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

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 D₃concentrations increased (P < 0.05) in a quadratic fashion over time with vitamin D₃supplementation, 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.

Key words: Calcium homeostasis, vitamin D₃, cattle, beef tenderness.