Energy metabolism of growing blue foxes (Alopex lagopus)

Abstract

The blue fox (Alopex lagopus) is presently the most important species in fur farming in Finland. In recent years selective breeding programmes have focused on producing large and heavy animals. Ten years ago the average weight at pelting of blue fox vixens varied between 7 and 10 kg. Nowadays individuals weighing over 20 kg can be found. At present blue foxes are fed ad libitum during the growing - furring period. The blue fox exhibits seasonal fluctuations in feed intake and accretion of body fat, feed intake and body fat retention being very high during autumn and early winter if given free access to feed. Unrestricted feeding hence often leads to animals being very fat or even obese at the time of pelting. The accumulation of body fat may have dual purposes, both of crucial importance for animals living in the wild: first to provide insulation of the body and protection from excessive heat loss when ambient temperature is very low and second, to serve as an energy reserve in situations of scarcity. The main objective of this project was to establish baseline data on the energy requirement of growing blue foxes, by measuring feed intake, energy expenditure, and protein and fat retention. This project is based on the main hypothesis that the energy requirement of the blue fox is strongly regulated by photoperiod, and that voluntary feed intake and energy expenditure reflect seasonal changes. Animals, diet and treatment groups: Sixteen juvenile blue fox vixens were used. All animals were fed the same conventional fox diet. The average chemical composition of the diet was 37.8 % dry matter, 1.9 % ash, 11.7 % crude protein, 8.7 % fat and 15.5 % carbohydrate as fed. The feed for the experiment was produced as one batch and stored frozen until use. The animals were allocated to four different treatment groups and given different energy supply: (1) ad libitum, target body condition "very high"; (2) 20 - 30 % below the ad libitum group, target body condition "high"; (3) 35 - 45 % below ad libitum, target condition "ideal mating condition";(4) 50 - 60 % below the ad libitum group, target condition "lean". The differentiation of the energy supply was used in order to establish a population of animals with very different body condition/body fat contents. Experimental techniques: The experiment was performed in five 7d balance periods (3d adaptation and 4d quantitative collection of faeces and urine), starting when the animals were about 10 weeks old and ending when the animals were about 30 weeks old. Each period included a 22 hr respiration experiment by means of indirect calorimetry in an open-air circulation system. Heat production was calculated according to the formula by Brouwer (1965). For a detailed description of the system, including instrumentation, calibration and measurement procedures see Chwalibog et al. (2004). During the balance periods the animals were kept in metabolic cages in an intensive animal care unit, under natural daylight conditions. Between balance periods the animals were kept under conventional farm conditions. The animals were weighed at the start and at the end of each balance period. Statistical analyses were carried out using the repeated MIXED Model (SAS release 9.1) with treatment group (1-4), and period (1-5) as the main effects, and interactions between the main effects. The covariance structure was AR(1). Mean live weights in groups were similar and ranged between 4.81 -5.09 kg at the beginning of the trial. Feed intake was affected by the group and by the period (p<0.001). At the end of the trial the mean weights in the groups were (1) 12.83 kg, (2) 12.52 kg, (3) 10.93 kg and (4) 9.23 kg. The final individual weights varied between 8.9 to 16.9 kg. The final body weights in groups (1) and (2) were similar. The final weights in groups (3) and (4) differed from all the other groups (p<0.001). Feed efficiency was affected only by period (p<0.001). Feed efficiency was highest at the beginning of the trial (mean for all groups 0.15) and lowest at the end of the trial (mean for all groups 0.02). Animals were growing during the trial so the period had an effect on mean live weights in all groups. The feed consumption was different in the groups and the animals' live weight increased according to the energy supply. Further results on heat production, energy and protein metabolism will be presented at the symposium.