P < 0.1), in the tissue content of DNA, the ratio of N to DNA was decreased (P < 0.05) or tended to be decreased (P < 0.1) in the small intestinal segments of the chlortetracycline-treated animals. The observed decrease in small intestinal epithelial mass does not appear to be due to alterations in cell proliferation rate but rather cell size. Consistent with this finding, cell proliferation, as determined by Ki67 antigen staining, was not affected by dietary treatment. Chlortetracycline administration decreased small intestinal mass that may be a result of decreased cell size.     

Effects of thyrotropin-releasing hormone and a thyrotropin releasing hormone analog on growth and selected plasma hormones in lambs

An 8-wk growth trial was conducted to assess the effects of continuous infusion of thyrotropin-releasing hormone (TRH) and an active TRH analog less than Aad-His-Pro-NH2 (the less than Aad is L-pyro-alpha-aminoadipic acid) on growth trial performance, carcass composition and hormone profiles of growing lambs. Both drugs were infused at 600 micrograms X lamb -1 X d -1 with 16 lambs/treatment. Both TRH and less than Aad-His-Pro-NH2 decreased average daily gain (ADG; P less than .01) and increased feed conversion (FC; P less than .01) compared with saline infused controls. Average daily feed intake was not altered. Carcasses of lambs given TRH or less than Aad-His-Pro-NH2 contained fewer kilograms of moisture (P less than .05) and appeared to contain fewer kilograms of protein. Thyrotropin-releasing hormone and less than Aad-His-Pro-NH2 increased thyroid gland weights (P less than .05), but pituitary gland weights were not different. Plasma thyrotropin (TSH) concentrations were increased by both drugs compared with control lambs, peaking at 4 to 7 d after initiating infusion. However, by 14 d, TSH concentrations returned to control levels. Triiodothyronine (T3) and thyroxine (T4) were elevated by both drugs over the entire 8-wk trial, with peak levels reached at 10 d and maintained for the duration of the study. Both TRH and less than Aad-His-Pro-NH2 increased prolactin over the entire period. Growth hormone levels were not altered by either drug. The effects of less than Aad-His-Pro-NH2 infusion on growth trial performance, carcass composition and hormone profiles of growing lambs were very similar to TRH. The negative effects of TRH and less than Aad-His-Pro-NH2 infusion on ADG, FC and carcass protein appear to be the result of elevated T3 and T4 levels.     

Effects of rapeseed silage variety and dietary level on digestion and growth performance of beef steers

A digestion and ruminal fermentation trial involving five ruminally cannulated steers assigned to a 5 x 5 Latin square with a 2 x 2 + 1 arrangement of dietary treatments was conducted to evaluate the effects of variety of rapeseed silage (RS) containing either a high (HG) or a low (LG) glucosinolate concentration when fed at 100 or 50% of diet DM. A bromegrass hay-corn-soybean meal mixture, which was expected to be isocaloric and isonitrogenous with the RS, was used as the control (C) treatment and replaced RS in the 50% diets. In situ disappearance of substrate from both RS varieties was measured in ruminal environments created by each diet. No dietary treatment x RS substrate interactions were observed for any in situ variable. Total tract digestibility and extent of in situ disappearance of both DM and NDF were greater (P less than .01) for HG than for LG. In situ DM and NDF disappearance at 8, 16 and 24 h was greater (P less than .01) for RS than for the C diet. Similarly, total tract DM digestibility was greater (P less than .01) for RS (61.0%) vs C (56.0%) diets. Diets with 50% RS had greater (P less than .01) NDF digestibility (50.4%) than 100% RS (43.6%) diets. Variety of RS had no effect on particulate passage rate. In a 77-d growth trial with 60 beef steers, ADG was greater (P less than .01) for HG vs LG (.46 vs .36 kg), 50 vs 100% (.52 vs .31) and C vs RS (.64 vs .41) diets.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dietary protein and growth hormone-releasing peptide (GHRP-2) on plasma IGF-1 and IGFBPs in Holstein steers

