Low dietary sodium content affects the digestibility of nutrients and fattening performance in growing rabbits

An experiment was conducted to study the effect of a reduction in dietary sodium content on performance and nutrient digestibility, in rabbits weaned at 25 days of age. Two diets with the same composition and only differing in the supply of sodium chloride content (0.5 or 0%) were used. A reduction on the dietary sodium content from 2.7 to 0.6 g kg DM decreased the ileal and faecal apparent digestibility of dry matter, crude protein and the ileal apparent digestibility of every amino acid studied with the exception of the phenylalanine, and reduced daily weight gain, feed intake and feed efficiency. These results indicate that a reduction of the dietary sodium content up to 0.6 g kg DM results excessive and insufficient to assure a correct absorption of nutrients and to maintain the growth performance in growing rabbit. Additional key words: amino acids, faecal digestibility, feed intake, ileal digestibility, weight gain.

Palabras clave adicionales: aminoácidos, consumo de pienso, digestibilidad fecal, digestibilidad ileal, ganancia de peso.from De Blas and Mateos (1998).Its ingredients and chemical composition is shown in Table 1.Another diet was made by adding 0.5% of sodium chloride to the basal diet, to achieve a dietary sodium content (2.7 g kg -1 DM) and chloride content (5.7 g kg -1 DM) above the current recommendation, maintaining the supply of the rest of nutrients as well as the dietary electrolyte balance (calculated as Na + K ± Cl meq kg -1 DM) at similar levels.The diets were pelleted and included 5 g kg -1 of lucerne hay marked with an indigestible marker (ytterbium) according to the procedure described by García et al. (1999).Experimental trials were carried using 68 New Zealand White × California mixed-sex rabbits weaned at 25 days of age.Animals were housed in wire metabolism cages measuring 250 × 600 × 330 mm in a building with heating and forced ventilation system to maintain the temperature between 18 and 23°C, and received a cycle of 12 h of light and 12 h of dark throughout the experiment.Rabbits were handled according to the principles for the care of animals in experimentation published by the Spanish Royal Decree 1201/2005 (BOE, 2005).Another 28 rabbits weighing 484 ± 11 (SE) g at 25 days of age were blocked by litter and assigned at random to the experimental diets to determine the apparent ileal digestibility (14 per diet) of DM, crude protein (CP) and AA.Twenty of them were also used to determine the faecal apparent digestibility of DM, CP and energy from 32 to 35 days of age.The faeces were collected daily and stored at -20°C, dried at 80°C during 48 h, and ground at 1 mm to be analysed.At 35 days of age, all the animals were slaughtered by cervical dislocation between 19:00 and 21:00 h.The last 20 cm of the ileum were taken and the ileal contents were removed, frozen and freezedried.The samples were then ground and, because of the small quantity available, they were pooled in groups of two rabbits of the same treatment to analyse CP and ytterbium.To determine the AA content of the ileal digesta, a fixed amount (0.05 g) of all the samples belonging to each treatment were pooled.Ytterbium content of experimental diets and ileal digesta were analysed to calculate apparent ileal digestibility of CP and amino acid (CP id and AA id ) according to the following equation: A finishing performance trial was carried using 40 rabbits (20 per diet), weaned at 25 days old and weighing 486 × 12 g.Animals were individually caged, blocked by litter and assigned at random to the experimental diets throughout a two-week period.After 39 days of age, all the animals received a commercial feed (CUNIUNIC ® , NANTA, S.A.: 170 g CP, 5 g NaCl, 144 g starch, 373 g neutral detergent fibre and 49 g acid detergent lignin per kg) until they reached 56 days of age.Animals had ad libitum access to the feed and water throughout the whole experimental period.Feed intake, weight gain at day 14 after weaning and at the end of the experimental period were recorded.
Chemical analysis of diets and ileal digesta was performed using the procedures of AOAC (2000) for dry matter (930.15),ash (923.03),Dumas N (968.06)and starch (according to the alpha-amyloglucosidase method, 996.11).Neutral-detergent fibre, acid-detergent fibre and acid-detergent lignin were determined according to the sequential method of Van Soest et al. (1991).Gross energy (GE) was measured by adiabatic calorimetry.The minerals (Na, Cl, K, Ca and Mg) of the diets were determined by ion chromatography.Amino acids and Ytterbium content of diets and ileal digesta were analyzed as described by Chamorro et al. (2007b).
Data were analyzed as a completely randomized block design using type of diet as main effect and litter as block effect, by using the GLM procedure of Statistical Analysis Systems Institute (SAS, 1991).Weaning weight was used as a linear covariate in the growth traits analysed.
The effect of diets on the apparent ileal and faecal digestibility (AID and AFD, respectively) of nutrients is shown in Table 2.A reduction on the dietary sodium content from 2.7 to 0.6 g kg -1 DM decreased the AID of DM by 31% (P < 0.001), CP by 17% (P < 0.01), and that of every AA studied with the exception of the  phenylalanine.Previous work (Chamorro et al., 2007a) reported a lower and less significant effect of a decrease of dietary sodium concentration from 2.6 to 1.6 g kg -1 DM on the AID of cystine, methionine, arginine, tyrosine and phenylalanine, whereas the AID of DM and CP and those of other amino acids remained unaltered.
The requirements of sodium in the mucosal solution for the absorption of amino acids demonstrated in earlier in vitro studies (Schultz et al., 1966;Frizzeli et al., 1973) might explain our results.
The lower ileal DM and CP digestibility in low sodium diets in the present study was partially compensated by a higher microbial caecal digestion.Accordingly, AFD of DM, CP and energy were closer than AID values between diets, although differences still reached a level of significance lower than 10%.
The effect of the dietary treatments on finishing performance is shown in Table 3.During the first two weeks after weaning, when rabbits received the experimental diets, a decrease of dietary sodium content from 2.6 to 0.6 g kg -1 reduced daily weight gain (P < 0.001), feed intake (P < 0.005) and feed efficiency (P < 0.001), by 30, 15 and 17% respectively.
The results of this study indicate that a reduction on the dietary sodium content up to 0.6 g kg -1 DM result excessive and insufficient to assure a high absorption of nutrients and to maintain the growth performance in growing rabbits.

Table 1 .
Ingredients and chemical composition of the basal diet 0.6 Na (g kg -1 )

Table 2 .
The effect of dietary treatments on the apparent ileal and faecal digestibility (%)

Table 3 .
The effect of dietary treatments on finishing performance a 0.6Na = dietary sodium content 0.6 g kg -1 DM; 2.7Na = dietary sodium content 2.7 g kg -1 DM. b n = 20 per treatment.