The ability of hens to regulate phosphorus intake when offered diets containing different levels of phosphorus

Two experiments were conducted to determine the extent and ability of laying hens to regulate their phosphorus (P) intake when offered a choice of diets containing different levels of phosphorus. In the first experiment, Babcock B-300 hens, 72 weeks old, received a choice of diets offered in each of two plastic cups. The dietary choices of P levels included: 0.75 percent P vs. 0.75 percent P diet (control), 0.19 percent vs. 0.46 percent P, 1.00 percent vs. 2.43 percent P and 0.19 percent vs. 2.43 percent P diet. The cups were weighed and replenished every three days. After one month, the calcium level in all diets was increased from 3.00 to 6.00 percent calcium and the experiment continued for a second month. The second experiment involved the same dietary choices of P with a constant level of 3.00 percent calcium. Hens in this experiment were 48 weeks old. In both Experiments 1 and 2 the daily feed consumption was determined by weighing all cups at two hour intervals for two days. This procedure was performed after hens had been on the experimental regime for one month. Hens in both experiments clearly regulated their intake of P when offered the above choices. Hens avoided the consumption of the 2.43 percent P diet in favor of the diet containing less P. Consumption of the 2.43 percent P diet, even when avoided to a large extent, reduced both egg production and egg specific gravity. Increased calcium levels in the diets resulted in an increase in consumption of the 2.43 percent P diet. Young hens demonstrated a significant increase in consumption of the 0.46 percent P diet over the 0.19 percent P diet. Both old and young hens in the 0.46 percent P vs. 0.19 percent P group in both experiments maintained egg specific gravity, egg weight and egg production at levels comparable to controls. Results of the two hour feed weighings showed that laying hens in the 0.19% vs. 2.43% P group exhibited a noon-time peak in preferential consumption of P, followed by a marked afternoon-evening decline. This decline occurs both for relative and absolute P intake.