True digestibility

Determination of endogenous intestinal losses of calcium and apparent and true total tract digestibility of calcium in canola meal fed to growing pigs

When formulating diets for pigs, it is more accurate to use values for standardized or true nutrient digestibility than values for apparent nutrient digestibility because the former are additive in mixed diets. No values for standardized or true total tract digestibility of calcium in pigs have been reported. The true total tract digestibility (TTTD) of a nutrient is calculated by correcting apparently total tract digestibility (ATTD) by total endogenous losses, which may be estimated using a regression procedure. To our knowledge, no measurements of the endogenous loss of calcium in pigs have been reported. An experiment was, therefore,  performed to measure endogenous loss of calcium and to determine TTTD of calcium in growing pigs, and to investigate if  the addition of microbial phytase to the diets affects TTTD of calcium. In addition, calcium retention was measured in pigs fed diets containing varying levels of calcium with or without microbial phytase.

Experimental design

Forty-eight growing pigs with an average initial body weight of 16.72 kg were allotted to a randomized complete block design with 8 dietary treatments and 6 pigs per treatment.

Eight experimental diets were formulated based on sucrose, cornstarch, potato protein isolate, corn gluten meal, vitamins, minerals, and canola meal. Canola meal provided all calcium in the diets. Four diets containing 12.3, 24.7, 37.0, or 50.0% canola meal were formulated to contain 0.08, 0.16, 0.24, or 0.32% calcium, respectively. Four additional diets were formulated that were identical to the first four with the exception that they also contained 1,500 units per kilogram of microbial phytase (Quantum, AB Vista, Marlborough, UK). All diets were formulated to contain 0.32% digestible phosphorus, with all phosphorus being supplied by canola meal and monosodium phosphate.

Apparent total tract digestibility of calcium in canola meal

Values for apparent total tract digestibility (ATTD) of calcium, expressed as a percentage of calcium intake, were calculated by subtracting calcium excreted in the feces from calcium intake, dividing by calcium intake, and multiplying the result by 100.

The ATTD of calcium increased (P < 0.05) as the calcium level in the diets increased (Table 1). In addition, ATTD of calcium was greater (P < 0.01) when phytase was added to the diets than in the diets with no added phytase. There was no interaction between dietary calcium level and addition of phytase for  the measured values for ATTD; however, calcium output in feces increased (P < 0.01) as dietary calcium increased to a greater extent in the diets with no added phytase than in the diets in which phytase was used (interaction, P < 0.05).

Endogenous losses of calcium and true total tract digestibility

Endogenous losses of calcium were determined by regressing apparent total tract digested calcium against dietary calcium intake. The negative y-intercept of the regression represented the estimated total endogenous loss of calcium, and the slope of the regression represented the estimated true total tract digestibility. The regression equations for diets containing canola meal without and with added phytase are presented in Table 2. The estimated total endogenous loss of calcium was 0.160 g/kg dry matter intake for pigs fed diets with no added phytase, and 0.189 g/kg DMI for pigs fed diets containing microbial phytase. These values were not significantly different.

The true total tract digestibility of calcium in each diet was calculated by correcting the ATTD for total endogenous losses. The TTTD of calcium did not differ among diets containing different levels of calcium. The TTTD of calcium was greater (P < 0.01) for diets containing microbial phytase than for diets with no added phytase.

Calcium retention

Values for retention of calcium, expressed as a percentage of calcium intake, were calculated by subtracting total calcium excreted (in both the feces and urine) from calcium intake, dividing by calcium intake, and multiplying the result by 100. Calcium retention increased (P < 0.05) with increasing levels of calcium in the diets. Calcium retention was also greater (P < 0.01) when phytase was added to the diets than if no phytase was used. Calcium retention ranged from 9.75% in diets containing 0.08% calcium with no phytase, to 56.9% in diets containing 0.32% calcium with added phytase.

Key points

  • Values for the apparent total tract digestibility of calcium are influenced by the level of calcium in the diet. This indicates that there is a fecal loss of endogenous calcium from pigs. This endogenous calcium loss can be estimated using a regression procedure.
  • True total tract digestibility of calcium can be calculated by correcting ATTD for the endogenous calcium loss. Unlike values for ATTD, values for TTTD of Ca are not influenced by the levels of calcium in the diet.
  • Addition of 1500 units of microbial phytase per kilogram to the diet increases the digestibility of calcium in canola meal.
  • Calcium retention is improved by addition of microbial phytase to the diet.

 

Table 1. Calcium balance, apparent total tract digestibility (ATTD) and true total tract digestibility (TTTD) of calcium for pigs fed canola meal without or with microbial phytase at different levels of calcium

 

Canola meal without phytase

Canola meal with phytase

P-value

Item                   Ca, %:

0.08

0.16

0.24

0.32

0.08

0.16

0.24

0.32

Ca level

Phytase

Ca level ×

phytase

Feed intake, g/d

682

723

734

786

710

718

740

765

0.033

0.918

0.823

Ca intake, g/d

0.52

1.16

1.76

2.52

0.58

1.15

1.77

2.45

<0.01

0.990

0.767

Fecal Ca output, g/d

0.35

0.76

1.07

1.45

0.32

0.48

0.61

0.89

<0.01

<0.01

0.012

Urine Ca output, g/d

0.13

0.12

0.16

0.21

0.12

0.13

0.14

0.18

0.006

0.516

0.857

Absorbed Ca, g/d

0.18

0.40

0.69

1.07

0.26

0.67

1.17

1.56

<0.01

<0.01

0.011

Ca retention, g/d

0.06

0.27

0.53

0.87

0.14

0.54

1.03

1.37

<0.01

<0.01

<0.01

Ca retention, %

9.75

24.15

30.52

34.77

24.89

45.56

58.35

56.90

<0.01

<0.01

0.733

Ca excretion, g/d

0.47

0.88

1.23

1.65

0.44

0.61

0.75

1.08

<0.01

<0.01

0.015

Ca excretion, %

90.25

75.85

69.48

65.23

75.11

54.44

41.65

43.10

<0.01

<0.01

0.733

ATTD of Ca, %

33.71

34.65

39.60

42.96

45.89

57.30

65.91

64.19

0.030

<0.01

0.560

TTTD of Ca,1 %

53.95

44.65

46.28

47.93

65.96

67.34

72.59

69.18

0.862

<0.01

0.548

1TTTD of Ca was calculated by correcting the ATTD values for the average (0.175 g/kg) of the endogenous calcium loss estimated in the linear regression.

 

Table 2. Regression of apparent total tract digested calcium (g/kg DMI) on dietary calcium intake (g/kg DM)

Item

Regression equation

R2

Estimated TTTD of Ca, %

Estimated endogenous Ca loss, g/kg DMI

Canola meal

y = 0.4661x - 0.1598

0.85

46.6b

0.160

Canola meal + phytase

y = 0.7026x - 0.1892

0.92

70.3a

0.189

 a,b Means within a column with no common superscript are different (P < 0.05).

 

This report is based on unpublished research by J. C. González-Vega and H. H. Stein.

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