This study was conduct to determine the influence of dietary protein on the response of plasma insulin-like growth factor-1 (IGF-1) and insulin-like growth factor binding proteins (IGFBPs) to exogenous growth hormone releasing peptide-2 (GHRP-2 or KP 102) in Holstein steers. Eight 16-month-old Holstein steers were grouped by liveweight to two feeding treatments; high protein (HP; CP 1.38 kg/day and TDN 4.5 kg/day DM intake, n=4) or low protein (LP; CP 0.66 kg/day and TDN 4.42 kg/day DM intake, n=4). The experiment was a single reverse design whereby each group was injected twice daily with GHRP-2 (12.5 microg/kg body weight (BW)/day) or saline solution into the jugular vein for a 6-day period. Plasma IGF-1 in the HP group were higher than in the LP group (P<0.05), but plasma 34 kDa IGFBP-2 was lower in the HP than the LP group (P<0.05). The amplitude of the maximum growth hormone (GH) peaks responding to GHRP-2 injection were higher at day 1 than at day 6 of saline or GHRP-2 treatment in both LP and HP steers (P<0.05). The area under the GH response curve for 180 min after the GHRP-2 injection was not significantly different between the LP and the HP groups at days 1 and 6. A response in plasma IGF-1 concentration to GHRP-2 treatment in the HP group was observed at day 1 (198.9+/-18.1 ng/ml), day 2 (195.2+/-21.1 ng/ml) and day 6 (201.3+/-14.8 ng/ml) (P<0.05). No increase in plasma IGF-1 was observed from GHRP-2 administration in the LP group. Although the response of plasma IGF-1 concentration to GHRP-2 administration was increased in the HP group (P<0.05), there was no apparent effect of GHRP-2 treatment on plasma 38-43 kDa IGFBP-3 and 34 kDa IGFBP-2 at days 2 and 6 of treatment. In conclusion, it is proposed that the 34 kDa IGFBP-2 is sensitive to dietary protein level and may play an important role in the regulation of circulating IGF-1 in ruminant. In addition, increased plasma IGF-1 concentration observed in the HP group in response to the GHRP-2 treatment did not appear to affect plasma IGFBPs.     

Influence of feeding level during postweaning growth on circulating concentrations of thyroid hormones and extrathyroidal 5'-deiodination in steers.

An experiment was conducted with 42 growing Montbéliard steers to study the effect of feed restriction, followed by refeeding, on circulating concentrations of thyroxine (T4) and triiodothyronine (T3) and on hepatic and muscle activities of 5'-deiodinase (5'D). At 9 mo of age, 21 steers were diet-restricted for 3 mo (ADG, 641 g/d), prior to a 4-mo compensatory growth period with ad libitum access to the same diet (ADG, 1,240 g/d). They were compared to 21 control steers continuously gaining 1,100 g/d between 9 and 16 mo of age. Blood samples were collected every 14 d and samples of liver and semitendinosus and triceps brachii (triceps) muscles were obtained at slaughter at the end of the restriction and refeeding periods (12 and 16 mo of age, respectively). Compared to control steers, feed restriction decreased plasma concentrations of T4 after 56 to 83 d of feed restriction (P < 0.05), whereas T3 concentration decreased only after 83 d of feed restriction (P < 0.05). No differences in hepatic and muscle 5'D activities were observed after 87 d of feed restriction and decreased growth rate (12 mo of age). During the refeeding period (compensatory growth), circulating concentrations of T4 and T3 were restored to control levels within 14 d. Moreover, T3 concentration rose above that of control steers after 56 d of refeeding and remained higher for the duration of the experiment (P < 0.05). Hepatic 5'D activity was higher (P = 0.07) in compensated than in control steers at the end of refeeding period (16 mo of age) and higher (P < 0.01) after compensation at 16 mo than during restriction at 12 mo. Activities of 5'D in semitendinosus and triceps muscles were higher (P < 0.001) in 16-mo-old than in 12-mo-old steers, but no differences were observed due to feed restriction or compensatory growth. These results indicate that nutritional status regulates both thyroidal secretion and extrathyroidal T3 production in cattle. The data also suggest that extrathyroidal T3 production may be involved in the mechanism of compensatory growth in cattle.     

