Growth And Development Of Broilers With Different Hatching Weight And Its Regulation By Dietary Methionine

Extensive studies have demonstrated that broiler chicks with high hatching weight (HW) have better growth performance than those with lower HW, but up to date little attention has been paid to their physiological differences and nutritional regulations. Therefore, the objective of this study was to investigate the growth and development of broilers with different HW, and to evaluate the regulatory effects of dietary methionine (Met) and underlying mechanism. The study was composed of5trials as below:Trial1was conducted to investigate the HW distribution of broiler chicks from breeders of different age groups. Three batches (500birds each) of one-day-old Arbor Acres (AA) broiler chicks were selected from peak (32wk), late (53wk) and molted (68wk) breeders. They were weighed individually to analyze the HW distribution. The HW of chicks were normally distributed, and the mean value was42.87,48.24and47.33g, respectively, and CV was7%-8%. Among the three batches of chicks,74.4%,17.0%and28.8%of chicks were below45g,24.4%,52.0%and49.0%were between45and50g, and1.2%,31.0%and22.2%were above50g.Trial2was conducted to determine the growth performance, organ development, serum physiological and biochemical parameters, nutrient utilization, pancreatic enzyme activities of broilers with different HW and genders. A total of384day-old AA broiler chicks (half male and half female) with different HW (heavy:51.2±0.1g, and light:42.3±0.3g) were allocated to a randomized block design with a2×2factorial arrangement with4treatments. Each treatment had8replicates and12chicks per replicate. Chicks were fed the same starter and finisher mash diets for42days. The BW at21and42d and ADG were greater in heavy chicks than in light chicks during whole growth period (P<0.05). Male birds had a greater BW than female birds at42d (P<0.05). The ADG and ADFI of male birds were greater than those of female birds during finisher and overall phase (P<0.05). The interaction between HW and gender was not significant for growth performance of broilers. Heavy chicks had greater yolk sac weight and yolk-free BW than light chicks (P<0.05). The relative weight of breast muscle at21and42d was greater in heavy than in light birds (P<0.05), and the same trend was observed at28and35d. However, the relative weights of other organs did not differ. Gender and HW did not affect serum parameters tested, nutrient utilization or pancreatic enzyme activities.Trial3was conducted to evaluate the effect of dietary Met levels on growth and development of broilers with different HW. A total of192one-day-old AA broiler chicks with different HW (heavy:48.30±0.04g, and light:41.73±0.04g) were allocated to a2×2factorial arrangement with6replicates of8chicks (half male and half female). Met in the control diets was formulated to meet NRC recommendations, and high Met diets were manufactured by supplementing0.1%Met to the control diets. Light chicks had lower BW and ADG than heavy chicks when both were fed the control diets (P<0.05), whereas serum biochemical parameters or relative organ weights did not differ. Compared with light chicks, heavy chicks had lower DNA content in breast muscle at42d and higher RNA/DNA and IGF-I content (P<0.05). High Met diets improved42-d BW and ADG during finisher and whole phase as well as F/G in light chicks (P<0.05). Serum uric acid and triglyceride concentrations were increased by Met in heavy chicks at42d (P<0.05). Increasing Met levels increased42-d breast muscle yield and decreased its DNA concentration in light chicks at21and42d (P<0.05), whereas RNA/DNA, protein/DNA and IGF-I concentrations were increased at42d (P<0.05).Trial4was conducted to determine the effect of dietary Met levels on expression of genes related to muscle development and MSTN methylation in broilers with different HW. Experiment design was the same as that of Trial3. No difference was observed in the parameters tested when the birds were fed the control diets. High Met diets upregulated the mRNA levels of Myf5and MEF2B and the MSTN methylation and downregulated that of MSTN in breast muscle of light chicks (P<0.05).Trial5was conducted to explore the effect of dietary Met levels on the IGF-I signaling pathway in broilers with different HW. Experiment design was the same as that of Trial3. Heavy chicks had higher IGF-I mRNA expression in breast muscle than light chicks when both were fed the control diets (P<0.05). High Met diets increased the mRNA expressions of IGF-I, TOR and p-TOR/TOR in breast muscle of light chicks (P<0.05). Inversely, the mRNA expressions of4EBP1, FOXO4, atrogin-1and p-FOXO4/FOXO4were decreased in light chicks (P<0.05).It can be concluded as follows:(1) The HW of chicks increased as breeders ages before molting. Most of chicks from peak breeders had HW less than45g, and most of chicks from late breeders had HW greater than45g.(2) Heavy chicks had a greater ADG than light chicks, which may be due to different growth rate of breast muscles, and there was no interaction between HW and gender.(3) Adding0.1%Met to the control diets did not affect the performance of heavy chicks, but improved that of light chicks and breast muscle development, which may be related to increased IGF-I concentration in breast muscle.(4) Increasing Met levels may promote breast muscle development of light chicks by upregulating mRNA expression of Myf5and MEF2B genes and downregulating that of MSTN gene, which be associated with increased MSTN DNA methylation.(5) Increasing Met levels may improve breast muscle growth of light chicks by upregulating IGF-I mRNA expression, which further activated TOR/4EBP1and FOXO4/atrogin-1pathway.