Digestibility of prairie hay diets supplemented with different levels of magnesium-mica by beef heifers

Four ruminally fistulated, nonpregnant, nonlactating heifers (515 +/- 7.9 k g) were offered chopped (10-cm screen size) prairie hay for ad Libitum cons umption with 1.8 kg/d of supplements to provide 0, 45, 113, or 181 g/d of m agnesium-mica (MM) in a 4 x 4 Latin square design experiment. Heifers were adapted to diets for 7 d in drylot followed by a 19-d confinement period in individual stalls within a metabolism facility. Total feces and urine were collected far 5 d, daily intake and water consumption were monitored for 1 6 d, and in situ disappearance of fiber (prairie hay) and CP (soybean meal) were determined during the confinement period. Ruminal samples for ferment ation product analyses were collected at feeding and every 2 h thereafter f or 12 h on d 18, and rumens were evacuated at 1100 on d 19 to determine rum en fill. Cubic responses (P < .05) to MM level were observed for DM, OM, ND F, and ADF digestion and were characterized by a tendency for improved dige stion with 45 g/d compared with the control diet, followed by a suppressive effect on digestion with 113 g/d, then little effect with 181 g/d MM. In s itu NDF disappearance at 72 and 96 h tended (P < .10) to be influenced quad ratically by level of MM, but MM level had no apparent effect on ruminal fi ll and passage rate, ciliated protozoa counts, in situ rate of disappearanc e of fiber or CP, or nitrogen balance. Therefore, feeding MM at low levels to heifers consuming a prairie hay diet with a 20% CP supplement seemed to be beneficial for feed digestion with no measurable negative impact on inta ke or ruminal protein or fiber digestion.