Tibiotarsus bone characteristics and tibial dyschondroplasia incidence of broilers fed diets supplemented with leucine and valine

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Leg problems represent a major cause of broiler mortality in modern lines compromising animal welfare and carcass quality, leading to high economic losses in the poultry industry (Knowles et al., 2008). Tibial dyschondroplasia, a major cause of lameness in broilers, is characterized by the presence of an avascular lesion with abnormal cartilage that is not calcified, resulting in an accumulation of immature chondrocytes with an extended life at the proximal tibiotarsus (Leach & Monsonego‐Ornan, 2007). Even though the proper ratio of branched‐chain amino acids (BCAA) is essential for osteoblasts to form bone matrix as well as to facilitate calcification and proliferation of bone cells (Farran & Thomas, 1992), there has been limited information on the role of BCAA on bone metabolism of broiler chickens.
Leucine restriction in rats has shown to reduce proliferation and differentiation of chondrocytes in the growth plate (Kim et al., 2009). Similarly, Phornphutkul, Wu, Auyeung, Chen, and Gruppuso (2008) reported that the metatarsal cells of mice with rapamycin resulted to a decrease in bone length by inhibition of the mechanistic target of rapamycin (mTOR), suggesting that mTOR is an important protein to modulate bone growth. Leucine is the most effective nutrient to activate mTOR (Kimball & Jefferson, 2006). Therefore, the restriction of this amino acid can impair bone development, as it blocks the mTOR signalling pathway (Kim et al., 2009). In addition to leucine, valine may also influence bone metabolism. Farran and Thomas (1992) reported a reduction in bone calcium and ash concentrations and lower bone weight of birds fed a valine‐deficient diet, suggesting a decrease in osteoblast activity by the low levels of this amino acid. The role of BCAA on bone metabolism has been seldom studied and it is unclear whether the leucine and valine interaction may interfere with bone development in birds as well as their mechanism of action. Therefore, two experiments were conducted in this study to determine the effect of leucine and valine supplementation on tibiotarsus bone characteristics and tibial dyschondroplasia incidence and also to ascertain whether leucine and valine amino acids are important for reducing the hypertrophic cartilage zone of the proximal tibiotarsus growth plate from day 1 to 21 (Experiment I) and day 21 to 42 (Experiment II) post‐hatch.
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