In plant-based feed ingredients there is a considerable amount of P bound to phytate, limiting the amount of P that is available for utilization, but inclusion of microbial phytase in pig diets increases the digestibility of P. The negatively charged phytate molecule can chelate Ca cations resulting in formation of insoluble Ca-phytate complexes. Degradation of phytate by microbial phytase may prevent formation of these non-digestible complexes, resulting in increased Ca digestibility. It is also possible that use of exogenous phytase reduces endogenous loss of Ca. If indeed the reduced endogenous loss of Ca is a result of degradation of phytate, it is expected that increased doses of dietary phytase will linearly reduce endogenous losses of Ca, but this hypothesis has not been experimentally verified. Therefore, this experiment was conducted to test the hypothesis that increasing dietary phytase reduces basal endogenous loss of Ca and increases digestibility of P in growing pigs.

Experimental design

Seventy barrows (initial body weight: 17.66 ± 1.69 kg) were allotted to seven Ca-free diets using a randomized complete block design with two blocks. Pigs were house individually in metabolism crates. There were five replicate pigs per diet in each block, for a total of 10 replicate pigs per diet. All diets were based on corn, potato protein concentrate, and full-fat rice bran. A positive control diet was formulated to contain no Ca but P was included to meet the requirement for standardized total tract digestible (STTD) P by 11 to 25 kg pigs; this diet did not contain microbial phytase. The negative control diet was identical to the positive control diet with the exception that the provision of digestible P was reduced by 0.15% unit. The 5 treatment diets were formulated by adding 250, 500, 1,000, 2,000, or 4,000 phytase units (FTU)/kg (a novel consensus bacterial 6-phytase variant, Danisco Animal Nutrition) to the negative control diet.

Pigs were fed experimental diets for 12 days with the initial 5 days considered the adaptation period to diets. Feces and urine samples were collected separately for 4 d following the adaptation period using the marker-to-maker procedure. Fecal and urine samples were stored at – 20 °C immediately after collection.

Data were analyzed using a model that included diet as fixed effect. Positive control and negative control diets were compared using a contrast statement and linear and exponential effects of microbial phytase were analyzed as well.

Results

Feed intake, weights of fecal and urine excretion, and the apparent total tract digestibility (ATTD) of dry matter were not affected by experimental diets. Calcium excretion in feces expressed in % and g/d was not different between the positive and the negative control diets, but Ca excretion in feces linearly (P < 0.05) decreased as phytase inclusion increased in the diets. Basal endogenous loss of Ca was not different between the positive control diet and the negative control diet. However, as phytase level increased in the diets, the basal endogenous loss of Ca exponentially decreased (P = 0.030) with the basal endogenous loss of Ca in the diet containing 4,000 FTU/kg being the least among all diets. Calcium excretion in urine expressed in % and in g/d was not affected by experimental diets, but Ca in urine tended (P = 0.06) to be less from pigs fed the negative control diet compared with pigs fed the positive control diet.

Phosphorus intake, concentration of P in feces, absorbed P, ATTD of P, and STTD of P were greater (P < 0.05) for pigs fed the positive control diet compared with the negative control diet. Concentration of P in the urine and P retention (g/d) were greater (P < 0.001) for pigs fed the positive control diet compared with the negative control diet. However, P retention calculated as % of intake and absorbed P was not different between the two diets.

As phytase levels increased in the diets, P excretion in feces, as expressed as % of feces or as g/d, exponentially decreased (P < 0.001), which resulted in an exponential increase (P < 0.001) in the ATTD and STTD of P. Retention of P (% of intake) was not affected by dietary phytase, but retention of P calculated as % of absorbed P linearly decreased (P = 0.006) as dietary phytase increased, which is likely because of the lack of Ca to support synthesis of bone tissue and as a consequence, the absorbed P could not be retained in the body.

Key points

• Concentration of P did not affect the basal endogenous loss of Ca in pigs fed Ca-free diets.
• Increasing concentrations of microbial phytase in diets reduced the basal endogenous loss of Ca demonstrating that without phytase in the diets, some of the endogenous Ca will be bound to phytate in the intestinal tract, but if phytase is added to the diets, the chelation of endogenous Ca to phytate can be prevented.
• Digestibility of P was increased by increasing phytase in Ca-free diets, but the extra absorbed P could not be retained in the body because of the lack of Ca to support bone tissue synthesis. Therefore, P retention calculated as a percent of absorbed P was reduced as microbial phytase increased in the Ca-free diets.

Table 1.  Basal endogenous loss of Ca and P-balance by pigs fed Ca-free diets

¹The exponential analysis did not provide data that fit the model.