Efficiency of utilization of L-(15N) threonine in broiler tissues and estimation of threonine requirement from 1 to 7 days of age

A traditional technique to estimate the efficiency of utilization in the body is the comparative slaughter technique, but the efficiencies in different tissues is hardly known. The aim of this study was to calculate the efficiencies of utilization of L-(¹⁵N) threonine in different broiler?s tissues and use the efficiencies of feathers and body free of feathers in a mechanistic model developed by the UNESP nutrition group to estimate threonine requirement (Thr_req). Fifty-one male broilers were individually housed. The diet was formulated according to the recommendations of the Brazilian Tables and then enriched with L-(¹⁵N) threonine based on the daily body weight (69.6 mmol/kg bird) during five days. The birds were slaughtered and samples of tissues were collected. The isotope concentrations at 4 and 9 days (Atm%) and the percentage of nitrogen (%N) were determined by an IRMS. The ¹⁵N depositions (¹⁵N_D, in mg) were calculated by the equation: ¹⁵N_D = W_dt*(%N)*(Atm%-Atm%₀), where: W_dt is dried tissue?s weight (mg). The efficiencies of utilization were determined by the equation: Ef = [(¹⁵N_D/W_t)/(¹⁵NI/FI)], where: W_t is tissue?s weight (mg); ¹⁵NI is L-(¹⁵N) threonine intake (mg) and FIis feed intake (g). The efficiencies of utilization calculated were: breast 78.27%; feathers 73.02%; body free of feathers 59.48%; sartorium 49.17%; kidneys 47.79%; liver 47.47%; jejunum 43.55%; and plasma 17.89%. Thr_req was determined according to the model: Thr_req = ((((Thr_b*PD_b)/Ef_b) + ((Thr_f*PD_f)/Ef_f))) + (((BPm⁰.73)*(BPt/BPm))*75,5) + (LR_f*FPW*Thr_f)) * 1000, where: Thr_b is the threonine concentration of body protein (mg/g); PD_b is body protein deposition (g/d); Ef_b is the efficiency of utilization in the body; Thr_f is the threonine concentration of feather protein (mg/g); PD_f is the protein deposition in the feathers (g/d); Ef_f is the efficiency of utilization in the feathers; BPm is the body protein at maturity (kg); BPt is body protein weight at age t (kg); LR_f is the loss rate of feathers; FPW is the feather protein weight. It was estimated a requirement of 247 mg threonine/day/bird in a period of 1 to 7 days according to the model. It is expected this value is closer to the threonine requirement, since this model do not consider only a single efficiency for the body.