Standardized total tract digestibility (STTD) of Ca has been determined for most Ca containing ingredients fed to growing pigs, but the basal endogenous loss of Ca is greater and the STTD of Ca in sows in mid-gestation are less than in growing pigs. As a consequence, if diets for gestating sows are formulated using STTD values determined in growing pigs, the provisions of digestible Ca may be less than anticipated.

The efficacy of phytase to release Ca and P is believed to be influenced by the physiological state of pigs with phytase fed to sows in mid-gestation releasing less Ca and P compared with growing pigs or sows in late-gestation. However, it is not known if values for STTD of Ca or retention of Ca and P that are measured in one period of gestation are representative of the entire gestation period.

Therefore, an experiment was conducted to test the hypothesis that the STTD of Ca from calcium carbonate and the response to microbial phytase on STTD of Ca and apparent total tract digestibility (ATTD) of P in corn-based diets fed to gestating sows are constant throughout the gestating period for sows. The second objective was to test the hypothesis that retention of Ca and P does not change during gestation.

Experimental design

Thirty six gestating sows (parity = 3.3 ± 1.5; d of gestation = 7 d) were allotted to 3 blocks of 12 sows using a randomized complete block design. Four diets were fed to the 12 sows in each block; thus, there was a total of 9 replicate sows for each treatment. Experimental diets included a corn-based diet in which calcium carbonate was the sole source of Ca and a Ca-free diet. Each diet was prepared with no microbial phytase and also with addition of 500 units of phytase (Quantum Blue®; AB Vista, Marlborough, United Kingdom). Daily feed allowance was 1.5 times the maintenance energy requirement for gestating sows based on the body weight of sows. Daily feed allotments were provided in 2 equal meals. Water was available at all times.

Sows were housed individually in gestation stalls throughout gestation with the exception that from d 7 to 20, d 49 to 62, and again from d 91 to 104, sows were housed individually in metabolism crates, where they were fed 1 of the 4 experimental diets. Sows were fed the same experimental diet every time they were placed in the metabolism crates. However, when sows were placed in the gestation stalls during periods between housing in metabolism crates, they were fed a common conventional gestation diet that was formulated to meet the requirement estimates for all nutrients including Ca.

Every time sows were placed in the metabolism crates, feces and urine were quantitatively collected over a 4-d period following a 4-d adaptation period. The basal endogenous losses of Ca were calculated from sows fed the Ca-free diets and STTD of Ca, ATTD of P, and retention of Ca and P in the calcium-carbonate containing diets were calculated based on intake and fecal and urine output of Ca and P.

Results

Interactions between period of gestation and phytase were not observed. Supplementation of microbial phytase decreased (P = 0.002) the basal endogenous loss of Ca (Table 1). The basal endogenous loss of Ca was greatest (P < 0.05) by sows in early-gestation, followed by sows in mid- and late-gestation periods, respectively.

Supplementation of microbial phytase did not affect the ATTD of DM, the ATTD of Ca, the STTD of Ca, Ca retention, ATTD of P, or P retention by gestating sows fed diets formulated based on corn and calcium carbonate (Table 2). The ATTD of DM was not affected by the period of gestation. However, the ATTD of Ca and retention of Ca were least (P < 0.05) in mid-gestation, followed by early- and late-gestation, respectively and the STTD of Ca for sows in mid-gestation was also lower (P < 0.05) than for sows in early- or late-gestation. The ATTD of P was least (P < 0.05) in mid-gestation, followed by early- and late-gestation periods, respectively. Phosphorus retention was also greater (P < 0.05) in late-gestation than in the earlier periods.

Key points

• Supplementation of microbial phytase decreases the basal endogenous loss of Ca regardless of gestation period. The basal endogenous loss of Ca is greatest by sows in early-gestation, followed by sows in mid- and late-gestation periods, respectively.
• The response to microbial phytase on STTD of Ca and ATTD of P in corn-based diets fed to gestating sows is less predictable than in diets fed to growing pigs.
• Values for STTD of Ca, ATTD of P, and retention of Ca and P differ among sows in different trimester of gestation.
• To accurately predict Ca and P metabolism in gestating sows, it appears that it is necessary to assume different digestibility values for Ca and P in late-gestation compared with early- or mid-gestation.
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Table 1. Basal endogenous losses of Ca from early-, mid-, and late-gestating sows fed Ca-free diet

Table 2. Standardized total tract digestibility (STTD) of Ca and retention of Ca and P in experimental diets fed to gestating sows and growing pigs

^a-cWithin a row, means without a common superscript differ (P < 0.05).