Minisymposium: Effect of Partitioning Agents on Nutrient Requirements

ROBERTJ. COLLIER,2 JOHNL VICINI,CHRISTOPHER D. KNIGHT, CAROLL MCLAUGHLIN ANDCLIFTON A. BAILE Monsanto Agricultural Company, Animal Sciences Diuision, St. Louis, MO 63198 been reviewed recently (1). Likewise, effects of ST on nutrient requirements of lactating animals have been reviewed (2) and will not be the subject of this paper.

ABSTRACT Somatotropins increase nitrogen reten tion, decrease carcass fat and increase lean/fat ratio in growing ruminants and swine. However, responses are much more variable in growing ruminants. Maintenance requirements in both ruminants and swine are increased slightly with somatotropin treatment in conjunction with increased lean mass. This is associated with in creases in partial efficiency of protein accretion, re sulting in increased efficiency of growth. Additional in formation is required to accurately assess effects of somatotropin on nutrient requirements for growing ru minants and swine. Future studies should target degradable and nondegradable protein requirements for growing ruminants and the impact of somatotropin on these requirements; definition of dose, pattern and for mulation of somatotropin for both ruminants and swine and interaction of somatotropin with other factors af fecting feed intake in swine and ruminants. J. Nutr. 122: 855-860, 1992.

STs AND GROWTH IN MQNOGASTRICS Three major effects of porcine somatotropin (pST) in swine are increased gain, increased lean: fat ratio and decreased feed intake. This suggests that some changes of nutritional recommendations for swine might occur with commercial introduction. However, specific recommendations would require better defi nition of the protein and fat accretion that occurs at a given dose and pattern of a commercial pST product. It is not likely that the approved dose range for pST in swine will be the one that produces the maximum effect because of limiting returns with maximal doses. Results obtained with maximally effective doses will have utility primarily as they relate to understanding the biological processes that are affected (3, 4). Daily pattern of ST injections has not been shown to affect milk yield response when the dose is held constant (5). However, this is not true of ST pattern effects on growth where a pulsatile ST pattern results in a larger body growth response than a more contin-


•somatotropin •nutrient requirements •ruminants •swine

Objectives of use of partitioning agents in livestock are to increase efficiency of growth or lactation. Im proved efficiency is achieved by shifting the pattern of gain (i.e., lean vs. fat), resulting in lowered cost of food production and diluting maintenance costs when expressed as a portion of maintenance plus lean gain. Some applications alter composition of salable prod ucts, which lowers losses in fat and results in more healthy foods. Key efficacy questions center on the impact of somatotropin (ST)3on maintenance cost and nutrient requirements. Altering either could reduce positive benefits obtained by increased productive output, possibly requiring formulation of specific diets and adversely influencing adoption into general use. Effects of beta agonists on nutrient requirements have 0022-3166/92

1 Presented as part of a minisymposium:

Effect of Partitioning

Agents on Nutrient Requirements, given at the 75th Annual Meeting of the Federation of American Societies for Experimental Biology, Atlanta, G A, April 24, 1991. Guest editor for this review was G. B. Huntington, Ruminant Nutrition Laboratory, USDA-ARS, Beltsville, MD. 2To whom correspondence should be addressed: Animal Sciences Division, Monsanto Agricultural Company BB3F, 700 Chesterfield Village Parkway, St. Louis, MO 63198. 3 Abbreviations: ST, somatotropin; pST, porcine somatotropin; EGF, epidermal growth factor; IGF-I, insulin-like growth factor-I; bST, bovine somatotropin; DE, digestible energy; oST, ovine so matotropin.

$3.00 ©1992 American Institute of Nutrition.


