Resumen de Nutritional strategies to improve carcass yield and carcass composition in fattening rabbits
The main objective of this PhD was to study the effect of different nutritional strategies on carcass traits. Secondary, it was also researched the effect of those strategies on growth performance, carcass chemical composition and nitrogen and energy balance in growing rabbits. For this purpose, four experiments were conducted. In experiment 1 was studied the effect of supplementation of a multi-enzyme complex at different doses on a standard diet on performance parameters, carcass traits, nutrient digestibility and nitrogen and energy balance in growing rabbits. In the experiment 2, same multi-enzyme complex than in experiment 1 was used, but two different diets were formulated, with different DP, DE and NDF content. Experiment 3 studied the effect of three levels and two sources of lignin. And, in the last one, experiment 4, feeding restriction on growth performance, carcass traits, nutrient digestibility and nitrogen and energy balance in growing rabbits was studied. All the feeds supplied to the rabbits were non-medicated. In experiment 1, the purpose was to compare the inclusion of a fungal multi-enzyme complex (activity of 100 XU/g pentosanase, 40 CMC/g cellulase, 30 FAU/g amylase, 100 XU/g xylanase, 700 HUT/g protease, 4000 AJDU/g pectinase and 200 BGU/g β-glucanase) in the feed at different doses (0, 100, 200, 400 and 800 ppm), on productive parameters, carcass traits and nitrogen and energy balance in growing rabbits. A total of 225 rabbits, weaned at 34 d of age, were used. They were housed in individual cages and randomly assigned to five treatments. A basal feed was formulated (CP: 17.6%; starch: 17.4%, NDF: 35.9%, DE: 2797 kcal/DM kg) and supplemented with different doses of the enzyme complex. Feed intake and animal weight (from 34 to 48 d of age and from 49 to 60 d) were controlled. In 11 rabbits per treatment, body composition was estimated by Bioelectrical Impedance technique (BIA) at 34 and 60 d of age to estimate the nitrogen balance. At the end of the trial, 24 rabbits per treatment were slaughtered and the carcass, the entire digestive tract, the full and empty stomach, and the full caecum were weighted. Between days 53 and 57 of life, a digestibility trial was done (for DM, OM, CP, EE, NDF, GE; n=11), except for the 100-ppm dose. During the first period (34-48 d) enzyme supplementation linearly improved the feed conversion ratio (FCR) (P<0.05), without affecting feed intake (DFI) or growth (DWG). In the second period (49-60 d), both FCR and DWG varied quadratically (P<0.05) with enzyme supplementation, being the optimum level for both: 400 ppm/kg. The digestibility of DM, OM, EE and GE decreased linearly (P<0.05) and the weight of the digestive tract (% BW) increased linearly (P<0.05) with the increase of enzyme complex dose, without affecting the carcass yield, which was 57.8%, on average. Enzymes had no effect on nitrogen and energy balance, except on energy losses in faeces (P<0.05) which increased with the dose due to the worse GE digestibility. Carcass composition was neither affected (P>0.05) by the level of the enzyme complex supplementation. In experiment 2, the objective was to compare the effect of supplementation with the same multi-enzyme complex in two feeds with different levels of digestible energy (DE), digestible protein (DP) and NDF on growth performance, carcass characteristics and nitrogen and energy balance in growing rabbits. Three doses (0, 100 and 200 ppm) of the same enzyme complex were added to two different basal diets (Diet A: CP: 17.6% DM; starch: 17.4% DM; NDF 35.9% DM, DE: 2797 kcal/kg DM and Diet B: CP: 15.8% DM, starch: 17.5% DM, NDF: 37.4% DM, DE: 2585 kcal/kg DM). Therefore, a total of 270 rabbits were weaned at 34 d and housed in individual cages. The parameters measured were the same as in the previous experiment. The digestibility trial (between days 53 and 57 of age) was performed on both diets (A and B) but only to the 0 and 200 ppm doses. The addition of enzymes did not affect the digestibility, or any other productive parameters studied in the growing period. Diet B decreased (P<0.01) the digestibility of DM, OM, NDF and GE vs. diet A (5.98, 5.66, 4.91 and 