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of grain and grain products for chickens. Poultry Sci. 39: 573-579. Lodhi, G. N., R. Renner and D. R. Clandinin, 1969. Studies on the metabolizable energy of rapeseed meal for growing chickens and laying hens. Poultry Sci. 48: 964-970. March, B. E., and J. Biely, 1971. Factors affecting the response of chicks to diets of different protein value: breed and age. Poultry Sci. 50: 1036-1040. Potter, L. M., L. D. Matterson, A. W. Arnold, W. J. Pudelkiewicz and E. P. Singsen, 1960. Studies in evaluating energy content of feeds for the chick. 1. The evaluation of the metabolizable energy and productive energy of alpha-cellulose. Poultry Sci. 39: 1166-1178.

Sibbald, I. R., 1975. The effect of level of feed intake on metabolizable energy values measured with adult roosters. Poultry Sci. 54: 1990-1998. Sibbald, I. R., and S. J. Slinger, 1963. A biological assay for metabolizable energy in poultry feed ingredients together with findings which demonstrate some of the problems associated with the evaluation of fats. Poultry Sci. 42: 313-325. Slinger, S. J., I. R. Sibbald and W. F. Pepper, 1964. The relative abilities of two breeds of chickens and two varieties of turkeys to metabolize dietary energy and dietary nitrogen. Poultry Sci. 43: 329-333. Zelenka, J., 1968. Influence of age of the chicken on the metabolizable energy values of poultry diets. Br. Poultry Sci. 9: 135-142.

D . J. HOLCOMBE, D . A. ROLAND, S R . AND R. H . HARMS

Department of Poultry Science, Florida Agricultural Experiment Station, Gainesville, Florida 32611 (Received for publication May 15, 1975)

ABSTRACT 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 afternoonevening decline. This decline occurs both for relative and absolute P intake. POULTRY SCIENCE 55: 308-317, 1976

INTRODUCTION

HE effect of phosphorus on the parameters of egg production, egg specific gravity and serum calcium levels has been an area of considerable research. Walter and Aitken

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1. Florida Agr. Exp. Sta. Journal Series No. 5920.

(1962) found that increasing the phosphorus level in the diet from 0.4 percent to either 0.6 or 0.7 percent, in diets adequate in other respects, resulted in increased egg production. Evans et al. (1944) and Mostert and Swart (1968) reported similar findings when using similar levels of phosphorus. Other

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The Ability of Hens to Regulate Phosphorus Intake When Offered Diets Containing Different Levels of Phosphorus1

REGULATION OF P INTAKE

EXPERIMENTAL PROCEDURE Two experiments were conducted following the same general procedure. In each experiment, 144 Babcock B-300 hens were

housed, one per cage, in a continous row of back-to-back cages. The hens were divided into four treatment groups of 36 hens each. All hens received a choice of two diets presented in two plastic cups using the system of Roland et al. (1971). One half of each group of 36 hens received the diet choices in a right to left orientation while the other half received them in a left to right orientation. The hens in the control group received a 0.75 percent phosphorus diet in each of the two cups. The second group was offered a 0.19 percent phosphorus diet in one cup and a 0.46 percent phosphorus diet in the other. The third group could choose between a 1.00 percent phosphorus diet in one cup and a 2.43 percent phosphorus diet in the other. The fourth group received a choice between a diet containing a 0.19 percent phosphorus in one cup and a 2.43 percent phosphorus diet in the other. All diets contained 16.2 percent protein and 2.875 kilocalories of metabolizable energy per kilogram of feed. The respective diets were weighed and replenished every three days. Hens in the first experiment were 72 weeks old. The second phase of the first experiment used the same hens with the same dietary phosphorus choices, but with an increase in the calcium level of the diets from 3.00 to 6.00 percent. Hens in the second experiment were 48 weeks old. They were fed the same dietary phosphorus choices containing 3.00 calcium throughout the duration of the experiment. Feed was weighed every two hours for two consecutive days for both old and young hens after each had been on the experimental regime for a minimum of one month. In both experiments, hens were weighed weekly. Specific gravity of eggs, egg weights and egg production per hen-day were determined. The data from all experiments were subjected to statistical analysis using the multiple range test of Duncan (1955).

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researchers reported either no increase or decreases in egg production with phosphorus levels in the diet ranging from 0.55 (Davidson and Boyne, 1970) up to 1.018 percent (Hinners et al., 1963). Significantly depressed egg production has been reported both with low phosphorus levels, 0.19 percent (O'Rourke et al., 1954), or high levels, 1.28 percent phosphorus (Harms et al., 1965) in the diet. This contrasts with reports of no adverse effect on egg production with over 1.00 percent phosphorus in the diet (Evans et al., 1944; Hinners et al., 1963). Hinners et al. (1963) reported that dietary phosphorus levels up to 1.018 percent did not affect egg shell quality. Other researchers found that 1.0 percent phosphorus in the diet resulted in a significant reduction in egg shell quality (Arscott et al., 1962; Taylor, 1965; Hunt and Chancey, 1970) over lower levels. Mostert and Swart (1968) found that increasing levels from 0.45 to 0.65 percent phosphorus in the feed resulted in reduced egg shell quality. These reports compliment studies by Salvesen et al. (1924) and Gardiner (1973) who found that high dietary phosphorus resulted in depressed serum calcium levels. Bachra et al. (1963) reported the retarding effects of high phosphate concentrations on the in vitro formation of calcium carbonate. The numerous studies cited point to the importance of dietary phosphorus to egg production and egg shell quality. The often contradictory results indicate the confusion which may exist in this area. Despite the obvious interest in phosphorus, virtually nothing has been reported on the hen's, or any other animal's, ability to regulate phosphorus intake when offered diets containing different levels of phosphorus.

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D. J. HOLCOMBE, D. A. ROLAND, SR. AND R. H. HARMS

RESULTS Experiment 1, Phase 1. Hens of the control group, receiving 0.75 percent phosphorus diet in each cup, ate approximately 50 percent from each of the two cups (Fig. 1). The hens receiving a choice between the 0.19 percent and 0.46 percent phosphorus diets exhibited a slight tendency to eat more of the higher phosphorus diet (Fig. 2). In those groups of hens receiving a choice between the 2.43

percent phosphorus diet and a lower level, both groups drastically avoided the 2.43 percent phosphorus diet (Fig. 3 and Fig. 4). Of the criteria of body weight, egg production, total feed consumption, egg weight and serum calcium levels, only egg specific gravity showed any significant differences (Table 1). Only those criteria which exhibited significant differences are presented in tabular form. Both groups of hens receiving the 2.43

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308 I. R. SIBBALD of grain and grain products for chickens. Poultry Sci. 39: 573-579. Lodhi, G. N., R. Renner and D. R. Clandinin, 1969. Studies on...
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