GROWTH AND HORMONAL RESPONSE OF INTACT AND CASTRATE MALE CAlTLE TO TRENBOLONE ACETATE AND ESTRADIOL',*

Transcription

1 GROWTH AND HORMONAL RESPONSE OF INTACT AND CASTRATE MALE CAlTLE TO TRENBOLONE ACETATE AND ESTRADIOL',* C. Y. Lee3, D. M. Henricks4, G. C. Skellefl and L. W. Grimes5 Clemson University, Clemson, SC ABSTRACT Effects of castration and anabolic implants on weight gain, rib soft tissue composition and serum hormones were studied in cattle using a completely random design with a 2 x 2 factorial arrangement. Half of 16 bulls and 16 steers (Angus or Angus x Brahman) aged 9 mo and weighing 290 kg were treated with an implant (200 mg trenbolone acetate and 24 mg estradiol). Half of each group were not treated with an implant. A growing diet was fed for 95 d and half the animals in each group were slaughtered. Animals in the treated groups were reimplanted with trenbolone acetate and fed a finishing diet for 84 d and slaughtered. Percentage dry matter, fat and protein were determined on soft tissue from the th rib. Two blood samples were collected from each animal every 2 wk. Serum was assayed for five hormones. During the growing phase, unteated and treated bulls and treated steers gained more! weight and had leaner rib sections that untreated steers (P e.05); after the finishing phase, there were no differences among groups. Untreated steers had lower insulin-like growth factor (IGF-I) and higher cortisol concentrations during both phases of growth than untreated bulls did (P e.05). Treatment with implants increased IGF-I concentrations in steers during both phases and reduced cortisol during the finishing phase. In bulls, the implant did not affect weight gain, rib composition, cortisol or IGF-I, but it decreased testosterone. We conclude that bulls over 9 mo of age have sufficient endogenous anabolic hormones for maximum growth; steers have insufficient hormones for maximal growth. (Key Words: Beef Cattle, Growth, Hormones, Carcass Composition, Anabolic Steroids.) lntroductlon Bulls grow faster and are leaner than steers. The growth rate of steers can be increased by various androgenic or estrogenic steroids; the hblished with the approval of the Director of the South Carolina Ag~ic. Exp. Station as technical contribution No %e authors gratefully acknowledge L. E. underwood and J. J. Van Wyk for providing IGF-I antibody, the National Hormone and Pituitary Program for distributing the antibod, and S. Gray and A. Whelchel for technical assistance. %mreat address: Dept. of Dairy Sci., Univ. of Florida, Gainesville. 'bept. of Anim. Sci. Address reprint requests to Dr. Henricks. hp. statistics unit. Received July 5, Accepted January 4, Anim. Sci : effect of androgenestrogen combinations is additive (see review by Galbraith and Topps, 1981; Van der Wal and Berende, 1983; Roche and Quirke, 1986). Trenbolone acetate and estradiol, like other androgenestrogen combinations, have been reported to have an additive anabolic effect in steers (Heitzman et al., 1977) but little effect in bulls (Henricks et al., 1988). In general, the anabolic compounds are not as effective in bulls as in steers. These differences in growth and responsiveness to anabolic compounds may be expressed as differences in the hormonal status between the two sex phenotypes. Understanding how the endocrine system changes following castration or treatment with anabolic implants may allow growth in cattle to be manipulated. To explain this sexdependent effect on hormonal status, the

2 ANABOLIC IMPLANTS IN BULLS AND STEERS 2683 TABLE 1. COMPOSITION OF DIETSa ON A DRY MATER BASIS Ingredient Growing Pinishing Corn, % O Corn silage, % Soybean meal, '% Nutrient content DM, % ME, Mcavkgb CP. %b *A mineral-vitamin supplement was fed free choice. It contained 12% Ca, 12% P, 13% salt, trace minerals (I, Mg, K, S, Fe. Mn, Co, CU, Se and Zn) and vitamins (A, 22,000 USP Units&; D3,55,000 USP dtdkg). kstimated composition from NRC (1988) on dry matter basis. objective of this experiment was to investigate the effects of castration and trenbolone acetateestradiol implants on weight gain and body composition in