Studies Concerning Possible Explanations for the Varying Response of Different Phosphorus Levels on Eggshell Quality

Four experiments were conducted to determine if differences in phosphorus (P) source or differences in dietary calcium (Ca) level were responsible for the changes in egg specific gravity observed when dietary P levels are increased or reduced. Individually caged hens (685) and 23 dietary treatments involving various Ca, P levels, and P sources (dicalcium phosphate and phosphoric acid) were used. The criteria were egg specific gravity, egg weight, egg production, feed consumption, serum calcium, and serum P. The results indicate that hens consuming various quantities of P (288 to 384 mg/hen/day) had similar, reduced, or increased egg specific gravities compared to that from hens receiving 714 to 819 mg P/hen/day. Hens with higher P intakes (1455 to 1710 mg/hen/day) had reduced egg specific gravity. There was no indication that P source or dietary calcium level influenced the observed response. Phosphorus intakes of 384, and 307 mg/hen/day maintained production in two experiments, but P intakes of 288, 310, 322, and 329 mg/hen/day did not in two other experiments. Although all low P treatments in which serum P was measured reduced serum P, there was little or no indication that this influenced egg specific gravity. Egg weight, feed consumption, and serum calcium varied among treatments. Results indicate that differences in ability of hens to utilize Ca from limestone vs. phosphate source or different calcium levels would not explain the variation in response to different P levels previously reported. It was concluded that until mechanisms responsible for varying responses to different P levels are known, reducing P levels to improve shell quality should be used cautiously.