Vitamin D regulates a wide spectrum of genes responsible for Ca and P homeostasis and cell differentiation. Cholecalciferol, commonly known as vitamin D₃, is a primary source of vitamin D₃ in diets for growing pigs; however, it needs to be hydroxylated twice to be active. The first hydroxylation occurs in the liver at the 25-position, resulting in 25-hydroxycholecalciferol [25(OH)D₃], whereas the second hydroxylation occurs in the kidneys at the 1-position, resulting in 1,25 dihydroxycholecalciferol [1,25(OH)₂D₃], which is the active form of vitamin D₃ in the body. Supplementation of 25(OH)D₃ to diets for sows in late gestation may increase the apparent total tract digestibility (ATTD) and retention of Ca and P, but there are no data to demonstrate this effect in growing pigs.

Cereal grains commonly used in diets for pigs have low digestibility of P because P is bound to phytate. Exogenous phytase increases the digestibility of both Ca and P in pigs by releasing the P from the phytate molecule within the gastrointestinal tract of pigs. However, there is limited information about the interaction between 25(OH)D₃ and supplemental phytase in diets fed to growing pigs. Therefore, the objective of this experiment was to test the hypothesis that both 25(OH)D₃ and microbial phytase independently and in combination may increase standardized total tract digestibility (STTD) of Ca and P by growing pigs.

Experimental design

Sixty growing male pigs with a body weight of 25.98 ± 2.01 kg were used. Pigs were housed individually in metabolism crates and allotted to one of five diets in 3 blocks. Each block had 20 pigs (4 pigs per diet) for a total of 12 replicate pigs per treatment for the 3 blocks.

A positive control diet that met all nutrient requirements for growing pigs was formulated. A negative control diet with reduced dietary concentrations of Ca and P was also formulated and three additional diets in which 25(OH)D_3, microbial phytase, or both 25(OH)D₃ and microbial phytase were added to the negative control diet were also formulated. The concentration of vitamin D₃ was identical among diets.

Daily feed allotments were provided in two daily meals that were fed at 0730 and 1530 h. The daily feed allowance was 3.2 times the maintenance requirement for metabolizable energy. Diets were fed for 13 days with the initial 7 days being the adaptation period and feces being collected from the feed provided during the following 4 days according to the marker-to-marker procedure. Collected fecal samples were stored at -20 ^oC as soon as collected and at the conclusion of the experiment, samples were thawed, ground, and analyzed for dry matter (DM), Ca, and P.

The ATTD of DM, Ca, and P in experimental diets was calculated based on intake and fecal output of DM, Ca, and P. The ATTD of Ca and P were then corrected for basal endogenous losses of Ca and P using standard values, and STTD of Ca and P in each diet was calculated.

The statistical model included diet as fixed effect and block and replicate pig within block as random effects. Least squares means were calculated using the LSMeans statement in SAS. Single degree of freedom contrasts were used to detect differences between the positive and the negative control diets as well as effects of microbial phytase and 25(OH)D₃.

Results

Feed intake of pigs was not affected by dietary treatments (Table 1). There were no interactions between 25(OH)D₃ and microbial phytase for Ca intake, Ca absorbed, Ca excreted in feces, ATTD of DM, or ATTD or STTD of Ca. The ATTD and STTD of P were increased (P < 0.05) by 25(OH)D₃ if phytase was not included in the diet, but that was not the case if microbial phytase was used (interaction; P < 0.001). The ATTD of DM was not affected by supplemental 25(OH)D₃ or microbial phytase, but the ATTD of DM in the positive control diet was less (P = 0.030) than in the negative control diet.

 Intake of Ca and absorbed Ca were greater (P < 0.05) in the positive than in the negative control diet. Excretion of Ca in feces was not different between pigs fed positive and negative control diets, but fecal Ca excretion was less (P < 0.05) from pigs fed the diets containing 25(OH)D₃ or microbial phytase compared with the positive control diet. Likewise, excretion of Ca was less (P < 0.05) from pigs fed one of the two diets with phytase compared with all other diets.

The ATTD and STTD of Ca were not different between the positive and the negative control diets, but the STTD of Ca increased (P < 0.05) if 25(OH)D₃ was added to the diet and both ATTD and STTD of Ca increased (P < 0.05) if phytase was used. Intake of P was less (P < 0.05) for the negative control diet than for the positive control diet, and P excreted in feces was greater (P < 0.05) for the positive control diet than for all other diets. Absorbed P was not different between the positive control diet and the 2 phytase diets, but the negative control diet and the 25(OH)D₃ diet had reduced (P < 0.05) absorption of P compared with the other diets. The ATTD and STTD of P were greater (P < 0.05) in pigs fed diets with phytase compared with diets without phytase, and diets containing 25(OH)D₃, tended (P < 0.10) to have greater ATTD and STTD of P than the diets without 25(OH)D₃.

Key points

• Supplementation of diets for growing pigs with microbial phytase increased STTD of Ca and P.
•  Supplementation of diets with 25(OH)D₃ tended to increase the STTD of Ca in diets.  
• The STTD of P was increased by 25(OH)D₃, if phytase was not used whereas 25(OH)D₃ did not increase STTD of P in diets containing phytase.
• STTD of Ca and P was greater in diets containing microbial phytase than in the control diet.
• Under the conditions of this experiment, the effects of 25(OH)D₃ and microbial phytase were not additive.

Table 1. Apparent total tract digestibility (ATTD) of dry matter (DM), and ATTD and standardized digestibility (STTD) of Ca and P in experimental diets fed to growing pigs^1,2

^a-dWithin a row, means without a common superscript differ (P < 0.05); the statistical model included diet as fixed variable.

¹n = 12.

²Values for the STTD of Ca were calculated by correcting the ATTD of Ca with the average BEL of Ca (i.e., 433 mg/kg DM intake, Lee and Stein, 2023); values for the STTD of P were calculated by correcting the ATTD of P with the average BEL of Ca (i.e., 190 mg/kg DM intake; NRC, 2012).

³NC = negative control; BEL = basal endogenous loss; BEL was calculated by multiplying daily DM intake of pigs by BEL of Ca or P.

⁴Vit D = effects of 25(OH)D₃; Phy = effects of microbial phytase; Vit D × Phy = interaction between 25(OH)D₃ and microbial phytase.

⁵Although the model P-value was significant, none of the pairwise comparisons were significant.