EFFECT OF DRAFT WORK ON PERFORMANCE AND METABOLISM OF CROSSBRED COWS .2. EFFECT OF WORK ON ROUGHAGE INTAKE, DIGESTION, DIGESTA KINETICS ANDPLASMA METABOLITES

Forty F-1 crossbred dairy cows (20 Friesian X Boran and 20 Simmental X Boran) in a 2 X 2 factorial arrangement (not working-not supplemented , NWNS; not working-supplemented, NWS; working-not supplemented, WNS a nd working-supplemented, WS) were used to study the effect of work and diet supplementation on roughage intake, apparent digestibility of nu trients, digesta kinetics and plasma metabolites. Dry matter (DM) and organic matter (OM) intakes were greater for working than for non-work ing cotes. Work and supplementation increased DM and OM in vivo appare nt digestibility. This effect was particularly significant when compar ing working cotes at vest and at work. Passage rate and rumen mean ret ention of Cr-mordanted hay were sin milar for working and for non-work ing cows. Liquid turn-over rate of Co-ethylene diamine tetra acetic ac id (EDTA) was lower and rumen mean retention time of Co-EDTA was great er in working than in non-working cows whether supplemented or not. Wo rking cows had lower plasma glucose during and immediately after worki ng than non-working cows. Plasma non-esterified fatty acid (NEFA) was higher in working cows at the end of the working period and after vest between working hours. This effect was greater in non-supplemented th an in supplemented cotes. In working cows, beta-hydroxybutyrate was lo wer at the end of working hours than in non-working cows. Lactate was greater in working than in non-working cows after the 3rd and 4th hour s of work. Digesta kinetics could explain only partially the possible mechanisms responsible for greater roughage intake and apparent digest ibility in working cows. The decrease of plasma glucose and the increa se in NEFA during work indicates an increasing utilization of NEFA by muscle during consecutive working hours.