Resumen de Consequences of dietary supplementation with n-3 polyunsaturated fatty acids on reproductive, endocrine and metabolic parameters of rabbit does and on carcass quality of growing rabbits
The n-3 polyunsaturated fatty acids (PUFA) play a decisive role in the development of the brain and the nervous, immune and reproductive systems of animals. These essential fatty acids (FA) are particularly important with regard to high production animals; where their greater energetic demands may not be covered with a "basic" alimentation, and consequently they need a direct supplementation through the diet. Focusing on reproductive level, PUFA are precursor molecules of different hormones (steroid and prostaglandin) that regulate the ovarian activity, the embryo implantation or the parturition time among other physiological processes. Moreover, dietary PUFA can alter the lipidemic profile affecting the gene expression that regulates the metabolism and increases peroxidation. When PUFA are included in diets, carcasses with a healthy lipidemic profile are obtained. Most of studies assessed in rabbits have used PUFA from vegetal origin. Nevertheless, there is a relatively low number of researchs which have used PUFA from marine origin, relating reproductive, metabolic, and digestive, as well as growing and carcass quality parameters.
The first specific aim was to evaluate the productive, endocrine, and metabolic responses as well as oxidative stress of rabbit does and their offspring when fed a diet supplemented with n-3 PUFA (15 g/kg) during their first productive cycle. To this purpose, a total of 105 rabbit does were fed ad libitum from d 60 to 172 of age 2 isoenergetic and isoproteic diets differing in FA composition. The control diet (n = 52 does) contained 45.9 g/kg of n-3 of the total FA and the enriched diet (n = 53 does) contained 149.2 g/kg of n-3 of the total FA. Both experimental groups had similar feed intake during rearing, pregnancy, and lactation. The enrichment of diet had no effect on ultrasonographic assessment of does on d 9 and 16 of pregnancy, with an embryonic vesicle number, foetus and placenta size similar between groups (P > 0.05). Even though there were no major effects (P > 0.05) on fertility, duration of gestation, and number born alive and stillborn kits at parturition, live kits from enriched does were longer (71.6 ± 2.42 vs. 79.5 ± 2.13 mm; P < 0.05) and tended to be heavier (42.5 ± 3.94 vs. 50.8 ± 3.47 g; P = 0.07) than those from control does (P < 0.05). The 2 groups had similar milk production and mortality values during lactation; consequently, there were no differences between diets in average daily gain (ADG), litter weight, and number of weaned kits (P > 0.05). In enriched does, higher plasma leptin and estradiol concentrations than in control does (P < 0.05) were observed. In addition, enriched females also had lower total and high-density lipoprotein cholesterol (HDL-c) than control females during lactation (P < 0.05). Regarding offspring, the enrichment of diet with PUFA caused a hyperlipidemic status (greater values of plasma triglycerides, total cholesterol, and HDL-c; P < 0.05) at 1 d postpartum (dpp), compared with the control group, that disappeared at 32 dpp. Supplemented does before parturition and their offspring at 1 dpp had greater oxidative stress than those in the control group.
