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 ea rly-maturing (EM) and lata-maturing (LM) Single Comb White Leghorn pul lets within a flock to marginal or low dietary phosphorus. Various lev els of dietary phosphorus were fed to EM and LM Leghorn pullets from 1 8 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 phospho rus (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 urin e calcium concentrations did not differ between EM and LM pullets, whe reas LM pullets had a better bone status than EM pullets as reflected by bone mineral content, bone density and bone breaking strength. As d ietary phosphorus was lowered from 0.55 to 0.4 g tP/100 g, the plasma concentration of inorganic phosphorus dropped and that of Ca++ increas ed at greater rates in LEI pullets than in EM pullets. The magnitude o f decline in bone status was also greater in LM than in EM pullets whe n dietary phosphorus was lowered from 0.55 to 0.40 g tP/100 g. The max imum incidences of osteoporosis and mortality were observed in LM pull ets fed 0.40 g tp/100 g, followed by LM pullets fed 0.45 g tP/100 g di et. We conclude that when early layer diets contain marginal or low le vels of phosphorus, the severity of adverse effects are greater in LM pullets than in EM pullets.