Weanling pigs have reduced secretion of HCl in the stomach needed for appropriate protein digestion. Therefore, acidifiers are sometimes used in weaning diets as alternatives to antibiotic growth promoters because these products may create a favorable environment in the stomach for proper pepsin activity. However, the presence of limestone and monocalcium phosphate (MCP) in phase 1 diets, which have a high buffer capacity, may contribute to the inability for pigs to secrete enough HCl in the stomach, and lowering the inclusion level of these ingredients in starter diets may be beneficial to young pigs. Therefore, the objective of this experiment was to test the hypothesis that reducing the amount of limestone and MCP in diets for weanling pigs, by lowering the concentration of dietary Ca and P and(or) by including microbial phytase in the diet, will reduce stomach pH and fecal score and therefore improve growth performance of pigs.

Experimental Procedures

A total of 160 newly weaned pigs (5.75 ± 1.04 kg of body weight) were allotted to 4 corn-soybean meal-based diets in a completely randomized design with 5 pigs per pen. The experiment was conducted for 5 weeks. A 3-phase feeding program was used with d 0 to 15 as phase 1, d 16 to 21 as phase 2, and d 22 to 35 as phase 3. Pigs were fed one of 4 diets during phase 1, whereas a common diet was fed in phase 2 and a common diet was also fed in phase 3. Diets were formulated using a 2 × 2 factorial design with 2 concentrations of Ca and P (100 or 50% of total Ca and digestible P requirements) and 2 inclusion levels of phytase [0 or 2,000 units of phytase/kg feed (FTU)]. Phytase was assumed to release 0.16% total Ca and 0.11% digestible P and Ca and digestible P were, therefore, reduced by these amounts in the phytase-containing diets. However, after removal of all MCP from the diet with 50% Ca and 2,000 FTU of phytase, digestible P was only reduced by 70% relative to the P requirement, which was the value obtained after MCP was fully removed. Fecal scores were recorded in phase 1 and on d 15, gastric pH was measured and the right femur was collected from 1 pig per pen. Growth performance data were recorded within each phase. Data were analyzed using the fixed effects of concentrations of Ca and P, phytase inclusion, and the interaction between concentrations of Ca and P and phytase inclusion, but if the interaction was not significant, only main effects were included in the model.

Results

For average daily gain (ADG) and gain to feed ratio (G:F) in phase 1, an interaction (P < 0.05) between concentrations of Ca and P and phytase inclusion was observed (Table 1). At 100% Ca and P requirement, no difference was observed in the ADG or G:F of pigs fed diets without or with 2,000 FTU. However, for pigs fed diets with 50% Ca and P relative to the requirement, the ADG and G:F were greater (P < 0.05) in diets containing phytase than in non-phytase diets. No difference between concentrations of Ca and P was observed for the G:F of pigs fed diets with phytase, but pigs fed diets without phytase had lower (P < 0.05) G:F if diets contained 50% Ca and P compared with pigs fed diets with 100% Ca and P. For phases 2 and 3 and for the overall experiment, no effect of Ca and P concentrations or phytase inclusion were observed.

There was no interaction between main effects for bone ash, stomach pH, or fecal score, and therefore, only main effects were included in the final model (Table 2). The concentration and percentage of bone ash was greater (P < 0.05) in pigs fed diets formulated to have 100% Ca and P than in pigs fed diets with 50% Ca and P. However, no effect of phytase inclusion on the concentration or percentage of bone ash of pigs was observed. There was no effect of dietary concentration of Ca and P on stomach pH of pigs, but pigs fed diets with 2,000 FTU tended (P < 0.10) to have reduced stomach pH compared with pigs fed diets without phytase. Pigs fed diets with phytase also tended (P < 0.10) to have lower fecal score and diarrhea frequency than pigs fed non-phytase diets, but no effect of Ca and P concentration on fecal score or diarrhea frequency was observed.

Key points:

• Average daily gain and G:F was improved by phytase in diets with low Ca and P.
• Pigs fed diets with phytase and low concentration of Ca and P had greater G:F than pigs fed this diet without phytase.  
• The reduction in growth performance that were induced by reducing the Ca and P in phase 1 diets did not extend to the following phases and for the overall experimental period, no differences were observed.
• Lowering Ca and P does not reduce stomach pH or fecal score, but compromises bone ash of pigs.
• Phytase inclusion appears to reduce diarrhea during the initial 14 days post weaning. 

Table 1. Growth performance of pigs fed diets formulated with 100 or 50% of the requirement for Ca and P (Req.) without microbial phytase or with 2,000 units of microbial phytase per kilogram of feed (FTU)¹

¹Data are least square means of 8 observations.

Table 2. Bone mineralization, stomach pH, feces evaluation, and serum metabolites from pigs fed diets formulated with 100 or 50% of the requirement for Ca and P (Req.) without microbial phytase or with 2,000 units of microbial phytase per kilogram of feed (FTU)

¹Data are least square means of 16 observations.

²Data are least square means of 13 to 16 observations.

³Average of in-situ and ex-situ stomach pH.

⁴Frequency = number of days with fecal score ≥ 3 ÷ total number of days × 100.