relation to serum hormones. Materials and Methods Animals. Angus or Angus x Brahman bull calves of similar age (6 to 7 mo) and weight were selected from a group weaned in October from the University herd; half the animals were then castrated. At approximately 8 to 9 mo of age (November) and 290 kg BW, 16 bulls and 16 steers were selected for the experiment. Eight bulls and eight steers were implanted with 200 mg trenbolone acetate6 (TBA) in the right ear and 24 mg estradiol- 17p7 in the left ear. The other eight bulls and eight steers received no implants. Angus and Angus x Brahman calves were distributed randomly across all treatment groups. The bulls and steers were equally distributed in two adjacent feed lots of.8 ha provided with shades and automatic waterers. The animals had ad libitum access to a growing (G) type diet (Table 1) for 95 d. Sixteen animals (half of each group) were adjusted to a finishing (F) diet (Table 1) for 2 wk and fed for 84 d. Upon initiation of the finishing phase, the implanted cattle received only TBA with no additional estrogen implant. Salt and mineral-vitamin mixture (Table 1) were available free choice. 6Fiplix-200 Roussel UCLAF, France. 'IC~mpudose~, Eli LiUy and Company, Greenfield, IN. Rib Sofr Tissue Analysis. Half the animals from each of the bull and steer groups were designated randomly to be slaughtered at the end of the G phase and at the end of the F phase. The 9th through the 11th rib sections were taken 48 h after slaughter and the soft tissue samples were prepared following the procedure of Hankins and Howe (1946) and stored at -20 C until they were analyzed. Dry matter, protein and fat were determined by freeze-drying, Kjeldahl method and Soxhlet extraction with ethyl ether, respectively, following the procedures of AOAC (1980). Serum Hormone Assays. Two 15-ml blood samples were collected at d 0, 12, 26, 40, 70, 84 and 96 during the growing phase and at 2-wk intervals of the finishing phase. Samples were taken from right and left jugular veins via vacutainer tube commencing at approximately 0830 and was usually completed within 2 h. Blood samples were clotted in warm water and immediately placed on ice for temporary storage until they were centrifuged. Serum was harvested and stored at -20'C until it was analyzed. Samples obtained form the left vein (same side as the estradiol implant) were used for estradiol-17p assay; right vein samples were used for the assay of the other hormones. Trenbolone-17fl. estradiol-l7& and both cortisol and testosterone were assayed following the procedures of Hoffmann and Oettel (1976), Henricks and Torrence (1978) and Henricks et al. (1984), respectively. Assay for Insulin-Like Growth Factor-I. The hormone was obtained from IMCERA (Terre Haute, IN). Five micrograms of insulinlike growth factor-i (IGF-I) were iodinated by mixing it with 1.0 mci Na 1251 and 5 pg chloramine-t for 20 s; this reaction was quenched with 50 mg sodium metabisulfite. Iodinated IGF-I was purified with a G-50 column (1 x 50 cm) and kept at 4 C. For serum IGF-I radioimmunoassay, serum was extracted with acidethanol and neutralized with Tris base as described by Daughaday et al. (1980). Following extraction with acidethanol and neutralization with Tris, the serum was diluted 57-fold (400-fold dilution of native serum) with assay buffer (.03 M phosphate, ph 7.5;.02% [wh] protarninesulfate;.05% [v/vl tween 20;.01 M EDTA,.02% [w/v] NaN3). To 100 pl of treated neutralized, diluted serum were added 100 pl of 1:2500 rabbit anti-human somatomedin-c/igf-i serum* and 100 pl of iodinated IGF-I (12,000 cpm); tubes were

