Electromyographic evaluation of masseter muscle activity in horses fed different types of roughage

The aim of the study was to obtain information about masseter muscle activity, the main determinant of salivary flow during the chewing process. Four horses were offered the following diets: cracked corn (dry matter intake (DMI): 0.26% of BW), hay ad libitum (DMI: 2.8±0.5% of BW); haylage ad libitum (DMI: 2.4±0.4% of BW) and a straw/alfalfa chaff ([SAC] (DMI: 3.1±0.5% of BW). Feed intake time (min/kg) and chewing frequency (chews/kg) were recorded by direct observation or by using a modified halter. The activity of the masseter muscle was determined by EMG (IED®) and the following measurements made: Amplitude (muscle action potential = MAP, expressed in V) and duration of MAP (expressed in sec). Feed intake time and chewing frequency for roughages were different compared to cracked corn in the following order: haylage > hay > SAC > cracked corn. The intake of hay or haylage was associated with intense masseter muscle activity (MAP: hay 9.82±1.7 V, haylage 11.4±3.3 V, duration MAP: hay 0.31±0.04 sec, haylage 0.30±0.04 sec). Similar intense chewing was measured for the SAC (MAP 12.6±3.8 V), although duration of the chewing cycle was relatively short (0.22±0.03 sec) which is possibly related to limited fibre length. In contrast to roughages, concentrates are consumed rapidly, with less intense masseter muscle activity as reflected by the low amplitude of EMG (MAP 4.9±1.5 V). This is associated with low salivary flow rates that are likely to negatively affect gastric digestion