High doses of phytase on growth performance and apparent ileal amino acid digestibility of broilers fed diets with graded concentrations of digestible lysine

Abstract

Two experiments of the same design were conducted to determine the influence of phytase on performance and apparent ileal digestibility (AID) of amino acids in broilers fed graded concentrations of digestible lysine (dgLys). Cobb 400, male broilers were allocated to 1 of 16 diets consisting of 4 basal diets formulated at 80, 88, 96, or 104% of the Cobb 400 dgLys requirements for each feeding phase. Phytase was included in each basal diet at 0, 750, 1,500, or 3,000 phytase units (FTU)/kg. In Exp. 1, 33 birds/pen from hatch to day 42 were fed a 2-phase feeding program with 12 replicate pens/diet. In Exp. 2, there were 25 birds/pen from hatch to day 21 and 8 replicate pens/diet. Data were analyzed as a 4 × 4 factorial and means separated using orthogonal contrasts. In Exp. 1, feed intake (FI) increased (quadratic, P < 0.05) as dgLys increased in the diet. Body weight gain (BWG) increased (quadratic, P < 0.05) as dgLys concentration or phytase dose increased in the diet. As phytase dose increased in the diet, feed conversion ratio (FCR) was improved in a linear or quadratic (P < 0.05) manner depending on the dgLys concentration of the diet (dgLys × phytase, P<0.05). In Exp. 2, FI linearly (P < 0.05) increased as dgLys increased in the diet. Increasing the concentration of dgLys or phytase in the diet increased (quadratic, P < 0.05) BWG and improved (quadratic, P < 0.05) FCR. The AID of most amino acids was influenced by a dgLys × phytase interaction (P < 0.05), except threonine, valine, tryptophan, serine, cysteine, or leucine (linear or quadratic effect of phytase, P < 0.05), where phytase improved the AID in birds fed diets containing 80, 88, or 96% of the dgLys requirement, but not birds fed 104%. The predicted dgLys requirement to maximize performance, carcass, and digestible lysine intake was 97.6 to ≥ 104%. The predicted dose of phytase to maximize BWG or FCR was between 1,990 and 2,308 FTU/kg, regardless of the dgLys concentration in the diet. The predicted dose of phytase to maximize carcass weight was between 1,527 and 2,658 FTU/kg of diet and to maximize breast weight was 0 to ≥ 3,000 FTU/kg diet, depending on the dgLys concentration in the diet. In conclusion, optimal performance in the absence of phytase could be achieved at much lower levels of lysine in the presence of phytase.

Figure 1.

Predicted effect of phytase dose and digestible lysine requirement on body weight gain (grams) of broilers from hatch to 42 d posthatch (Exp. 1). The model was determined as:!$y$ = −2,080 + 93.71!$x$– 0.4768!$x^2$+0.0599!$v$– 1.3982e-5!$v^2$R² = 0.59. P < 0.0001. Diets were formulated to contain digestible lysine at ● 80%, ■ 88%, ▲ 96%, or × 104% of the requirement according to Cobb 400 breed guidelines (VenCobb 400 Broiler Management Guide, Cobb-Vantress Inc., Siloam Spring, AR); this corresponds to 1.02, 1.13, 1.23 and 1.33% dgLys, respectively, in the starter diets and 0.84, 0.92, 1.01 and 1.09% dgLys, respectively, in the grower diets. The digestible lysine concentration to maximize body weight gain was predicted at 98.2% of the requirement (1.23% dgLys in the starter diet and 1.03% dgLys in the grower diet), regardless of the dose of phytase supplied in the diet. The dose of phytase to maximize body weight gain was predicted at 2,146 FTU/kg of diet, regardless of the digestible lysine concentration of the diet.

Figure 2.

Predicted effect of phytase dose and digestible lysine requirement on feed conversion ratio of broilers from hatch to 42 d posthatch (Exp. 1). The model was determined as: !$y$ = 3.7748– 0.0431!$x$ + 0.0002!$x^2$ – 3.5667e-5!$v$+ 7.7502e-9!$v^2.$R² = 0.69. P < 0.0001. Diets were formulated to contain digestible lysine at ● 80%, ■ 88%, ▲ 96%, or × 104% of the requirement according to Cobb 400 breed guidelines (VenCobb 400 Broiler Management Guide, Cobb-Vantress Inc., Siloam Spring, AR); this corresponds to 1.02, 1.13, 1.23, and 1.33% dgLys, respectively, in the starter diets and 0.84, 0.92, 1.01, and 1.09% dgLys, respectively, in the grower diets. The digestible lysine concentration to maximize (reduce) feed conversion ratio was predicted at 98.9% of the requirement (1.24% dgLys in the starter diet and 1.04% dgLys in the grower diet), regardless of the dose of phytase supplied in the diet. The dose of phytase to minimize feed conversion ratio was predicted at 2,308 FTU/kg of diet, regardless of the digestible lysine concentration of the diet.

Figure 3.

Predicted effect of phytase dose × digestible lysine concentration on carcass weight (grams) of broilers at 42 d posthatch (Exp. 1). The model was determined as !$y$ = 1,465 + 7.9611!$x$ + 0.3794!$v$ – 2.9318e-5!$v$² – 0.0028!$xv$. R² = 0.14. P < 0.0001. Diets were formulated to contain digestible lysine at ● 80%, ■ 88%, ▲ 96%, or × 104% of the requirement according to Cobb 400 breed guidelines (VenCobb 400 Broiler Management Guide, Cobb-Vantress Inc., Siloam Spring, AR); this corresponds to 1.02, 1.13, 1.23 and 1.33% dgLys, respectively, in the starter diets and 0.84, 0.92, 1.01, and 1.09% dgLys, respectively, in the grower diets. Carcass weight was greatest (maximum) in birds fed 104% of the digestible lysine requirement (1.33% dgLys in the starter diet and 1.09% dgLys in the grower diet) with 1,527 FTU of phytase/kg diet. At 80, 88, or 96% of the digestible lysine requirement (1.02 or 0.84% dgLys, 1.13 or 0.92% dgLys, or 1.23 or 1.01% dgLys in the starter or grower, respectively) the predicted dose of phytase required to maximize carcass weight was 2,658, 2,281, or 1,904 FTU/kg of diet, respectively.

Figure 4.

Predicted effect of phytase dose × digestible lysine concentration on breast meat weight of broilers at 42 d posthatch (Exp. 1). The model was determined as: !$y$ = 218 + 5.2906!$x$ + 0.1405!$v$ – 0.0014!$xv$R² = 0.18. P < 0.0001. Diets were formulated to contain digestible lysine at ● 80%, ■ 88%, ▲ 96%, or × 104% of the requirement according to Cobb 400 breed guidelines (VenCobb 400 Broiler Management Guide, Cobb-Vantress Inc., Siloam Spring, AR); this corresponds to 1.02, 1.13, 1.23, and 1.33% dgLys, respectively, in the starter diets and 0.84, 0.92, 1.01, and 1.09% dgLys, respectively, in the grower diets. The predicted optimum breast weight was achieved at ≥ 104% of the digestible lysine requirement (> 1.33% dgLy in the starter and > 1.09% dgLys in the grower) with 0 FTU of phytase/kg diet. At 80, 88 or 96% of the digestible lysine requirement (1.02 or 0.84% dgLys, 1.13 or 0.92% dgLys, or 1.23 or 1.01% dgLys in the starter or grower, respectively) the predicted dose of phytase to maximize breast weight was ≥ 3,000 FTU/kg of diet.