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Impact of Somatotropins on Nutrient Requirements in Domestic Animals1



TABLE 1 Responses of finishing barrows receiving porcine sontatotropin (pST) for 42 d as daily 2 mg injections or front one or two continuous release implants front 71 to 107 kg live weight1



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uous plasma ST (see réf. 6 for review). Furthermore, the difference in plasma ST patterns between males (more pulses) and females (fewer pulses) is postulated to be responsible for the gender difference in growth patterns. A pulsatile ST pattern is markedly more ef fective in enhancing hepatic epidermal growth factor (EGF) receptor mRNA levels and EGF binding than a continuous feminine ST pattern. Pulsatile ST patterns also result in increased plasma insulin-like growth factor I (IGF-I) levels, higher skeletal tissue IGF-I message and lower plasma corticosteroid-binding globulin than a continuous ST pattern (6). This dif ference in growth response due to pattern of plasma ST has an impact on assessment of nutrient require ments of animals given exogenous ST. Commercial feasibility of daily injection for regulation of growth in domestic animals is less desirable. Estimates of nu trient requirements based on daily injection studies for a given dose may not reflect responses to the same dose by using a prolonged release delivery system. Correct estimates of the nutrient requirements for growing animals given exogenous ST will require identification of the specific dose, plasma ST pattern and growth response a given ST product produces. We have worked extensively with two types of de livery systems for pST, a pellet that produces a firstorder delivery for 1 wk and an implant designed to deliver pST in a zero-order manner for 6 w (7, 8). The delivery kinetics of these systems are quite different from that of a daily injection and not surprisingly re sult in differences in response. Daily weight gain is not consistently increased with the 1- or 6-wk systems despite changes in carcass protein and fat. This is demonstrated in Table 1, comparing responses of fin ishing barrows that received one or two implants de signed to deliver 2 or 4 mg pST/d with barrows treated with 2 mg pST by daily injection and untreated con trols (8). Only barrows given daily injection had in creases in daily gain compared with controls. Perfor mance responses of barrows to a 1-wk pST delivery system are similar to those of barrows with the implant in that daily gain was not affected by dose; daily feed intake, however, was linearly reduced with pST dose, and gain:feed improved linearly (7). Despite the lack of a significant gain response, carcass protein was in creased and carcass fat was reduced with increasing pST doses. The magnitude of the shift in the pattern and rate of gain for protein and fat together with changes in voluntary intake will contribute to the magnitude of potential alterations of nutritional rec ommendations for pigs receiving these types of for mulations. Implications for protein requirements. An in crease in protein accretion alone does not dictate an increase in dietary protein requirement; however, re sponses of pigs to pST have been shown to be reduced or prevented when fed inadequate dietary protein (3). Apart from effects on protein accretion, the impact of

*3.01 g/dFeed gain, b309" kg/dGain:feed, intake, g/kgControl821"3.49'235'Dailyinjection2mg/d954'3.14"304"1Implant2mg/d86 1Adapted from Knight et al. (8). *bMeans bearing different superscripts

differ (P < 0.05).

pST on dietary protein requirement can be affected by a combination of several factors, including feed con sumption, digestion, absorption and efficiency of pro tein utilization for lean growth. The effects of geno type, gender and stage of growth on the capacity for protein accretion must also be considered when de termining dietary protein requirements. Taken in to tal, the many factors that impact dietary protein re quirement for swine substantially diminish the utility of any single recommendation for finishing pigs. Several studies have reported effects of genotype and gender on the relative response to pST and have produced seemingly conflicting data (3, 9, 10). Pres ently, it is clear that pST will elicit positive perfor mance responses in finishing pigs regardless of gender or genotype; however, the magnitude of the effects may differ. For purposes of this review it is assumed that the impact of pST on nutritional parameters is simply additive to those of gender and genotype. Digestion and absorption of nutrients do not appear to be markedly altered by pST treatment. Similar to results reported for growing sheep and steers, major effects of pST in swine are on postabsorptive nutrient use (10). Because pST reduces ad libitum feed consumption (11) and maintains or increases protein deposition, unless efficiency of protein utilization is improved substantially, an increase in dietary protein percentage would be necessary, if only to equalize amino acid in take to the pretreatment condition. Clearly, a thorough understanding of the level of feed consumption is critical to providing adequate nutrition for the animal. Numerous factors other than pST affect rates of lean and fat accretion, such as body weight, gender, ge notype, energy density, protein level, amino acid pat tern, effective ambient temperature and stocking den sity; however, current recommendations for swine (12) make allowances for only one, body weight. Many swine producers in the United States have been rela tively unsuccessful at providing adequate nutrition to their pigs as evidenced by a 3-y evaluation of feed con sumption on 38 midwestern swine farms (13). The re-



30-60 ko boars

150 .

