Proximate chemical composition, amino acids profile and minerals content of meat depending on carcass part, sire genotype and sex of meat rabbits

Citation: Gál, R.; Zapletal, D.; Jakešová, P.; Straková, E. Proximate Chemical Composition, Amino Acids Profile and Minerals Content of Meat Depending on Carcass Part, Sire Genotype and Sex of Meat Rabbits. Animals 2022,12, 1537. https://doi.org/10.3390/ ani12121537 Academic Editors: Pietro Lombardi, Pietro Medica and Nadia Musco Received: 28 April 2022 Accepted: 11 June 2022 Published: 14 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). animals Article Proximate Chemical Composition, Amino Acids Profile and Minerals Content of Meat Depending on Carcass Part, Sire Genotype and Sex of Meat Rabbits Robert Gál1, David Zapletal 2,* , Petra Jakešová2and Eva Straková2 1Department of Food Technology, Faculty of Technology, Tomas Bata University in Zlín, 760 01 Zlín, Czech Republic; [email protected] 2Department of Animal Breeding, Animal Nutrition and Biochemistry, Faculty of Veterinary Hygiene and Ecology, University of Veterinary Sciences Brno, 612 42 Brno, Czech Republic; [email protected] (P.J.); [email protected] (E.S.) *Correspondence: [email protected]; Tel.: +420-541-56-2676 Simple Summary: Rabbit meat is popular with consumers mainly for its high-quality protein. Among the main factors influencing the characteristics of rabbit meat proteins to appertain the breed, genotype, carcass part and age. Conventional production of rabbit meat in many European countries is mainly ensured by intensive production systems, when commercial meat-type albinotic rabbit crossbreds are used. However, spotted and solidly coloured lines of meat rabbit breeds have begun to be used in rabbit breeding schemes as some consumers have begun to negatively perceive meat from albinotic coloured rabbits. The aim of the present study was to assess the effects of the sire genotype, sex and carcass part on the composition of meats of rabbits fattened under conditions where no synthetic drugs were used. Crossbreeding of Mecklenburger Schecke sires with a commercial dam line of HYLA rabbits resulted in a worse nutritional quality of meat proteins in progeny. These findings point to a possible risk of alterations in the nutritional quality of meat proteins when using different rabbit sire genotypes than those originally intended for the specific commercial crossbreeding scheme. Abstract: The aim of the study was to assess the effects of the sire genotype, sex and carcass part on the composition of the meat of rabbits, which were fattened under conditions where no synthetic drugs were used. As for carcass parts, the higher content of both total amino acids (AA) and all essential AA (EAA) monitored was found in the Longissimus thoracis et lumborum (LTL) muscle as compared to hind leg meat (p< 0.001). Significant effects of the rabbit sire genotype and the genotype x sex interaction on proportions of some AA in meat were found (p< 0.001). Crossbreeding of the Mecklenburger Schecke (MS) sires with a commercial dam line of HYLA rabbits resulted in a lower proportion of the total AA and all EAA monitored in meats of MS sired males as compared to MS sired females and HYLA rabbits (p< 0.05). The sex-related effect on AA profile was not so noticeable in final commercial crossbreds of HYLA rabbits when compared to MS sired progeny. These findings