Digestibility and nitrogen utilisation of diets containing protein at different levels and supplemented with dl-methionine, l-methionine and l-lysine in blue fox (Alopex lagopus)

Digestibility and nitrogen (N) utilisation in diets with low protein levels and supplemented with some amino acids were studied in two experiments with 21-week-old blue foxes (Alopex lagopus). The dietary protein levels studied, expressed as percentage of metabolisable energy (ME) from protein, were in both experiments: 30, 22.5 and 15, corresponding on average 306, 244 and 174 g protein kg−1 dry matter (DM), respectively. At the two lowest levels the diets were fed as such or as supplemented with dl-methionine (experiment 1) or l-methionine or l-lysine (experiment 2). The added amount brought the level of the respective amino acid in the diet to that of diet containing 30% of ME from protein, and was verified by analyses. parent digestibility of crude protein (p<0.001), ether extract (EE, p<0.001), crude carbohydrate (CC, p<0.001), DM (p<0.001) and amino acids (p<0.001) excluding cystine (in the first experiment p<0.05 and in the second experiment the effect was not significant) was impaired linearly when the dietary protein level decreased. However, both dl- and l-methionine supplementation increased the apparent digestibility of EE (p<0.01 and p<0.001) and dl-methionine (experiment 1) also the digestibility of DM (p<0.01) and CC (p<0.05). A similar trend was found in experiment 2 where l-methionine tended to increase the digestibility of DM (p<0.10) and CC (p<0.10). In addition, dl- and l-methionine and l-lysine supplementation improved the apparent digestibility of the respective amino acids (for methionine, p<0.001 in experiment 1 and p<0.01 in experiment 2, and for lysine, p<0.001). N retention, as % of intake or of absorption, increased linearly with decreasing protein level in the diet (p<0.10 and p<0.01). However, no significant differences in daily N retention (g per animal) were found. concluded that lowering the dietary protein level decreases the digestibility of the diets. Supplementary dietary methionine improves the digestibility of low protein diets, irrespective of whether it is added as dl- or l-methionine. The lower the protein level, the more efficient the utilisation of nitrogen and thus the lower the percentage of N excreted. On the basis of N retention, even diets with a very low protein level (15–16% of ME from protein) which are composed of raw materials rich in methionine and lysine can satisfy the blue fox’s requirement for protein at the stage where the animal has already developed its full body length and passed the culmination phase in the cycle of growing winter hairs.