The aim of this study was to evaluate the effects of thermal manipulation (TM) during pre and post-hatch periods on thermotolerance of male broiler chickens exposed to chronic heat stress (CHS) during the finisher phase (34 ± 2°C, 6 h/day). Seven hundred fertile eggs of Ross 308 were assigned to the following groups: 1) control group incubated and housed in standard conditions, 2) pre-hatch treatment (PRE), the eggs were exposed to 39.5°C and 65% RH for 12 h, d from embryonic d 7 to 16 and after hatching the chicks where housed in standard conditions; 3 and 4) post-hatch TM at d 3 (PO3) and post-hatch TM at d 5 (PO5), which had the same incubation conditions as control and exposed to 36 to 38°C for 24 h at 3 and 5 days of age, respectively. TM in PRE group resulted in delay in the hatch time (6 h) along with reduction in body weight compared to control (P = 0.02). TM caused a significant reduction of facial surface temperature (FST) until d 28 (P < 0.02), but not significant during CHS. Body weight gain was suppressed in PO3 and PO5 groups at d 14 (P = 0.007) and compensated at d 28. However, TM led to higher BWG (P = 0.000) but lower FCR (P = 0.03) and mortality at the first week of CHS compared to control. European production efficiency index was higher in TM-treated chickens compared to control (P = 0.01). TM reduced the blood concentration of uric acid, total protein, T3, and T4 in which thyroid hormones in PO3 and PO5 treatments showed more reduction rather than other groups. In PRE group, chickens had lower abdominal fat pad than control (P = 0.0001). The relative weight of heart was decreased in TM groups (P = 0.001). It was concluded that TM may induce thermotolerance in growing broilers, possibly through the modification of physiological parameters of broilers especially during the first week of CHS.

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In a dose-response study, dietary levels of standardized ileal digestible methionine (dMet), lysine (dLys), and threonine (dThr) were optimized by response surface methodology (RSM) using a central composite rotatable design in starting broiler chicks that were feed-deprived 2 d posthatch. In total, 60 floor pens of 6 birds each were assigned to 15 diets of central composite rotatable design containing 5 levels of dMet (0.42-0.58%), dLys (0.88-1.32%), and dThr (0.53-0.87%) from 3 to 16 d of age. Experimental levels of dMet, dLys, and dThr significantly affected bird performance. The second-order models for BW gain and feed conversion ratio (FCR) were generated. The response surface analysis showed significant quadratic models for BW gain and FCR. The crossproduct of amino acids was significant for FCR but not significant for BW gain. Canonical analyses on BW gain and FCR models showed that the maximum BW gain at the stationary point may be obtained with 0.54, 1.12, and 0.78% of dMet, dLys, and dThr in the diet, respectively, and the minimum FCR at the stationary point may be obtained with 0.53, 1.13, and 0.75% of dMet, dLys, and dThr in diet, respectively. Estimated ideal ratios of dMet and dThr to dLys were 48 and 70% for BW gain and 47 and 66% for FCR. Canonical analysis revealed the most important amino acids in the models of BW gain and FCR were Lys and Thr, respectively.

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