point to a possible risk of alterations in the nutritional quality of meat proteins when using different rabbit sire genotypes than those originally intended for the specific commercial crossbreeding scheme. However, on the contrary, higher contents of magnesium (p< 0.05), manganese (p< 0.001) and zinc (p< 0.05) were found in meats of MS sired progeny as compared to HYLA rabbits. Keywords: meat-type rabbit; sire genotype; gender; meat quality; amino acid; mineral content 1. Introduction Rabbit meat shows excellent nutritional and dietetic properties; moreover, it can also be effectively fortified with bioactive compounds to provide consumers an outstanding functional food [ 1 , 2 ]. Rabbit meat possesses a very low content of fat and cholesterol, a high level of proteins with essential amino acids (AA), no uric acid and low purine Animals 2022,12, 1537. https://doi.org/10.3390/ani12121537 https://www.mdpi.com/journal/animals Animals 2022,12, 1537 2 of 15 content [ 3 ]. Furthermore, rabbit meat is low in monounsaturated fatty acids, high in n-3 polyunsaturated fatty acids and it is a significant source of vitamin B (vitamins B 2 , B 3 , B5, B6, B12) [4]. It is low in sodium (Na) and rich in phosphorus (P) and selenium [5,6]. Rabbit meat is often popular with consumers mainly for its high-quality protein, which shows a higher digestibility value compared to other meats such as beef or pork [ 7 , 8 ]. The quality of meat proteins is affected by various factors and complex interactions among the biological traits of an animal [ 9 , 10 ]. The main factors influencing the characteristics of rabbit meat proteins, particularly an intrinsic AA composition, are breed [ 8 ], genotype [ 11 ], carcass part [ 12 ] and age [ 8 ]. Studies focusing on the evaluation of the influence of gender on AA profile in rabbit meat are still limited, whereas no sex effect on essential amino acids (EAA) proportion in rabbit meat was reported earlier by [13]. Conventional production of rabbit meat in many European countries is mainly ensured by intensive fattening of commercial meat-type rabbit crossbreds [ 14 ]. In addition to high growth rates and favourable feed conversions, these broiler rabbits also show high carcass value and meat quality, whereas the issue is that mostly albinotic hybrid genotypes are often reared for meat production. Recently; however, spottedand solidly-coloured lines of meat rabbit breeds have begun to be used in rabbit breeding schemes as some consumers have begun to negatively perceive meat from albinotic coloured rabbits, which allegedly evoke laboratory-bred experimental rabbits [ 15 ]. A potential of coloured rabbit breeds and genotypes for meat production was demonstrated recently in some studies [ 16 – 18 ]. Due to their typical production traits, a lot of these rabbit genotypes can be included into specific crossbreeding schemes and utilised in the European organic and alternative production systems [17,19,20]. We found in our previous study that the use of the Mecklenburger Schecke (MS) males as terminal sires in a rabbit crossbreeding scheme led to favourable growth performance and some of the carcass characteristics in their progeny. The MS breed is medium-sized, and these rabbits show a well-muscled cylindrical body, with wide foreand hindquarters. The breed displays three typical colour genotypes, which also differ in growth performance and