Relation of growth hormone response to growth hormone-releasing hormone before weaning and postweaning growth performance in beef calves

Previously, GH response to GHRH challenge at weaning has been shown to be indicative of ADG during a standard postweaning growth performance test in Angus cattle. In this study, we tested the hypothesis that GH response to GHRH before weaning would predict postweaning ADG. Bulls with the highest and lowest GH responses to GHRH over a 3-yr period, relative to their contemporaries, were used as sires, to allow for examination of the persistence of GH response to GHRH through selection. The selected calves in this study were sired by one of four Angus bulls chosen based on their GH response to GHRH (high response, n = 2; low response, n = 2). Forty-nine Angus calves (bulls, n = 24; heifers, n = 25) were challenged with GHRH at approximately 60, 105, and 150 d of age and at weaning (219 d; SD = 25). Blood samples were taken immediately prior to and 10 min following an i.v. clearance dose of 4.5 microg of GHRH/100 kg BW and, 2 h later, immediately prior to and 10 min following a challenge dose of either 1.5 or 4.5 microg of GHRH/100 kg BW. Two hours later, the procedure was repeated, with each calf receiving the other challenge dose. Body weight was measured every 28 d and ADG was calculated over a 140-d growth performance test (heifers and bulls maintained separately). Data were log-transformed for statistical analyses. In the selected bulls and heifers, response of GH to 1.5 microg of GHRH/100 kg BW at 60 and 105 d of age was positively related (P < 0.05) to postweaning ADG. Response to 4.5 microg of GHRH/100 kg BW at 105 d of age and at weaning was positively related (P < 0.01) to postweaning ADG. Inclusion of sire in the analysis improved the relationship between GH response and ADG for calves of sires with high GH responses from R2 = 0.18 (P = 0.01) to R2 = 0.33 (P = 0.02). When the GH response to GHRH of the unselected calves at weaning was added to the data from the selected animals and analyzed, the GH response of the bulls was related to postweaning ADG (R2 = 0.09; P = 0.04). In conclusion, GH response to GHRH as early as 60 d of age is indicative of postweaning ADG in beef cattle. In addition, the relationship between GH response to GHRH and postweaning ADG is improved with selection for greater GH response to GHRH.     

Effects of dietary antioxidant supplementation before and after oral acetaminophen challenge in cats

OBJECTIVE: To determine effects of lipoic acid, vitamin E, and cysteine before and after oxidant challenge in cats. ANIMALS: 24 sexually intact adult cats. PROCEDURE: Cats were allocated into 4 equal groups. For 25 weeks, group A was fed a control dry diet and groups B, C, and D received this diet supplemented with vitamin E (2200 U/kg [dry matter basis {DMB}]) plus cysteine (9.5 g/kg [DMB]), lipoate (150 mg/kg [DMB]), or all 3 antioxidants together, respectively. Weights were measured every 3 days and venous blood obtained every 5 weeks for CBC; serum biochemical analyses; lymphocyte blastogenesis; thiobarbituric acid reactive substances concentration; and concentrations of plasma protein carbonyl, 8-OH d-guanosine, blood glutathione, plasma amino acid, lipoate, and dihydrolipoate. At 15 weeks, all cats received acetaminophen (9 mg/kg, PO, once), clinical effects were observed, and methemoglobin concentrations were measured. RESULTS: Lymphocyte blastogenesis increased transiently in group C and D cats. After acetaminophen administration, all groups had transient increases in methemoglobin within 4 hours and mild, brief facial edema; group C had decreased glutathione concentration and increased 8-OH d-guanosine concentration versus controls; and protein carbonyl concentration increased least for group B. Plasma lipoate and dihydrolipoate concentrations peaked by week 10 for groups C and D. Conclusions and Clinical Relevance-Lipoate, vitamin E, and cysteine did not have synergistic effects. Lipoate supplementation (150 mg/kg [DMB]) did not act as an antioxidant but appeared to enhance oxidant effects of acetaminophen. Vitamin E plus cysteine had protective effects.     

Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares

OBJECTIVE: To determine the effects of levothyroxine sodium (L-T4) on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone (TRH) in euthyroid horses. ANIMALS: 12 healthy adult mares. PROCEDURE: 8 horses received an incrementally increasing dosage of L-T4 (24, 48, 72, or 96 mg of L-T4/d) for weeks 1 to 8. Each dose was provided for 2 weeks. Four additional horses remained untreated. Serum concentrations of total triiodothyronine (tT3), total thyroxine (tT4), free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH) were measured in samples obtained at weeks 0, 2, 4, 6, and 8; 1.2 mg of TRH was then administered i.v., and serum concentrations of thyroid gland hormones were measured 2 and 4 hours after injection. Serum reverseT3 (rT3) concentration was also measured in the samples collected at weeks 0 and 8. RESULTS: Treated horses lost a significant amount of weight (median, 19 kg). Significant treatment-by-time effects were detected for serum tT3, tT4, fT3, fT4, and TSH concentrations, and serum tT4 concentrations were positively correlated (r, 0.95) with time (and therefore dosage) in treated horses. Mean +/- SD serum rT3 concentration significantly increased in treated horses (3.06 +/- 0.51 nmol/L for week 8 vs 0.74 +/- 0.22 nmol/L for week 0). Serum tT3, tT4, fT3, and TSH concentrations in response to TRH injections differed significantly between treated and untreated horses. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of levothyroxine sodium increased serum tT4 concentrations and blunted responses toTRH injection in healthy euthyroid horses.     

Effects of human pancreatic and rat hypothalamic growth hormone-releasing factors on growth hormone secretion in steers

Potencies of human pancreatic growth hormone-releasing factor [hpGRF(1–40)-OH] and of a peptide corresponding to the N-terminal 29 residues of rat hypothalamic GRF, [rGRF(1–29)-NH₂] were compared in two experiments. Eight Angus steers averaging 297 days of age and 290 kg in February 1984 were used in Exp. 1. Five months later six of the steers, weighing 391 kg, were used in Exp. 2.In Exp. 1, hpGRF(1–40)-OH and rGRF(1–29)-NH₂ were infused for 5 min at rates of 0, 1.3, 2.6, 5.2, 7.8 and 13.3 pmol/min/kg. Two steers were infused simultaneously, one received hpGRF(1–40)-OH and the other the equivalent dose of rGRF(1–29)-NH₂. Four pairs of steers received each dose. Both peptides elicited rapid GH release. Plasma GH concentrations peaked 15 to 20 min following onset of GRF administration, and returned to baseline levels 60 to 90 min later. Minimum effective doses, the lowest dose tested that resulted in a statistically significant GH reponse, were 5.2 pmol/min/kg hpGRF(1–40)-OH and 13.3 pmol/min/kg rGRF(1–29)-NH₂. Magnitudes of GH responses to 5.2, 7.8 and 13.3 pmol/min/kg hpGRF(1–40)-OH and 13.3 pmol/min/kg rGRF(1–29)-NH₂ were similar; corresponding to respective peak concentrations of 79, 66, 57 and 56 ng/ml. Growth hormone levels before GRF administration averaged 16 ng/ml.Experiment two was designed like the first except steers were infused for 6 hr with hpGRF(1–40)-OH and rGRF(1–29)-NH₂ at rates of 0, .5 and 1 pmol/min/kg. Both peptides at both rates raised (P<.05) GH concentrations during the 6 hr infusion period. Mean GH levels were 7 ng/ml during saline infusion, 30 and 23 ng/ml during infusion of .5 pmol/min/kg hpGRF(1–40)-OH and rGRF(1–29)-NH₂, and 41 and 27 ng/ml during infusion of 1 pmol/min/kg of the respective peptides. The initial GH response was biphasic, after which GH levels decreased temporarily and then one or two more GH surges occurred during the latter portion of the infusion period. Results demonstrate that hpGRF(1–40)-OH and rGRF(1–29)-NH₂ are potent GH secretagogues in steers. Potency of rGRF(1–29)-NH₂ is about 40% of hpGRF(1–40)-OH. Intrinsic activities, their ability to stimulate maximum GH secretion, appear to be similar. Both peptides are effective in raising GH levels over a 6 hr constant infusion period.     

