Valine deficiency has a greater impact on broiler growth and bone health than isoleucine deficiency

Valine and isoleucine are not only two of the indispensable amino acids (AAs) in chickens but also have special mechanisms with leucine within the branched-chain AA (BCAA) category. Therefore, we aimed to investigate how valine or isoleucine deficiency could specifically affect growth performance in broilers through various analyses. A total of 252 seven-day (d)-old male Cobb 500 broilers were allotted to three treatments with six replicates and reared until d 21. The three treatments were as follows: (1) Control group (CON; 1.31 leucine:lysine ratio), (2) valine deficiency group (-VAL; 0.62 valine:lysine ratio and 85% valine level compared to the CON group), and (3) isoleucine deficiency group (-ILE; 0.54 isoleucine:lysine ratio and 85% isoleucine level compared to the CON group). The -VAL group had significantly decreased d 7, 14, and 21 body weight (BW), BW gain (BWG), feed intake (FI), and feed efficiency from d 7 to 21 compared to the CON and -ILE groups (P < 0.001). The -ILE group showed no difference in d 14 and 18 BW; however, they showed significantly reduced BW and BWG at d 21 and feed efficiency from d 7 to 21 compared to the CON group (P < 0.001). Daily FI in the -VAL group significantly decreased from the beginning compared to the two groups, and this gap further expanded until d 21. The -VAL group also had significantly decreased breast muscle weight, total tissue weight, bone mineral density, bone mineral content, and walking ability (P < 0.01). The expression levels of mechanistic target of rapamycin and BCAA catabolism-related genes were highest in the -VAL group (P < 0.05), whereas the -ILE group did not show any difference compared to the CON group (P > 0.05). In conclusion, about 85% valine deficiency is accompanied by a substantial reduction in chicken growth, which has a much greater effect than isoleucine. Valine deficiency can also lead to increased utilization of leucine, which may result in BCAA antagonism.