Food intake and growth of red deer is lower in winter than in spring and this reduces the efficiency of venison production. Rumen capacity is also lower during winter and this may contribute to the reduced food intake and therefore growth. In the present study, we investigated the ability of deer to regulate food intake during winter and spring by feeding diets of differing energy densities.
Six groups of eight male red deer calves were housed indoors in separate pens. Each group was given, ad libitum, a pelleted diet of a different energy density (8·5, 9·0, 9·5, 10·0, 10·5 and 11·0 MJ metabolizable energy (ME) per kg dry matter (DM) for groups 1 to 6 respectively) but the same amount of protein (156 g/kg DM). Food intake of each group was recorded every 2nd day and animals were weighed every 6 days from 17 May to 9 December. For seasonal comparisons, winter was defined as 24 May to 31 August and spring as 1 September to 9 December.
There was no difference (P > 0·05) between the mean live weights of the groups at any time during the study. Live-weight gain (LWG) reached a minimum on 4 July and was lower in winter than spring (161 v. 308 g/day, s.e.d. = 10·0, P < 0·001). LWG was positively related (P < 0·001) to diet ME during winter. DM intake (g/kg M0·75 per day) and ME intake (MJ ME per kg M0·75 per day) decreased until 16 July and increased thereafter. Mean DM intake was lower in winter than spring (83·5 v. 97·2 g/kg M0·75 per day, s.e.d. = 2·05, P < 0·001). DM intake increased as diet energy decreased (P < 0·001) in winter and spring with a steeper slope (P < 0·05) in spring than winter. ME intake was not related to diet ME (P > 0·005) and was lower in winter than spring (0·82 v. 0·95 MJ/kg M0·75 per day, s.e.d. = 0·25, P < 0·001). Maintenance energy requirement (MEm) across groups and seasons was calculated to be 0·45 (s.e. 0·22) MJ ME per kg M0·75 and the energy requirement for LWG (MEf) was 53 (s.e. 8·5) MJ/kg LWG. MEf was related (P < 0·01) negatively to diet ME during winter.
In summary, deer consuming diets with a wide range of energy densities, altered their DM intake, resulting in similar energy intakes and growth rates on all diets. Animals seemed less able to achieve this compensation in winter compared with spring when food intake increased to support the natural rise in growth rate at that time. These results indicate that deer have target growth rates and/or energy intakes that change with season, and are defended by adjusting food intake.