Electromyographic evaluation of masseter muscle activity in horses fed (i) different types of roughage and (ii) maize after different hay allocations

The aims of this study were to monitor electromyographic (EMG) activity of masseter muscle in healthy horses fed (i) different types of roughage and (ii) maize after different hay allocations. Four horses were offered the following three diets ad libitum: hay, haylage or straw/alfalfa chaff (SAC). In a second trial, four horses were fed cracked maize (CM) and hay in three different orders: (i) CM after a 12-h overnight fast; (ii) CM immediately after restricted hay intake (0.6kg hay/100kg BW); or 3) CM after hay intake ad libitum. The activity of the masseter muscle was determined by EMG (IED (R)), and the following were measured: amplitude (muscle action potential=MAP, maximum voltage) and duration of MAP (s). The intake of hay or haylage was associated with intense masseter muscle activity (MAP: hay, 10 +/- 1.7V; haylage, 11 +/- 3.3V; and duration of MAP: hay, 0.31 +/- 0.04s; haylage, 0.30 +/- 0.04s). Similar intense chewing was measured for SAC (MAP 13 +/- 3.8V), although duration of the chewing cycle was relatively short (0.22 +/- 0.03s, diet p<0.05), which is possibly related to the shorter fibre length. CM was consumed rapidly, with less intense masseter muscle activity (MAP 6.0 +/- 1.5V). Hay intake before CM did not affect chewing force of CM, but duration of chewing cycle was significantly prolonged by feeding hay ad libitum before CM was fed. The consumption of hay, haylage or SAC was associated with intensive masseter muscle activity that was likely to stimulate salivary flow rate. In contrast to roughage, concentrates like CM are consumed rapidly with less intensive masseter muscle activity. This situation is associated with a low salivary flow that may have an adverse effect on gastric function.