Hematocrit (%) in blood of broiler breeder hens fed diets supplemented with Fe. Regression equations obtained using the increasing dietary Fe in the current study (44, 58, 72, 86, and 100 mg/kg). Dashed line is the quadratic polynomial (QP) regression (Y = 8.46 + 0.583 × X − 0.003627 × X2; the maximum response arrow pointing at 80.4 mg/kg Fe, R2 = 0.407, P = 0.002); solid line is the 2-slope broken-line estimation [Y = 22.1 + 0.126 × X (X ≤ 86.4) and Y = 52.3 − 0.224 × X (X > 86.4), the maximum response at 86.4 mg/kg Fe, R2 = 0.475, P < 0.001].