Several experiments were conducted to estimate Ca digestibility in different feed ingredients in the presence or absence of microbial phytase to allow formulation of diets for pigs to be based on standardized total tract digestible (STTD) Ca instead of total Ca. Thus, 4 experiments aimed at determining Ca requirements expressed as a ratio between STTD Ca and STTD P in pigs from 11 to 130 kg. A follow-up study was later conducted to validate those data and to evaluate the effect of using ratios that maximize growth performance on bone development because maximum bone ash requires more Ca than maximum growth performance. However, data indicate that STTD Ca to STTD P ratios to maximize Ca retention are greater than to maximize bone ash synthesis. The use of STTD Ca to STTD P ratios in diet formulation may result in a reduction in excess dietary Ca, which is beneficial because excess dietary Ca is detrimental to P digestibility and growth performance of pigs. Therefore, the objective of this experiment was to test the hypothesis that formulating diets for growing pigs based on a ratio between STTD Ca and STTD P instead of total Ca and STTD P does not decrease Ca retention, but increases P utilization.

Experimental Procedures

Forty barrows [body weight (BW): 59.4 ± 3.8 kg] were allotted to 4 corn-soybean meal based diets and 2 periods of 11 d in a randomized complete block design. Pigs were individually housed in metabolism crates and there were 5 replicate pigs per diet in each block. Diet formulation was based on a 2 × 2 factorial design with 2 Ca requirement estimates (total Ca or STTD Ca) and 2 inclusion levels of microbial phytase [0 or 500 units/kg feed (FTU)]. Phytase (Quantum Blue; AB Vista Feed Ingredients, Marlborough, UK) was assumed to release 0.11% STTD P and 0.16% total Ca. The total Ca diets were formulated based on requirements for total Ca and STTD P whereas a ratio between STTD Ca and STTD P of 1.25:1 was used in diets formulated based on STTD Ca. The first 5 days of each period were considered the adaptation period to the diet and fecal and urine samples were collected from feed provided on day 6, 7, 8, and 9 following the marker-to-marker approach. Diet, fecal, and urine samples were analyzed for Ca and P to calculate values for the apparent total tract digestibility (ATTD) and retention of Ca and P.

Results

Phytase increased (P < 0.05) the ATTD of Ca in both total Ca and STTD Ca diets. Regardless of phytase, Ca excretion in urine was lower (P < 0.05) from pigs fed STTD Ca diets than from pigs fed total Ca diets, but no differences in Ca retention were observed between pigs fed STTD Ca diets and pigs fed total Ca diets (Table 1). In the absence of microbial phytase, the ATTD of P and the amount of P excreted in urine was greater (P < 0.05) in pigs fed STTD Ca diets than in pigs fed total Ca diets (Table 2). However, in the presence of microbial phytase, no differences in the ATTD of P or the excretion of P in urine were observed between the 2 Ca requirement estimates. Phosphorus retention was greater (P < 0.05) in pigs fed diets without phytase than in pigs fed diets with phytase, but no differences in P retention between pigs fed STTD Ca or total Ca diets were observed.

Key points

• Pigs fed STTD Ca diets excrete less Ca in urine than pigs fed total Ca diets because diets formulated based on STTD Ca contain less Ca.
• The retention of Ca by pigs is not affected if diets are based on STTD Ca.
• Formulating diets in absence of phytase and based on values for STTD Ca result in increased P absorption, which confirms the negative effect of excess dietary Ca on P digestibility.
• The retention of P by pigs does not improve if diets are based on STTD Ca.

Table 1. Calcium balance for pigs fed diets formulated based on requirements for total Ca or standardized total tract digestible (STTD) Ca, without or with 500 units of microbial phytase per kilogram of feed (FTU)^1,2

^a-cMeans within a row lacking a common superscript letter are different (P < 0.05).

¹Each least squares mean represents 10 observations.

²Values for daily intake and output are the average of a 4-d collection.

³Percentage of intake.

Table 2. Phosphorus balance for pigs fed diets formulated based on requirements for total Ca or standardized total tract digestible (STTD) Ca, without or with 500 units of microbial phytase per kilogram of feed (FTU)^1,2

^a-cMeans within a row lacking a common superscript letter are different (P < 0.05).

¹Each least squares mean represents 10 observations.

²Values for daily intake and output are the average of a 4-d collection.

³Percentage of intake.