Predicting bull growth performance and carcass composition from growth hormone response to growth hormone-releasing hormone.

Development of practical, physiologically based methods that provide an early, yet accurate, evaluation of a bull's genetic merit could benefit the beef industry. The use of GH response to a single, acute dose of GHRH was evaluated as a predictor of future growth performance and carcass characteristics of weanling bulls. Fifty-six Angus bulls averaging 229 d (SD = 27) of age were administered three doses i.v. (0, 1.5, and 4.5 microg/100 kg BW) of human GHRH (1-29) analog in a Latin square design balanced for residual effects. Blood samples were collected via jugular catheter at -60, -45, -30, -15, 0, 5, 10, 15, 30, 45, 60, 90 and 120 min relative to GHRH injection. Serum concentrations of GH were plotted over time. Response to GHRH was calculated as the area under the GH response curve (AUC-GH) using the trapezoidal approximation. Relationships between AUC-GH, weaning weight adjusted to 205 d of age (205-d WW), and direct weaning weight EPD (WWEPD) versus age-adjusted BW (BWadj), ADG, and carcass measurements from a 140-d growth performance test were evaluated using simple linear regression. A positive correlation between AUC-GH and ADG and an inverse relationship between AUC-GH and carcass fat were observed. The present study provides evidence that AUC-GH is a better predictor of future growth performance in beef bulls than 205-d WW or WWEPD values. Thus, GH response to GHRH is associated with subsequent growth and may be a useful tool for sire selection in beef production.     

Use of growth hormone response to growth hormone-releasing hormone to determine growth potential in beef heifers.

The response of GH to GHRH at weaning is known to predict postweaning growth and body composition in beef bulls. The objective of this study was to determine whether GH response to a challenge of GHRH and plasma IGF-I can predict growth rate and body composition in the beef heifer. Growth hormone response to a challenge with two doses of GHRH was measured in 67 Angus heifers averaging 225 d of age (SD = 21) and 217 kg BW (SD = 32). Blood samples were collected at 0 and 10 min relative to an initial "clearance dose" (4.5 micrograms GHRH/100 kg BW) and again, 3 h later, relative to a challenge dose (1.5 or 4.5 micrograms GHRH/100 kg BW). Each animal received each of the two challenge doses, which were randomly assigned across 2 d of blood collection. Serum GH concentration was measured by RIA. Plasma was collected every 28 d during a 140-d growth test and assayed for IGF-I by RIA. Body weight was measured every 28 d and hip height was measured at weaning and at the end of a 140-d growth test. Average daily gain was calculated on d 140 of the growth test and body composition measurements were estimated by ultrasound 2 wk after completion of the growth test. Responses to the two GHRH challenges were dose-dependent (P < 0.05). Average daily gain tended to be related to GH response to the 1.5 micrograms GHRH/100 kg BW dose (R2 = 0.05; P = 0.06), but no relationship was observed at the 4.5 micrograms GHRH/100 kg BW dose (R2 = 0.00; P = 0.93). An inverse relationship (R2 = 0.06; P = 0.02) was observed between response to the 1.5 micrograms GHRH/100 kg BW dose and intramuscular fat percentage. Mean plasma IGF-I concentration was positively associated with ADG (R2 = 0.06; P < 0.01). Growth hormone response to GHRH is modestly related to body composition but not to ADG in weanling beef heifers and likely has limited use in evaluation of growth performance in replacement beef heifers.     

Effects of restricted feeding, low-energy diet, and implantation of trenbolone acetate plus estradiol on growth, carcass traits, and circulating concentrations of insulin-like growth factor (IGF)-I and IGF-binding protein-3 in finishing barrows