3 2684 LEE ET AL. incubated at 4'C for 16 h. Each tube received 100 pl of 1:30 sheep anti-rabbit gamma globulin serum and was placed in ice-water for 1 h; 50 pl of 1:25 normal rabbit serum was added and tubes were incubated for an additional 1 h in icewater. Finally, 1 ml of assay buffer was added to each tube and the tubes were centrifuged for 30 min at 2,400 x g. The supernatant fluid was aspirated and the pellet was counted. The intra- and interassay coefficients of variation of the assay were 4.1% and 19.6%, respectively. To evaluate the validity of the assay, a composite bovine serum treated by either acidethanol extraction or glycine HC1-incubation (see the legend to Figure 1 for details) was subjected to a parallelism trial. Serum treated by either method was nearly parallel to IGF-I standard in the displacement curve (Figure l), with little potency difference between the acidethanol extract and acid-incubation serum. Recovery of various doses of serum-added IGF-I standard measured in the RIA was 80% to 90% by both treatment methods. Based on these data and further unpublished results (Lee and Henricks), IGF-I RIA data reported in this paper are relative measurements of serum IGF- I concentrations. Statistical Analysis. The experiment was a completely random design with a 2 x 2 factorial arrangement between castration and implantation. Analyses of variance were performed using the General Linear Model procedure of SAS (1982). Main effect and interaction components were fitted and certain pairs of interaction means were compared using preplanned linear contrasts. Specifically, these contrasts included comparisons between implanted and control steers, implanted and control bulls, and implanted steers and control bulls. Differences between means were tested at the 5% significance level. Blood measurements were analyzed as a completely random split plot in time. Results and Discussion Growing Phase. During this phase, the trenbolone and estradiol-17p implants increased the weight gain in steers (P <.05), but *Provided by L. E. Underwood and J. J. Van Wyk, University of North Carolina, and distributed through the National Hormone and Pituitary Program. they did not increase gain in bulls (Table 2). Feed intake was not measured in this experiment. According to Hunt and Henricks in an experiment immediately following this one (unpublished), feed intake was not changed by castration and the trenbolone acetate-estradiol- 17P (TBA-kP) implants had no effect on feed intake in either growing bulls or steers. Published data (Heitzman et al., 1977, 1981) also indicate that the TBA-E;?P implants increased weight gain with little effect on feed intake by steers. As assessed from these data and a normal growth response of each group in this experiment, the increased weight gain in implanted steers probably is attributable to improved metabolic efficiency rather than to an increased feed intake. Untreated steers had ribs with greater fat content than ribs of untreated bulls (Table 2). The implants caused a decrease in dry matter and fat content of the rib section in steers but not in bulls (Table 2). The only difference in protein content was between control steers and implanted bulls (P <.OS). The mean E;? concentrations were not different (P >.05) between nonimplanted bulls and steers, nor was trenbolone-17p (TBOH) detectable (400 pg/ml) in either group. The TBA implant gave mean serum levels of 600 to 800 pgml, tending to be higher in bulls. The E;? implant increased (P <.05) E;? concentration. Concentration was higher (P <.05) in steers than in bulls (22.2 pdml vs 16.9 pg/ml). Serum levels indicate the so-called burst effect that lasted for about 30 d followed by a slowly decreasing E;? level (Figure 2). Steers may have had a slower clearance rate, as reported previously (Henricks et al., 1983). An analysis of the data detected neither a linear nor a quadratic relationship between weight gain and mean TBOH and(or) mean E$ concentrations of individual implanted steers. These results suggest that there is some minimal amount of these steroids required for maximum growth in each steer. The effect of these implants on growth in the bull is more complex. The mean serum cortisol concentration (Table 2) was greater in control steers than in control bulls, as observed previously (Henricks et al., 1988). Because cortisol generally is regarded as a catabolic hormone, the greater cortisol concentration of steers than of bulls may explain part of the reduced weight gain and higher rib fat content of steers. Though

