BackgroundLosses in poultry production due to heat stress have considerable negative economic consequences. Previous studies in poultry have elucidated a genetic influence on response to heat. Using a unique chicken genetic resource, we identified genomic regions associated with body temperature (BT), body weight (BW), breast yield, and digestibility measured during heat stress. Identifying genes associated with a favorable response during high ambient temperature can facilitate genetic selection of heat-resilient chickens.MethodsGenerations F18 and F19 of a broiler (heat-susceptible) × Fayoumi (heat-resistant) advanced intercross line (AIL) were used to fine-map quantitative trait loci (QTL). Six hundred and thirty-one birds were exposed to daily heat cycles from 22 to 28 days of age, and phenotypes were measured before heat treatment, on the 1st day and after 1 week of heat treatment. BT was measured at these three phases and BW at pre-heat treatment and after 1 week of heat treatment. Breast muscle yield was calculated as the percentage of BW at day 28. Ileal feed digestibility was assayed from digesta collected from the ileum at day 28. Four hundred and sixty-eight AIL were genotyped using the 600 K Affymetrix chicken SNP (single nucleotide polymorphism) array. Trait heritabilities were estimated using an animal model. A genome-wide association study (GWAS) for these traits and changes in BT and BW was conducted using Bayesian analyses. Candidate genes were identified within 200-kb regions around SNPs with significant association signals.ResultsHeritabilities were low to moderate (0.03 to 0.35). We identified QTL for BT on Gallus gallus chromosome (GGA)14, 15, 26, and 27; BW on GGA1 to 8, 10, 14, and 21; dry matter digestibility on GGA19, 20 and 21; and QTL of very large effect for breast muscle yield on GGA1, 15, and 22 with a single 1-Mb window on GGA1 explaining more than 15 % of the genetic variation.ConclusionsThis is the first study to estimate heritabilities and perform GWAS using this AIL for traits measured during heat stress. Significant QTL as well as low to moderate heritabilities were found for each trait, and these QTL may facilitate selection for improved animal performance in hot climatic conditions.

and Implications An experiment was conducted to test methionine source and concentration on the growth performance and physiological responses of heat-stressed broiler chickens. Birds were fed either DL methionine or 2-hydroxy-4methylthiobutanoic acid (HMB) at two concentrations (adequate: starter 0.94%, grower 0.84% or superadequate: adequate concentrations +20% methionine) over a six day period at normal or elevated environmental temperatures. Heat treatment was initiated at 21 days of age within controlled environmental chambers. Blood samples were taken 6 hours and 6 days post treatment to determine the effects of acute and chronic heat stress on various blood parameters. Heat treatment significantly impaired growth performance over the 6 day period. There were no effects of methionine source or concentration on the growth performance of broilers within any of the environmental treatments. Blood parameters changed during acute heat stress, but normalized over chronic heat exposure. Birds were apparently able to adapt blood chemistry to heat exposure subsequent to chronic heat stress, but were still negatively affected from a performance standpoint.

Experiments were conducted exploring potential strategies to mitigate the negative effects of heat stress in chickens. In the first experiment, broilers were placed in environments which were heat treated (HT), thermoneutral (TN), or thermo-neutral coupled with pair-feeding (TN-PF). Birds were fed diets containing either DL-methionine (DLM) or 2-hydroxy-4-methylthiobutanoic acid (HMTBA) formulated to adequate or super-adequate levels of digestible sulfur amino acids (DSAA). Reduced (GSH), oxidized (GSSG), and total (TGSH) glutathione as well glutathione peroxidase (GPx) activity were measured to assess antioxidant capacity. Thiobarbituric acid reactive substances (TBARS) were measured to assess lipid peroxidation. Several blood variables were also measured. Responses were measured at 6 hours (acute) and 6 days (chronic). Growth over 6 days was impaired by HT (P < 0.01). Blood variables were altered during the acute phase. Birds fed DLM during the acute phase exhibited lower TBARS concentrations than those fed HMTBA (P = 0.02). Concentrations of GSSG were lower (P < 0.01) in birds receiving super-adequate DSAA than those receiving adequate DSAA in the acute phase. During the chronic phase, GSH and TGSH concentrations were higher (P = 0.02) in the HT group compared to the TN group. There were no effects on GPx activity. During the second experiment, broiler x Fayoumi crosses were incubated at a normal incubation temperature (NI; 37.8°C) or elevated incubation temperature (EI; 39.5°C) from embryonic day-12 to-4. Birds were then placed in TN or cyclical HT from 22 to 28 days of age. Growth performance and feed dry matter digestibility were assessed from 21 to 28 days of age. An interaction occurred where birds in the EI group in HT environments had higher xi digestibility than those birds in the NI group held at TN temperatures. The third experiment followed the same procedures from the second experiment, except all were incubated normally, then divided into groups in which birds had parents that were incubated normally (PN) or parents incubated at elevated temperatures (PE) from the previous experiment. Heat treatment during the third experiment increased BWG. In summary, neither methionine supplementation nor embryonic thermal conditioning improved growth performance during heat stress. performance, but generally did not affect lipid peroxidation or antioxidant status. Results indicated that source of methionine and concentration of DSAA did not consistently impact lipid peroxidation, antioxidant activity, or performance.

Immunoglobulin knockout chickens have a unique potential for use as a research tool. One could examine the contribution of immunoglobulins to pathogen resistance or further characterize the nutritional implications of mounting an immune response. Researchers have attempted to examine the effects of eliminating antibody production in chickens by ablation of the bursa of Fabricius. These techniques cause a loss of patency of the cloaca causing leakage of rectal contents. This results in continuous diarrhea and complications with dehydration and hygiene. Thus, knockout chickens are a superior model because the bursa is maintained, which eliminates the hygienic complications, while also eliminating immunoglobulin production. We examined chickens that were either heterozygous or homozygous for a knockout of the immunoglobulin heavy chain J gene segment. It was previously shown that heterozygotes but not homozygotes produce circulating IgY and IgM in response to vaccination, but IgA was not characterized. The goal of the present study was to affirm that IgA producing cells are also absent in the homozygotes. At 4 weeks of age, birds were injected intramuscularly with 300µg of keyhole limpet hemocyanin mixed 1:1 with complete Freund's adjuvant. A booster was administered at 5 and 6 weeks of age using incomplete Freund's adjuvant. Serum was collected via wing vein puncture at 6 and 8 weeks of age for IgA detection using a sandwich ELISA. It was found that circulating IgA was present in heterozygotes, but homozygotes had no detectable IgA concentrations. In conjunction with previous research showing homozygotes also lack circulating IgY and IgM, these results confirm that homozygotes lack all major classes of circulating immunoglobulin known in chickens.

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