Effects of restricted feeding (80% ad libitum), feeding a low-energy diet containing 84% DE (2.95 Mcal/kg) of the control diet, and implantation of Revalor H (140 mg trenbolone acetate plus 14 mg estradiol-17beta) on growth, carcass traits, and serum concentrations of insulin-like growth factor (IGF)-I and IGFbinding protein-3 (IGFBP-3) were studied in crossbred finishing barrows beginning from 59 +/- 0.9 kg of body weight. Blood samples were taken every 3 wk and the animals were slaughtered at approximately 105 kg body weight. Restricted feeding caused a decrease (P < 0.01) in ADG; feeding the low-energy diet was effective in reducing backfat thickness but decreased gain:feed; the implantation caused a decrease in ADG, feed intake, and backfat thickness and increased gain:feed. Overall pork quality based on pH, drip loss, and the lightness in color of longissimus muscle was not affected by any of the treatments. Serum IGF-I concentration increased following the implantation but did not change (P > 0.05) due to other treatments. Immunoreactive IGFBP-3 concentration was not changed by any of the treatments. Overall ADG was positively correlated with early-stage (d 21) IGF-I and IGFBP-3 concentrations only in unimplanted barrows, whereas backfat thickness was negatively correlated with d-42 IGF-I concentration in all but unimplanted barrows with ad libitum intake. A strong positive correlation (P < 0.01) between IGF-I and IGFBP-3 concentrations was apparent with increasing age of the animals. Results suggest that growth rate and backfat thickness are decreased by a moderate restriction of feed or energy intake with no accompanying changes in circulating IGF-I and IGFBP-3 concentrations and that the beneficial effect of Revalor H implantation on feed efficiency may be mediated, in part, by IGF-I. Moreover, both IGF-I and IGFBP-3 concentrations may be useful as growth indices in pigs.     

Genetic parameter estimates for serum insulin-like growth factor-I concentration and carcass traits in Angus beef cattle

Divergent selection for serum insulin-like growth factor-I (IGF-I) concentration began at the Eastern Ohio Resource Development Center (EORDC) in 1989 using 100 spring-calving (50 high line and 50 low line) and 100 fall-calving (50 high line and 50 low line) purebred Angus cows. Following weaning, bull and heifer calves were fed in drylot for a 140-d postweaning period. At the conclusion of the postweaning test, bulls not selected for breeding were slaughtered and carcass data were collected at a commercial abbatoir. At the time of this analysis, IGF-I measurements were available for 1,283 bull and heifer calves, and carcass data were available for 452 bulls. A set of multiple-trait, derivative-free, restricted maximum likelihood (MTDFREML) computer programs were used for data analysis. Estimates of direct heritability for IGF-I concentration at d 28, 42, and 56 of the postweaning period, and for mean IGF-I concentration were .32, .59, .31, and .42, respectively. Direct heritabilities for carcass traits ranged from .27 to 1.0, .26 to 1.0, and .23 to 1.0 when the age-, fat-, and weight-constant end points, respectively, were used, with marbling score having the smallest heritability and longissimus muscle area having the highest heritability in each case. Maternal heritability and the proportion of phenotypic variance due to permanent environmental effect of dam generally were < or = .21 for IGF-I concentrations and for carcass traits other than longissimus muscle area. Additive genetic correlations of IGF-I concentrations with backfat thickness, longissimus muscle area, hot carcass weight, marbling score, quality grade, and yield grade averaged -.26, .19, -.04, -.53, -.45, and -.27, respectively, when carcass data were adjusted to an age-constant end point. Bulls with lower IGF-I concentrations had higher marbling scores and quality grades, but also had higher backfat thickness and yield grades regardless of the slaughter end point. Serum IGF-I concentration may be a useful selection criterion when efforts are directed toward improvement of marbling scores and quality grades of beef 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 growth hormone and feeding level on endocrine measurements,hormone receptors,muscle growth and performance of prepubertal heifers