Plasma concentrations of thyroid hormone in steers treated with Synovex-S and 3,5,3'-triiodothyronine.

This experiment examined the effect of daily administration of 3,5,3'-triiodothyronine (T3) on plasma profiles of T3, thyroxine (T4), 3,3',5'-triiodothyronine (reverse T3; rT3) and thyrotropin (TSH) in beef steers in which protein accretion was increased by using implants of Synovex-S (SYN). Twenty-four Angus-Hereford steers (302 +/- 16 kg) were individually fed a diet of a corn-based concentrate and silage mixture for 56 d at equal energy intake per steer (ME/unit BW.75). A 2 x 2 factorial arrangement of treatments was used in which treatments were SYN ear implants (200 mg of progesterone and 20 mg of estradiol benzoate) or no implants and s.c. injections of T3 in polyethylene glycol (2 micrograms of T3/kg BW every 48 h) or no injections of T3. Blood samples were collected every 2 wk. Plasma T3 concentration during the experimental period was increased in T3-treated steers (3.0 +/- .1 vs 2.2 +/- .1 ng/mL, P < .01) and was decreased in SYN-implanted steers (2.4 +/- .1 vs 2.7 +/- .1 ng/mL, P < .01). Plasma T4 and rT3 concentrations were reduced (22 +/- 4 vs 75 +/- 2 and .04 +/- .01 vs .12 +/- .01 ng/mL, respectively, P < .01) in T3-treated steers. Concurrently, plasma TSH concentration was decreased in T3-treated steers (.37 +/- .01 vs .51 +/- .02 ng/mL, P < .02). Synovex-S increased BW gain (21.0%, P < .01) and protein gain (35.6%, P < .01) compared with that of nonimplanted steers. Body weight gain and protein gain were not affected by treatment with T3.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of immunization against luteinizing hormone-releasing hormone and treatment with trenbolone acetate on reproductive function of beef bulls and steers

The objectives of this study were 1) to evaluate the ability of trenbolone acetate (TBA) administered in tandem with LHRH immunization to suppress reproductive function in bulls and 2) to examine the effects of LHRH and androgen (TBA) signaling on pituitary gland function. Forty-four Angus × Hereford crossbred calves (BW=225 ± 2 kg; age=187 ± 6 d) received castration, LHRH immunization, or TBA administration in a 2 × 2 × 2 factorial design. Treatment groups receiving LHRH immunization contained 6 animals, whereas other treatment groups contained 5 animals. Animals immunized against LHRH received a primary injection and 2 booster injections of ovalbumin-LHRH-7 fusion protein on d 0, 42, and 196, respectively. Animals treated with TBA were implanted on d 224. Serum LHRH antibodies increased (P<0.05) after each booster for immunized animals, but were negligible in nonimmunized animals throughout the experiment. Serum testosterone concentration (P<0.001) and scrotal circumference (P<0.05) were depressed in LHRH-immunized bulls compared with nonimmunized bulls by d 84 and 168 of the experiment, respectively. Treatment with TBA tended (P=0.08) to decrease serum testosterone concentrations of nonimmunized bulls. Weights of testes at slaughter were decreased (P<0.001) for LHRH-immunized (232 ± 41 g) compared with nonimmunized (752 ± 45 g) bulls, but did not differ (P=0.80) between TBA-implanted (500 ± 49 g) and nonimplanted bulls (484 ± 36 g). Both LHRH immunization and castration decreased pituitary gland stores of LH and FSH (P<0. 001). There was no effect (P>0.10) of TBA on pituitary gland FSH content and only a tendency (P=0.09) to increase pituitary gland LH content. Immunization against LHRH decreased expression of LH β-subunit and common α-subunit genes (P<0.001). Castration increased expression of LH β-subunit and common α-subunit genes (P=0.02). Treatment with TBA further suppressed (P=0.04) α-subunit mRNA expression in LHRH-immunized steers. In summary, LHRH immunization decreased synthesis and storage of LH and decreased storage, but not synthesis of FSH in bulls. The increased synthesis of LH and FSH in nonimmunized, but not LHRH-immunized steers suggests that castration removes the negative feedback on gonadotropin synthesis but that LHRH is still needed for release of these hormones. Androgen replacement with TBA did not restore the negative feedback control of gonadotropin synthesis.     

