Discipline: digestion; Keywords:  diurnal variation, enzymatic activity, digestibility, rumen fluid, dairy cows

There is reason to believe that diurnal patterns for enzymatic activity exist, as bacterial numbers have been shown to be influenced by time of feeding. Also, when performing ruminal in vitro studies, little attention is given to the time of day when rumen fluid is sampled, thereby not controlling variation caused by diurnal patterns and the effect it might have on starch, NDF, and protein digestibility. Additionally, often results are compared between in vitro studies without considering time of rumen fluid collection or feeding time. It was therefore hypothesized by the authors cited below that rumen enzymatic activities in dairy cows are characterized by diurnal patterns, and thus, the objective of their study was to determine whether diurnal patterns in starch, NDF, and protein digestibility and amylolytic, fibrolytic, and proteolytic activity exist in the rumen fluid of the cows.

Rumen fluid was collected from four ruminally cannulated Holstein dairy cows before the morning feeding and subsequently every four hours for a 24-hour period. Two of the cows were restricted from feed for eight hours overnight, and the other two continued to receive their feed ad libitum, to isolate and quantify the effects of changes in feeding behaviour at night. After two runs the cows were crossed over between night feeding treatments. Rumen fluid was analysed for enzymatic activity and in vitro starch, NDF, and nitrogen digestibility. Circadian rhythm analyses of enzymatic activity and in vitro digestibility were conducted by fitting an applicable function to a 24-hour period.

Trends were observed in activity for amylase, lichenase, endoglucanase, and xylanase, with the highest activities observed at the time points subsequent to milking and feed delivery. Protease activity was unaffected by either feeding treatment or possible feeding behaviour. When the data were fitted to the function, all the parameters tested followed a daily trend that was sensitive to the overnight availability of feed, although the parameters responded differently to the feeding treatment. The patterns displayed by in vitro digestibility results of starch, NDF, and nitrogen, across the various fluid collection time points, were highly variable. The time at peak observed in the enzymatic analysis did not correspond to those observed in the in vitro analysis.

The results suggest that different interpretations should be given to enzymatic activities and in vitro digestibility values, and the time of rumen fluid collection relative to feeding time should be considered and reported when rumen fluid is used for research or commercial purposes. Maximum digestibility appears in fact to be reached around four to five hours after the main ration delivery for NDF and starch and around ration delivery time for protein.