Prepubertal Friesian heifer calves (n = 24, initial BW = 195 +/- 5 kg) were assigned to a 2 x 2 factorial block design and used to evaluate the effects of daily GH treatment (0 or 15 mg/d) at either a low or a high feeding level in a 5-wk treatment period on endocrine measurements, hormone receptors, muscle growth, and overall performance. In the pretreatment period, a low feeding level was employed for all calves. During the treatment period, animals at the low feeding level had free access to a roughage-based mixture, whereas animals at the high feeding level had free access to a concentrate mixture and were offered 2 kg/d of the roughage-based mixture. Blood samples were collected weekly starting 3 wk before treatment. Longissimus (LM) and supraspinatus (SS) muscles were obtained at slaughter. Metabolizable energy intake was 81% higher, digestible CP intake was 140% higher, and ADG was 115% higher (all P < 0.001) at the high vs. low feeding level. Feed (DMI, ME, and protein) intake was not affected by GH treatment, but ADG was 18% higher (P < 0.13) in GH-treated than in control heifers at both feeding levels. Although of different magnitudes, the muscle anabolic effects of GH treatment and high vs. low feeding level were additive, and both treatments increased carcass weights (P < 0.02 and P < 0.001, respectively), LM (P < 0.05 and P < 0.001), and SS (P < 0.06 and P < 0.003). The anabolic effect of GH treatment was similar in both muscles, whereas the effect of feeding level was most pronounced in LM. Overall, GH treatment increased plasma GH, IGF-I (both P < 0.001), and IGFBP-3 (P < 0.02); however, GH treatment increased total IGF-I, free IGF-I, and IGFBP-3, and decreased IGFBP-2 mainly at the high feeding level (GH x feeding level interaction; P < 0.02, 0.01, 0.03, and 0.10, respectively). The high feeding level increased insulin, free and total IGF-I, and IGFBP-3 (all P < 0.001), but decreased GH and IGFBP-2 (both P < 0.001). High feeding increased type-1 IGF receptor density (P < 0.02), mainly in LM, in accordance with the largest anabolic response in this muscle, whereas GH treatment had no effect on type-1 IGF receptors. The results suggest that in skeletal muscle, the anabolic effects of exogenous GH are related to endocrine changes in the GH-IGF axis, whereas the effects of feeding level also seem to rely on IGF receptor density in the muscles.     

Genetic parameter estimates for serum insulin-like growth factor-I concentration and ultrasound measurements of backfat thickness and longissimus muscle area in Angus beef cattle

A divergent selection experiment for serum IGF-I concentration began at the Eastern Ohio Resource Development Center in 1989 using 100 spring-calving (50 high line and 50 low line) and 100 fall-calving (50 high line and 50 low line) purebred Angus cows. Following weaning, bull and heifer calves were fed in drylot for a 140-d period. Real-time ultrasound measurements of backfat thickness and longissimus muscle area were taken on d 56 and 140 of the postweaning test. Only ultrasound data from calves born from fall 1995 through spring 1999 were included in the analysis. At the time of this study, IGF-I measurements were available for 1,521 bull and heifer calves, and ultrasound data were available for 636 bull and heifer calves. Data were analyzed by multiple-trait, derivative-free, restricted maximum likelihood methods. Estimates of direct heritability for IGF-I concentration at d 28, 42, and 56 of the postweaning period, and for mean IGF-I concentration were 0.26 +/- 0.07, 0.32 +/- 0.08, 0.26 +/- 0.07, and 0.32 +/- 0.08, respectively. Direct heritabilities for ultrasound estimates of backfat thickness ranged from 0.17 +/- 0.11 to 0.28 +/- 0.12, whereas direct heritabilities for longissimus muscle area ranged from 0.20 +/- 0.10 to 0.36 +/- 0.12, depending on the time of measurement and the covariate used for adjustment (age vs. weight). Direct genetic correlations of IGF-I concentrations with backfat thickness at d 56 and 140 and with longissiumus muscle area at d 56 and 140 averaged 0.02, 0.20, -0.08, and 0.23, respectively, when age was used as the covariate for both IGF-I and ultrasound measurements. Corresponding genetic correlations when age was used as the covariate for IGF-I and weight was used as the covariate for ultrasound measurements were 0.05, -0.07, -0.22, and -0.04, respectively. Therefore, the positive associations of serum IGF-I concentration with backfat thickness and longissimus muscle area at d 140 seem to have been partially mediated by weight. Results of this study do not indicate strong associations of serum IGF-I concentration with fat thickness or muscling of bulls and heifers during the postweaning feedlot period.     