meatiness. In this regard, solidly- (genotype kk) and spotted- (genotype Kk) coloured rabbits excel in important production traits. When they are crossed with white-coloured rabbits, they pass a spotted or solidly dark colour of coat on their progeny [15]. As the effect of inclusion of the MS breed in a crossbreeding scheme on the meat composition of their progeny has not yet been examined, the aim of the present study was to assess the effect of crossing of MS sires with the commercial dam line of HYLA rabbits on meat composition in their progeny fattened under intensive production systems where no synthetic veterinary and anticoccidial drugs were used. Further, an integral part of the present study was to evaluate the effects of sex and carcass part on the composition of rabbit meat. 2. Materials and Methods 2.1. Animals and Management The study was performed in the commercial Centre of HYLA rabbits (Jaromˇeˇrice nad Rokytnou–Ratiboˇrice, Czech Republic). On the farm, which focuses mainly on the production of breeding HYLA crossbred rabbits used in other commercial farms, neither synthetic allopathic veterinary drugs nor synthetic anticoccidial drugs are used. A total of 112 crossbred rabbits (control and experimental groups) were used in the experiment. The control group (H; 28 males + 28 females) consisted of albino crossbred rabbits of the HYLA combination AB sires (n= 7 sires) × CD dams. The experimental group ( 28 males + 28 females ) consisted of MS sired progeny, when the semen of MS males was inseminated to females of the same HYLA CD line as in the H group. The used MS sires (n= 6 sires) originated from small-scale hobby breeding stocks; all of them were kk or Kk genotypes. Females of the HYLA CD line were inseminated with the heterospermic insemination dose, which contained spermatozoa of all involved MS sires. Animals 2022,12, 1537 3 of 15 2.2. Growing of Rabbits The experimental design of the study was conducted according to [ 21 ], which respected guidelines for experiments with rabbits reared for meat. During the whole experimental period, rabbits of both groups were raised and fattened under identical management conditions. After weaning (35 days of age), rabbits were housed in wire cages (2 rabbits per cage) with the floor density of 0.18 m 2 per rabbit. The cage size was 90 cm ×40 cm ×35 cm (length × width × height). The lighting period was 12 h light/12 h darkness, temperature ranged from 17 ◦ C to 20 ◦ C and relative humidity was 55% to 60%. Rabbits were fed ad libitum by commercial compound pelleted feeds (De Heus a.s., Bˇestovice, Czech Republic) and the grower (K-Optimum) and finisher (K-Finisher) diets were used from the 35th to 64th day of age and from the 65th day of age to slaughter, respectively. The nutrient composition of the diets used is shown in Table 1; the grower diet contained an anticoccidial agent Emanox which is an extract of aromatic plants. Table 1. Chemical composition (g/kg) of the diets as-fed basis. Grower Finisher Item (Day 35 to 64) (After Day 65) Crude protein 157.4 154.1 Crude fibre 125.8 128.1 Crude fat 42.1 33.6 Crude starch 151.8 150.3 Ash 68.6 63.2 Calcium 8.68 6.94 Inorganic phosphorus 7.2 6.6 Asparagine 12.5 12.6 Threonine 5.3 5.4 Serine 6.3 6.4 Glutamine 28.8 27.6 Proline 8.7 9.0 Glycine 7.1 7.1 Alanine 7.1 7.3 Valine 7.5 7.4 Methionine 1.6 1.5 Isoleucine 5.4 5.4 Leucine 9.8 9.7 Tyrosine 4.7 4.3 Phenylalanine 