Effects of growth hormone-releasing factor and feed intake on energy metabolism in growing beef steers: net hormone metabolism by portal-drained viscera and liver.

Effects of growth hormone-releasing factor (GRF) and intake on arterial concentrations and net visceral metabolism of hormones were measured in six growing Hereford x Angus steers using a split-plot design with 4-wk injection periods within 8-wk intake periods. Steers were fed a 75% concentrate diet at two intakes and were injected s.c. twice daily with saline or GRF (10 micrograms/kg of BW). Arterial concentrations of growth hormone (GH) were measured on d 1 and d 8 to 10 of injections. Eleven measurements, obtained at 30-min intervals, of arterial concentration and net flux of hormones across portal-drained viscera (PDV) and liver were obtained on d 8 to 10 of injections (six hourly measurements were used for insulin-like growth factor-I [IGF-I] and somatostatin). The area under the GH curve and average and peak GH concentrations were increased (P less than .01) by GRF and were greater (P less than .10) at low than at high intake. Liver removal of GH was not affected by GRF or intake. Arterial IGF-I concentration was increased (P less than .05) by GRF and not affected by intake. Treatments did not affect IGF-I flux across the liver. Arterial insulin concentration was greater (P less than .05) at high than at low intake, in part because of greater (P less than .01) PDV release. Increased (P less than .10) arterial insulin concentration in GRF-treated steers was not attributable to significant changes in PDV or liver net flux. Arterial glucagon concentration was greater (P less than .01) at high than at low intake, in part because of greater (P less than .05) PDV glucagon release and decreased (P less than .10) liver extraction ratio. Effects of intake on arterial concentration of insulin and glucagon were in part due to changes in visceral metabolism, but GRF did not affect PDV or liver hormone metabolism.     

Dose effect of human growth hormone-releasing factor and thyrotropin-releasing factor on hormone concentrations in lactating dairy cows

Two experiments were conducted to study the effects of growth hormone-releasing factor (GRF) and thyrotropin-releasing factor (TRF) administration on hormone concentrations in dairy cows. In the first trial, 12 cows were used on 5 consecutive days to determine the effect of four sc doses of GRF (0, 1.1, 3.3 and 10 μg•kg⁻¹ BW) and three sc doses of TRF (0, 1.1 and 3.3 μg•kg⁻¹ BW) combined in a factorial arrangement. GRF and TRF acted in synergy (P = .02) on serum growth hormone (GH) concentration even at the lowest dose tested and GH response to the two releasing factors was higher than the maximal response observed with each factor alone. TRF increased (P<.01) prolactin (Prl), thyrotropin (TSH), triiodothyronine (T₃) and thyroxine (T₄) concentrations similarly at the 1.1 and 3.3 μg•kg⁻¹ doses and GRF did not interact (P>.40) with TRF on the release of these hormones. In the second trial, the effect of GRF (3.3 μg•kg⁻¹ BW, sc) and TRF (1.1 μg•kg⁻¹ BW, sc) was tested at three stages (18, 72 and 210 days) of lactation on serum Prl and TSH concentrations. Eighteen cows (n = 6 per stage of lactation) were used in two replicates of a 3 × 3 latin square. The TRF and GRF-TRF treatments were equipotent (P>.05) in increasing Prl and TSH concentrations. Prl and TSH responses were similar (P>.40) throughout lactation. In summary, GRF at doses ranging from 1.1 to 10.0 μg•kg⁻¹ and TRF at doses ranging from 1.1 to 3.3 μg•kg⁻¹ act in synergy on GH release and do not interact on Prl, TSH, T₃ and T₄ concentrations in dairy cows. Furthermore, Prl and TSH response to TRF are not affected by stage of lactation.     

