Effects of Monensin on Voluntary Intake, Eating and Ruminating Behavior and Ruminal Motility in Heifers Fed Corn Silage

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Abstract
Eating and ruminating behavior and associated ruminal motility of six heifers (1/4 Brahman × 1/4 Jersey × 1/2 Angus, 290 kg average weight) given ad libitum access to corn silage with or without 100 mg monensin·head·−1 ·d−1 were examined according to a two-period crossover design. There was no effect (P>.05) of monensin on level of intake, daily and unitary eating, ruminating and masticating times [min·g dry matter−1 ·(kg body weight.75)−1], duration or number of these activity periods, duration of main meals or latency time for onset of rumination following cessation of main eating activities. With the monensin treatment, daily numbers of normal boli and total boli were decreased (P<.05) and mean duration of one rumination bolus cycle was longer (P<.05). Analysis of covariance indicated relationships between intake of corn silage and duration of the main morning meal, duration or number of rumination boli and total ruminal contractions were affected (P<.01) by monensin. Frequency and unitary number of strong cranio-dorsal ruminal contractions were similar for both treatments. During eating, number of contractions per minute was about twice (2.55/min) that during idling and rumination activity (1.43/min and 1.22/min, respectively). The unitary daily number of contractions was negatively (P<.05) related to level of intake. Total daily ruminal contractions were slightly reduced (−3.96%, P>.05) by monensin. Results are interpreted to suggest that monensin indirectly affects rumination through a lowered motility and thereby affects turnover, gut fill and intake. Copyright © 1987. American Society of Animal Science . Copyright 1987 by American Society of Animal Science.