6.3 6.0 Histidine 4.3 4.1 Lysine 8.1 7.5 Arginine 9.6 9.0 2.3. Slaughter of Rabbits and Meat Samples At the end of fattening (108 days of age), 24 rabbits per genotype (12 males and 12 females ), were randomly selected (one rabbit per cage), weighed and slaughtered after a previous 12 h fasting in the abattoir. The rabbits were mechanically stunned with a captive bolt gun and bled, then the skin, distal parts of the tail, gastrointestinal and urogenital tracts and the distal part of the legs were removed according to the methodology described by Blasco and Ouhayoun [ 22 ] for rabbit meat research. Thereafter rabbit carcasses were placed in a cold storage chamber at a temperature of four ◦ C. After 24 h, from a chilled carcass, both LTL muscles and hind legs were dissected. Then, both hind legs were deboned. Finally, samples of the LTL and hind leg meat were packed and stored at −20 ◦C until analysed. 2.4. Laboratory Methods All samples were analysed in triplicate. The dry matter (DM) of the samples was determined by weight upon drying the sample at 105 ◦ C under the prescribed conditions. The Kjeldahl method using a Buchi analyser (Centec Automatika, spol. s.r.o., Prague, Animals 2022,12, 1537 4 of 15 Czech Republic) was performed to determine the crude protein (CP) content. A Soxhlet method was used to determine the ether extract by a Soxtec apparatus (Thermo Stientific, Warrington, UK). The ash was determined by weighing the sample after incineration at 550 ◦ C. Water/protein (W/P) ratio was calculated from the formula: (1000–dry matter content)/CP content. AA contents were determined following acid hydrolysis in 6 N HCl at 110 ◦ C for 24 h using the Automatic Amino acid Analyzer AAA 400 (Ingos a.s., Prague, Czech Republic), based on the colour-forming reaction of AA with the oxidative agent ninhydrin according to procedures used by Strakováet al. [ 23 ]. The AA analysis was used to determine the value of pure protein, expressed as the sum of EAA–lysine (Lys), leucine (Leu), isoleucine (Ile), threonine (Thr), arginine (Arg), histidine (His), phenylalanine (Phe), valine (Val), methionine (Met) and of non-essential AA (NEAA)–serine (Ser), asparagine (Asp), glutamine (Glu) , proline (Pro), glycine (Gly), alanine (Ala), tyrosine (Tyr). Because it was not possible to validly determine the cysteine content due to the used AA analysis, and the tryptophan content was not determined for the objective assessment of the quality of rabbit meat protein either, the representation of individual AAs assessed as their proportion from the total CP content in the analysed meat is stated in this work. The levels of mineral elements were determined through incinerating and leaching the sample by extraction and the subsequent titration according to the Association of Official Agricultural Chemists [ 24 ]. The content of the potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) was determined using an atomic absorption spectrometer Agilent Technologies 200 Series AA (HPST, s.r.o., Prague, Czech Republic). The phosphorus (P) content was determined using a spectrophotometer Helios α (Thermo Scientific, Great Britain). 