120 .

90 . O


• pST

60 250 -,

60-90 ko boars


SO 100

200 300 400 500 Dietary "Ideal" Protein Intake (g/d)


FIGURE 1 Relationship between dietary ideal protein in take and body weight on protein deposition in control and porcine somatotropin (pST)-treated (0.09 mg'kg~'-d~') in tact male pigs (Adapted from refs. 4 and 15).

feed (energy) intake and the imposition of an insulinresistant state with pST treatment it has been postu lated that alteration of energy availability by dietary means may affect the magnitude of response to pST (16). This was examined by Jewell and Knight (17), using three levels of pST treatment (0, 2 and 4 mg/d) and two energy densities [13.4 and 15.5 MJ digestible energy (DE)/kg], with both diets containing 0.9 g lysine/MJ DE (Table 2). Gain:feed was improved 20 and 33% relative to controls when pigs were given 2 and 4 mg/d pST and by 10% with higher nutrient density. Energetic efficiency was improved with pST but was not affected by nutrient density. These results are consistent with the concept that finishing pigs overconsume energy required for maximum protein de position by 15-25% (18) and that reduced feed intake and increased protein gain at those pST doses was not great enough to limit protein accretion. Recently re ported research in which the feed intake depression associated with 3 mg pST/d treatment was overcome by addition of feed through gastric fistulas (19) indi cates that the increase in feed intake resulted in greater weight gains but the increase was due to greater fat and not to greater protein accretion. Higher doses of pST are probably required to increase protein depo sition rates under these conditions of surfeit feed in take. STs AND GROWTH IN RUMINANTS Several studies have reported effects of bovine (bST) and ovine ST (oST)on growth and carcass composition

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suits of this study indicated that pigs were consistently consuming up to 15% less feed than assumed. Simi larly, lysine utilization was 10-15% below recom mended levels, particularly in summer months, further indicating that nutrient density had not been increased to account for reduced consumption. Because a good response to pST is predicated on adequate protein in take, a good understanding of on-farm feed consump tion is required to make effective recommendations for dietary changes. To determine impact of partitioning agents on di etary protein requirement, one must distinguish be tween the tissue requirement for amino acids and the efficiency with which amino acids are used for various metabolic processes, including protein accretion (11). Current data indicate that there is an increase in the dietary lysine requirement in finishing pigs (>60 kg) treated with maximally effective doses of pST. How ever, efficiency of lysine use for protein accretion is improved in growing pigs to an extent that they may not require any increase in dietary lysine content to elicit the expected increase in protein deposition. A series of trials was conducted by Campbell and coworkers (4) using fast growing boars (30-60 kg) treated with a maximally effective daily dose of pST (0.09 mg/kg body weight) or excipient and fed diets ranging from 83 to 238 g dietary "ideal protein"/kg (14). Protein deposition rates were not improved with pST treatment relative to controls for the first two dietary protein levels (83 and 114 g/kg), but were in creased for all other levels to a maximum response of 18-24% (Fig. 1.). Despite this increase in protein de position rate, the estimated protein requirement for the pST-treated boars was only increased by 4%, in dicating a major impact of pST on the efficiency of protein utilization for lean growth. A similar protein titration experiment conducted with boars from 60 to 90 kg yielded somewhat different results (15). In this case, an 81% increase in protein deposition rate over controls (Responses used throughout the manuscript refer to "treated divided by control X 100".) was ob served with pST treatment (215 vs. 119 g/d) with cor responding lysine intakes of 27.3 and 16.2 g/d. These results clearly indicated that pST treatment of finishing pigs at this level increased the daily need for dietary protein (lysine). Definitive data to quantify the daily protein needs of pigs treated with pST at expected commercial dose levels are at this time unavailable. Recently reported results using daily pST injections of 3 mg/d would suggest that a 16% crude protein ration fed to average U.S. finishing pigs would provide adequate nutrients to support the 20% improvement in feed efficiency expected with pST treatment (12). Nutrient density and response to ST. Treatment of pigs with pST has been shown to increase the es timated maintenance energy expenditure (3, 9). This effect appears in large part to be due to the greater lean body mass. Taken in combination with the reduced





