The role of rumen fill in terminating grazing bouts of dairy cows under continuous stocking

This experiment was aimed at studying the role of rumen fill in signalling the termination of grazing bouts of dairy cows using a combination of experimental and modelling approaches. The day was divided into three main periods (06:00-12:00 h, 12:00-18:00 h and 18:00-24:00 h) during which the three main grazing bouts (dawn, afternoon and dusk) of dairy cows usually occur. Four rumen-cannulated dairy cows were used in a repeated measurements design, with time of day as the within subjects factor. The cows were rumen-evacuated at 6:00, 12:00, 18:00 and 23:30 h. Data on grazing behaviour of the cows were obtained using jaw recorders fitted to the cows. Fractional clearance rate (Kcl) was estimated from the difference in rumen fill between 23:30 and 8:00 h next morning, during which period cows were deprived of feed. Because rumen evacuations were performed at fixed times during the day and not immediately when grazing ceased, the fluctuation in rumen fill between the measured points had to be estimated in order to draw valid conclusions concerning its role in regulating the cessation of grazing. To estimate rumen fill fluctuation, a simple dynamic model was constructed. There is one state variable, the rumen neutral detergent fibre (NDF) pool. The model was based on the measured total eating time, the exact timing of each individual grazing event, bite rate, bite mass and Kcl. The diurnal fluctuation in rumen pool sizes of dairy cows was predicted satisfactorily. The mean squared prediction error (MSPE) was small (5.1% of observed mean), and the major part of the error was due to random variation. The simulated pool sizes indicated that cows stopped grazing during the morning and afternoon at a point where rumen pool size was much lower than what the rumen could hold at 23:30 h. This illustrated that dairy cows interrupted grazing bouts at dawn and in the afternoon long before reaching their maximum rumen fill capacity.