The limited capacity for weanling pigs to secret HCl in the stomach may be exacerbated by inclusion of ingredients with high acid binding capacity such as limestone and monocalcium phosphate. As a consequence, reducing the amount of these 2 ingredients in diets for weanling pigs may contribute to a stable low pH for proper pepsin activity and increased action of microbial phytase. Inclusion of high doses of phytase that results in increased phytate degradation and increased release of Ca, P, and inositol may also be beneficial to newly weaned pigs. Therefore, an experiment was conducted to test the hypothesis that lowering dietary Ca and P reduces gastric pH and diarrhea of weanling pigs, but microbial phytase may overcome negative effects of low Ca and P on growth performance and bone ash.

Experimental Procedures

A total of 320 weanling pigs (6.35 ± 0.87 kg body weight) were allotted to 8 corn-soybean meal-based diets in a randomized complete block design with 5 pigs per pen and 8 replicate pens per diet. Two phase 1 (d 1 to 14) control diets contained 100 or 50% of total Ca and digestible P relative to the requirement, and 6 diets in which 500, 2,000, or 16,000 phytase units per kg of feed (FTU) was added to each control diet were formulated. In the 6 diets that contained phytase, provisions of total Ca and digestible P were reduced by 0.16% and 0.11%, respectively, to account for the expected release of Ca and P by phytase. Common diets without phytase were fed in phases 2 (d 15 to 27) and 3 (d 28 to 42). Fecal scores were recorded in phase 1 and growth performance data were recorded within each phase. Plasma samples were collected from 1 pig per pen at the beginning and the end of each phase, and the same pig was used for blood collection throughout the experiment. From 1 pig per pen, stomach pH was measured on d 14 and the right femur was collected from one pig per pen on d 14 and 42. Data were analyzed using contrast statements in SAS.

Results

In phase 1, lowering Ca and P did not reduce gastric pH or fecal score, but pigs fed the diets containing only 50% of required Ca and P had reduced (P < 0.05) average daily gain (ADG) and average daily feed intake (ADFI) compared with pigs fed diets with 100% of the required Ca and P (Table 1). Regardless of dietary Ca and P, phytase above 500 FTU increased (P < 0.05) gain:feed ratio (G:F) and tended (P < 0.10) to reduce gastric pH of pigs. From d 1 to 42, pigs fed the diets in phase 1 with only 50% of the required Ca and P tended (P < 0.10) to have reduced ADG and ADFI compared with pigs fed the diets in phase 1 that contained 100% of required Ca and P. However, among pigs fed the 100% diets, there was a tendency (P < 0.10) for a linear increase in G:F as inclusion of dietary phytase increased. At d 14 and 42, pigs fed deits in phase 1 with 50% of required Ca and P had reduced (P < 0.05) bone ash compared with pigs fed the 100% diets, but phytase did not influence bone ash despite the fact that Ca and P were reduced in diets containing phytase. On d 14, phytase  inclusion resulted in increased (P < 0.05) plasma inositol in pigs regardless of the quantity of Ca and P in the diet, and in pigs fed the diets that contained Ca and P to meet the requirement, plasma inositol linearly increased (P < 0.05) with increasing levels of phytase. There was also an interaction (P < 0.05) between diet and day for inositol in plasma (Fig. 1). Regardless of Ca and P in the phase 1 diet, plasma concentrations of inositol on d 14 were much greater in pigs fed diets containing phytase compared with pigs fed diets without phytase, but that was not the case on d 27 and d 42. This observation indicates that during the initial 2 weeks after weaning, pigs were not able to synthesize sufficient quantities of inositol to maintain pre-weaning levels of plasma inositol but if phytase was added to the diets, pre-weaning levels of inositol were maintained.

Key points:

• Reducing Ca and P in diets for weanling pigs did not influence gastric pH or fecal score
• Reducing dietary Ca and P in phase 1 diets resulted in reduced growth performance and bone ash.
• Regardless of dietary Ca and P, high doses of phytase increased inositol absorption and consequently G:F of pigs.
• Weanling pigs appear not to be able to synthesize enough inositol during the initial 2 weeks post-weaning to maintain pre-weaning inositol levels, but microbial phytase can help pigs maintain pre-weaning plasma phytase levels.
• Inositol may be a conditionally essential nutrient during the immediate post-weaning period.

Table 1. Growth performance, gastric pH, plasma inositol, and bone ash of pigs fed the experimental diets¹

¹Significance (P < 0.05) and tendency (P < 0.10) for contrasts were expressed by a letter as follows: a = 100% vs. 50%; b = 100%: control vs. 500 FTU; c = 100%: control vs. 2,000+16,000 FTU; d = 100% linear effect of phytase (500 to 16,000 FTU); e = 50%: control vs. 500 FTU; f = 50%: control vs. 2,000+16,000 FTU; g = 50% linear effect of phytase (500 to 16,000 FTU).

²Concentration of inositol in plasma samples.

Figure 1. Concentration of inositol in plasma of pigs fed diets formulated with 100 or 50% of the requirement for Ca and P and 0 (CON), 500, 2,000, or 16,000 FTU during d -1 to 42.