Differences in responses to ST between ruminants and monogastrics may be related to nutrient avail The effects of porcine somatotropin ¡pSTtanJ dietary ability and energy intake as related to maintenance nutrient density on rate and composition of growth requirements (11, 21). Energy intake of growing and in finishing pigs front 73 to 108 kg live weight1 finishing pigs exceeds their maintenance requirement and ST-treated pigs have elevated concentrations of Lipid density1DE, pSTmg/d024024Diet blood glucose and nonesterified fatty acids. Feed intake feeda/kg. efficiencyMJ/kg340.1332.1829.5040.6835.6631.35Protein gain gainZ/d15520423416620123222725510438119790 depression is greater in pST-treated pigs than ST/kg13.413.413.415.515.515.5Cainkg/d1. M] treated ruminants and the temporal pattern of the in crease in blood glucose and nonesterified fatty acids occurs before the decrease in feed intake (28). The ex act cause and effect relationship of these events has not been established. Lactating cows, which have much higher intakes than growing steers, also do not dem onstrate a decrease in feed intake when given exoge nous bST even at supraphysiological doses (29). In this ANOVA case, the mammary gland is using the increased glucose pST 0.09 0.001 0.001 0.001 0.001 and fatty acids for increased milk yield during bST Diet NS 0.01 NS NS NS pST X Diet NS NS treatment, thus creating a diminished body energy NS NS 0.02 status and thereby eliminating the decrease in feed ' Adapted from Jewell and Knight (17). intake. Feed intake is much lower relative to mainte 2All diets contained 3.75 g lysine/MJ digestible energy (DE). nance requirements in growing steers and appears to 3 MJ body gain/kg feed. be only slightly reduced in steers given doses of bST below 50 Mg'kg body weight""1-d"1. Feed intake was TABLEZ

decreased in a linear manner in steers given 33, 100 or 300 ßg • kg body weight"1 • d"1, demonstrating that high doses of ST, which increase plasma glucose and fatty acids (30), will decrease feed intake in a manner similar to results reported for growing pigs (7, 8). In the same study, percent fat in the carcass was decreased 54% at the high dose. An increased protein requirement has not been demonstrated consistently in studies evaluating effects of dietary protein percent on responses to ST in grow ing ruminants (11, 20, 22). However, the protein re quirement of a ruminant is complicated by rumen degradation of dietary protein. Lactating dairy cows given exogenous bST did not have an additional in crease in milk yield when given abomasal casein in fusions (31). This indicated that feed intake and body protein stores did not limit the milk production re sponse to bST. However, growing sheep and steers with lower feed intake than lactating cows had greatly improved growth responses to ST when given aboma sal casein infusions (11, 21). These authors indicated that amino acid supply to the lower gut should be evaluated to maximize lean tissue accretion in growing and ST-treated animals. Specifically, the degradable and undegradable protein requirement of ruminant diets require further study. In support of these obser vations, the effects of abomasal casein infusion were additive to the effect of ST in both sheep and steers, indicating that growth of animals not given exogenous ST was also limited by availability of amino acids at the lower gut (11, 21). Pell (32) observed that feed restriction improved average daily gain response to oST when compared with control lambs fed ad libitum. However, the oST

