| Abstract: | In Exp. 1, 2 continuous culture fermentation trials were used to determine rumen microbial volatile fatty acid (VFA) production in the presence of monensin (trial 1; 0 and 20 ppm) or lasalocid (trial 2; 0 and 20 ppm) with two levels of Mg (.07 and .14%) and K (.6 and 2.5%) in a 2 X 2 X 2 factorial arrangement using eight 500-ml continuous culture fermentors. Digesta was collected every 2 h (20 h total) for VFA analysis on d 6. Propionate (mol/100 mol) increased (trial 1) 14% when monensin was fed with.07% Mg and 32% (P<.001) when fed with.14% Mg compared to diets without monensin. In trial 2, lasalocid lowered (P<.001) butyrate (mol/100 ml) 35% when fed with.07% Mg and 53% when fed with.14% Mg. Lasalocid increased (P<.10) propionate concentration 30% when fed with.6% K and 41% when fed with 2.5% K compared to diets without lasalocid. In Exp. 2, 16 wether lambs (avg wt, 30 kg) were fed lasalocid (0 and 20 mg/kg) and Ca (.4 and 1.6%) in a 2 X 2 factorial arrangement and used in two trials to determine Ca, P, Mg, Na, K and Zn balance. The lambs (blocked by weight) were fed an oat hay diet (.44% Ca; ad libitum) and a corn-soybean meal supplement (350 g/head daily) containing the two levels of lasalocid for an 8-d adaptation period. There was a 3-d stall adjustment, 4-d feces, orts and urine collection, and serum and ruminal fluid collected at 0, 4 and 8 h postfeeding. During trial 2, lambs were fed the level of Ca they did not receive in trial 1. Lambs fed 1.6% Ca supplement without lasalocid had higher (P<.05) serum, ruminal fluid, fecal Ca, lower (P<.05) serum, ruminal fluid inorganic P, and 10% increase in serum Zn concentrations compared to those fed.4% Ca. The 1.6% Ca supplements with lasalocid increased Mg retention by 42% and decreased {P<.01) serum Na concentrations. Lasalocid increased (P<.01) ruminal fluid K concentrations and absorption. In Exp. 3, 32 mature, pregnant beef cows grazing oat pastures were used to determine the effects of Ca and lasalocid on serum minerals and ruminal VFA concentrations. The cows (blocked by breed) were randomly assigned to eight, 2-ha pastures containing oat forage and bled by jugular puncture every 28 d. Mineral supplements containing Ca (6 or 18%), Mg (4%) and P (12%) were assigned to the pastures in a 2 x 2 factorial arrangement and cows fed .25 kg of ground corn with or without 200 mg of lasalocid daily. Forage samples were taken every 28 d. Daily mineral supplement intake for all cow groups was low (<10 g/d) and variable. Calcium supplementation did not alter VFA concentrations during the study. By March 11, lasalocid decreased (P<.05) acetate and butyrate (mol/100 mol) which decreased (P<.05) the A:P ratio. In conclusion, these results suggest that the efficacy of monensin and lasalocid in altering the VFA concentrations of rumen fermentation might be enhanced by optimizing other nutrients and that lasalocid is effective in altering ruminal VFA concentrations in cows grazing small grain forages. |
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