22.59%, respectively). Digestible nitrogen (DNi) and DE intake were lower for diet B than diet A (10% and 7% respectively, P<0.01). The lower DN intake of animals fed with diet B led to a lower DN in carcass (4%; P<0.05) and a reduction of the nitrogen excreted in urine (19.4%; P<0.05) and in faeces (6.3%; P<0.05). Digestible energy intake of diet B was lower (P<0.05) compared with diet A (0.998 vs. 0.947 MJ/kg BW0.75.d. Accordingly, animals of diet B had lower energy retention efficacy, both in whole body (0.389 vs. 0.360 kJ/kg BW0.75.d) and in carcass (0.231 vs. 0.211 kJ/kg BW0.75.d). Due to the lower energy ingestion and efficacy, energy excreted in faeces was higher (0.660 vs. 0.705 MJ/kg BW0.75.d; P<0.05) in animals of diet B, whereas energy excreted in skin and viscera was higher in diet A (0.157 vs. 0.148 MJ/kg BW0.75.d). The lower retention of N and energy of diet B, resulted in a lower weight at 48 d of age (1650 g vs. 1622 g, P<0.05), due to a lower DWG in the first period (58.2 vs. 60.2; P<0.05), although second fattening period and final weight did not differ significantly. Feed intake during the whole growing period was 2.92% higher for diet B, so FCR increased (2.52 vs. 2.63, P<0.05). The type of diet did not affect carcass yield, although the rabbits of diet B had heavier digestive tract and full and empty stomach (P<0.05). Carcass chemical composition of animals fed diet A showed a higher EE and energy content (P<0.05), whereas diet B had higher water, CP, and ash content (P<0.05). In the third trial, the aim was to study the effect of including different levels of acid detergent lignin (ADL) (Low (L): 5% DM, Medium (M): 6% DM and High (H): 7% DM) and different sources of lignin (Arbocel® (A) vs. defatted grape seed meal (G)) on performance, carcass characteristics and nitrogen and energy balance in growing rabbits. A total of 936 rabbits were used, 216 were weaned at 34 d of age and housed in individual cages, and the remaining 720 were weaned at 35 d of age and housed in collective cages (5 rabbits/cage). Five different feeds were formulated: one control diet for both sources (AL and GL: DE: 2939 kcal/kg DM, CP: 18.4% DM, NDF: 36.4% DM; ADF: 21.3% DM; ADL: 4.78% DM; Starch: 15.5% DM), two diets with an average level for each source (AM (DE: 2825 kcal/kg DM, CP: 17.7% DM , NDF: 37.8% DM; ADF: 23.8% DM; ADL: 5.87% DM; Starch: 15.1% DM) and GM (DE: 2909 kcal/kg DM, CP: 17.9% DM , NDF: 36.2% DM; ADF: 21.8% DM; ADL: 6.03% DM; Starch: 15.6% DM) ) and other two diets with a high level for each source AH (DE: 2848 kcal/kg DM; CP: 18.1% DM, NDF: 37.7% DM; ADF: 23.8% DM; ADL: 6.23% DM; Starch: 14.5% DM) and GH (ED: 2908 kcal/kg DM, CP: 17.6% DM, NDF: 35.4% DM; ADF: 22.0% DM; ADL: 6.72% DM; Starch: 15.9% DM)). In rabbits housed in collective cages, weight and intake were measured at the beginning and at the end of the growing period (35 and 61 d of age), feed conversion ratio (FCR) was calculated for that period, and mortality was recorded daily. For rabbits housed in individual cages average daily intake and weight were controlled in two periods from 34-48 d, and 49-60 d of life, as well as the FCR and daily mortality during growing period. In 11 rabbits per treatment, body composition was estimated by Bioelectrical Impedance technique at 34 and 60 d of age to estimate the nitrogen and energy balance. At the end of the trial, 24 rabbits per treatment were slaughtered and the carcass weight, the entire digestive tract, the full and the empty stomach and full caecum were measured. Between days 53 and 57 of life a digestibility trial was done (DM, OM, CP, EE, NDF and GE, n=11). For rabbits in collective cages, the highest level of lignin affected the FCR compared with the other two levels (2.65 on average vs. 2.69 g/g, P<0.05) without affecting daily feed intake (139 g/d) or growth (52.2 g/d). The digestive tract weight (%BW) decreased with medium and high levels of lignin, compared with the control one (17.4 on average vs. 17.9%; P=0.05) but carcass yield was not affected (58.2% on average). The apparent feed digestibility of DM, OM and GE was higher with the lowest level of ADL compared to the average of the other two: DM (58.3 vs. 56.8%), OM (60.2 vs. 58.8%), and GE (60.2 vs. 58.9%). Apparent digestibility of CP for the medium level was lower