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in cattle and sheep. Collectively these studies dem onstrate an increase in nitrogen retention, decrease in carcass fat and increased lean/fat ratio (20, 21). Fur thermore, ability of ST to enhance nitrogen utilization was due to postabsorptive changes in nitrogen utili zation (22). Studies evaluating the dose range of ST treatment by using daily injection report a maximum anabolic response at doses between 40 and 60 Mg/kg body weight (23). This is similar to the maximum dose range for lactating dairy cows and growing swine given daily injections. Average daily gain responses in ruminants have been more variable than those reported for monogastric animals using daily injection. bST studies have been conducted in veal calves over a wide range of ages and doses (24-26). In studies where weight gain response was obtained, animals were fed on a per centage body weight basis that may have limited in take. However, protein and energy content of the ra tions in all of the studies were typical of those fed to calves. Despite the fact that there was large variability in the growth response in studies where nitrogen re tention was reported, it was increased. Similar variability in body weight gain has been re ported in ST-treated steers and sheep (11, 20, 22, 23). Additionally, the decreased feed intake and body fat are often not as dramatic in ST-treated ruminants compared with swine (27). However, blood endocrine and metabolic analytes consistently support increased muscle and bone and decreased fat accretion. Weights of noncarcass (liver, kidney, digestive tract, blood etc.) components also increase in ST-treated animals, re sulting in a reduced dressing percentage (27).






12. 13.




LITERATURE CITED 1. Reeds, P. J. & Mersmann, H. J. (1991) Protein and energy re quirements of animals treated with B-adrenergic agonists: A discussion. J. Anim. Sci. 69: 1532-1550. 2. Bauman, D. E., Dunshea, F. R., Boisclair, Y. R., McGuire, M. A., Harris, D. M. S. Houseknecht, K. L. (1989) Regulation of nutrient partitioning: Homeostasis, homeorhesis and exogenous somatotropin. Proceedings Seventh International Conference on Production Disease in Farm Animals (Callfelz, F. A., ed.), Cornell University Press, Ithaca, NY. pp. 306-323. 3. Campbell, R. G., Steele, N. C., Caperna, T. J., McMurtry, J. P., Solomon, M. B. & Mitchell, A. D. (1989) Interrelationships between sex and exogenous porcine growth hormone adminis tration on performance, body composition and protein and fat accretion of growing pigs. J. Anim. Sci. 67: 177-186. 4. Campbell, R. G., Johnson, R. J., King, R. H., Taverner, M. R. & Meisinger, D. J. (l990) Interaction of dietary protein content and exogenous porcine growth hormone administration on pro tein and lipid accretion rates in growing pigs. J. Anim. Sci. 68: 3217-3225. 5. McCutcheon, S. N. & Bauman, D. E. (1986) Effect of pattern of administration of bovine growth hormone on lactational per formances of dairy cows. J. Dairy Sci. 69: 38-43. 4. Janssen, J-O., Isaksson, G. P., Eden, S., Isgaard, J., Carlsson, L. &. Ekberg, S. (1989) Effects of plasma GH pattern on growth factors and body growth. In: Hormonal Regulation of Growth (Frisch, H. & Thorner, M. O., eds.) vol. 58, pp. 185-199, Serono Symposia Publication, Raven Press, New York, NY. 7. Knight, C. D., Kasser, T. R., Swenson, G. H. Hintz, R. L. Azain, M. J., Bates, R. O., Cline, T. R., Crenshaw, J. D., Cromwell, G. L., Hedrick, H. B., Jones, S. J., Kröpf,D. H., Lewis, A. J., Mahan, D. C., McKeith, F. M., McLaughlin, C. L., Nelssen, J. L., Novakofski, J. E., Orcutt, M. W. & Parrett, N. A. (1991) The per formance and carcass composition responses of finishing swine