A two-sample method for assessing growth hormone response to growth hormone-releasing hormone challenge: use as a predictor of gain in beef bulls

In two experiments, Black Angus bulls were challenged at weaning with GHRH analog and evaluated for their GH response to determine whether GH response can predict subsequent growth characteristics. The GH response was determined by measuring GH in blood serum collected 0 and 10 min after GHRH injection (Exp. 1: 1.5 microg/100 kg BW human GHRH, n = 34; Exp. 2: 1.5 and 4.5 microg/100 kg BW bovine GHRH [treatments LGHRH and HGHRH, respectively] administered 3 h after a 4.5 microg/100 kg BW "clearance dose" of GHRH, n = 38]. In Exp. 1, GH response did not predict growth or carcass measurements. In Exp. 2, GH response to LGHRH was positively related to ADG (R2 = .18; P = .007) during a 112-d controlled feeding trial. In addition, there was a tendency for bulls with a greater GH response to HGHRH to exhibit greater ADG than animals with a low response. However, GH response to GHRH was not related to changes in hip height (HH) or carcass ultrasound measurements at d 112 of the growth performance trial. Response of GH to repeated GHRH challenges was consistent within animal over time (r = .47; P = .003). The use of a clearance dose 3 h prior to GHRH challenge improved the relationship between GH response and ADG. Results of this study suggest that GH response to GHRH challenge is a useful tool for identifying beef bulls with superior growth potential.     

Effects of cimaterol administration on plasma concentrations of various hormones and metabolites in friesian steers

The aim of the experiment was to determine the acute and chronic effects of the β-agonist, cimaterol, on plasma hormone and metabolite concentrations in steers. Twelve Friesian steers (liveweight = 488 ± 3 kg) were randomly assigned to receive either 0 (control; n=6) or .09 mg cimaterol/kg body weight/day (treated; n=6). Steers were fed grass silage ad libitum. Cimaterol, dissolved in 140 ml of acidified distilled water (pH 4.2), was administered orally at 1400 hr each d. After 13 d of treatment with cimaterol or vehicle (days 1 to 13), all animals were treated with vehicle for a further 7 d (days 14 to 20). On days 1, 13 and 20, blood samples were collected at 20 min-intervals for 4 hr before and 8 hr after cimaterol or vehicle dosing. All samples were assayed for growth hormone (GH) and insulin, while samples taken at −4, −2, 0, +2, +4, +6 and +8 hr relative to dosing were assayed for thyroxine (T₄), triiodothyronine (T₃), cortisol, urea, glucose and non-esterified fatty acids (NEFA). Samples taken at −3 and +3 hr relative to dosing were assayed for IGF-I only. On day 1, cimaterol acutely reduced (P<.05) GH and urea concentrations (7.6 vs 2.9 ± 1.4 ng/ml; and 6.0 vs 4.9 ± 0.45 mmol/l, respectively; mean control vs mean treated ± pooled standard error of difference), and increased (P<.05) NEFA, glucose and insulin concentrations (160 vs 276 ± 22 μmol/l, 4.1 vs 6.2 ± 0.15 mmol/l and 29.9 vs 179.7 ± 13.9 μU/ml, respectively). Plasma IGF-I, T₃, T₄ and cortisol concentrations were not altered by treatment. On day 13, cimaterol increased (P<.05) GH and NEFA concentrations (7.7 vs 14.5 ± 1.4 ng/ml and 202 vs 310 ± 22 mEq/l, respectively) and reduced (P<.05) plasma IGF-I concentrations (1296 vs 776 ± 227 ng/ml). Seven-d withdrawal of cimaterol (day 20) returned hormone and metabolite concentrations to control values. It is concluded that : 1) cimaterol acutely increased insulin, glucose and NEFA and decreased GH and urea concentrations, 2) cimaterol chronically increased GH and NEFA and decreased IGF-I concentrations, and 3) there was no residual effect of cimaterol following a 7-d withdrawal period.     