4 1.o ANABOLIC IMPLANTS IN BULLS AND STEERS 2685.ti -0 ocl z a =. I z a 0 m a 2 0 LO eo 10 81) ,280 Sm-C/IGF-I STANDARD (PP).L I, I 1 I.e.I.ti h. 1 SERUM (1111 Figure 1. Dose-response displacement cwes of raw serum and acid-treated sera in the insulin-like growth factor-i (IGP- I) RL4. A composite bovine serum obtained from seven growing beef cattle was treated by acid-ethanol extraction as described in Materials and Methods or incubated with an equal volume of.1 M glycine-glycine HCI, ph 2.0. at 37 C for 1 d and neutralized with 30% incubation volume of.1 MNaOH. Sm-C = Somatomedin-C. there was a trend toward lower cortisol concentration in implanted than in control steers, this difference was not significant. In bulls, the implants did not change the cortisol concentration. Testosterone was not detectable in steers. The implants decreased the testosterone concentration in bulls (Figure 3). The decreased concentration in implanted bulls may be from a feedback suppression of testosterone secretion by the implanted steroids, Gettys et al. (1984) have reported that both TBA and & decreased the LH secretion in castrated cattle. In a subsequent study, Henricks et al. (1988) reported that the TJ3A-Q implants decreased the systemic testosterone concentration without diminishing the LH response to a GnRH challenge. Based on these data, the authors concluded that the implanted steroids suppressed testosterone secretion through hypothalamic feedback. The IGF-I concentration increased progressively with age in the two untreated groups of bulls and steers (Figure 4); this trend was more pronounced in bulls. Thus control bulls had a higher mean IGF-I concentration during the growing period than control steers (340 vs 261 f 25ng/ml, P <.05). The anabolic implants erased this difference between bulls and steers. The mean IGF concentrations in implanted bulls and steers were 336 and 368 f 25 ng/ml. The nonimplanted bulls had mean concentrations of 340 f 25 ng/ml. Because IGF-I increased in parallel with an increase in testosterone in the bulls but not in steers, serum IGF-I concentration may be increased by gonadal hormones. Testosterone promotes IGF-I secretion in the human (Parker et al., 1984) and potentiates GH action on somatomedin A secretion in rats (Kawai et al., 1982). Estradiol stimulates GH secretion in steers (Grigsby and Trenkle, 1986), which in

5 ~ ~~ ~ ~ ~~~ 2686 LEE ET AL. TABLE 2. WEIGHT GAIN, RIB SOm TISSUE COMPOSITION AND SERUM HORMONE CONCENTRATIONS Bulls Steers Item Control Implanteda Control Implanted* SE GfOwingphaseb Initial age, mo Initial wt, kg 292.6' 287.8' 283.8' Final wt, kg 430.9' 432.6' 389.P 432.1' 11.8 ADG, Wd 1.46d 1.53' 1.11" 1.48' llthrib composition, % Dry matter 37.0' 36.3d 45.6e 37.5d 1.9 Fat 17.9' 16.8' 28.4e 19.2d 2.5 Protein 18.lk 19d 15.9e 17.9".9 Trenbolone-17j3, pghnl 839d 676' 174 Estradiol-17j3, p@d 8.3d 16.p 6.9' 22.2f 1.8 Cortisol, ndml 13.Od & 2.6 Fini~hingphase~ Initial wt, kg 457.0' ' 447.4d 21.9 Final wt, kg 548.9' d 539.gd 23.7 Wd th rib composition, % 1.09'.9' 36' 1.d.09 Dry matter 46.3d 48.4d 51.2d 51.4' 1.8 Fat 29.od 31 Sd 35.2d 36.1' 2.5 Protein # 15.3' 14Sd.8 Trenbolone-17j3, pghnl 1012' 987d 325 Estradiol- 17J3, pg/ml 6.8' 8.Zd 4.2d 14.v 1.7 Cortisol,nglml 12.7d 16.9& 24.9" 14.3' 3.0 'Animals were implanted with 200 mg trenbolone acetate and 24 mg ahadiol on d 0 of growing phase followed by a reimplantation of 200 mg trenbolone acetate on d 0 of f&shing phase. bdata are means of eight animals except for rib composition. Rib composition data are means of four animals. CData are means of four animals. d.evfmeans in the same row with superscripts that do not have a common letter differ (P <.05). - A E - ṁ Q Y -.- E 0 '0 c v) W Growing Phase Finishing Phase + Control Bulls -E- Implanted Bulls Control Steers + Implanted Steers ,. I.,.,.,.,.,.,.,.,., Days Pipre 2. Serum estradiol-l7b concentrations. Implanted bulls and steers received 24 mg estradiol and 200 mg trenbolone acetate at d 0 of growing phase and 200mg trenbolone acetate only at d 0 of finishing phase. Data are means f 1.8 pg/ml SE of eight animals for the growing phase and means f 1.7 pglml SE of four animals for the finishing phase.

