Calcium and phosphorus in late-gestation

By Su A Lee and Hans H. Stein

Absorption of calcium (Ca) and phosphorus (P) from the small intestine and renal reabsorption of Ca and P are regulated by parathyroid hormone, calcitonin and calcitriol (i.e., 1,25-dihydroxycholecalciferol or 1,25-dihydroxy-vitamin D3), which is the active form of vitamin D. Vitamin D in commercial vitamin premixes is provided mostly by one of the vitamin D3 forms (i.e., cholecalciferol).To be utilized in the body, cholecalciferol needs to be hydroxylated in two steps. In the first step, vitamin D3 is hydroxylated at the 25 position to yield 25-hydroxycholecalciferol (25-OH-D3).In the second step, 25-OH-D3 is hydroxylated at the 1-position to yield 1,25-dihydroxycholecalciferol. One-alpha-hydroxycholecalciferol (1-α-OH-D3) is a vitamin D analog that does not require the second hydroxylation step for vitamin D3 to be active.It is possible that supplementation of diets with 1-α-OH-D3 increases the conversion efficiency to 1,25-dihydroxycholecalciferol compared with using traditional cholecalciferol, which may increase Ca and P balance by increasing intestinal absorption and renal reabsorption, but this hypothesis has not been experimentally verified.It is also possible that 1-α-OH-D3 increases absorption and retention of Ca and P, but this may work differently with different levels of Ca and P in late gestating sows. However, data to demonstrate this have not been reported.Increasing ATTD, retaining calcium and phosphorus
Therefore, the objective of this experiment was to test the hypothesis that supplementation of 1-α-OH-D3 to diets for gestating sows containing Ca and P at or below the requirement will increase apparent total tract digestibility (ATTD) and retention of Ca and P as well as the ATTD of gross energy (GE), digestible energy (DE) and metabolizable energy (ME).A total of 36 gestating sows, parity 2 to 6, were fed experimental diets from days 91 to 105 of gestation. Sows were housed individually in metabolism crates and randomly allotted to four diets. Two diets were formulated to contain 75 or 100% of the required Ca and P with a Ca to P ratio of 1.3:1.0 in both diets.Two additional diets were formulated by adding 12.5 mg/kg of 1-α-OH-D3 product premix (Savint, Iluma Alliance, Durham, NC) to each of the initial diets. Concentrations of 1-α-OH-D3 in the two diets were 4.96 and 3.46 μg/kg, respectively.Feed was provided daily in two meals and a feeding level equivalent to 1.5 times the daily maintenance ME requirement was used.Fecal and urine samples were collected quantitatively, and the ATTD and retention of Ca and P in experimental diets were calculated based on intake and fecal and urine output of Ca and P. The ATTD of GE and concentrations of DE and ME in diets were also calculated.No interactions between Ca and P concentrations and use of supplemental 1-α-OH-D3 were observed for any of the response variables (See Table 1- page 3). The ATTD of DM and concentrations of DE and ME in diets, the ATTD of Ca and P, and retention of Ca and P were not affected by the level of Ca and P in the diets.Although feed intake was not different among treatments, fecal excretion was less (P = 0.001) from sows fed diets supplemented with 1-α-OH-D3 compared with sows fed diets with no 1-α-OH-D3, which resulted in greater (P = 0.003) ATTD of DM in sows fed diets supplemented with 1-α-OH-D3 compared with sows fed no supplemental 1-α-OH-D3. Concentrations of DE and ME in diets, the ATTD of Ca and P, and retention of Ca and P by sows were greater (P < 0.05) in sows fed diets with supplemental 1-α-OH-D3 compared with sows fed no supplemental 1-α-OH-D3.In conclusion, the ATTD and retention of Ca and P and concentrations of DE and ME in diets fed to sows in late gestation were not affected by dietary Ca and P, but supplementation of 1-α-OH-D3 increased the ATTD of Ca and P by almost 100%. The increased ATTD of DM and GE resulted in increased DE and ME of approximately 100 kcal/kg in diets containing 1-α-OH-D3 compared with diets without 1-α-OH-D3.

Lee is a postdoctoral research associate and Stein is a professor of animal science, both at the University of Illinois.

Item	Ca and P levels¹Normal Low		1-α-OH-D₃, mg/kg diet 0 12.5		SEM	P-value Ca,P levels Vit D₃
Feed intake, kg/d	2.95	2.95	2.98	2.93	0.05	0.997	0.223
Fecal excretion, kg/d	0.33	0.36	0.37	0.31	0.06	0.096	0.001
Urine excretion, kg/d	13.00	10.89	11.31	12.58	3.40	0.327	0.554
ATTD of DM, %	87.56	86.38	85.92	88.02	2.17	0.076	0.003
Ca balance
ATTD of Ca, %	24.35	20.78	14.76	30.37	12.17	0.407	0.001
Ca retention, g/d	3.02	4.13	2.30	4.85	2.24	0.212	0.007
Ca retention, % of intake	19.74	18.64	12.60	25.78	11.97	0.796	0.005
P balance
ATTD of P, %	26.50	25.13	19.47	32.16	11.16	0.731	0.003
P retention, g/d	2.71	3.06	2.15	3.62	1.52	0.541	0.016
P retention, % of intake	25.22	20.23	17.18	28.27	11.52	0.223	0.010
Energy concentrations
ATTD of GE, %	86.66	85.96	85.07	87.54	2.3	0.291	0.001
DE, kcal/kg	3331	3300	3257	3374	90	0.227	<0.001
ME, kcal/kg	3,244	3,215	3,180	3,280	98	0.357	0.003
¹Normal level of Ca and P = 100% of the requirement for late gestation sows; low level of Ca and P = 75% of the requirement for late gestation sows (NRC, 2012).