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Metabolic Modifiers: Effects on the Nutrient Requirements of Food-Producing Animals Executive Summary Historically, humankind has been confronted with the need to increase food production to meet the growing demands imposed by the expanding world population. And, as societies become more economically developed, per capita demand also increases. The ability to increase world food production has been the result of remarkable scientific advances that increase not only production but also the productive efficiency of agriculture. During the last decade, several new technologies have emerged that increase the efficiency of livestock production as well as meat and milk production to an extent not envisioned 10 years ago. The emerging technologies that are the focus of this report are somatotropin (ST) and β-adrenergic agonists. Because of the extent to which these metabolic modifiers improve productive efficiency (body weight gain or milk yield per unit of feed consumed), improve carcass composition (ratio of muscle:fat) in growing animals, and increase milk yield, it is important to establish whether dietary nutrient requirements are altered and, if so, how diet formulation and nutrient intake might need modification to achieve the maximum beneficial response. Although metabolic modifiers exert their effects on animal performance in a dose-related manner, it is important to appreciate that nutrient requirements are dynamically affected by many other factors (such as gender, age, weight, and feed management system). Thus, the task of the subcommittee was to review the available literature and, where possible, determine the extent to which treatment with metabolic modifiers affected dietary nutrient requirements of dairy cattle, growing ruminants, poultry, and swine. DAIRY CATTLE The subcommittee concluded that current feeding recommendations can be used for cows treated with bovine somatotropin (bST). Milk composition, diet digestibility, maintenance requirements, and the partial energetic efficiency of lactation are not affected by bST. Numerous metabolic adaptations result from bST administration with a consequent increase in nutrient availability for milk synthesis. Depending on nutritional status, nutrients derived from body reserves can provide support for the increase in milk synthesis; but, subsequently, feed intake increases, which provides the nutrients necessary to sustain the increase in milk yield. Cows treated with bST need to be fed and managed like other cows at similar levels of milk production. When ration density is increased by feeding grain, alteration of ruminal hydrogen-ion imbalance should be managed by the addition of buffers like sodium bicarbonate. Rations should be balanced for rumen degradable and nondegradable protein and ration adjustments with respect to nutrient density should be made on the basis of body weight and condition. GROWING RUMINANTS There is some evidence to suggest that amino acid availability at the site of absorption may limit the response of the growing ruminant to metabolic modifiers. Results from studies in which protein was infused abomasally in cattle and lambs and in which sheep were fed fishmeal have demonstrated a marked increase in the growth response (nitrogen retention) to exogenous ST. This suggests that amino acid availability may limit the response to exogenous ST. The subcommittee did not make any recommendations regarding whether β-adrenergic agonists or ST affected nutrient requirements of growing cattle and sheep because few systematic studies have been reported that examined the effects of metabolic modifiers on protein and energy metabolism. The subcommittee concluded that advances must be made in formulating diets that deliver the appropriate profile and quantity of amino acids postruminally to match the tissue requirement for nutrients when metabolic modifiers are administered
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Metabolic Modifiers: Effects on the Nutrient Requirements of Food-Producing Animals (and alter tissue requirements). This will require adjustments in experimental design to quantify accretion rates of body protein and lipid as a function of dietary inputs. SWINE In contrast to other livestock species, swine are quite responsive to the anabolic effects of porcine ST (pST). Considerable information exists in the literature about the effects of pST on nutrient requirements. The available information suggests that protein deposition in the pig is constrained by dietary energy intake. Furthermore, with optimal energy nutriture, lysine, as the first limiting amino acid for growth, will require fortification compared to conventional diet formulations. The subcommittee recommends the adoption of feeding standards formulated on the basis of target tissue growth and composition. This requires adoption of the ''ideal" protein concept to diet formulation or some modification of this concept. In this way, significant improvement in the realization of lean tissue growth potential can be achieved. Diet modifications associated with the use of metabolic modifiers can then be logically calculated on a biological basis. At the present time the subcommittee does not believe it appropriate to recommend a generalized increase in dietary protein intake in conjunction with the use of metabolic modifiers. Rather the use of feeding standards that reference rate and composition of gain of desired tissue (i.e., ideal protein concept) would improve productive efficiency of animals treated with metabolic modifiers, in addition to those not so treated. There is some evidence which suggests that pST improves the efficiency of dietary protein (amino acid) utilization for tissue growth; however, the extent of this improvement and the underlying mechanisms remain obscure. The subcommittee, however, emphasizes that there is a need for additional research to determine the effects of metabolic modifiers on dietary protein (amino acid) utilization and requirements. POULTRY Studies conducted with metabolic modifiers have not convincingly shown that they have a positive effect on growth performance in poultry. It is of concern whether using diets formulated using contemporary National Research Council (NRC) recommendations will be adequate to support enhanced growth or to permit efficacy testing of the metabolic modifiers. Diets fortified in amino acids, macronutrients, and enriched in energy content may be required to demonstrate growth enhancement of lean tissue. For research and development purposes, the subcommittee recommends adoption of a standardized diet that does not limit the biological response of poultry to metabolic modifiers. SUMMARY New technologies targeted to improve production and productive efficiency of domestic livestock have the potential to markedly benefit animal production. These technologies, depending on the target species, may modify contemporary strategies used for diet formulation. These technologies will, however, amplify inadequacies in either diet design or animal management practices. The subcommittee identified a need for additional research to determine the appropriate profile and quantity of amino acids that must be available postruminally to match the tissue requirements for nutrients when metabolic modifiers are administered to growing ruminants. The subcommittee believes that additional research is needed to determine the effects of metabolic modifiers on dietary protein utilization and requirements in swine. The subcommittee concluded that new feeding standards formulated on the basis of target tissue growth rate and composition must be adopted. This will require the incorporation of the "ideal protein" concept or some modification of this concept to diet formulation. The subcommittee has pointed out the need for additional information in a variety of areas. As such, this report is constrained in some respects because of the limited scientific information available. It is our belief that this report will stimulate research to expand our knowledge base about the effects that metabolic modifiers have on nutrient requirements of domestic livestock. This will be essential in order for the advantages of the technology to be fully realized in practice.
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