Genetic parameter estimates for serum insulin-like growth factor I concentrations, and body weight and weight gains in Angus beef cattle divergently selected for serum insulin-like growth factor I concentration

Data for the current study were obtained from a divergent selection experiment in which the selection criterion was the average serum IGF-I concentrations of 3 postweaning blood samples collected from purebred Angus calves. Multiple-trait derivative-free REML procedures were used to obtain genetic parameter estimates for IGF-I concentrations and for BW and BW gains measured from birth to the conclusion of a 140-d postweaning performance test. Included in the analysis were 2,674 animals in the A(-1) matrix, 1,761 of which had valid records for IGF-I concentrations. Direct heritability estimates +/- SE for IGF-I concentration at d 28, 42, and 56 of the postweaning period and for mean IGF-I concentrations were 0.44 +/- 0.07, 0.51 +/- 0.08, 0.42 +/- 0.07, and 0.52 +/- 0.08, respectively. Heritability estimates for maternal genetic effects ranged from 0.10 +/- 0.05 to 0.20 +/- 0.06. The proportion of total phenotypic variance due to the maternal permanent environmental effect was essentially zero for all measures of IGF-I concentrations. Genetic correlations of IGF-I concentrations with weaning and post-weaning BW ranged from 0.07 +/- 0.12 to 0.32 +/- 0.11 and generally demonstrated an increasing trend during the postweaning period. Averaged across the various measures of IGF-I, the genetic correlation of IGF-I with preweaning gain was 0.14, whereas the genetic correlation with postweaning gain was 0.29. Genetic correlations between IGF-I and BW gain were positive during all time intervals, except between weaning and the beginning of the postweaning test and from d 84 to 112 of the postweaning period. Environmental and phenotypic correlations of IGF-I with BW and BW gains were generally positive, but small. These results indicate that postweaning serum IGF-I concentration is moderately to highly heritable and has small positive genetic, environmental, and phenotypic correlations with BW other than birth weight and with pre- and postweaning gain. Therefore, if IGF-I proves to be a biological indicator of an economically important trait (e.g., efficiency of feed use for growth) in beef cattle, it should be possible to rapidly change IGF-I concentrations via selection without significantly altering live weight or rate of gain.     

Immunohistochemical detection of components of the insulin-like growth factor system during skeletal muscle growth in the pig

The insulin-like growth factor (IGF) system plays an important role in postnatal somatic and skeletal muscle growth in pigs. There is little information on the occurrence and distribution of components of the IGF system in postnatal porcine skeletal muscle. IGF-I, IGF receptor 1 (IGF1R) and the IGF-binding proteins IGFBP-1 and -3 in longissimus dorsi and triceps brachii were localized in muscle biopsies from 12 commercially crossbred pigs aged from 28 to 199 days as well as from the sire generation, by immunohistochemistry. Plasma IGF-I concentrations were also determined using radio-immunoassays. Unlike other species, IGF-I was localized in porcine skeletal muscle fibres. Staining intensity correlated with the highest plasma IGF-I levels and phases of intensive muscle growth from the 11th to 22nd week. The pattern of IGF1R immunostaining, which was strong, correlated with that of IGF-I, IGF1R was also localized in endomysial tissues. IGFBP-1 was not detected within muscle fibres, but was found in the endomysium and vessel walls, while IGFBP-3 was localized with IGF-1 and its receptor. Higher magnification revealed that IGF1R, IGFBP-3 and probably IGF-I appeared in the tubular system. Inhibitory as well as stimulating controls of IGFBP-1 and -3 on IGF functions are discussed, which may maintain a balance between autocrine growth promoting activities of IGF-I and IGF1R.     