2.5. Statistical Analysis The arithmetic mean and standard error of the mean (SEM) were determined for all assessed traits in respective evaluated groups. A Shapiro-Wilk test was used to test the normality of data distribution within the assessed groups. The normality was found in all the variables. Statistical evaluation of data followed basic procedures [ 25 ]. Differences in proximate chemical composition, AA proportion and mineral elements content between assessed carcass parts were carried out by ANOVA, with carcass part as a fixed effect and a random term for cage. Regarding assessment of genotype and sex effects, general linear model (GLM) procedure was used, where genotype and sex were included as fixed effects and their interaction as an interaction term. Random terms included slaughter weight (SW) and cage. Differences among groups were tested by Tukey’s post-hoc test. Significance was considered at the level p< 0.05. All statistical procedures were performed by the STATISTICA CZ version 10 software. 3. Results 3.1. Slaughter Traits and Proximate Chemical Composition of Meat The higher dry matter content of rabbit LTL was related to the higher CP content and lower W/P ratio (p< 0.001) as compared to hind leg meat (Table 2). The hind leg meat displayed the higher content of ether extract and ash than the LTL muscles (p< 0.001). When compared to the control (final crossbreds of H rabbits), a higher slaughter weight of MS sired progeny at 108 days of age (p< 0.001; Table 3) was associated with a higher weight of carcass (p< 0.001), hind leg meat and LTL muscles (p< 0.05). On the contrary, H rabbits displayed a higher hind legs yield as compared to the MS sired rabbits (p< 0.001). The sex of rabbits affected the carcass dressing and a higher level was found in males compared to females (p< 0.05). Concerning basic indicators of the proximate chemical composition of meat in both assessed carcass parts, the rabbit genotype influenced only the ash content in the hind leg meat; the higher value of ash content was observed in H rabbits (p< 0.01). The sex of rabbits affected only the content of ether extract in LTL, the lower value of ether extract was found in females compared to males (p< 0.05). In addition, Animals 2022,12, 1537 5 of 15 a significant effect of genotype x sex interaction on the content of CP and ash (p< 0.001) and on the W/P ratio (p< 0.01) in hind leg meat was found. Table 2. Effect of the carcass part on meat composition of rabbits. Item Meat p-Value Hind Leg LTL Proximate chemical composition (g/kg of fresh meat) Dry matter 241.7 ±1.65 250.7 ±1.74 <0.001 Crude protein 215.4 ±1.16 227.4 ±1.11 <0.001 Ether extract 24.6 ±0.91 16.5 ±0.67 <0.001 Ash 12.3 ±0.05 11.9 ±0.06 <0.001 W/P 3.53 ±0.025 3.30 ±0.022 <0.001 Amino acids (g/100 g of total crude protein) Lysine 8.86 ±0.193 8.88 ±0.172 0.844 Leucine 7.29 ±0.097 7.71 ±0.093 <0.001 Isoleucine 4.29 ±0.055 4.52 ±0.056 <0.001 Threonine 4.00 ±0.055 4.27 ±0.052 <0.001 Arginine 5.64 ±0.107 6.33 ±0.182 <0.001 Histidine 3.76 ±0.063 4.15 ±0.063 <0.001 Phenylalanine 4.26 ±0.138 3.90 ±0.055 0.004 Valine 4.74 ±0.070 4.97 ±0.071 <0.001 Methionine 1.84 ±0.038 2.07 ±0.042 <0.001 ∑essential AA 44.7 ±0.05 49.8 ±0.54 <0.001 Serine 3.44 ±0.043 3.67 ±0.042 <0.001 Asparagine 8.57 ±0.097 8.95 ±0.105 <0.001 Glutamine 14.4 ±0.16 14.6 ±0.16 0.150 Proline 3.76 ±0.050 3.70 ±0.058 0.223 Glycine 4.63 ±0.062 4.29 ±0.065 <0.001 Alanine 5.38 ±0.094 5.33 ±0.094 0.307 Tyrosine 3.37 ±0.079 3.54 ±0.081 <0.001 ∑non-essential AA 43.6 ±0.46 44.1 ±0.472 0.095 ∑all AA 88.3 ±0.07 90.9 ±0.91 <0.001 Mineral elements (mg/100 g of fresh meat) Phosphorus 229.8 ±4.24 224.8 ±9.60 0.558 Potassium 232.9 ±4.35 250.3 ±5.85 0.005 Sodium 55.4 ±0.78 58.3 ±1.61 0.113 Calcium 27.8 ±1.39 24.4 ±1.12 0.003 Magnesium 21.9 ±0.43 22.9 ±0.42 0.063 Copper 0.05 ±0.002 0.06 ±0.002 0.003 Iron 1.01 ±0.032 1.03 ±0.028 0.574 Manganese 0.10 ±0.006 0.10 ±0.005 0.514 Zinc 0.92 ±0.014 0.96 ±0.015 0.019 Data are means ± standard error of the mean. ∑ : sum. AA: amino acid. LTL:Longissimus thoracis et lumborum. W/P: water/protein ration. 