6 :: Growing Phase ANABOLIC IMPLANTS IN BULLS AND STEERS 2687 U 1. 1, I ' I ' I ' I ' I ' I. 1 ' I. I Days Figure 3. Serum testosterone concentration of bulls. Implanted bulls received 200 mg trenbolone acetate and 24 mg estradiol at d 0 of growing phase and 200 mg trenbolone acetate only at d 0 of finishing phase. Data are means (lt.5 pghnl SE) of eight and four animals for the growing phase and finishing phase, respectively. turn is likely to stimulate IGF-I secretion. According to Breier et al. (1988) and unpublished data cited by them, estradiol increases GH binding to hepatic membrane receptors and promotes IGF-I secretion in steers. Perhaps the fact that the steer had been deprived of its endogenous androgen causes a decrease in the circulating IGF-I, a hormone supporting growth. Thus the steer benefits from an exogenously delivered androgen whereas the bull does not. Restricted feeding of cattle is known to decrease circulating IGF-I concentration also (Breier et al., 1986; Elsasser et al., 1989). As noted previously, the decreased IGF-I concentration in control steers in this experiment is not likely from a reduced feed intake as assessed from the normal growth response of this group. The synthesis and secretion of IGFI is believed to be partially regulated by GH in cattle (Fabry et al., 1987; Elsasser et al., 1989) and other animal species (Baxter, 1986). Recently Elsasser et al. (1989) reported that the IGF-I response to GH in cattle can be attenuated by undernutrition in the form of either energy or protein intake. Hepatic IGF-I gene expression and serum IGF-I concentration are increased in GH-transgenic mice (Mathews et al., 1988) and GH-injected, ovariectomized, hypophysectomized rats (Murphy and Friesen, 1988). Because the amino acid sequences of the human, bovine and porcine IGF-I are identical (Honegger and Humbel, 1986; Tavakkol et al., 1988). and the N-terminal half is nearly identical between human and rat (Rubin et al., 1982), these reports suggest that IGF-I may be a mediator of GH in stimulating somatic growth in animal species. Finishing Phase. There were no differences (P >.05) in weight gain among groups (Table 2), but bulls and implanted steers tended to gain weight more rapidly than untreated steers. After continuing for almost 3 mo on a finishing ration, carcass composition as determined from rib analysis for dry matter, fat and protein was not different (P >.05) between unimplanted bulls and steers, nor did the steroid implants have any effect. Because these animals continued into the finishing phase, the changes in composition measured at the end of this phase suggests that fat deposition during the finishing phase was greater for implanted steers and for bulls than for unimplanted steers. The lack of a difference in composition among groups at slaughter may have been due to heavier weights of the bulls and implanted steers, which would result in increased deposition of fat. Analysis of serum levels of trenbolone and during this phase (Table 2) indicated that

7 2688 LEE ET AL. Growing Phase + Control Bulls 4- Implanted Bulls 4- Control Steers Finishing Phase I. /.,. /.,.,.,. /., Days Figure 4. Seaum insulin-lie growth factor-i (IGF-I) concentlations. Implanted bulls and steers received 200 mg trenbolone acetate and 24 mg estradiol at d 0 of growing phase and 200 mg trenbolone acetate only at d 0 of f~sbing phase. Data represent means f 25 pg/ml SE of eight animals for the growing phase and means f 43 pg/ml SE for four animals for the fdshiag phase, respectively. the implants were releasing trenbolone in both bulls and steers. Serum Q was less in both phenotypes than during the growing phase; the animals