Quantitative analysis of nycterohemeral eating and ruminating patterns in heifers with different voluntary intakes and effects of monensin.

The nycterohemeral pattern of eating and ruminating behavior was examined in six heifers given ad libitum access to a corn silage-based diet with or without 100 mg monensin.hd-1.d-1 in a two-period crossover design. Rhythm components (no. of cycles/24 h) were characterized by the finite Fourier transform of 24-h mastication activities series measured over 7 d. Analysis of variance of the daily mean of hourly activities and rhythm components 1 to 12 showed significant effects of heifer (H), monensin treatment (T), period (P) and T x P x H interactions. A reparameterization of the finite Fourier transform yielded the amplitude and the phase for each rhythm component and allowed the plot of periodograms and phase diagrams, respectively. Rhythm components 1, 2 and 3 contributed primarily in explaining the total dispersion of 24-h eating and ruminating mastication series. The major effect of monensin was to increase the phase at rhythm component 1, delaying by 1 h in the onset of rumination after the morning feeding. Heifer effects were significant and appeared related to variations in daily voluntary intake (VI) of individual heifers. Heifer 5, with the largest VI, had two main eating periods and the highest rate of eating. Heifer 4, with the smallest VI, partitioned mastication activity throughout the day into four main eating and ruminating periods. Differences among animals in their nycterohemeral chewing behavior patterns may be related to differences in forestomach structure associated with a larger VI capacity by animals having mastication rhythm components 1 and 3.