Influence of improved genetics on the nutrient and environmental requirements of broilers

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ABSTRACT
Consumer demand for lean poultry products necessitates that broiler leanness and uniformity be improved. As a result, technologies resulting in greater protein production, not overall bird mass, will be emphasized. Knowledge related to bird energetics, stress management and waste production appear to be evolving towards new management approaches. For example, recent and ongoing studies directed at evaluating the metabolizable energy (ME) system indicated that cellular energy supply is not necessarily reflective of ME consumption. Increased understanding and application of cellular energy-nutrient relationships will be required. Identification of growth limiting processes, following a period of stress mediated growth depression, may make it possible to minimize stress effects on overall bird performance. The focus in this area will likely be on intracellular environmental-stress mediated perturbations and how to assist in bird recovery.


INTRODUCTION
The trend towards consumer demand for leaner poultry products, at nominal cost, will necessitate that product leanness, uniformity and supply reach new highs. "Ice pack" will decrease and "value added" products will increase. As a result, technologies resulting in greater protein production, not overall bird mass, will be emphasized. The shift of focus to the profit center of proteinaceous tissue mass necessitates that nutritional advances occur which enable muscle growth at optimal rates while minimizing fat accretion. Though this direction may change the way we grow broilers, research directed at broiler management may help offset this dilemma. Nonetheless, technological developments must occur within the bounds of increasing environmental restrictions, which are becoming more intense.


ENERGY CONSIDERATIONS
Today's commercial broiler is the fastest growing and most efficient bird ever produced. It represents the combined efforts of genetics and management. However, with this tremendous potential also comes greater susceptibility to different types of stress. Because growth taxes numerous physiological systems and because stress consequences typically are additive, we should not be surprised that modern day birds frequently require added attention. Knowledge related to bird energetics, stress management and waste production are evolving and new management approaches are being employed. More thorough understanding of energy metabolism and amino acid requirements are fundamental to improving profitability of production enterprises.

To date, the metabolizable energy (MEn) system has been accepted as the standard for ration formulation. However, the MEn system by definition, does not quantitatively predict bird feed energy deposition. Any heat increment change alters MEn utilization and thereby can affect the cellular energy/nutrient ratios. Alterations in the cellular energy/nutrient ratio may enhance fat deposition. For example, recent and ongoing studies directed at evaluating the MEn system indicate that cellular energy supply does not necessarily reflect MEn consumption. The greater heat increment from protein MEn calories vs. those from starch and fat make low protein diets lipogenic. Oxygen required per unit protein synthesis is 380% greater than that for fat (Teeter and Wiernusz, 1994). Deeper understanding and application of cellular energy-nutrient relationships will be required to produce breeders with optimum body composition.

Energetic efficiency of MEn use for tissue gain depends upon numerous variables. Efficiency varies with substrate source, for lipogenesis being approximately 75, 84, and 61% for carbohydrates, fats and proteins, respectively (De Groote, 1969; Chudy and Schiemann, 1971; Hoffmann and Schiemann, 1971). The high availability of fat MEn for tissue gain, however, requires that fat is used for lipogenesis (Bossard and Combs, 1961). Utilization of protein for tissue energy gain depends upon the biological value of the protein source and should not be constant (De Groote, 1973). Indeed, one could summarize that the bird's energetic efficiency for use of protein or any substrate is the net result of partitioning consumed substrate energy into maintenance needs verses accretion of protein and fat.

Recommendations for dietary protein concentration for optimum rates of lean tissue accretion range from high (Kubena et al., 1972) to low levels complemented with specific amino acids (Waldroup et al., 1976). Whether the carcass leanness associated with feeding high protein diets is attributable to substrate limitations (amino acids), or due to greater heat production per kcal MEn for dietary amino acids carbohydrate and fat is subject to debate. Research conducted at Oklahoma State Univeristy by Mittelstaedt (1990) examined the true metabolizable energy (TME) utilization of carbohydrate, protein and fat sources for energy, protein and fat gain. Despite similar TME consumption among the energy supplemented groups, carcass energy was impacted significantly. Total carcass energy gain was 17, 27, and 30% greater for the gelatin, starch, and corn oil groups, than for birds fed the basal diet. Estimated energy gain from the basal ration was similar among the energy supplemented groups due to nearly identical feed consumptions. However, total calories gained differed (P
An additional consequence of low protein MEn utilization efficiency is that the birds heat load is increased. Elevated heat load has little consequence when birds are housed at or below thermoneutral temperatures. However, if the bird's heat load is elevated by high ambient temperature stress, without a concomitant increase in heat dissipation, elevated heat load can be devastating (Wiernusz and Teeter, 1993). Belay and Teeter (1992) fed birds various protein levels and calorie/protein ratios. Increasing dietary energy and (or) narrowing calorie-protein ratios by relaxing restrictions on amino acid balance (which necessitated increased dietary protein) significantly impacted bird carcass composition. Improving amino acid balance and lowering dietary crude protein concentration increased survival both in the thermoneutral environment (4.4%) and within the heat stressed environment (10.8%; P < .05). Lowering crude protein (at adequate amino acid balance) for birds subjected to heat stress can prove beneficial. Research is needed to identify which amino acid excess cause the greatest risk.

