Free diet selection by broilers as influenced by dietary macronutrient ratio and corticosterone supplementation. 1. Diet selection, organ weights, and plasma metabolites.

Male broiler chickens (aged 21 d) were allowed to chose freely for 14 d between three diets in which only one specific macronutrient (protein, lipid, or carbohydrate) was isocalorically substituted for one other macronutrient, but otherwise (nearly) isocaloric and composed of the same ingredients. The three diets were low protein (LowCP; 15.81% CP; 6.56% lipid; 50.78% carbohydrate), low lipid (LowL; 19.63% CP; 3.01% lipid; 51.12% carbohydrate), and low carbohydrate (LowCHO; 19.50% CP; 7.72% lipid; 44.00% carbohydrate). The chickens either received 0, 30, or 45 mg of corticosterone (CORT) per kg diet. As a percentage of their total intake, unsupplemented chickens consumed 24.0, 71.4, and 4.6% of the LowCP, LowL, and LowCHO diets, respectively, giving a total CP, L, and CHO intake of 282, 61, and 765 g, respectively. The addition of CORT significantly changed the diet selection, as compared to the unsupplemented chickens, CORT chickens consumed a greater percentage from the LowCP (35%), less from the LowL (55%), and again more from the Low-CHO (10%) diet. On the other hand, total feed consumption, macronutrient, and ME intake were not altered significantly by CORT supplementation, probably because of the close similarity of the diets. Corticosterone-supplemented chickens manifested hyperglycemia, hyperlipidemia, and uric acidemia suggesting insulin resistance, increased lipogenesis and protein catabolism, respectively. The elevated plasma creatine kinase (CK) activities of CORT chickens are also suggestive for decreased muscle cell membrane stability. Furthermore, CORT chickens were characterized by increased proportional weights of liver, abdominal fat pad, proventriculus, and gizzard, whereas an involution of spleen and bursa was observed. In conclusion, the present results suggest that high circulating levels of CORT as in the case of stress results in metabolic alterations, which in turn, affects diet preference as a compensatory mechanism to adapt energy and nutrient metabolism.

Somatotrophic and thyroid hormones around the onset of lay in broiler breeders under different conditions.

Somatotrophic and thyroid hormones were determined around the onset of reproduction in broiler breeders reared in two different housing systems [dark, close-sided house (CH) and conventional, open-sided house (OH)]. In both groups age-related changes were obvious for thyroxine (T4), growth hormone (GH) and insulin-like growth factor (IGF-1); levels of T4 decreased, especially between 24 and 28 weeks in both groups; concomitantly GH sharply increased over the same period. A transient peak in triiodothyronine (T3) occurred between 25 and 27 weeks. The effect of housing was only present after the onset of lay. Between weeks 27-28 and the end of the period studied, the CH group showed higher levels of GH and T3 but lower T4 levels as compared to the OH group. A significant increase in GH after onset of lay, without any significant rise in T3 or in IGF-I, could point to a relative insensitivity to high plasma GH levels. Changes at GH receptor level, together with an increased pituitary GH secretion and/or decreased GH turnover may be expected. This may indicate that hypothalamo-pituitary changes at the onset of lay not only imply changes of gonadotrophic cell function, but also other hormonal axes. The relatively decrease in T4 without changes in T3, may point to a decrease in the activity of the thyrotropic axis.

Metabolic disturbances in male broilers of different strains. 2. Relationship between the thyroid and somatotropic axes with growth rate and mortality.

