SEASONALITY IN DIGESTION AND RUMEN METABOLISM IN RED DEER (CERVUS-ELAPHUS) FED ON A FORAGE DIET

Six adult castrated male red deer (Cervus elaphus), fitted with rumen cannulas, were offered chaffed lucerne hay ad lib. during winter and s ummer, with voluntary food intake (VFI) being respectively 59 and 89 g dry matter intake (DMI)/kg bodyweight 0.75 per d. The same animals we re also offered the same feed during summer, with intake restricted to that of winter VFI. The apparent digestibility of gross energy (0.60) or fibre (0.41) and the total capacity (volume) of the rumen were una ffected by season or level of intake. Relative to winter ad lib. feedi ng, N retention, total rumen pool size (DM + water), rumen pool size a s a proportion of capacity, and rumen total volatile fatty acid (VFA) pool size were increased during summer ad lib. feeding. Relative to wi nter ad lib. feeding, N retention, rumen ammonia irreversible loss rat e (IRL), total rumen pool size, rumen pool size as a proportion of cap acity, and rumen ammonia and total VFA pool sizes were also increased during summer restricted feeding. Rumen lignin fractional disappearanc e rate (FDPR),vas lower in summer than in winter, and there was a non- significant trend for rumen fractional outflow rate (FOR) of liquid to follow the same trend. Molar proportions of acetate and propionate we re unaffected by season, proportions of n-butyrate were slightly highe r in summer, and proportions of iso-butyrate and iso-valerate were hig her for summer restricted than for winter ad lib. feeding. When intake s were equalized there were no seasonal changes in rate of rumen water outflow, net rumen water balance or intestinal water absorption. It i s concluded that there is a seasonal change in rumen physiology in red deer during summer causing increased total rumen pool size (DM + wate r), an increase in rumen ammonia production and pool size, and an incr ease in rumen total VFA pool size which are all independent of the inc rease in VFI. The increased total rumen pool size in the summer restri cted group may indicate an increased mean retention time (MRT) of dige sta in the rumen. MRT for particulate matter was calculated to be 29.2 and 34.8 h during winter and summer respectively. This, together with increased rumen ammonia production, mag function to maintain rumen fi bre digestion when VFI normally increases during summer. The increased rumen VFA pool size may indicate increased VPA production during summ er, in the same way as ammonia IRL was increased.