Effect of supplementing high levels of vitamin D3 on calcium homeostasis of steers fed barley-based finishing diets

Abstract

The objective of this study was to evaluate the effect of supplementing high levels of vitamin D3 to steers fed barley-grain-based finishing diets on Ca metabolism. Fifteen Hereford steers (607 ± 12 kg) were individually penned and fed at 0800 and 1600 with a ration consisting of 90% barley-grain-based concentrate and 10% barley silage [dry matter (DM) basis]. Steers were randomly assigned to one of three treatments: 0, 2.5 or 5 million IU (MIU) of vitamin D3 steer-1 d-1 for 7 d. Daily blood samples were obtained for measurement of serum for total and ionized Ca, parathyroid hormone (PTH) and calcitonin and plasma for vitamin D3 and 25(OH)D3. Data were analyzed by repeated measures analysis and single degree of freedom contrasts. Feed intake was depressed relative to control animals by 18 and 37% for the 2.5 and 5 MIU D3 treatments, respectively. Relative to controls, total and ionized Ca increased (P < 0.05) by 8 and 19% and 6 and 18% for the 2.5 and 5 MIU D3 treatments, respectively. Serum PTH concentrations were reduced (P < 0.05) by vitamin D3 feeding with the greatest reduction seen with animals fed the 5 MIU treatment. Calcitonin values were not (P > 0.05) affected. Relative to controls, plasma vitamin D3 concentrations increased (P < 0.05) in a quadratic fashion over time with vitamin D3 supplementation, while that of 25(OH)D3 increased (P < 0.05) in a linear fashion. It was concluded that the increase in serum calcium was stimulated by a rise in plasma vitamin D3 and the resulting increased 25(OH)D3 concentrations, which have been shown to influence calcium absorption either directly or via induced synthesis of 1,25(OH)2D3.