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-1 d-1 for 3 and 7 d, respectively. The cattle were also supplemented with one of four vitamin D3 treatments: 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-1 for 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