LAUSR

Twelve Angus steers (BW 452.8 ±21.8 kg) fitted with ruminal cannulae were used to determine the impact of trace mineral source on neutral detergent fiber (NDF) digestibility, short chain fatty acid (SCFA) production, and ruminal solubility of Cu, Mn, and Zn. Steers were fed a low-quality grass hay diet (10.8% CP, 63.1% NDF, 6.9 mg Cu/kg, 65.5 mg Mn/kg, and 39.4 mg Zn/kg) supplemented with protein for 21 d. Treatments consisted of 20, 40, and 60 mg supplemental Cu, Mn, and Zn/kg DM, respectively from either sulfate (STM) or hydroxy (HTM) sources (n = 6 steers/treatment). Following a 21-d adaptation period total fecal output was collected for 5 d. Dry matter digestibility tended (P < 0.07) to be reduced (51.9 vs. 53.4 ±0.52%) and NDF digestibility was reduced (P < 0.04; 40.4 vs. 42.7 ±0.67%) in STM vs. HTM steers. On d 6, rumen fluid was collected at 0, 2, and 4h post feeding. Steers receiving HTM had lesser (P < 0.02) butyric acid and greater (P < 0.05) total SCFA than STM steers. Steers were then fed the same grass hay diet without supplemental Cu, Zn, or Mn for 14 d. On d 15 steers received a pulse dose of 20, 40, and 60 mg Cu, Mn, and Zn/kg DM, respectively from either STM or HTM. Ruminal samples were obtained at 2h intervals starting at -4 and ending at 24h relative to dosing. Ruminal soluble mineral concentrations were greater (P < 0.05) for Cu at 4, 6, 8, 10, 12, and 14h; for Mn at 4 and 6h; and for Zn at 4, 6, and 8h post dosing in STM compared to HTM steers. Results indicate that ruminal solubility of Cu, Mn, and Zn differs between STM and HTM sources and rumen Cu, Mn, and Zn solubility may impact ruminal fermentation.