3.2. Amino Acids Profile As for carcass parts assessed (Table 2), the higher content of both total AA and all EAA (p< 0.001) monitored was found in the LTL muscle as compared to hind leg meat. In particular, a higher proportion of Leu, Iso, Thr, Arg, His, Val, Met, Ser, Asp and Tyr was found in the CP of LTL compared to the CP of hind leg meat (p< 0.001). By contrast, higher proportions of Gly (p< 0.001) and Phe (p< 0.01) were found in the CP of hind leg meat than in LTL meat. Concerning a rabbit genotype (Table 4), a higher proportion of the total AA and all NEAA monitored (p< 0.001) as well as all EAA (p< 0.05) was found in the CP of hind leg meat of H rabbits as compared to MS sired rabbits. Similarly, in the CP of LTL meat, the higher proportion of total AA (p< 0.05) and all NEAA monitored (p< 0.01) was found in H rabbits as compared to MS sired rabbits. As for the proportion of particular AA in the CP of hind leg meat, higher values of Leu, Iso, Thr, Pro, Gly and Tyr (p< 0.001), of Val and Ser Animals 2022,12, 1537 6 of 15 (p< 0.01) and also of Asp and Glu (p< 0.05) were found in H rabbits as compared to MS sired rabbits. Concerning the proportion of particular AA in the CP of LTL meat, higher values of Gly and Tyr (p< 0.001), then of Leu, Thr, Val and Ser (p< 0.01) and also of Iso, Asp and Glu (p< 0.05) were observed in H rabbits than in MS sired rabbits. Table 3. Carcass traits and proximate chemical composition (g/kg of fresh meat) of rabbit meats in relation to the genotype and sex. Item Genotype p-Value H MS ×H Gen. Sex Gen. ×Sex M F M F Slaughter weight (g) 2754 ±81.3 2941 ±70.2 3321 ±63.9 3331 ±72.3 <0.001 0.167 0.213 ADG (g) 23.9 ±1.39 26.4 ±1.56 32.3 ±1.05 31.9 ±1.11 <0.001 0.490 0.267 Carcass weight (g) 1677 ±64.1 1735 ±78.0 2059 ±46.4 1983 ±48.2 <0.001 0.715 0.421 Carcass dressing (%) 60.9 ±0.80 58.8 ±0.97 61.1 ±0.63 60.0 ±0.27 0.137 0.012 0.453 Hind legs yield (%) 33.7 ±0.29 34.0 ±0.17 32.7 ±0.24 32.9 ±0.25 <0.001 0.209 0.956 Hind legs meat (g) 378.2 ±16.40 390.7 ±16.86 434.8 ±11.82 432.1 ±11.84 0.026 0.105 0.371 LTL (g) 210.9 ±10.86 228.0 ±13.75 256.5 ±10.68 241.5 ±7.16 0.036 0.108 0.173 Hind leg meat Dry matter 237.3 ±3.26 240.1 ±4.16 245.0 ±2.95 244.4 ±2.54 0.362 0.815 0.637 Crude protein 210.0 ±2.11 b216.5 ±2.44 a,b 221.2 ±1.39 a213.7 ±2.09 a,b 0.241 0.704 <0.001 Ether extract 22.8 ±2.32 22.5 ±1.51 24.0 ±1.64 28.9 ±1.18 0.404 0.229 0.077 Ash 12.3 ±0.11 a12.5 ±0.09 a12.4 ±0.07 a12.0 ±0.08 b0.002 0.152 <0.001 W/P 3.64 ±0.049 a3.52 ±0.055 a,b 3.41 ±0.024 b3.54 ±0.046 a,b 0.238 0.824 0.005 LTL Dry matter 247.8 ±4.05 249.5 ±3.12 255.8 ±2.63 249.7 ±3.88 0.827 0.436 0.336 Crude protein 223.2 ±2.25 227.6 ±2.75 230.1 ±1.10 228.7 ±2.15 0.799 0.701 0.275 Ether extract 18.3 ±2.08 15.1 ±0.87 17.3 ±1.07 15.3 ±0.86 0.166 0.028 0.451 Ash 11.7 ±0.18 12.0 ±0.10 12.0 ±0.05 11.9 ±0.07 0.860 0.358 0.121 W/P 3.38 ±0.044 3.30 ±0.051 3.23 ±0.021 3.29 ±0.046 0.740 0.960 0.239 Data are means ± standard error of the mean. a,b : In rows, means