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The objective of this study was to evaluate the effects of genotype x temperature interactions on hormonal heat stress indicators and performance traits. Two-hundred forty female oneday-old chicks were randomly assigned to a completely randomized design in a 5×2 factorial arrangement with 5 genotypes (Naked-neck, Na, from Ethiopia; New Hampshire, NH; Lohmann White, LW and F 1 crosses of Na with NH and LW) and 2 ambient temperatures (thermo-neutral, 18-20 o C; heat stress, 30-32 o C). Blood samples were taken from 12 randomly selected birds per genotype and ambient temperature at 22, 38, 51 and 65 weeks of age. Levels of corticosterone (CS) and 3,5,3′-triiodothyronine (T 3 ) were determined in blood plasma. Heat stress effects on egg production traits were most severe in LW and NH, least severe in Na and Na × LW, intermediate in Na × NH. Plasma T 3 level was significantly reduced by 27.9% in heat stressed genotypes. Plasma CS increased by 12.6% in heat exposed genotypes. However, inconsistent responses of CS and T 3 levels were observed at different ages. The Na and their F 1 crosses demonstrated relatively better thermotolerance than LW and NH hens. Levels of plasma T 3 hormone might be considered as indicator of long-term heat stress in hens. HEAT STRESS -LAYING PERFORMANCE

Assessing the thermo-tolerance potentials of five commercial layer chicken genotypes under long-term heat stress environment as measured by their performance traits This study was conducted to test the thermo-tolerance ability of five commercial chicken genotypes (Lohmann Brown, LB; Lohmann White, LW; new hampshire, nh; White Leghorn selected for low feed expenditure, WL-fe and White Leghorn with sex-linked dwarf gene, WL-dw) under long-term heat exposure. two-hundred forty female chickens were assigned to a completely randomized design in a 5 × 2 factorial arrangements (five genetic groups and two ambient temperatures [thermo-neutral, 18-20 °C; heat stress, 30-32 °C]). Individual eggs were collected on daily basis while egg weight and feed intake were determined on individual and group basis at 28-days intervals, respectively. Shell quality traits were determined at 25, 40 and 56 weeks age. no genotype × ambient temperature interactions were found except for body weight and egg deformation. Chickens at thermo-neutral temperature produced significantly heavier eggs than those of heat-exposed (60 g vs. 54 g). hen-housed egg production of chickens in thermo-neutral temperature was significantly higher than those of heat-stressed (76.8 % vs. 66.2 %). daily egg mass production at thermo-neutral and heat stressed chickens was 46 g and 35.8 g, respectively. feed consumption in heat-stressed and thermo-neutral chickens was 109 and 80.8 g, respectively. Shell thickness, breaking strength and haugh unit values were significantly reduced in heat-stressed chickens. Among heat-exposed chickens, the nh had the highest body weight while the LW produced 10 % more eggs than the group average. The heat-induced effect on shell quality traits was lowest in LW chickens. The results indicated that the magnitude of heat stress was breed dependent in which the LB showed poor adaptability to heat stress while both nh and LW genotypes demonstrated better thermo-tolerance ability. Key words: poultry / laying hens / egg quality / egg production / heat stress / genotype Ocena tolerančnega potenciala petih komercianih genotipov kokoši nesnic na osnovi proizvodnih lastnosti pod pogoji dolgotrajnega toplotnega stresa v študiji smo testirali toplotno toleranco petih komercialnih genotipov kokoši (Lohmann Brown, LB; Lohmann White, LW; new hampshire, nh; beli leghorn, selekcioniran na nizko porabo krme, WL-fe in beli leghorn s spolno vezanim genom za pritlikavost, WLdw) pod pogoji dolgotrajne izpostavljenosti visokim temperaturam. Za naključno zasnovan 5 × 2 faktorski poskus (pet genetskih skupin in dve ambientalni temperaturi [termo nevtralna, 18-20 °C; toplotni stres, 30-32 °C]) smo uporabili 240 kokoši. Jajca smo zbirali individualno vsak dan, poraba krme pa je bila ocenjena individualno in za posamezne skupine v 28-dnevnih intervalih. kakovost jajčne lupine smo ocenili pri starosti 25, 40 in 56 tednov. med genotipi in okoljskimi temperaturami nismo našli interakcij, razen za telesno maso in deformacije jajc. kokoši so v termo nevtralnem okolj...