Effect of dietary Ca manipulation, anionic salts and supplemental vitamin D3 on calcium homeostasis of finishing steers

The objective of this study was to evaluate the effects of a low-Ca diet, followed by a high-Ca diet in combination with feeding of anionic salts and/or vitamin D₃ on Ca and acid-base homeostasis of finishing steers. Twenty Hereford steers (448 _ 26 kg) were individually penned and fed at 0800 and 1600 with a ration consisting of 90% barley grain-based concentrate and 10% barley silage (DM basis). The experimental protocol included a 19-d adaptation period, a 14-d period of low Ca feeding, a 10-d supplemental period and a 5-d withdrawal period. During the supplemental period, anionic salts (MgSO₄ and NH₄Cl) were incorporated into the diet at -1500 and -3000 mEq steer^-1d^-1 for 3 and 7 d, respectively. The cattle were also supplemented with one of four vitamin D₃treatments: 0, 0.6, 1.2 and 2.4 million IU (MIU) of vitamin D₃ steer^-1 d^-1. Blood samples were obtained for measurement of total and ionized Ca, parathyroid hormone (PTH) and calcitonin, vitamin D₃, 25(OH)D₃ and 1,25(OH)₂D₃. Feed intake was depressed (P < 0.05) by anionic salt feeding but not vitamin D₃ treatment. Cattle supplemented with vitamin D₃ had higher total (P < 0.05) and ionized (P < 0.05) serum Ca levels than the control cattle that were only supplemented with anionic salts. Higher (P < 0.04) ionized Ca levels were seen in cattle fed 2.4 versus those fed 0.6 MIU vitamin D₃. Maximum total serum Ca values were 10.11, 10.97, 11.43 and 12.24 mg dL^-1for the 0, 0.6, 1.2 and 2.4 MIU vitamin D₃ treatments, respectively. Respective maximum values for ionized Ca were 5.43, 5.90, 5.98 and 6.25 mg dL^-1. These values represent increases, relative to the adaptation period of 3, 12, 16 and 25% for total and 9, 18, 20 and 31% for ionized Ca for the 0, 0.6, 1.2 and 2.4 MIU D₃ treatments, respectively. Circulating concentrations of vitamin D₃, 25(OH)D₃, and 1,25(OH)₂D₃ were increased (P < 0.05) by vitamin D₃ feeding, however, PTH was decreased (P < 0.05), while calcitonin was not affected (P > 0.05). Anionic salt feeding induced (P< 0.05) a mild systemic acidosis. The results indicate that anionic salt feeding enhanced the response of serum Ca to vitamin D₃ supplementation. Higher serum Ca levels prior to slaughter may increase intra-muscular Ca levels and enhance postmortem activity of Ca-dependant proteases responsible for myofibril degradation. The results point to a dietary strategy that with further research may prove useful in enhancing beef tenderness. Of particular interest is that serum Ca levels were elevated when vitamin D₃ was supplemented at levels 25 to 50% or less than levels used in previous research.

Key words: Calcium homeostasis, vitamin D₃, anionic salts, beef tenderness.