Calcium requirements by pigs are expressed as total Ca because of a lack of data for the digestibility of Ca in feed ingredients, but it is believed that a ratio between standardized total tract digestible (STTD) Ca and STTD P is a more appropriate way to express requirements for Ca by pigs. Values for Ca digestibility in different Ca-containing feed ingredients were recently generated using diets without or with microbial phytase, which allowed for the formulation of diets based on STTD Ca values. A number of experiments were also conducted to determine STTD Ca to STTD P requirements to optimize growth performance and bone mineralization of pigs from 11 to 25 kg, 25 to 50 kg, 50 to 85 kg, and 100 to 130 kg. However, these experiments were performed independently and in experiments lasting only 3 to 5 weeks. Therefore, a follow-up experiment was conducted to test the hypothesis that the requirement for Ca expressed as a ratio between STTD Ca and STTD P obtained in short-term experiments may be applied to pigs fed diets without or with microbial phytase from 11 to 130 kg.
Experimental Procedures
In a 5-phase program, 160 pigs (initial body weight: 11.2 ± 1.8 kg) were randomly allotted to 4 corn-soybean meal based diets in a 2 × 2 factorial design with 2 diet formulation principles (total Ca or STTD Ca), and 2 phytase inclusion levels [0 or 500 phytase units/kg of feed (FTU)]. There were 32 pens and 5 pigs per pen for a total of 8 replicate pens per diet. In each phase, one diet was formulated based on the requirement for total Ca and STTD P. The second diet was formulated based on a ratio between STTD Ca and STTD P of 1.40:1, 1.35:1, 1.25:1, 1.18:1, and 1.10:1 for phases 1, 2, 3, 4, and 5, respectively, and STTD P was at the requirement. A third diet was formulated based on the requirement for total Ca and STTD P, but 500 FTU of microbial phytase (Quantum Blue; AB Vista Feed Ingredients, Marlborough, UK) were included assuming that phytase released 0.11% STTD P and 0.16% total Ca. The last diet also contained microbial phytase (500 FTU) and it was assumed a release of 0.11% STTD P by phytase. However, Ca was included to meet the above ratios between STTD Ca and STTD P and STTD Ca values for each ingredient were based on values previously determined in the presence of phytase. Pigs were allowed ad libitum intake of feed and at the end of each phase, feed left in feeders and BW of pigs were recorded to calculate growth performance parameters. At the end of phase 1 (d 24), 1 pig per pen was euthanized and a blood sample and the right femur were collected. A blood sample was collected from the same pig in each pen at the end of phases 2 to 5. At the conclusion of the experiment (d 126), all pigs were euthanized, but from 1 pig per pen, the right femur was collected and carcass characteristics were also determined in this pig.
Results
No interactions between diet formulation principle and phytase inclusion were observed. Likewise, no differences among treatments were observed for growth performance (at any phase or the entire experimental period) or carcass characteristics (Table 1). Bone ash was not affected by dietary treatments at the end of phase 1, but on d 126, pigs fed non-phytase diets based on total Ca had greater bone ash than pigs fed STTD Ca based diets, but if phytase was used, no differences were observed between diet formulation principles (interaction P < 0.05; Table 2). The reason for this observation most likely is that the Ca requirements to maximize bone ash are greater than the requirements to maximize growth performance of pigs and STTD Ca requirements used in diet formulations were based on the requirements to maximize growth performance.
At the end of phase 1, 4, and 5 there were no differences in plasma Ca and P among dietary treatments (Table 3). However, at the end of phases 2 and 3, pigs fed diets without phytase had greater (P < 0.05) concentration of plasma P than pigs fed diets with phytase, which may indicate that the balance between Ca and P was more ideal in diets with phytase, which subsequently resulted in more of the absorbed P being used for bone tissue synthesis.
Key points
- The ratios between STTD Ca and STTD P obtained in previous experiments can be used to formulate diets for growing-finishing pigs without affecting growth performance or carcass characteristics of pigs.
- Inclusion of 500 FTU of microbial phytase in diets results in release of more Ca than P, which subsequently increases bone tissue synthesis.
Table 1. Growth performance (d 1 to 126) and carcass characteristics (d 126) of pigs fed diets formulated based on total Ca (tCa) or standardized total tract digestible Ca (dCa), without microbial phytase or with 500 units of microbial phytase per kg of feed (FTU)1
1Data are least squares means of 8 observations.
2BW = body weight; ADG = average daily gain; ADFI = average daily feed intake; G:F = gain to feed ratio; HWC = hot carcass weight; CY = carcass yield; BFT = back fat thickness; LEA = loin eye area; CL = carcass lean.
Table 2. Concentration of ash in femurs collected at the end of phase 1 (d 24) and 5 (d 126) from pigs fed diets formulated based on total Ca (tCa) or standardized total tract digestible Ca (dCa), without microbial phytase or with 500 units of microbial phytase per kg of feed (FTU)1
a-bMeans within a row lacking a common superscript letter are different (P < 0.05).
1Data are least squares means of 7 or 8 observations.
Table 3. Concentration of Ca and P in plasma of pigs fed diets formulated based on total Ca (tCa) or standardized total tract digestible Ca (dCa), without microbial phytase or with 500 units of microbial phytase per kg of feed (FTU)1,2
a-bMeans within a row lacking a common superscript letter are different (P < 0.05).
1Data are least squares means of 7 or 8 observations.