Effect of pituitary somatotropin injections on plasma insulin-like growth factor I and somatotropin profiles in growing heifers

Effects of daily injections of pituitary-derived bovine somatotropin (bST) for 6 wk were evaluated in 10 growing heifers and compared to 9 placebo-treated control animals. Bovine somatotropin was injected at 50 micrograms/kg BW each day. Body weight and growth, plasma concentrations of insulin-like growth factor I (IGF-I) and somatotropin (ST) were assessed. To measure plasma concentrations of IGF-I, we validated a RIA in which bovine plasma samples were extracted with acid-ethanol, a method that resulted in greater than 90% recovery of IGF-I. Average daily gain was similar during the first 4 wk of the experiment in both control and bST-treated groups; however, at the end of the experimental period (wk 4 and 6) ADG was greater (P less than .05) in bST-treated heifers (1.24 +/- .21 kg/d vs .75 +/- .25 kg/d). Plasma IGF-I from wk 2 to wk 6 were increased in bST-treated animals (452 +/- 97 ng/ml at wk 2; 683 +/- 106 ng/ml at wk 6) compared with controls (293 +/- 62 ng/ml at wk 2 (P less than .01) and 293 +/- 115 ng/ml at wk 6 (P less than .001). Moreover, ADG over the 6-wk experimental period was correlated with mean IGF-I concentrations determined over the same period (r = .55; P less than .01). As expected, mean plasma ST concentrations were increased in bST-injected animals from wk 1 to 6. Gel chromatographic profiles of bovine plasma exhibit a 150,000 molecular weight ST-dependent binding protein-IGF-I complex and a 30,000 molecular weight non-ST-dependent complex. This study validates a method for measuring IGF-I in cattle, and shows a positive relationship among IGF-I and ADG after ST treatment. No correlation, however, was found between plasma ST and growth performance.     

The relationship between endogenous insulin-like growth factors and growth in pigs.

Previous studies have reported conflicting data on gender differences in plasma IGF-I in postnatal pigs. There is also debate over the role of IGF-II in regulation of postnatal growth. We have, therefore, determined the concentrations of plasma IGF-I, IGF-II, and IGF-binding protein-3 (IGFBP-3) in boars, barrows, and gilts and related these to postnatal growth characteristics. Plasma concentrations of IGF-I were higher in boars than in gilts or barrows from 13 wk. of age, and plasma IGF-II levels were generally higher in barrows than in boars or gilts. Plasma IGFBP-3 levels were higher in boars than in gilts or barrows at most ages. Between 15 and 23 wk. of age, IGF-I and IGFBP-3, but not IGF-II, were positively associated with growth rate, voluntary feed intake, and gain:feed ratio. Plasma IGF-II, but not IGF-I or IGFBP-3, was positively associated with backfat depth during this period. These results support the hypothesis that circulating IGF-I and IGF-II are regulators of lean and adipose tissue growth, respectively.     

The influence of level of feeding on growth and serum insulin-like growth factor I and insulin-like growth factor-binding proteins in growing beef cattle supplemented with somatotropin

The objective of this study was to determine the effects of level of feeding on growth, feed efficiency (gain:feed; G:F), body composition (BC), and serum concentrations of somatotropin (ST), IGF-I, and IGF-binding proteins (BP) in growing beef cattle supplemented with bovine (b) ST. In each of two consecutive years, 40 growing beef cattle were blocked by weight (average BW: yr 1 = 316 kg, yr 2 = 305 kg) and used in a 2 x 2 factorial arrangement with main effects of bST (0 or 33 microg x kg BW(-1) x d(-1)) and level of feed intake (ad libitum [AL] or 0.75 AL). Relative to uninjected cattle, treatment with bST increased ADG 9.6% (1.14 vs 1.25 kg/d; P < 0.05) and increased G:F 8.1% (12.3 vs 13.3 gain [g]:feed [kg]; P < 0.05), whereas ADG in AL animals was 39% greater than that in 0.75 AL animals (1.39 vs 1.00 kg/d; P < 0.05). There was a tendency (P = 0.10) for a bST x level of feeding interaction, such that the increase in ADG with bST was greater in AL cattle than in 0.75 AL cattle (10.6 vs 7.8%; P = 0.10). Serum concentrations of ST were greater in 0.75 AL cattle than in AL cattle (13.0 vs 8.6 ng/mL; P < 0.05) and in bST-treated cattle than in uninjected cattle (16.3 vs 5.2 ng/mL; P < 0.05). Due to a bST x level of feeding interaction (P < 0.01), the magnitude of the increase in serum ST to exogenous bST was greater (P < 0.01) in 0.75 AL cattle than in AL cattle. Relative to uninjected cattle, treatment with bST increased (P < 0.05) serum concentrations of IGF-I and IGFBP-3 and reduced (P < 0.05) concentrations of IGFBP-2. Similarly, AL cattle had greater (P < 0.05) serum concentrations of IGF-I and IGFBP-3 and reduced (P < 0.05) IGFBP-2 compared with 0.75 AL cattle. In summary, treatment with bST increased growth rate and G:F and stimulated serum IGF-I and IGFBP-3 while reducing IGFBP-2. Feeding at 0.75 ad libitum intake reduced the magnitude of response for each of these variables. Thus, limit-feeding may reduce the effect of exogenous bST on growth rate by blunting bST-induced increases in IGF-I and IGFBP-3 and bST-induced decreases in IGFBP-2.     

