Several bioassays were conducted with young chicks and pigs fed phosphorus (P)-deficient corn-soybean meal diets. With diets for chicks containing .62% Ca and .42% P (.10% available P), graded doses of a citric acid + sodium citrate (1:1, wt:wt) mixture (0, 1, 2, 4, or 6% of diet) resulted in linear (P < .01) increases in both weight gain and tibia ash. Relative to chicks fed no citric acid, tibia ash (%) and weight gain (g/d) were increased by 43 and 22%, respectively, in chicks fed 6% citric acid. Additional chick trials showed that 6% citric acid alone or sodium citrate alone was as efficacious as the citric acid + sodium citrate mixture and that 1,450 U/kg of phytase produced a positive response in bone ash and weight gain in chicks fed a diet containing 6% citrate. Varying the Ca:available P ratio with and without citrate supplementation indicated that citric acid primarily affected phytate-P utilization, not Ca, in chicks. Moreover, chicks did not respond to citrate supplementation when fed a P-deficient (.13% available P), phytate-free casein-dextrose diet. Young pigs averaging 10 to 11 kg also were used to evaluate citric acid efficacy in two experiments. A P-deficient corn-soybean meal basal diet was used to construct five treatment diets that contained 1) no additive, 2) 3% citric acid, 3) 6% citric acid, 4) 1,450 U/kg phytase, and 5) 6% citric acid + 1,450 U/kg phytase. Phytase supplementation increased (P < .01) weight gain, gain:feed, and metatarsal ash, whereas citric acid addition increased only gain:feed (P < .05) and metatarsal ash (P < .08). A subsequent 22-d pig experiment was conducted to evaluate the effect of lower levels of citric acid (0, 1, 2, or 3%) or 1,450 U/kg phytase addition to a P-deficient corn-soybean meal diet. Phytase supplementation improved (P < .01) all criteria measured. Weight gain and gain:feed data suggested a response to citric acid addition, but this was not supported by fibula ash results (P > .10). The positive responses to phytase were much greater than those to citric acid in both pig experiments. Thus, dietary citric acid effectively improved phytate P utilization in chicks but had a much smaller effect in pigs.

In the first of two experiments (20 to 70 wk of age), eight treatments consisted of corn-soybean meal diets (0.34% total P, 3.8% Ca, 17% CP, 2,758 kcal ME/kg) containing 0.10, 0.15, 0.20, 0.25, or 0.45% available P (AP), with the three lowest AP diets supplemented with 300 units of phytase/kg of diet. A second experiment evaluated the effect of feeding the 0.10% AP diet with and without phytase on performance and time required for onset of P deficiency in older hens (70 to 76 wk of age). In Experiment 1, the 0.10% AP diet with no supplemental phytase depressed performance by 28 wk of age. No other significant differences in performance were observed among treatments for the entire 20- to 70-wk period except that the 0.15% AP diet with no phytase resulted in body weights and tibia ash that were lower (P < 0.05) than those of hens fed the 0.45% AP diet during the last 30 wk. The mean daily AP intake of hens fed the 0.10% AP, 0.15% AP, 0.45% AP, or 0.10% AP + phytase diets was 94,159, 499, or 108 mg, respectively. Excreta P concentration was decreased by approximately 50% in birds consuming 0.10% AP + 300 U/kg phytase compared with those consuming 0.45% AP. In the second experiment, P deficiency signs occurred within 3 wk of consuming the unsupplemented corn-soybean meal diet (0.10% AP) compared with 8 wk in Experiment 1. The results of this study indicate that phytase improves P utilization in corn-soybean meal diets for laying hens and that a corn-soybean meal diet containing 0.15% AP (159 mg AP/d) or containing 0.10% AP + 300 units of phytase/kg (108 mg AP/d) supported optimal egg production from 20 to 70 wk of age. Additionally, results suggested that older hens may exhibit P deficiency symptoms sooner than younger hens.

Data previously obtained from our laboratory indicated that addition of 300 U of phytase/kg diet supported optimal long-term performance of laying hens (20 to 70 wk) fed a corn-soybean meal (SBM) diet containing 0.10% available phosphorus (AP). Our primary objective was to determine if a phytase level lower than 300 units/kg is adequate for a commercial strain of Single Comb White Leghorn laying hens (n = 504) fed a corn-SBM diet containing no supplemental P (0.10% AP). Dietary treatments consisted of the corn-SBM basal diet (0.10% AP, 3.8% Ca, and 17% CP) supplemented with 0, 100, 200, 250, or 300 U of phytase/kg, 0.05% inorganic P (0.15% AP), and a positive control diet containing 0.45% AP. Each of the seven dietary treatments was fed to six replicate groups of 12 hens from 20 to 60 wk of age. No significant differences in performance were observed among treatments during the first 8 wk of the experiment. By 28 wk of age, the 0.10% AP diet, with no supplemental phytase or P, resulted in significantly lower (P < 0.05) egg production and body weight compared with all other dietary treatments. Feed consumption, feed efficiency, and egg yield were subsequently depressed by 32 wk of age in hens fed the 0.10% AP diet. No other significant differences among treatments were observed for performance averaged over the entire 40-wk experimental period. The mean daily AP intake of hens fed the 0.15% AP, 0.45% AP, and 0.10% AP + phytase diets was 155, 474, and 103 mg, respectively. The results of this study indicate that phytase improves the utilization of P in corn-SBM diets for laying hens and that corn-SBM diets containing 0.10% AP + 100 units of phytase/kg diet or 0.15% AP supported egg production performance that was not significantly different (P > 0.05) from that of hens fed a corn-SBM diet containing 0.45% AP.

Three chick experiments and two cecectomized rooster experiments were conducted to determine P bioavailability and amino acid (AA) digestibility in two low phytate corns (LP), a high protein corn (HP), and a corn containing both low phytate and high protein content (HP/LP) compared with conventional corn (CONV). From 8 to 20 or 21 d of age, 1-wk-old New Hampshire x Columbian chicks were fed a cornstarch-dextrose-soybean meal (SBM) basal diet containing 0.10% available P or the basal diet supplemented with two concentrations of P (0.05 or 0.06% and 0.10 or 0.12%) from KH2PO4 or two concentrations of the corns (20 or 21% and 40 or 42%). Bioavailability of P based on tibia bone ash was much higher for LP than for CONV; values ranged from 21 to 40% for CONV and from 59 to 95% for LP. Digestibility of AA in cecectomized roosters indicated no significant differences (P > 0.05) between CONV and HP in the first rooster experiment. Digestibilities of eight AA, including lysine, methionine, and arginine, in LP and HP/LP were higher (P < 0.05) than those in CONV in the second rooster experiment. The results of this study indicated that the P in LP was two to three times more bioavailable than the P in CONV and that the digestibilities of AA in HP/LP were equal to or higher than those in CONV.