Comparisons between species and seasons in nitrogen (N) retention, rumen ammonia concentration, rumen volatile fatty acid (VFA) proportions, and water flows along the digestive tract were measured with red deer, goats, and sheep fed a chaffed lucerne hay diet ad libitum. Measurements were made in summer (S) and winter (W). Rumen ammonia kinetics were measured in the three species during W only using continuous infusions of15N. Red deer showed greater N retention in S than in W (P 0.05), whereas goats showed no seasonal trends (P 0.05) in N retention, with sheep being intermediate. Sheep and goats showed no (P 0.05) seasonal differences in rumen ammonia (NH3-N) concentration (158#x2013;181 mg N/litre), whereas deer showed an increase from W to S (110 versus 172 mg N/litre;P 0.01). The irreversible loss rate (IRL) of NH3-N from the rumen (mg N/g N intake) during W was in the order goats sheep deer. Sheep showed no seasonal differences in rumen VFA patterns, whereas both deer (P 0.05) and goats (P 0.05) showed greater acetate/propionate (Ac/Pr) ratios in S than in W. In S, the Ac/Pr ratio was in the order deer sheep goats whereas there were no species differences (P 0.05) in W. There were no (P 0.05) seasonal differences in drinking water or in total water intakes (g/g DMI or g/kgW1.0per day) in deer, goats, and sheep. Sheep showed no significant seasonal differences (P 0.05) in rumen water outflow or rumen net water balance (g/kgW1.0per day). In contrast, deer and goats showed greater (P 0.01) rumen water outflow, net rumen water balance (including saliva flow), and intestinal water absorption in S compared to W. It was calculated that both recycling of N to the rumen and rumen NH3absorption during W were in the order goats sheep deer. It was concluded that the high rumen NH3IRL in goats may be a factor in their superior rumen fibre digestion, especially when consuming low-quality roughage diets, and that high Ac/Pr ratios in deer may be a contributing cause of their increased metabolic heat production during S.
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