than the low level (70.1 vs. 72.6%, P=0.05) while the diet with the highest level of ADL showed an intermediate value (71.0%). EE digestibility increased with level inclusion of ADL (84.7, 85.8, and 88.0%, respectively). The animals fed high levels of ADL consumed less DN than low and medium levels (1.901 vs. 1.992 g NDi/kg BW0.75.d, on average, P<0.05), and tended to improve the retention efficiency (0.348 vs. 0.338%, on average, P=0.07), so the N retained in all lignin levels was similar (0.669 g NR carcass/kg BW0.75.d). N excretion in urine was lower for M and H lignin levels (average of 0.894 vs. 1.003 g N/kg BW0.75.d). Otherwise the energy losses in faeces increased in these two diets (+5%) compared with the low level of ADL. Arbocel® increased FCR during the growing period in individual cages (2.56 vs. 2.52 g/g, P <0.05), as well as in group cages (+1.42%, P<0.05). The Arbocel® OM digestibility was worse than defatted grape seed meal (56.9 vs. 57.7%, P<0.05). The carcass chemical composition of the animals was not affected neither by the source nor by the level of feed ADL. The aim of the experiment 4 was to study the effect of different feeding restriction strategies in growing rabbits. A total of 600 rabbits were weaned at 34 d, caged in groups (5 animals/cage; 24 cages/treatment) and randomly assigned to the 5 treatments. Control group (AL) was fed ad libitum and the others four were restricted 80 and 70% of theoretical ad libitum intake (R80 and R70, respectively), during the whole fattening period or until 48 d (R80AL and R70AL, respectively). Only one diet was formulated (CP: 159 g/kg DM; NDF: 324 g/kg DM; DE: 2689 kcal/kg DM). At the end of the fattening period, 24 rabbits per treatment were slaughtered and the weight of carcasses and digestive organs were measured. A digestibility trial was done in individual metabolic cages between 53 and 57 d (12 rabbits/treatment). Feed restriction impaired (P<0.001) the feed conversion ratio (1.93 vs. 1.82 g/g, on average) and daily weight gain (42.4 vs. 60.3 g, on average) from 34 to 48 d. However, from 48 to 61 d there was a compensatory growth in rabbits fed ad libitum (R80AL and R70AL) once they finished the restriction period (61.1 vs. 46.4 g/d, on average). On the whole fattening period, the compensatory growth led to a better feed conversion ratio (2.25 vs. 2.33 g/g; P<0.001), and rabbits fed R80AL finished the fattening period with a similar final body weight than those fed the control diet. Internal organs increased (P<0.05) their size, when rabbits were fed ad libitum after the restriction period, especially in those restricted at 70% in the first phase. Average mortality was 12.8% and it was not affected by treatments. Protein digestibility in treatments R80 and R70 tended to be higher than those restricted until 48 d. The digestibility of the other constituents was not affected by any of the restriction strategies. The nitrogen and ashes concentration in carcass increased, whereas fat and energy content decreased in whole fattening period of restricted groups (P<0.001). Efficacy of nitrogen retention tended to increase (P=0.074) and the nitrogen and energy excretion through the faeces, and urine+heat losses decreased (P<0.001) in restricted animals. From the results of these 4 experiments, it can be concluded that in growing rabbits the supplementation with this multi-enzyme complex did not improve digestibility, nitrogen balance or reduce the weight of the digestive tract, and had no effect on the carcass yield. The use of enzymes improved the feed conversion ratio in the first period, so its use would be interesting in animals after weaning. Under the conditions of this study, the addition of enzymes in a lower nutritional value feed, neither improved the feed efficiency, the N balance, nor the carcass yield. Including ADL levels above 5% in isoenergetic diets decreased gastrointestinal weight, although this effect was not reflected in carcass yield. Arbocel® got worse FCR without affecting the growth. Restriction until 48 d could be a useful tool to reduce the environmental impact of the farms without an impairment of growth performance and carcass yield in the whole fattening period. In non-enteropathy conditions, none of the strategies studied had an effect on mortality.
