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Abstract
This research aimed to study the effects of growth rate on the growth physiology, behavior, and welfare of modern broilers. Male broilers were allocated to standard (STD), 10% (RES10), or 20% feed-restricted (RES20) groups using the paired-feeding method. Tissues were collected when STD and RES20 reached 1kg, 2.5kg, and 4kg target weights. Similarly, behavioral and welfare parameters were assessed each week from Wk 4 to Wk 8. Growth restriction decreased the diameter, cortical thickness, and bone-breaking strength of the tibiotarsus during the same weight and age comparison. Furthermore, increased hepatic mRNA levels of IGFBP2 and decreased muscle expression of IGFs, IGF1R, and IGFBPs were observed in feed restricted group. Restricting growth reduced incidences of lameness and hockburn while promoting preening and dustbathing behaviors. In conclusion, restricting growth by feed restriction (20%) alters growth physiology, and improves behavior, and welfare outcomes, but might increase abnormal object pecking and preening.