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.     

Stimulation of swine growth by porcine growth hormone

Highly purified porcine growth hormone (pGH; USDA-B1) was administered by im injection (22 micrograms X kg body weight-1 X d-1) to rapidly growing Yorkshire barrows for 30 d. Growth hormone significantly increased growth rate (10%), feed efficiency (4%), cartilage growth and muscle mass. However, pGH did not affect carcass adipose tissue mass. Intramuscular lipid content of the longissimus was increased 50% by pGH administration. Plasma pGH concentration was elevated (7- to 11-fold) for 3 to 5 h post-injection. Chronic administration of pGH depressed pituitary GH content and concentration approximately 45%. No GH antibodies were detected in the plasma of GH-treated swine. Plasma somatomedin-C concentration was increased 55% by GH treatment 3 h post-injection. Plasma glucose and insulin concentrations were both significantly increased in GH-treated swine, suggesting that the animals had developed a state of insulin resistance. Plasma-free fatty acid concentration tended to be higher in GH-treated animals. Treatment of swine with pGH significantly decreased plasma blood urea nitrogen. Assessment of animal health during the trial and postmortem indicated that pGH administration did not have any adverse effects. In summary, treatment of young, rapidly growing swine with pGH stimulated growth performance without affecting animal health or inducing the production of GH antibodies.     

Effects of short- and long-term fasting on plasma and stomach ghrelin,and the growth hormone/insulin-like growth factor I axis in the tilapia, Oreochromis mossambicus

Ghrelin is a highly conserved peptide hormone secreted by the stomach, which is involved in the regulation of food intake and energy expenditure. Ghrelin stimulates growth hormone (GH) release, and increases appetite in a variety of mammalian and non-mammalian vertebrates, including several fish species. Studies were conducted to investigate the effect of feeding and fasting on plasma and stomach ghrelin, and the growth hormone/insulin-like growth factor I (IGF-I) axis in the Mozambique tilapia, a euryhaline teleost. No postprandial changes in plasma and stomach ghrelin levels or stomach ghrelin mRNA levels were observed. Plasma levels of GH, IGF-I and glucose all increased postprandially which agrees with the anabolic roles of these factors. Fasting for 4 and 8 d did not affect ghrelin levels in plasma or stomach. Plasma GH was elevated significantly after 4 and 8 d of fasting, while plasma IGF-I levels were reduced. Plasma ghrelin levels were elevated significantly after 2 and 4 wk of fasting, but no change was detected in stomach ghrelin mRNA levels. Four weeks of fasting did not affect plasma GH levels, although plasma IGF-I and glucose were reduced significantly, indicating that GH resistance exists during a prolonged nutrient deficit (catabolic state). These results indicate that ghrelin may not be acting as a meal-initiated signal in tilapia, although it may be acting as a long-term indicator of negative energy balance.     

