Why does a deficiency of dietary Vitamin D3 (cholecalciferol) cause Tibial Dyschondroplasia (TD) and rickets in young brown replacement pullets?

Vitamin D3 (cholecalciferol) is a critical steroid hormone precursor required for calcium and phosphorus homeostasis in poultry. In young, rapidly growing brown replacement pullets, dietary Vitamin D3 is absorbed and hydroxylated first in the liver to 25-hydroxycholecalciferol ($25(OH)D_3$), and then in the kidneys to its highly active hormonal form, 1,25-dihydroxycholecalciferol (calcitriol). Calcitriol binds to mucosal receptors in the small intestine, stimulating the transcription of calbindin, the active transport protein that absorbs calcium. Without sufficient Vitamin D3, calcium absorption drops to near zero, regardless of dietary calcium levels. This halts the mineralization of the growth plates in long bones, causing rickets and Tibial Dyschondroplasia (TD)—a condition where a plug of unmineralized, hypertrophic cartilage accumulates in the proximal metaphysis of the tibia. This leads to severe bone bowing, lameness, skeletal deformities, and high pullet mortality. Supplementing feed with stabilized Vitamin D3 or advanced 25-hydroxycholecalciferol (Hy-D) metabolites is mandatory. Farmers can consult skeletal biochemistry in the Poultry Encyclopedia, buy certified liquid Vitamin D3 on Poultry Plaza, track daily raw material prices on Poultry Rates, and trade healthy pullets on Murghi Mandi.