EFFECTS OF PREGNANCY ON DIGESTA KINETICS AND RUMINAL FERMENTATION IN BEEF-COWS

Four pregnant and four nonpregnant, ruminally cannulated beef cows wer e used to evaluate the effects of the last trimester of pregnancy on d igesta kinetics and ruminal fermentation. Before breeding, cows were a llotted randomly either to pregnant (PR) or to nonpregnant (NP) groups ; PR cows were bred at the first estrus after synchronization. All cow s were fed long-stem fescue hay at 15 g of DM/kg of BW based on BW 120 d before parturition of pregnant cows (average BW of 642 kg). Collect ion periods, each lasting 9 d, began at 96, 68, 41, and 10 +/-1.3 d be fore parturition. Particulate passage rate was greater ( P < .0 5) and retention times and gastrointestinal fill were less (P < .05) for PR than for NP cows. Apparent total tract DM digestion, rate and extent ( 96-h in situ incubation) of NDF disappearance, and ruminal fluid kinet ics were not affected (P > .10) by pregnancy. A physiological state x sampling day interaction was noted (P < .05) for ruminal NH3 N, total VFA concentrations, and molar proportions of butyrate. Ruminal pH and individual VFA proportions, however, did not differ (P > .10) between PR and NP cows, except for valerate, which was less (P < .05) for PR t han NP cows. Results suggest that in late pregnancy with restricted fe eding, passage rate of particulates increases without substantial chan ges in fermentation or extent of digestion.