Energy exchanges associated with eating and rumination in sheep given grass diets of different physical forms.

1. Energy exchanges and other physiological functions associated with eating and rumination were determined in four experiments. Sheep were given chopped, dried grass (DGC), pelleted, dried grass (DGP) or fresh grass (FGC). 2. In Expt I a preliminary study was made using all three diets. The dry matter (DM) of DGP was eaten significantly faster than that of chopped diets. Sheep salivated most during eating and ruminated longest when given DGC. Rates of contraction for the reticulo-rumen did not differ significantly between diets during idling and rumination, but were significantly faster during eating with DGP. The apparent energy costs of eating were 17, 109 and 176 kJ/kg DM eaten for DGP, DGC and FGC respectively, but these probably underestimated the true energy cost. 3. Expt 2 compared DGP and DGC at two levels of intake. The mean energy costs of eating DGP and DGC were 23.5 and 267 kI/kg DM respectively. The mean energy costs of eating between the energy cost of eating and the duration of the meal. The proportion of time the sheep spent ruminating DGC was about 23% but less than 1% for DGP. There was no significant relationship between heat production and the time spent ruminating. 4. In Expt 3 four sheep were offered fresh grass and, later, an equivalent DM intake after the material had been dried. The sheep ate the dried meal significantly faster. The mean energy costs of eating were 208 and 346 kJ/kg DM for DGC and FGC respectively. In this experiment the sheep ruminated significantly longer when given FGC, and the energy cost of rumination was 0.11 kJ/min. 5. Increases in heat production during and after fistula-feeding only 2-8% of those obtained during eating, indicating that nearly all the increase in heat production during eating could be attributed to the energy cost of eating per se. 6. The contribution of the energy costs of eating and rumination to the heat increment of feeding and the energy requirement for maintenance of sheep are discussed.