EFFECTS OF INGESTIVE AND RUMINATIVE MASTICATION ON DIGESTION OF FORAGE BY CATTLE

The effects of chewing during eating and rumination on digestion of forage harvested in early and middle stages of maturity were studied. The experiment was conducted as a 4 X 4 Latin square using four Hereford yearling heifers. Heifers received once daily 5.45 kg of dry matter (DM) from: (1) alfalfa, 10% bloom; (2) alfalfa, 50% bloom; (3) orchardgrass, anthesis; (4) orchardgrass, post-anthesis. On a DM basis, crude protein and neutral detergent fibre (NDF) contents were: 21, 37%; 16, 47%; 17, 56%; and 14, 59% for the early, mid-bloom alfalfa, early and mature orchardgrass, respectively. Unmasticated forages, sampled using a hay corer, and masticated forages collected from the oesophageal fistula of each animal during periods of eating and rumination, were incubated in the rumen in sacco for predetermined intervals. Digestion of unmasticated alfalfa was initially rapid, whereas digestion of unmasticated orchardgrass was more gradual but continued over a longer period such that the extent of DM digestion in the rumen was greater (P < 0.01) for grass compared with alfalfa. Potential DM digestibility of both forage species declined (P < 0.01) with increased maturity. Potential DM digestibility of the forages was relatively high (more than 63%) and was increased (P < 0.05) by 1.5-7.0 units by ingestive mastication. After ingestive mastication, the proportion of DM in the soluble fraction was increased relative to the slowly digestible fraction, but rate of digestion was not affected. The potentially digestible NDF fraction increased after chewing during eating, and lag time prior to the onset of fibre digestion decreased, but rate of fibre digestion was not altered. Digestion kinetics of ruminated digesta varied considerably among animals, and kinetics were altered minimally after chewing during a single rumination event. It was concluded that chewing during eating increases rumen degradation of forage by increasing the potentially digestible DM and NDF fractions and by decreasing the lag time for fibre digestion, but not by altering rate of digestion. Chewing during a single rumination event has minimal effects on digestion kinetics of ruminated digesta.