Nutritional Considerations for Optimum Growth during the Broiler Chick's First Week of Life

摘要

With continued genetic progress, broilers reach market weight at younger ages each year and the first week represents an ever increasing proportion of the broiler's lifespan. During the immediate post-hatch period the chick must make the transition from dependence on endogenous lipid-rich yolk to exogenous carbohydrate and protein rich feed. Early feed intake is essential for maximal intestinal growth, which occurs at a 3 to 5-fold greater rate than the rest of the body. Research has shown that immediate access to feed is associated with increased intestinal trypsin, amylase and lipase activities and increased intestinal weight compared to chicks without access to feed for 48 hours. Even in starved chicks, intestinal growth commences but villi and enterocyte length is reduced. Early feed intake also increases brush border enzyme activities. Optimum early nutrition should not rely solely on the residual yolk but rather the contributions of both yolk and exogenous feed. Immediate feed intake resultsin more rapid utilization of the residual yolk. Apart from the supply of energy via the yolk lipids, a further benefit of this is the more rapid assimilation of maternal antibodies present in the yolk. Yolk provides precursors for intestinal growth and energy for maintenance immediately post-hatch. Fatty acids are taken up efficiently from yolk but carbohydrates and amino acids are absorbed from the gut only when adequate enzymes and Na are present. Glucose and several amino acids are absorbed by co-transport with Na, making sufficient Na intake essential during early life for optimum nutrient uptake and growth. Uptake of long chain and saturated fatty acids from the gut is less efficient compared to shorter, less saturated fatty acids. This is due inpart to insufficient bile production at young ages. Gut growth commences even if feed intake is delayed but optimal muscle growth can not be achieved without early access to feed. Several studies have highlighted the need for early feeding in order to maximize breast meat yield. Chicks without access to feed in the first 48 hours may catch up to their fed counterparts in body weight, but breast meat yield may be permanently decreased. Skeletal muscle growth relies on hypertrophy and addition of nuclei to muscle fibers. Addition of nuclei to muscle fibers is a function of satellite cells, which are myogenic precursor cells present in skeletal muscle beneath the basal lamina. In broiler chicks, the proliferation of satellite cells is limited to the firstdays after hatch, after which the satellite cell population declines dramatically. Good early nutrition enhances satellite cell proliferation and hence ultimate muscle size. Studies have shown that breast muscle weight is highly correlated to the numberof satellite cells per gram of breast muscle at 5 days of age. Studies have shown that high levels of balanced, digstible amino acids in the first week of life can lead to advantages in early growth that are maintained through 40 days. The mechanisms bywhich such high levels of amino acids can increase growth include elevated IGF-1 levels, alteration of lipogenesis and enhanced satellite cell activity. Formulating using digestible amino acid values allows nutritionists to more accurately meet the needs of young chicks at this crucial time. While mineral requirements in the immediate post-hatch period are not well defined, it is known that adequate breeder hen mineral intake is essential to development of healthy chicks. Sufficient Zn is essential forregulation of cell turnover, gene expression and development of the immune system. Response to vaccination is improved by adequate Zn intake. Zn is a part of keratin, collagen and several enzymes and also acts as an activator of many enzymes. Copper isalso an essential component of several metallo-enzymes and is involved in cross-linking of collagen and elastin. Deficiencies are associated with defects in the skeletal and cardiovascular systems. In