were not reimplanted with estrogen. Concentrations of were higher in the implanted steers than in the implanted bulls; implanted steers tended to have the highest ADG during this phase of growth. In bulls, serum testosterone concentrations (Figure 3) were similar to those measured in the G phase; the implants decreased testosterone (P e.05). Serum cortisol was lower (P <.05) in the implanted steers than in the control steers; between the implanted and control bulls and implanted steers there was no difference. These findings are somewhat similar to what was found during the growing phase. Insulinlike growth factor4 (pigure 4) showed a ranking similar to that of cortisol; the implant raised serum IGF-I in steers but not in bulls. In summary, untreated steers grew more slowly than their bull counterparts and had increased fat content of the th ribs during the growing phase. The TBA-F5# implants increased rate of weight gain and decreased the dry matter and fat content of the rib sections of steers during the G phase but not during the F phase. There were no differences between implanted steers and untreated bulls in weight gain and rib measurements (P >.05). The IGF-I concentration was greater and cortisol concentration was less in unimplanted bulls than in their counterpart steers during the G and F phases. The Qp concentration was least in control steers and greatest in implanted steers. The implants increased the IGF-I concentration and decreased the cortisol concentration, though not significantly, in steers during the growing phase. Implanting bulls affected neither weight gain nor rib composition during either phase. The implants caused a decrease in testosterone concentration, but they did not affect cortisol and IGF-I concentrations. lmpllcatlons The reduced growth efficiency of steers versus bulls may be due to an adverse effect of castration on the animal s hormonal status. Removal of testosterone was followed by decreased IGF-I and increased cortisol secretions. Likewise, the anabolic effects of trenbolone acetate and estradiol implants in steers caused a change in hormonal status to re-

8 ANABOLIC IMPLANTS IN BULLS AND STEERS 2689 semble that of bulls. The bulls had sufficient endogenous anabolic steroids for maximum growth. Additional steroids decreased testosterone secretion, perhaps through a feedback mechanism, but did not alter growth rate. Literature Cited AOAC Official Methods of Analysis (13th Ed.). Association of Official Analytical Chemists, Washington, DC. Baxter, R. C The somatomedins: insulin-like growth factors. Adv. Clin. Chem Breier, B. H., J. J. Bass, J. H. Butler and P. D. Gluckman l%e somatotrophic axis in young steers: influence of nutritional status on pulsatile release of growth hormone and circulating concentrations of insulin-like growth factor I. J. J h d ~ l :209. Breier, B. H., P. D. Gluckman and J. J. Bass The somatotrophic axis in young steers: influence of nutritional status and oestradiol-17p on hepatic highand low-aftinty somatotrophic binding sites. J. Endocrinol Daughday, W. H., I. K. Mark and S. L. Blethen Inhibition of access of bound somatomedin to membrane receptor and immunobinding sites: a comparison of radioreceptor and radioimmunoassay of somatomedin in native and acid-ethanol-extracted serum. J. Clin. Endocrinol. & Metab. 51:781. Elsasser, T. H.. T. S. Rnmsey and A. C. Hammond Influence of diet on basal and pwth hormone- stimulated pla~na co-ttation~ of IGF-I in beef cattle. J. Anim. Sci Fabry, J., D. Lemal, V. Claes and L. Ruelle meet of long-term exogenous pituitary growth hormone administration on body weight gains and plasmatic levels of growth hormone and somatomedin-c in heifers. J. Anim. Sci. 65 (Suppl. 