DIETARY EFFECT OF SHORT-CHAIN ORGANIC ACIDS ON GROWTH PERFORMANCE AND DEVELOPMENT OF INTESTINAL LYMPHOID TISSUES IN FATTENING RESTRICTED RABBITS

This work aimed to test the effect of a dietary inclusion of formic and citric acids on growth performance, mortality, jejunal histology and development of intestinal lymphoid tissues in growing non-medicated rabbits. To this end, a control diet (diet C; 35.9 and 34.7% NDF and 18.9 and 19.1% CP, on DM basis, in the 2855 d and 56-77 d periods, respectively) was compared with the same diet supplemented with a blend of microencapsulated formic and citric acids (diet A: diet C enriched with 0.4% and 0.2% in the 28-55 d and 56-77 d periods, respectively). Sixty rabbits weaned at 28 d were assigned to each diet. At 56 and 77 d, 5 rabbits per diet and age were slaughtered to assess caecal traits, jejunal histology and follicular development in the caudal ileal Peyer's patch and the appendix. No dietary effects were observed on growth performance in the 28-55 d period. In the 56-77 d period, average daily gain of rabbits fed diet A was greater than that of control rabbits (48.0 vs. 43.9 g; P=0.019). Mortality rate was not affected by the diet (11.8% on average). Caecal pH, volatile fatty acids and ammonia concentration, and appendix size were not affected by diet. Caecal pH was lower at 77 than at 56 d (6.02 vs 6.19; P=0.016). The concentrations of ammonia (P=0.003) and volatile fatty acids (P<0.001) in the caecal contents increased, respectively, from 9.62 and 70.5 to 14.2 and 81.9 mmol/L when rabbits reached 77 d of age. At 77 d the appendix was heavier (4.30 vs. 3.21 g/kg BW; P<0.001), longer (13.3 vs. 10.4 cm; P<0.001) and wider (1.74 vs. 1.45 cm; P=0.006) than at 56 d. At 56 d, villi of control rabbits were shorter than those of rabbits fed diet A and than those of rabbits aged 77 d, irrespective of the dietary treatment (662 vs. 807 pm; P<0.001). In the Peyer's patch, the average follicle area was greater at 77 than at 56 d of age (118 vs. 88.4 x 10(3) mu m(2); P<0.001) and was also greater in rabbits fed diet C than in those fed diet A (109 vs. 97.5 x 10(3) mu m(2); P=0.049). In the appendix, no differences in the average follicle area were found at 56 d of age (115x10(3) mu m(2)), whereas at 77 d the area increase was higher for rabbits fed diet C than for those fed diet A (95.5 vs. 50.8%; P<0.001). In conclusion, including formic and citric acids in growing rabbit diets improved weight gain in the 56-77 period, had a trophic effect on the jejunal mucosa at 56 d and controlled the hypertrophy of gut-associated lymphoid tissues.