Effect of human growth hormone-releasing factor and(or) thyrotropin-releasing factor on hormone concentrations in dairy calves

To determine the effect of chronic treatment with human growth hormone-releasing factor (1-29)NH2 (GRF) and(or) thyrotropin-releasing factor (TRF), 20 calves averaging 70.2 kg BW were divided into four groups (n = 5) according to a 2 X 2 factorial design. For 86 d, calves in each group received twice daily s.c. injections of either .9% NaCl, GRF (5 micrograms/kg BW), TRF (1 microgram/kg BW) or GRF (5 micrograms/kg BW) plus TRF (1 microgram/kg BW). On d 87, all calves received a s.c. injection of GRF (5 micrograms/kg BW) plus TRF (1 microgram/kg BW). Blood samples were collected every 20 min for 18 h on d 1, 29, 57 and 85, and for 8 h on d 87. Hormone responses were measured as area under the hormone concentration curve over time. GRF and TRF acted in synergy (P less than .10) on GH release throughout the treatment period. Growth hormone responsiveness to GRF and(or) TRF decreased (P less than .01) with days of treatment, but this decrease was due to aging rather than to chronic treatment, because GH response to GRF plus TRF was similar (P greater than .10) between control and treated calves on d 87. TRF increased prolactin (Prl) concentration until the end of the treatment period (P less than .01). The response of thyroid-stimulating hormone (TSH) to TRF disappeared (P greater than .10) after 1 mo of treatment, whereas the thyroxine (T4) response decreased (P less than .01) throughout the treatment period. GRF did not induce nor did it interact with TRF on TSH and T4 release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Performance and carcass merit of growing beef steers with chlortetracycline-modified sensitivity to pituitary releasing hormones and fed two dietary protein levels

This paper reports the effects of reduced sensitivity to growth hormone-releasing hormone and thyrotropin-releasing hormone through feeding a subtherapeutic level of chlortetracycline (CTC; 350 mg CTC/d) and two levels of dietary CP (10% and 13% of diet DM) on growth performance and carcass merit characteristics. Thirty-two steers (initial average BW, 286 kg) were adapted to a common 13% CP diet consisting primarily of grass hay, corn, and soybean meal fed to gain 1.25 kg/d. The steers were assigned to four treatments (with or without CTC and 10% or 13% dietary CP in a factorial arrangement) and fed ad libitum amounts of diet for 91 d. Feed intake was determined daily and steers were weighed weekly. Steers were killed at the end of the feeding period for carcass merit determinations. Efficiency of BW gain was greater (P < .05) for steers fed the 13% CP diet than for the 10% CP diet and tended to be less for CTC-steers when the 10% CP diet was fed and greater for the CTC-steers when the 13% CP diet was fed (CTC x dietary CP interaction, P < .10). Feeding CTC increased (P < .01) fat over the longissimus muscle and marbling. This study is interpreted to indicate that the sustained effect of subtherapeutic feeding of CTC to cattle appears to increase fat deposition consistent with a reduced growth hormone and thyroid status reported earlier for these same steers. This would tend to increase energy utilization but may not necessarily produce a measurable increase in BW gain.     

Effects of level and diurnal variation of dietary concentrate on digestion and passage rate in beef steers

Effects of dietary concentrate level and varying the level of concentrate diurnally on site of digestion and passage rate with high concentrate diets were determined with four mature beef steers. Increasing concentrate level from 65 to 80 and 95% decreased ruminal, duodenal and fecal pH, ruminal dilution rates for fluids and particulates and tended to reduce digestion of acid detergent fiber, especially post-ruminally. Ruminal escape of dietary protein tended to increase with concentrate level although microbial efficiency tended to decrease. Compared with an 80% concentrate diet, feeding 95% concentrate in the morning and 65% concentrate in the evening tended to increase ruminal digestion of acid detergent fiber and starch.