1):256 (Abstr.). Galbraith, H. and J. H. Topps Effect of hormones on the growth and body composition of animals. Nu&. Abstr. Rev. Ser. B Gettys, T. W., M. J. D Occhio, D. M. Henricks and B. D. Schanbacher Suppression of LH secretion by oestradiol, dihydrotestostemne and trenbolone acetate in the acutely castrated bull. J. Endocrinol. 1oO:lW. Grigsby, M. E. and A. TrenLle Plasma growth hormone, indin, glucocorticoids and thyroid hormones in large, medium and small breeds of steers with and without an estradiol implant. Domest. Anim. Endocrinol. 3:261. Hankins, 0. G. and P. E. Howe Estimation of the composition of beef carcass and cuts. USDA Tech. Bull Heitzman, R I., K. H. Chan and I. C. Hart Liveweight gains, blood levels of metabolites, proteins and hormones following implantation of anabolic agents in steers. Br. Vet. J Heitpaao, R J., D. N. Gibbons, W. Little and L. P. Harrison A note on the comparative performance of beef stem implanted with the anabolic steroids trenbolone acetate and esttadiol-l7p, alone or in combination. Anim. Rod Henricks, D. M., J. W. Cooper, J. C. SpitZer and L. W. Grimes Sex differences in plasma cortisol and growth in the bovine. J. Anim. Sci. 59:376. Henricks, D. M., T. Gimenez. T. W. Gettys and B. D. Schanbacher Effect of castration and an anabolic implant on growth and serum hormones in cattle. Anh. Prod. 46:35. Henricks. D. M, S. L. Gray and J.L.B. Hoover Residue levels of endogenous estrogens in beef tissues. J. Anim. Sci Hemicks, D. M., and A, K. Torrence Endogenous estradiol-17 in bovine tissues. J. Assoc. Off. Anal. Chem. 61:1280. Hoffman, B. and G. Oettel Radioimmunoassay for free and conjugated trenbolone and for trenbolone acetate in bovine tissue and plasma samples. Steroids Honegger, A. and R. E. Humbel Insulin-like growth factors I and II in fetal and adult bovine serum: purification, primary structures. and immunological cross-reactivities. J. Biol. Chem Kawai, K., E. Ogata, K. Takano, N. Hizuka, K. Yamashita and K. Schizume Effmts of testosterone and estradiol on serum somatomedin A and growth rate of rats. Endocrinol. Jpn Mathews, L. S., R. E. Hammer, R. L. Brinster and R. D. palmiter Expression of insulin-like growth factor I in transgenic mice with elevated levels of growth hormone is correlated with growth. Endocrinology 123:433. Murphy, L. J. and H. G. Friesea Differential effects of estrogen and growth hormone on uterine and hepatic insulin-like growth factor I gene expression in the Ovariectomized hypophysectomi7-ed rat. Endochology NRC Nutrient Requirements of Beef Cattle (6th Ed.). National Academy Press, Washingtoq DC. Parker, M. W., A. J. Johnson, A. D. Rogol, D. L. Kaiser and R. M. Blizzard Effect of testosterone on somatomedin-c concentrations in prepubertal boys. J. Clin. Endocrinol. & Metab. 58:87. Roche, J. F. and 1. F. Quiuke The effects of steroid hormones and xenobiotics on growth of farm animals. In: P. J. Buttery, N. B. I-Iaynes and D. B. Lindsay (Ed.) Control and Manipulations of Animal Growth. pp Butterwortbs, London. Rubin, J. S., I. Mariz, J. W. Jacob, W. H. Daughaday and R. A. Bradshaw Isolation and partial sequence analysis of rat basic somatomedin. Endocrinology SAS User s Guide: Statistics. SAS Inst., Inc.. Cary, NC. Tavakkol, A., F. A. Simmen and R.C.M. Simmen Porcine insulin-like growth complementary deoxyribonucleic acid cloning and uterine expression of messenger ribonucleic acid encoding evolutiody collserved IGF-I peptide. J. Mol. Endocrinol. 2:674. Van der Wal. P. and PL.M. Berende Effects of auabolic agents on food producing animals. In: E. Meissonnier (Ed.) Anabolics in Animal Production. pp Soregraph. Levallois, France.