Effects of mineral supplementation on growing cattle and in vitro fermentation by ruminal microbes

Abstract

Three studies were conducted to assess effects of mineral supplementation on growing cattle performance, mineral status, and in vitro fermentation. Exp. 1 was a 3-part study that measured effects of Cu source and concentration on in vitro fermentation by mixed ruminal microbes. An initial in vitro experiment was performed to identify a Cu concentration (0, 100, 200, 300, 400, or 500 mg Cu/kg substrate DM) that would yield a 50% decrease in gas production. This concentration (100 mg Cu/kg substrate) was then used to evaluate varying Cu sources in the 3rd part of Exp.1. Titration of Cu (0, 10, 20, 30, 40, 50, 60, and 70 mg Cu/kg DM substrate) linearly decreased (P < 0.01) in vitro gas production, acetate, and propionate production. Inhibition of ruminal fermentation by Cu sources (CuSO₄, CuCl₂, CuCO₃, CuO, and tribasic copper chloride) also was evaluated using an in vitro fermentation system. Sources were incorporated into cultures at 100 mg Cu/kg substrate DM, a concentration great enough to elicit an inhibitory response. Copper sulfate and CuCl₂ were more inhibitory to in vitro fermentation, as indicated by decreases in gas production, VFA, and IVDMD, and increases in pH (P < 0.01). In Exp. 2, heifers were fed 3 different free-choice minerals: salt (S), a dry mineral basemix with salt (M), and a cooked molasses block (B); M and utilized the identical basemix. Mineral source had no effect on DMI, G:F, or concentrations of plasma P and Zn (P > 0.10). Average daily gain was greatest for M (P = 0.03), and not different between S and B (P = 0.98). Liver Cu concentrations were different among treatments (P < 0.01), with M having the greatest, B intermediate, and S having the least. Total dietary mineral intake also was different among treatments (P < 0.01), and was greatest for M, intermediate for B, and the least for S (P < 0.01). Experiment 3 × 4 factorial design and evaluated minerals added as different supplement types and trace mineral concentrations (0, 1, 5, or 10×) in an in vitro batch culture fermentation. Cooked molasses mineral blocks were compared to a dry mineral premix, and a dry mineral premix + molasses block added separately. In vitro fermentation was not different between the two molasses block treatments (P > 0.01); however, addition of molasses blocks increased fermentation to a greater extent than dry mineral alone (P < 0.01). Increasing trace mineral concentration decreased fermentation linearly (P < 0.01). In conclusion, excesses of trace elements can adversely affect fermentation by ruminal microbes. Mineral status in growing cattle was reflective of mineral intake; however, block supplements may be a method to control mineral intake to minimize overconsumption.