Effects of divergent selection for serum insulin-like growth factor-I concentration on performance, feed efficiency, and ultrasound measures of carcass composition traits in Angus bulls and heifers

Angus bulls and heifers from lines divergently selected for serum IGF-I concentration were used to evaluate the effects of IGF-I selection line on growth performance and feed efficiency in 2 studies. In study 1, bulls (low line, n = 9; high line, n = 8; initial BW = 367.1 +/- 22.9 kg) and heifers (low line, n = 9; high line, n = 13; initial BW = 286.4 +/- 28.6 kg) were adapted to a roughage-based diet (ME = 1.95 Mcal/kg of DM) for 24 d and fed individually for 77 d by using Calan gate feeders. In study 2, bulls (low line, n = 15; high line, n = 12; initial BW = 297.5 +/- 34.4 kg) and heifers (low line, n = 9; high line, n = 20; initial BW = 256.0 +/- 25.1 kg) were adapted to a grain-based diet (ME = 2.85 Mcal/kg of DM) for 32 d and fed individually for 70 d by using Calan gate feeders. Blood samples were collected at weaning and at the start and end of each study, and serum IGF-I concentration was determined. Residual feed intake (RFI) was calculated, within study, as the residual from the linear regression of DMI on midtest BW(0.75), ADG, sex, sex by midtest BW(0.75) and sex by ADG. In study 1, calves from the low IGF-I selection line had similar initial and final BW and ADG, compared with calves from the high IGF-I selection line. In addition, DMI and feed conversion ratio were similar between IGF-I selection lines; however, calves from the low IGF-I selection line tended (P < 0.10) to have lesser RFI than calves from the high IGF-I selection line (-0.26 vs. 0.24 +/- 0.31 kg/d). In study 2, IGF-I selection line had no influence on performance or feed efficiency traits. However, there was a tendency (P = 0.15) for an IGF-I selection line x sex interaction for RFI. Bulls from the low IGF-I selection line had numerically lesser RFI than those from the high IGF-I selection line, whereas in heifers, the IGF-I selection line had no effect on RFI. In studies 1 and 2, weaning and initial IGF-I concentrations were not correlated with either feed conversion ratio or RFI. However, regression analysis revealed a sex x IGF-I concentration interaction for initial IGF-I concentration in study 1 and weaning IGF-I concentration in study 2 such that the regression coefficient was positive for bulls and negative for heifers. These data suggest that genetic selection for postweaning serum IGF-I concentration had a minimal effect on RFI in beef cattle.     

Insulin administration suppresses an increase in insulin-like growth factor binding protein-2 gene expression stimulated by fasting in the chicken

1. We examined the changes in plasma IGF-I concentration and tissue IGFBP-2 gene expression of young fasted chickens refed a commercial diet or administered bovine insulin intravenously. 2. Plasma IGF-I concentration was decreased by fasting for 2 d. Although plasma IGF-I concentration was increased by refeeding, it didn't recover to the level of chickens fed a commercial diet ad libitum. 3. Insulin administration lowered plasma IGF-I concentration compared to other groups. 4. Hepatic IGFBP-2 mRNA was increased by fasting for 2 d and decreased by refeeding for 6 h. Insulin administration also decreased hepatic IGFBP-2 gene expression stimulated by fasting to the level of refed chickens. 5. IGFBP-2 mRNA in the gizzard was increased by fasting for 2 d and tended to decrease after refeeding for 6 h. Insulin administration decreased gizzard IGFBP-2 gene expression to less than that in refed chickens. 6. There was no between-treatment difference in IGFBP-2 mRNA in the brain and kidney. 7. These results suggest that the changes in IGFBP-2 gene expression in the liver and gizzard by fasting and refeeding might be partly regulated by the alteration in plasma insulin concentration.     