Seven male broiler strains (Arbor Acres, Avian Farms, Cobb-500, Hubbard-Peterson, ISA, Naked Neck, and Ross) were compared for their growth rate, feed efficiency, and mortality due to sudden death and ascites. In addition, weekly plasma levels of thyroid hormones [3,3',5-triiodothyronine (T3) thyroxine (T4), T3: T4 ratio, growth hormone (GH), and insulin-like growth factor-I (IGF-I)] were determined. The highly productive, commercial strains were very similar in their endocrine profiles but differed markedly from the Naked Neck chickens. Naked Neck chickens were characterized by higher plasma T3 and lower T4 levels at similar ages as well as when compared on the same body weight basis. The present findings support the hypothesis that the slightly hypothyroid state of high productive broilers renders them more sensitive to metabolic disorders. Naked Neck chickens also had higher plasma GH levels than those of their age-matched commercial broilers. The coefficient of variation for GH was highest for Naked Neck chickens, which is indicative for an amplified GH burst amplitude. It may be stated that changes in plasma thyroid hormone concentration in indirect response to selection for low feed conversion and fast growth may be causatively linked to susceptibility for metabolic disturbances such as sudden death syndrome and ascites.

Metabolic disturbances in male broilers of different strains. 1. Performance, mortality, and right ventricular hypertrophy.

Two trials were carried out to test the susceptibility for metabolic disturbances of different strains of male broilers. In Trial 1, 1,890 male chickens were allotted in a randomized block design with seven treatments (Arbor Acres, Avian Farms, Cobb-500, Hubbard-Peterson, ISA, Naked Neck, and Ross) and six blocks of 45 chickens. Trial 2 involved 2,184 male chickens of six strains (Arbor Acres, Avian Farms, Cobb 500, Hubbard-Peterson, ISA Naked Neck, and Ross) allotted in seven complete blocks of 52 birds. The same management system was adopted for all birds, reared up to 42 d in an open house during late winter (Trial 1) or late autumn (Trial 2). The most marked differences observed among the strains tested was the lower BW and higher feed conversion of Naked Neck broilers. Total percentage mortalities were high among the most productive broilers, being more than 50% due to sudden death (SDS) and ascites syndrome (AS). No Naked Neck birds died as a consequence of these disturbances and the total mortalities were significantly lower (P < or = 0.05) than the other strains. The ratio of right ventricle weight to total ventricle weight of the dead birds was over 0.25, except for Naked Neck birds, which presented a nonhypertrophic ratio. The two trials confirmed the relationship between high productivity and high incidence of SDS and AS and indicated that Naked Neck male broilers are resistant to these metabolic disturbances.

Performance, incidence of metabolic disturbances and endocrine variables of food-restricted male broiler chickens.

1. This experiment was carried out to evaluate the productive and physiological consequences of a slight but long term food restriction of male broiler chickens from 2 commercial strains. 2. Cobb-500 and Ross chickens were submitted to a 20% food restriction from 8 to 21 d of age. Strain, food programme and their interactive effects were analysed in terms of consequences upon performance, mortality, incidence of sudden death syndrome (SDS) and ascites syndrome (AS), index of right cardiac hypertrophy and plasma concentrations of hormones related to metabolism and growth (T3, T4, T3:T4 ratio, IGF-I and GH). 3. Although some catch-up growth was observed by refeeding previously restricted birds after 22 d of rearing, food restriction decreased (P < or = 0.05) body weight at market age (42 d) irrespective of the strain, but improved (P < or = 0.05) food conversion. 4. The incidence of mortality was not high in non-restricted birds but SDS and AS caused more than 50% of deaths. Hypertrophic cardiac index was observed in chickens of both strains after 4 weeks of age and was higher in ad libitum fed birds. 5. During the period of food restriction, plasma T3 and IGF-I concentrations decreased whereas plasma T4 and GH concentrations increased compared to those of the age-matched ad libitum fed counterparts. During the subsequent ad libitum feeding period, few differences in circulating hormone concentrations were observed, except for the higher mean GH litres in previously food-restricted chickens at 35 d of age. 6. These results indicate that even a non-severe food restriction negatively affects body weight of 42-d-old male broilers but these are benefits with improved food efficiency and diminished mortality from metabolic disturbances. The hormone results suggest that the degree of food restriction applied was not severe because there was a very fast adaptive response with small and transient alterations in T3, T4 and GH plasma concentrations during the period of compensatory growth.