to a range of PST doses in a 1-week delivery system. J. Anim. Sci. 69: 4678-4689. Knight, C. D., Azain, M. J., Kasser, T. R., Sabacky, M. J., Baile, C. A., Buonomo, F. C. & McLaughlin, C. L. (1988) Function ality of an impiantatile 6-wk delivery system for porcine so matotropin (PST) in finishing hogs. J. Anim. Sci. 66(suppl. 1): 257(abs.|. Nossaman, D. A., Schinckel, A. P., Miller, L. F. & Mills, S. E. (1991) Interaction of somatotropin and genotype on the require ment for energy in two lines of finishing pigs. J. Nutr. 121: 223230. Wray-Cahen, D., Ross, D. A., Bauman, D. E. & Boyd, R. D. (1991 ) Metabolic effects of porcine somatotropin: Nitrogen and energy balance and characterization of the temporal pattern of blood metabolites and hormones. J. Anim. Sci. 69: 1503-1514. Boyd, R. D., Bauman, D. E., Fox, D. G. & Scanes, C. G. (1991) Impact of metabolism modifiers on protein accretion and pro tein-energy requirements in livestock. J. Anim. Sci. 69(suppl. 2):56-75. National Research Council (1988) Nutrient Requirements of Swine, 9th ed., National Academy Press, Washington, DC. Brumm, M. C., Gourley,G. &Greenley, W. M. (1990) Seasonal patterns of nutrient disappearance for growing finishing swine on midwest hog farms. J. Anim. Sci. 68(suppl. 1):113(abs.). Agricultural Research Council (1981) The Nutrient Require ments of Pigs, 2nd ed., Commonwealth Agriculture Bureaux, Farnham Royal, England. Campbell, R. G., Johnson, R. J., Taverner, M. R. & King, R. H. (1991) Interrelationships between exogenous porcine somato tropin (PST) administration and dietary protein and energy in take on protein deposition capacity and energy metabolism. J. Anim. Sci. 69: 1522-1531. Azain, M. J., Seerley, R. W., Reagan, J. O. & Anderson, M. K. (1991) Effect of a high fat-diet on the performance response to porcine somatotropin (PST) in finishing pigs. J. Anim. Sci. 69: 153-161. Jewell, D. E. & Knight, C. D. (1990) The effects of porcine somatotropin and dietary nutrient density on rate and compo sition of growth in pigs. J. Anim. Sci. 68(suppl. l):274(abs.). Campbell, R.G., Taverner, M. R. & Curie, D.M. (1984) Effect of feeding level and dietary protein content on the growth, body composition and rate of protein deposition in pigs growing from 45 to 90 kg. Anim. Prod. 38: 233-240. Newcomb, M. D., van Kempen, T., Bechtel, P. J., McKeith, F. K., Novakofski, J. & Easter, R. A. (1990) Responses of finishing pigs treated with porcine somatotropin (PST) to hyperalimentation. J. Anim. Sci. 68(suppl. 1):385 (abs.|. Enright, W. J., Quirke, J. F., Gluckman, P. D., Brier, B. H., Ken nedy, L. G., Hart, I. C., Roche, J. F., Coen, A. & Allen, P. (1990) Effects of long-term administration of pituitary-de rived bovine growth hormone and estradici on growth in steers. J. Anim. Sci. 68: 2345-2356. Beermann, D. H., Robinson, T. F., Byrem, T. M., Hogue, D. E., Bell, A. W. & McLaughlin, C. L. (1991) Abomasal casein in fusion and exogenous somatotropin enhance nitrogen utilization by growing lambs. J. Nutr. 121: 2020-2028. Eisemann, J. H., Tyrell, H. F., Hammond, A. C., Reynolds, P. J., Bauman, D. E., Haaland, G. L., McMurtry, J. P. & Varga, G. A. (1986) Effect of bovine growth hormone administration on metabolism of growing hereford heifers: Dietary digestibility, energy and nitrogen balance. J. Nutr. 116: 157-163. Hancock, D. L. & Preston, R. L. (1990) Titration of the recom binant bovine somatotropin dosage that maximizes the anabolic response in feedlot steers. J. Anim. Sci. 68: 4117-4121. Kirchgessner, V. M., Roth, F. X., Schams, D. & Karg, H. (1987) Influence of exogenous growth hormone (GH) on performance and plasma GH concentrations of female veal calves. J. Anim. Physiol. Nutr. 58: 50-59.

