The ascites syndrome, as a consequence of high oxygen demands of the fast-growing tissues in meat type chicken is accompanied by a marked increase in broiler mortality. Thyroid hormones concentration has been reported to be reduced in ascitic chickens prior to death and thyroxine (T4) administration was suggested to diminish the mortality rate from the ascites syndrome in broiler chickens. Therefore, the aim of the present study was to ascertain whether the long-term administration of T4 for 14 consecutive weeks might be associated with any changes in blood hematological and biochemical attributes in Cobb 500 broiler breeder hens to provide valuable recommendations for T4 administration in reducing the ascites incidence.
A total of seventy 47-wk-old Cobb 500 breeder hens were randomly allotted to control or extra-T4 treated (T4) groups. The hens in the T4 treatment group individually received an oral administration of pure T4 (0.3 mg/bird per day) dissolved in 1 mL of water for 14 successive weeks. The birds were reared under the same conditions and fed a corn-soybean based diet (155 g/d per bird; 2700 kcal metabolizable energy/kg, and 14.0, 2.99, and 0.36% crude protein, calcium, and phosphorus) with a free access to fresh drinking water. Body weight and egg production were recorded on a weekly basis. Blood samples (5 mL) were obtained from the brachial vein at weeks 47, 50, 53, 55, 57, 59, and 61 of age to determine the percentages of lymphocytes, monocytes, heterophils, eosinophils, basophils, and heterophil to lymphocyte (H:L) ratio. Plasma/serum samples were analyzed for T3 and T4. The concentrations of total protein, albumin, globulin, total cholesterol, high-(HDL), low-(LDL), very low-(VLDL) density lipoprotein, triglycerides, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP) were quantified in the plasma samples. The concentration of glucose, calcium, phosphorus, and ALP were determined in serum samples.
The concentration of T4 was elevated in the T4 hens (27.08 ng/mL) as compared with that in the control birds (10.24 ng/mL; P = 0.003); whereas plasma T3 concentration was not influenced by the long-term administration of T4 (P > 0.05). A decrease in body weight was recorded in T4 birds (4,369 and 4,257 g for control and T4 birds, respectively; P = 0.040). No differences in egg production were found between the experimental groups. Thyroxine administration increased the total number of leukocytes (11.5 × 103/mL) compared to the control (10.7 × 103/mL) birds (P < 0.001). Likewise, a higher total erythrocyte number was found in hyperthyroid hens (2600 vs. 2400 × 103/mL for control and T4 birds, respectively; P < 0.001).
Overall, the long-term administration of T4 in broiler breeder hens had no adverse effects on blood hematological and biochemical characteristics, and apart from increases in the concentration of some attributes, the recorded values were within their reference ranges. According to unaffected H:L ratio, the birds orally administered by extra T4 did not experience stress during the trial. The present study, however, did not evaluate the progeny chicks in term of ascites incidence. Application at the farm level requires further substantiation of the present findings, with an eye to immune- and growth-associated effects in progeny broilers.