Diets formulated, based on the MEn system do not necessarily correlate with bird energy retention; the calorie-nutrient ratios of depot tissue can vary independent of metabolizable energy. In order for the bird to achieve optimum carcass composition with maximum energetic efficiency, an energy-requirement scheme must account for the variation in substrate-mediated heat production.


ENVIRONMENTAL EFFECTS ON CARCASS COMPOSITION
Ambient Temperature is well documented to impact carcass composition with ambient temperatures below the themoneutral zone being inversely related to carcass fat content and directly related to carcass fat content at temperatures above the zone of thermoneutrality. The precise interrelationship between cellular energy supply and energy needs for maintenance and protein synthesis will, once understood, likely make this a definable process. Several laboratories are actively working on these relationships. However, another area that has not received research attention is related to the bird's oxygen requirement. The impact of insufficient atmospheric oxygen on bird ascites incidence is well documented, while its impact on carcass fat has not been addressed. Bird oxygen needs are elevated in lean birds for two reasons: the first being that maintenance requirements appear to vary directly with lean tissue mass, while fat content is poorly correlated, and second, that the synthesis of lean tissue requires more oxygen than lipid synthesis. Estimates of oxygen needs per gram of tissue gained, including maintenance needs at equalized fat accretion, has been estimated to be 3.1 L over a 28 day feeding period. In contrast, the oxygen required for lipid synthesis, including maintenance needs at equalized protein gain, is just 0.82 L/g. Future diet formulation schemes may need to consider the environmental impact to avoid excess fat accumulation.


REFERENCES
Belay, T. and R. G. Teeter, 1992. Caloric density and calorie/protein ratio effects on broiler growth rate, survivability and carcass composition estimated by direct and indirect methods. Poultry Sci. 71(Suppl. 1):138 (Abstract).

Bossard, E. H. and G. F. Combs, 1961. Studies on energy utilization by the growing chick. Poultry Sci. 40:930-938.

Chudy, A., and R. Schiemann, 1971. Energetische Verwertung der Futternahrstoffe beim Huhn. In: Energetische Futterbewertung und Energienormen, pp. 168-198. Ed. R. Schiemann, K. Nehring, L. Hoffman, W. Jentsch and A. Chudy. Berlin, VEB Deutscher Landwirtschaftsverlag.

De Groote, G., 1969. Experiments concerning the utilization of metabolizable energy by growing chicks. Annual Report 1969 of the Government Research Station for Small Stock Husbandry, Merebelke, Belgium, p. 145-146.

Hoffmann, L., and R. Schiemann, 1971. Verdaulichkeit und Energiekennzahlen von Futterstoffen beim Huhn. Archiv Tierrernahrung, 21:65-81.

Kubena, L. F., B. D. Lott, J. W. Deaton, F. N. Reece, and J. D. May, 1972. Body composition of chicks as influenced by environmental temperature and selected dietary factors. Poultry Sci. 51:517-522.

Mittelstaedt, C. W., 1990. Feed bioenergy evaluation: methodology as applies to growing broilers. M.S. thesis, Oklahoma State University, Stillwater, OK.

Teeter, R. G. and C. J. Wiernusz, 1994. New management approaches will provide opportunity. Feedstuffs, 66 (6):14-16.

Waldroup, P. W., R. J. Mitchell, J. R. Payne and Z. B. Johnson. 1976. Performance of chicks fed diets formulated to minimize excess levels of essential amino acids. Poultry Sci. 55:243-253.

Wiernusz, C. J., and R. G. Teeter, 1993. Feeding effects on broiler thermobalance during thermoneutral and high ambient temperature exposure. Poultry Sci. 72:1917-1924.