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response of the restricted-fed animals was measured against pair-fed controls, and both restricted controls and oST-treated animals demonstrated reduced growth rates compared with ad libitum-fed animals. This sug gests that the nitrogen-sparing action of ST is more easily demonstrated in restricted-fed ruminants (22) compared with restricted-fed controls. The reason gains may be more apparent when energy intake is limited is that the bST response in these animals may be to increase lean tissue accretion, whereas in ad li bitum-fed animals the effect of bST is to decrease fat and increase lean accretion. Additionally, energy in take must also be taken into account and evaluated separately from protein. Care must be taken not to overfeed energy when trying to maximize available protein. Otherwise, feed intake may be limited, re ducing responses to ST. In general, ST treatment may affect nutrient re quirements of growing ruminant and monogastric an imals, but this can only be determined when pattern and rates of protein and fat accretion to specific com mercially feasible treatment regimens are defined. Changes in requirements will likely be related to changes in feed intake and changes in fat and protein accretion. Nutritionists will need to formulate diets based on daily requirements for specific nutrients, such as protein and amino acids, in terms of daily utilization rather than as percentages of dry matter.



COLLIER ET AL. 19. Vicini, J. L., Hudson, S., Cole, W. J., Miller, M. A., Eppard, P. J., White, T. C. & Collier, R. J. (1990) Effect of acute challenge with an extreme dose of somatotropin in a prolonged-release formulation on milk production and health of dairy cattle. J. Dairy Sci. 73:2093-2102. ÃŽO.Moseley, W. M., Paulissen, J. B., Goodwin, M. C., Alaniz, G. R. &.Claflin, W. H. (1990) Growth performance in finishing beef steers administered recombinant bovine somatotropin (rbST). J. Anim. Sci. 68:(suppl. l):273(abs.). 31. Peel, C. J., Fronk, T. J., Bauman, D. E. & Gorewit, R. C. (1982) Lactational response to exogenous growth hormone and abomasal infusion of a glucose-sodium cascinale mixture in highyielding dairy cows. J. Nutr. 112: 1770-1778. 32. Pell, J. M. (1989) Growth promoting properties of recombinant growth hormone. In: Biotechnology in Growth Regulation (Heap, R. B., Presser, C. G. & Lamming, G. E., eds.), pp. 8596, Butterworths, London, England.

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25. Neathery, M. W., Crowe, C. T., Hartnell, G. F., Veenhuizen, J. J., Reagan, J. O. &. Blackmon, D. M. (1991) Effects of sometribove on performance, carcass composition and chemical blood characteristics of dairy calves. J. Dairy Sci. 74: 39333939. 16. Groenewegen, P. P., McBride, B. W., Burton, J. H. & Elsasser, T. H. (l990) Effect of bovine somatotropinon the growth rate, hormone profiles and carcass composition of Holstein bull calves. Domes. Anim. Endocrinol. 7: 43-54. 27. Early, R. J., McBride, B. W. & Ball, R. O. (1990) Growth and metabolism in somatotropin-treated steers: I. Growth, serum chemistry and carcass weights. J. Anim. Sci. 68: 41344143. 28. Dunshea, F. R., Bauman, D. E., Boyd, R. D. & Bell, A. W. (1992) Temporal response of circulating metabolites and hormones during somatotropin treatment of growing pigs. J. Anim. Sci. 70: 123-131.

Impact of somatotropins on nutrient requirements in domestic animals.

Somatotropins increase nitrogen retention, decrease carcass fat and increase lean/fat ratio in growing ruminants and swine. However, responses are muc...
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