PREGNANCY AND LACTATION IN BEEF HEIFERS GRAZING TALLGRASS PRAIRIE IN THE WINTER - INFLUENCE ON INTAKE, FORAGE UTILIZATION, AND GRAZING BEHAVIOR

Six ruminally and eight bifistulated (ruminal and esophageal) Hereford x Angus heifers were used to determine effects of pregnancy and early lactation on forage intake and utilization under grazing conditions. Three ruminally and three bifistulated heifers were bred (pregnant/lactating; P/L; average calving date was February 11, 1989); remaining heifers served as controls (C). The experiment consisted of three periods (P1 = average of 55 d before parturition; P2 = average of 12 d before parturition; P3 = average of 26 d after parturition). All heifers grazed the same 24-ha Flint Hills range pasture. Dehydrated alfalfa pellets were supplemented at .5% BW/heifer daily prepartum and at .8% BW/heifer daily postpartum. Treatment x period interactions were noted (P < .10) for forage OM intake, ruminal capacity, indigestible ADF (IADF) fill, and grazing time but not for OM digestibility or IADF passage rate (P > .10). In P1 and P3, P/L heifers had greater (P < .10) forage OM intake than C heifers, whereas no differences were noted (P > .10) in P2. Pregnant/lactating heifers spent less time (P < .10) grazing than did controls in P2 and more time (P < .10) than controls in P3. Ruminal IADF fill and ruminal capacity, as measured by water fill, tended (P = .14 and .16, respectively) to differ between the two groups in P2. Digestibility of OM was unaffected (P > .10) by physiological status, whereas IADF passage rate was greater (P < .10) in P/L heifers. Only minor differences were noted for the chemical composition of diets selected by the two groups. Ruminal fermentation patterns shifted only slightly; the largest effects were in P3, when P/L heifers had greater (P < .10) propionate and less (P < .10) acetate (mol/100 mol) than C heifers. In summary, during the period just before parturition, differences in forage OM intake between P/L and C heifers disappeared, and ruminal fill and capacity tended to be lower for P/L heifers. Intake was 16% greater for P/L than for C heifers during the early postpartum period. Oragnic matter digestibility was not influenced by physiological status, even though IADF passage rates were greater for P/L heifers.