“…In this study, we used several regression models, five types of broken line models and the quadratic polynomial, to estimate the lysine requirement of growing quails. Different models and design of experiments (DOE) have been used for estimation of nutritional requirements in birds and domestic animals (Baker, Batal, Parr, Augspurger, & Parsons, 2002;De Leon, Kidd, & Corzo, 2010;Ghazaghi, Mehri, Yousef-Elahi, & Rokouei, 2012;Mehri, 2012Mehri, , 2014Mehri, Davarpanah, & Mirzaei, 2012;Mehri & Ghazaghi, 2014;Mehri, Nassiri Moghaddam, Kermanshahi, & Danesh-Mesgaran, 2013;Mercer, 1992) but as Pesti et al (2009) suggested, each model may have some advantages and disadvantages. For instance, although second-order polynomials are easy to fit to data, the inclusion of input data below or above those points that are required for maximum or minimum responses, resulting in remarkable changes in requirement estimates ).…”