| Summary: | Supplementation of high levels of vitamin D<sub>3</sub> (D<sub>3</sub>) to cattle has enhanced beef tenderness. However, high levels of D<sub>3</sub> possess a food safety concern. Therefore, the objective of this thesis was to evaluate feeding strategies that would reduce the amount of D<sub>3</sub> required to increase serum calcium (Ca) to levels where tenderness has been improved. In experiment 1, 15 steers (452 ±30kg) were fed with 75% barley grain-base concentrate and 25% barley silage (as fed) and 1.25,2.5 or 5 MIU D<sub>3</sub>/hd/d for 7 d. Feed intake was depressed (P<0.05) while total (T) and ionized (I) Ca were increased (P<0.05) in relationship to the amount of D<sub>3</sub> fed. Feeding 5 MIU D<sub>3</sub> increased TCa and ICa by 2 and 1 mg/dl, respectively. Peak values were reached 3-5 d post-supplementation. Serum Ca levels were similar to those related with meat tenderization. In experiment 2, 15 steers (607 ± 12 kg) were supplemented with 0, 2.5 or 5 MIU D<sub>3</sub>/hd/d for 7 d. Feed intake, TCa, and ICa showed similar trends as in experiment 1. Plasma D<sub>3</sub>and 25(OH)D<sub>3</sub> were increased (P<0.05) quadratically and linearly, respectively. Parathyroid hormone (PTH) was depressed (P<0.05). In experiment 3, 19 steers (522 ± 33 kg) were fed with a low Ca diet for 10 d prior to supplementation with 0, 1.25, 2.5 or 5 MIU D<sub>3</sub>/hd/d for 7 d. Blood TCa, ICa, and D<sub>3</sub>decreased (P<0.05) during the first 2-4 d of the low Ca period. All variables responded as in experiment 2. Plasma 1,25(OH)<sub>2</sub>D<sub>3</sub> was increased (P<0.05). Calcium response was no higher. In experiment 4, 20 steers (448 ± 26 kg) were fed a low Ca for 14 d and then switched to a high Ca diet and anionic salts (NH<sub>4</sub>CI and MgS0<sub>4</sub>) at -1500 and -3000 mEq/hd/d for 3 and 7 d, respectively, and fed with 0, 0.6, 1.2 or 2.4 MIU D<sub>3</sub>/hd/d (10 d). All variables responded as in experiment 3. Similar total and ionized serum Ca levels were achieved with 30 to 80 % less D<sub>3</sub>, indicating that such a feeding strategy may be a successful approach to improving beef tenderness.
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