The second specific aim was to investigate the effect of continuing the same dietary supplementation during growing period. For this purpose, we evaluated growth performance, carcass characteristics and FA profile of muscle and fat tissues (at slaughter), as well as cecal fermentation and ileal mucosa morphology of growing rabbits. Similar diets to those used in the first objective were provided each to 24 does (12 per diet) and their offspring during pregnancy and lactation. From weaning (30 d of age) to slaughter (60 d), the litters (12 per diet; 8 kits each) continued fed the corresponding experimental diet. There were no differences (P > 0.05) between groups in average daily feed intake, ADG and G:F ratio during the growing period. At slaughter, body weight, full gastrointestinal tract weight, carcass yield, meat colour and pH, drip loss percentage, content of scapular fat and tissue composition of the left hind leg were similar between groups (P > 0.05), but perirenal fat was lower (P = 0.020) and skin weight and abdominal fat tended to be lower (P = 0.055 and P = 0.063, respectively) in enriched rabbits than in control ones. Total PUFA content in both Longissimus dorsi muscle (LM) and perirenal fat was greater (P = 0.021 and < 0.001, respectively) in enriched rabbits, that also showed lower n-6/n-3 ratios in LM (1.61 vs. 5.80; P < 0.001) and perirenal fat (4.71 vs. 12.0; P < 0.001) than those fed the control diet. Cecal concentrations of total volatile fatty acids (VFA) were greater (P < 0.001) in enriched than in control group at 30, 45 and 60 d of age, but diet did not affect (P ≥ 0.332) VFA profile, with the exception of a lower (P = 0.013) proportion of minor VFA (sum of isobutyrate, isovalerate, and valerate) in control group. Diet did not affect (P > 0.255) either pH and NH3-N concentrations in the cecum or ileal morphology (crypt depth and villi length).
Taking into account, the results obtained, the third specific aim of this thesis was to confirm the effect of an enriched diet with n-3 PUFA in a 4-folder higher level of inclusion (60 g/kg) than previous experiments. A total of 127 rabbit does were fed ad libitum throughout their two first cycles with two diets with different fat sources (in terms of n-3 of the total FA): the control diet had 31.9 g/kg (n = 63 rabbit does) and the enriched diet had 287.7 g/kg (n = 64 rabbit does). Feed intake was similar between groups (P > 0.05). Dietary treatment affected plasma progesterone concentration, which was higher than in control group on Days 7 (30.9 ± 2.18 vs. 23.9 ± 2.30 ng/ml, respectively; P = 0.029) and 14 (38.7 ± 2.18 vs. 28.2 ± 2.30 ng/ml, respectively; P = 0.001) of first gestation. Reproductive parameters of mothers (fertility, duration of gestation and prolificacy) and litter parameters (weight at parturition and weaning, mortality and ADG of kits during lactation) were similar in both groups. However, individual measurements of neonates of enriched group improved 5.87%, 7.10% and 18.01% (P < 0.05) in terms of crown-rump length, biparietal and thoracic diameters, respectively, compared to control ones at first parturition. It is noteworthy that at the second insemination, critical point in rabbit does, fertility rate of enriched group did not decline as sharply as in the control group (89.7 vs. 76.6%, respectively; P = 0.067), although ADG and littler weight were slightly lower at the second lactation after PUFA enrichment (P < 0.05). Moreover, total PUFA and unsaturated index of milk of enriched does group were significantly elevated than in control one (33.3 ± 0.02 vs. 23.2 ± 0.02 g/100g and 1.20 ± 0.00 vs. 0.86 ± 0.00, respectively; P < 0.05). Finally, plasma progesterone, ovulation rate and embryo development at 3.5 days after the artificial insemination were similar between diets (P > 0.05), but embryo apoptosis rate was almost twice higher in control group than in supplemented one (31.1 ± 4.56 vs. 17.1 ± 3.87%, respectively; P < 0.05).
In conclusion, an increase of n-3 PUFA concentration in the diet of rabbit does and, consequently, of their offspring during a productive cycle alters their lipid profile and the indicators of oxidative stress, without major endocrine modifications or improvements in the productive variables. The fact of continuing with the same diet during growing period showed that dietary fish oil supplementation enhanced beneficial long-chain n-3 FA and decreased n-6/n-3 ratio in rabbit meat and fat, being healthier for human consumption, without having negative effects on growth performance, cecal fermentation, and ileal morphology or carcass characteristics. Moreover, increasing the level of inclusion of the same supplement previously administrated from the rearing and throughout the first two productive cycles improved plasma progesterone during pregnancy, fertility, milk fatty acid profile and neonates development of primiparous does supporting the accumulative long-term beneficial effect of n-3 PUFA supplementation in rabbit does.
