Age at sexual maturity influences the response of single comb White Leghorn pullets to marginal and low levels of dietary phosphorus

This study was designed to determine the differences in response of early-maturing (EM) and late-maturing (LM) Single Comb White Leghorn pullets within a flock to marginal or low dietary phosphorus. Various levels of dietary phosphorus were fed to EM and LM Leghorn pullets from 18 wk of age until the age of peak egg production (24 wk). The dietary phosphorus levels were 0.40, 0.45, 0.50, 0.55 and 0.70 g total phosphorus (tP)/100 g diet, corresponding to calculated available phosphorus values of 0.18, 0.23, 0.28, 0.33 and 0.48 g/100 g, respectively. At 0.70 and 0.55 g tP/100 g, the plasma inorganic phosphorus, Ca++ and urine calcium concentrations did not differ between EM and LM pullets, whereas LM pullets had a better bone status than EM pullets as reflected by bone mineral content, bone density and bone breaking strength. As dietary phosphorus was lowered from 0.55 to 0.4 g tP/100 g, the plasma concentration of inorganic phosphorus dropped and that of Ca++ increased at greater rates in LM pullets than in EM pullets. The magnitude of decline in bone status was also greater in LM than in EM pullets when dietary phosphorus was lowered from 0.55 to 0.40 g tP/100 g. The maximum incidences of osteoporosis and mortality were observed in LM pullets fed 0.40 g tP/100 g, followed by LM pullets fed 0.45 g tP/100 g diet. We conclude that when early layer diets contain marginal or low levels of phosphorus, the severity of adverse effects are greater in LM pullets than in EM pullets