Effects of supplemental zinc and manganese on ruminal fermentation, forageintake, and digestion by cattle fed prairie hay and urea

One in vitro and one in vivo metabolism experiment were conducted to examin e the effects of supplemental Zn on ruminal parameters, digestion, and DMI by heifers fed low-quality prairie hay supplemented with urea. In. Exp. 1, prairie hay was incubated in vitro for 24 h with five different concentrati ons of supplemental Zn (0, 5, 10, 15, and 20 ppm) and two concentrations of supplemental Mn (0 and 100 ppm), both provided as chloride salts. Added Mn increased (P < 0.02) IVDMD, but added Zn linearly decreased (P < 0.03) TVD MD. Added Zn tended to increase the amount of residual urea Linearly (P < 0 .06) at 120 min and quadratically (P < 0.02) at 180 min of incubation, alth ough added Mn counteracted these effects of added Zn. Six 363-kg heifers in two simultaneous 3 x 3 Latin squares were fed prairie hay and dosed once d aily via ruminal cannulas with urea (45 or 90 g/d) and with Zn chloride to provide the equivalent of an additional 30 (the dietary requirement), 250, or 470 ppm of dietary Zn. After a 7-d adaptation period, ruminal contents w ere sampled 2, 4, 6, 12, 18, 21, and 24 h after the supplement was dosed. S upplemental Zn did not alter prairie hay DMI (mean = 4.9 kg/d) or digestibi lity, although 470 ppm added Zn tended to decrease (P < 0.06) intake of dig estible DM, primarily due to a trend for reduced digestibility with 470 ppm supplemental Zn. Zinc x time interactions were detected for both pH (P = 0 .06) and NH3 (P = 0.06). At 2 h after dosing, ruminal pH and ruminal ammoni a were linearly decreased (P < 0.05; P < 0.01) by added Zn. At 5 h after fe eding, ruminal pH was linearly increased (P < 0.05) by added Zn, suggesting that added Zn delayed ammonia release from urea. The molar proportion of p ropionate in ruminal fluid was linearly and quadratically increased (P < 0. 02; P < 0.01) whereas the acetate:propionate ratio was linearly and quadrat ically decreased (P = 0.02; P < 0.05) by added Zn. Through retarding ammoni a release from urea and increasing the proportion of propionate in ruminal VFA, Zn supplementation at a concentration of 250 ppm may decrease the like lihood of urea toxicity and increase energetic efficiency of ruminal fermen tation.