Carry-over effects of dietary crude protein and triiodothyronine (T₃) inbroiler chickens

Abstract

Indian River male broiler chickens growing from 7 to 30 d of age were fed on diets containing crude protein levels ranging from 120 to 3041 g/kg plus 0 or 1 mg triiodothyronine (Ta/kg diet. The purpose of this study was to examine the effects of these treatments on lipogenesis after a common diet was fed (180 g crude protein/kg diet from 30 to 56 d of age). Dietary treatment groups were sampled at 30 and 56 d. In vifro lipogenesis was determined by incubating liver explants for 2 h at 37" in Hanks' salts containing 25 mM-HEPES and 10 m~-[2-'~C]acetate and then measuring acetate incorporation into total lipid. Growth and feed consumption from 7 to 30 d increased (P < 0.01) as dietary protein increased from 120 to 210 g/kg diet. Both measurements decreased as crude protein increased from 210 to 300 g/kg diet.T, decreased (P < 001) growth and feed intake during this period. Low-protein (< 180 g/kg) diets increased (P < 0.05) and T, decreased lipogenesis in 30-d-old chickens. Although birds given T, from 7 to 30 d grew at the greatest rate from 30 to 56 d of age, the final body weight was still less than controls. In vifm lipogenesis at 56 d of age was not affected by either of the two dietary treatments. In contrast, the relative size of the abdominal fat pad (g/kg body weight) at 56 d was decreased by feeding T, from 7 to 30 d. Any changes in metabolism elicited by either dietary protein levels or hormone treatments may be specific to the particular dosing interval and are not sustained when a common diet is fed during a repletion period.Dietary protein : Triiodothyronine: Lipogenesis: Chicken Diets containing high energy :protein ratios promote de novo lipogenesis resulting in obese broiler chickens (Donaldson, 1985;Rosebrough & Steele, 1985). On the other hand, diets with low energy:protein ratios promote lean broiler carcasses (Donaldson et al. 1956;Thomas & Combs, 1967). Feeding a very low energy :protein diet (43 MJ/kg crude protein) results in a very lean carcass (crude protein as a percentage of DM) when compared with results attained by feeding a higher energy: protein diet (65 MJ/kg crude protein; Rosebrough & Steele, 1985). Bartov (1979) has proposed that the excretion of excess amino acid N from diets containing high levels of crude protein would require energy. This requirement for energy would come at the expense of that used for fat synthesis.Later work has suggested an interaction between thyroid status and dietary crude protein on lipid metabolism. Dietary triiodothyronine (T,) depressed lipogenesis in chickens fed on a diet containing 106 MJ/kg crude protein. Artificial changes in thyroid metabolism gave conflicting results in poultry (Leung et al. 1984a(Leung et al. , b, 1985. For example, a daily injection of thyrotropin-releasing hormone increased both plasma growth hormone and thyroid hormone levels (Cogburn et ai. 1989). These increases were accompanied by an increase in U S . Department of Agriculture and does not imply an approval to the exclusion of other products.