Effects of antimicrobials and weaning on porcine serum insulin-like growth factor binding protein levels

The effects of subtherapeutic antimicrobial supplementation and weaning on serum levels of IGF-I and insulin-like growth factor binding proteins (IGFBP)-2, -3 and -4 were determined in crossbred weanling pigs. At weaning, pigs were allotted to a diet containing 21.8% crude protein and 1.15% lysine with or without Aureozol (110 mg/kg of Aureomycin chlortetracycline, 110 mg/kg of sulfathiazole, and 55 mg/kg of penicillin) for 4 wk. Insulin-like growth factor-binding proteins and IGF-I analyses were performed on blood samples that were drawn weekly. Weaning decreased serum IGFBP-3 levels in both control and Aureozol-treated groups on d 6 and d 14 (P < 0.05) relative to preweaning levels. The IGFBP-3 values returned to preweaning levels by d 21. Although the circulating levels of both the 43-kDa and the 39-kDa glycosylation variants of IGFBP-3 were affected by weaning, the level of the 39-kDa IGFBP-3 was affected relatively more than that of the 43-kDa IGFBP-3 (P < 0.05). Compared with circulating IGFBP-3 levels in control pigs, Aureozol-treated pigs had higher circulating IGFBP-3 levels on d 21 (43%, P < 0.05) and d 27 (46%, P < 0.05). In direct contrast to the effect of weaning on serum IGFBP-3 level, serum IGFBP-2 levels increased on d 6 and d 14 after weaning (P < 0.05) and decreased to preweaning levels by d 21. The IGFBP-2 levels continued to decline and were less than preweaning levels by d 27 (P < 0.05). Aureozol treatment had no effect on serum IGFBP-2 levels at any time. Serum levels of nonglycosylated IGFBP-4 were not affected by either weaning or Aureozol supplementation. Weaning decreased circulating IGF-I concentration on d 6 in both control and Aureozol-treated pigs (76 and 73%, respectively, P < 0.05) and on d 14 (62%, P < 0.05) and d 21 (32%, P < 0.05) in control pigs. Aureozol-supplemented pigs had higher serum IGF-I concentrations than control pigs on d 14 (82%, P < 0.05), d 21 (55%, P < 0.05), and d 27 (36%, P < 0.05). The Aureozol-fed pigs had a 14.2% increase in BW gain (P < 0.05) and a 59.6% increase in ADG (P < 0.05) compared with pigs fed the control diet. Both Aureozol-supplementation and weaning cause changes in serum IGFBP levels and IGF-I concentrations that might be involved in regulating rate and efficiency of growth.     

Effects of breed and wintering diet on growth, puberty and plasma concentrations of growth hormone and insulin-like growth factor 1 in heifers

Twenty-five Brangus (BR) and 15 Angus (AN) heifers were used to study the effects of breed and wintering diet on average daily gain (ADG), onset of puberty and plasma concentrations of growth hormone (GH) and insulin-like growth factor 1 (IGF-1). Wintering diets (fed for 107 days beginning November 15) consisted of the following: 1) native grass hay (NGH), 2) ammoniated NGH, 3) NGH plus cottonseed meal, 4) Diet 3 plus corn and 5) Diet 4 plus monensin. After wintering, heifers were transferred to ryegrass pasture for 70 days. Mean ADG during the wintering phase were -.20, -.10, .17, .29 and .39 kg for heifers fed Diets 1 through 5, respectively (P less than .01). ADG was greater (P less than .05) for BR than for AN heifers. Plasma concentrations of GH were higher (P less than .05) in heifers fed Diets 1 and 2 than in heifers fed Diets 3, 4 or 5. Plasma concentrations of IGF-1 were lowest in heifers fed Diet 1 and highest in heifers fed Diets 4 and 5. During ryegrass grazing, GH concentrations were similar for all groups. However, concentrations of IGF-1 were higher (P less than .05) in heifers fed Diets 3, 4 and 5 than in heifers fed Diets 1 and 2. Age at puberty (onset of cyclic progesterone concentrations) was greatest in heifers fed Diet 1 and lowest in heifers fed Diet 5. Weight at puberty was not affected (P greater than .10) by wintering diet but was greater (P less than .01) in BR than in AN heifers. Therefore, negative ADG appears to be associated with elevated plasma GH concentrations in heifers, and plasma IGF-1 concentration appears to be a more accurate indication of nutritional status than plasma concentrations of GH.