Author
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Rosebrough, Robert |
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Submitted to: Growth Development and Aging
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/3/2002 Publication Date: 9/23/2002 Citation: Growth Development & Aging 66:95-106 Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 1000 to 1500 million dollars annually. The original source of this problem relates to selection genetic practices that emphasized rapid growth at the expense of other carcass characteristics. The literature is of limited value in determining methods to depress fat synthesis and allow lean tissue synthesis to remain at an elevated rate. The thyroid axis is more important in regulating intermediary metabolism in birds than in mammals because of a questionable role for insulin in birds. The purpose of this set of experiments was to 1) chemically inhibit thyroid hormone production and 2) examine protein nutritional status as a moderator of this inhibition. Although hypothyroidism decreased lipid synthesis in 28-day old chickens, restoration of thyroid function in these birds increased synthetic ability in market-age birds (49 days of age). Assigning a role to the thyroid hormone axis may be difficult because an interplay between protein nutrition and background thyroid status. Technical Abstract: The thyroid axis is one of the more controversial areas in growth and metabolism of the broiler chicken. Although chemical hypothyroidism decreased growth, artificial changes in thyroid hormone levels did not always change growth predictably. What is lacking from previous studies is any information concerning recovery from inhibition of T4 production. Male broiler chickens were fed diets containing 12, 18 or 24% crude protein + 0 or 1 g methimazole/kg diet from 7 to 28 d of age and then a diet containing 18% crude protein from 28 to 49 d of age. Birds were killed at 28 and 49 days to 1) determine effects of treatments at 28 d and 2) determine carry over effects of these treatments. In vitro lipogenesis was inversely related to dietary protein levels in control birds at 28 d. Dietary methimazole attenuated this effect, resulting in a common rate similar to that attained in the birds fed the highest level of protein without methimazole. In contrast, birds fed methimazole from 7 to 28 d had greater lipogenic rates at 49 d than did their control counterparts. It is unclear at this time if observations noted at 28 d can be traced to reduced feed intake or to changes in thyroid status. Previous pair-feeding studies from this laboratory confirmed that differences in metabolic parameters caused by differences in dietary protein were not attenuated by limit feeding. Observations at 49 d suggest that permutations in the thyroid of the young bird may substantially change metabolism in later life. |