with different superscript letters differ at p< 0.05 . H: HYLA. MS: Mecklenburger Schecke. M: male. F: female. Gen.: genotype. ADG: average daily gain from 35 to 108 days. LTL: Longissimus thoracis et lumborum. W/P: water/protein ratio. Table 4. Amino acid profile (g/100 g of total crude protein) of rabbit meats in relation to the genotype and sex. Item Genotype p-Value H MS ×H Gen. Sex Gen. ×Sex M F M F Hind leg meat Lysine 8.94 ±0.102 a,b 8.80 ±0.147 b,c 7.65 ±0.164 c10.0 ±0.57 a0.756 0.001 <0.001 Leucine 7.69 ±0.184 7.64 ±0.092 6.81 ±0.137 7.02 ±0.216 <0.001 0.895 0.270 Isoleucine 4.49 ±0.099 4.51 ±0.056 3.99 ±0.077 4.18 ±0.126 <0.001 0.453 0.219 Threonine 4.28 ±0.096 4.19 ±0.058 3.77 ±0.069 3.77 ±0.119 <0.001 0.401 0.439 Arginine 5.88 ±0.064 5.60 ±0.066 5.21 ±0.086 5.86 ±0.393 0.991 0.294 0.052 Histidine 3.97 ±0.059 a3.81 ±0.076 a3.26 ±0.065 b4.03 ±0.144 a0.091 0.003 <0.001 Phenylalanine 4.07 ±0.087 b3.99 ±0.202 b3.54 ±0.153 b5.43 ±0.267 a0.114 <0.001 <0.001 Valine 5.07 ±0.148 a4.87 ±0.071 a4.35 ±0.093 b4.66 ±0.144 0.002 0.883 0.034 Methionine 2.05 ±0.082 1.76 ±0.047 1.87 ±0.084 1.69 ±0.043 0.444 0.004 0.477 ∑essential AA 46.4 ±0.69 a45.2 ±0.67 a40.5 ±0.76 b46.7 ±0.73 a0.034 0.003 <0.001 Serine 3.63 ±0.069 3.57 ±0.055 3.31 ±0.056 3.26 ±0.107 0.001 0.285 0.718 Asparagine 8.90 ±0.124 a8.64 ±0.183 a,b 8.10 ±0.163 b8.36 ±0.231 a,b 0.011 0.694 0.015 Animals 2022,12, 1537 7 of 15 Table 4. Cont. Item Genotype p-Value H MS ×H Gen. Sex Gen. ×Sex M F M F Glutamine 14.8 ±0.20 a14.7 ±0.23 a13.4 ±0.28 b14.8 ±0.37 a0.012 0.105 0.006 Proline 3.83 ±0.079 3.99 ±0.094 3.62 ±0.075 3.60 ±0.113 <0.001 0.726 0.523 Glycine 5.05 ±0.072 a4.59 ±0.130 b4.41 ±0.084 b4.46 ±0.118 b<0.001 0.022 0.008 Alanine 4.97 ±0.259 5.73 ±0.081 5.16 ±0.111 5.67 ±0.157 0.477 0.003 0.666 Tyrosine 4.14 ±0.076 a3.42 ±0.093 b2.84 ±0.042 c3.09 ±0.053 c0.000 0.001 <0.001 ∑ non-essential AA 45.3 ±0.69 a44.6 ±0.73 a40.8 ±0.76 b43.5 ±0.98 a,b <0.001 0.459 0.020 ∑all AA 91.8 ±1.37 a89.8 ±1.36 a81.3 ±1.51 b90.2 ±0.87 a0.002 0.031 0.001 LTL Lysine 9.03 ±0.088 a,b 8.72 ±0.130 a,b 8.01 ±0.187 b9.75 ±0.558 a0.598 0.017 0.003 Leucine 7.97 ±0.129 a7.84 ±0.112 a,b 7.24 ±0.199 b7.78 ±0.225 a,b 0.005 0.518 0.027 Isoleucine 4.66 ±0.086 4.60 ±0.067 4.26 ±0.113 4.55 ±0.142 0.035 0.469 0.083 Threonine 4.48 ±0.074 a4.36 ±0.066 a3.99 ±0.110 b4.26 ±0.111 a,b 0.009 0.645 0.036 Arginine 6.46 ±0.060 a,b 5.89 ±0.097 b5.51 ±0.140 b7.46 ±0.583 a0.273 0.023 <0.001 Histidine 4.31 ±0.058 a,b 4.02 ±0.056 b,c 3.80 ±0.097 c4.49 ±0.160 a0.912 0.060 <0.001 Phenylalanine 4.15 ±0.048 a3.84 ±0.046 a,b 3.52 ±0.086 b4.11 ±0.132 a0.165 0.150 <0.001 Valine 5.40 ±0.066 a4.99 ±0.083 a,b 4.57 ±0.137 b4.94 ±0.158 b0.002 0.652 0.002 Methionine 2.14 ±0.073 1.97 ±0.095 2.04 ±0.069 2.12 ±0.098 0.806 0.647 0.215 ∑essential AA 48.6 ±0.44 a,b 46.2 ±0.64 b,c 42.9 ±1.08 d49.5 ±0.97 a0.209 0.028 <0.001 Serine 3.84 ±0.046 a3.71 ±0.060 a,b 3.45 ±0.091 b3.66 ±0.095 a,b 0.009 0.867 0.027 Asparagine 9.40 ±0.101 a8.90 ±0.144 a,b 8.47 ±0.241 b9.04 ±0.246 a,b 0.047 0.934 0.009 Glutamine 15.5 ±0.13 a14.4 ±0.22 a,b 13.8 ±0.34 b14.6 ±0.38 a,b 0.012 0.422 0.002 Proline 3.65 ±0.064 3.83 ±0.106 3.60 ±0.075 3.73 ±0.183 0.271 0.285 0.943 Glycine 4.76 ±0.052 a4.37 ±0.077 b3.91 ±0.130 c4.13 ±0.108 b,c <0.001 0.231 0.002 Alanine 4.92 ±0.263 5.63 ±0.077 5.22 ±0.147 5.57 ±0.155 0.532 0.019 0.506 Tyrosine 4.34 ±0.051 a3.49 ±0.068 b3.09 ±0.054 c3.25 ±0.130 b,c <0.001 <0.001 <0.001 ∑ non-essential AA 46.4 ±0.49 a44.3 ±0.62 a,b 41.6 ±1.06 b44.0 ±1.01 a,b 0.005 0.894 0.009 ∑all AA 95.0 ±0.92 a90.6 ±1.18 a84.5 ±2.14 b93.5 ±1.19 a0.018 0.227 <0.001 Data are means ± standard error of the mean. a,b,c : In rows, means with different superscript letters differ at p< 0.05 . H: HYLA. MS: Mecklenburger Schecke. M: male. F: female. Gen.: genotype. ∑ : sum. AA: amino acid. LTL:Longissimus thoracis et lumborum. Regarding the effect of gender (Table 4), in the CP of hind leg meat, a higher proportion of all EAA (p< 0.01) and the total AA monitored (p< 0.05), in particular of Phe (p< 0.001) and also of Lys, His and Ala (p< 0.01) was found in females as compared to males. By contrast, there was a higher proportion of Met and Tyr (p< 0.01), as well as Gly (p< 0.05) in the CP of male hind leg meat as compared to female hind leg meat. In addition, a significant effect of genotype x sex interaction on the proportion of all EAA (p< 0.001), total AA (p< 0.01 ), as well as NEAA (p< 0.05) monitored in CP of hind leg rabbit meat was found; specifically, on the proportion of Lys, His, Phe, Tyr, Glu, Val, Asp and Gly. Within LTL, the rabbit sex influenced the proportion of all EAA monitored (p< 0.05), of Tyr ( p< 0.001 ) and of Ala, Lys and Arg (p< 0.05) in CP of this meat; the higher proportion of total EAA, Ala, Lys and Arg and the lower Tyr proportion was found in females. Moreover, the rabbit genotype x sex interaction had a considerable effect on the proportion of the total AA and all EAA (p< 0.001) and of all NEAA monitored (p< 0.01) in LTL meat; specifically, on the proportion of Arg, His, Phe, Tyr, Lys, Val, Asp, Glu, Gly, Thr and Ser. 3.3. Mineral Elements As for rabbit carcass parts, the higher Ca level (p< 0.01) and lower K and Cu (p< 0.01) and Zn (p< 0.05) level was observed in hind leg meat as compared to LTL meat (Table 2). Regarding a rabbit genotype (Table 5), the higher content of Mg and Mn (p< 0.001), Zn (p< 0.01) and K (p< 0.05) in hind leg meat was found in MS sired rabbits as compared to H rabbits. In LTL meat, a higher level of Na and Mn (p< 0.001), Mg and Zn (p< 0.05) was observed in MS sired rabbits than in H rabbits. Animals 2022,12, 1537 8 of 15 Table 5. Content of mineral elements (mg/100 g of fresh meat) of rabbit meats in relation to the genotype and sex. Item Genotype p-Value H MS ×HGen. Sex Gen. ×Sex MFMF Hind leg meat Phosphorus 231.8 ±9.94 227.4 ±11.57 228.7 ±5.44 231.0 ±6.62 0.218 0.605 0.462 Potassium 243.8 ±12.33 212.5 ±6.15 240.5 ±4.78 235.1 ±7.55 0.014 0.129 0.328 Sodium 60.1 ±1.17 a52.9 ±1.40 b52.2 ±1.22 b56.5 ±1.41 a,b 0.238 0.271 <0.001 Calcium 25.0 ±1.48 b37.0 ±4.36 a26.1 ±0.73 b23.1 ±0.64 b0.341 0.038 0.002 Magnesium 20.5 ±0.61 19.3 ±0.46 25.3 ±0.80 22.7 ±0.35 <0.001 0.008 0.249 Copper 0.04 ±0.003 0.05 ±0.004 0.06 ±0.002 0.06 ±0.002 0.506 0.243 0.090 Iron 0.87 ±0.071 b1.16 ±0.090 a1.02 ±0.013 a,b 1.00 ±0.013 a,b 0.847 0.023 0.009 Manganese 0.06 ±0.009 0.07 ±0.011 0.14 ±0.004 0.13 ±0.004 <0.001 0.747 0.485 Zinc 0.87 ±0.026 0.88 ±0.017 0.92 ±0.029 1.02 ±0.013 0.003 0.016 0.094 LTL Phosphorus 219.4 ±9.00 228.9 ±16.75 228.1 ±7.01 223.0 ±5.46 0.172 0.735 0.967 Potassium 272.2 ±12.30 238.0 ±14.66 260.3 ±5.55 230.7 ±9.33 0.773 0.018 0.969 Sodium 52.1 ±1.26 48.5 ±1.62 63.5 ±2.86 69.0 ±2.63 <0.001 0.556 0.097 Calcium 24.9 ±1.49 23.6 ±2.56 25.2 ±0.47 23.9 ±1.09 0.415 0.607 0.846 Magnesium 22.0 ±0.69 21.1 ±0.57 24.4. ±0.79 24.0 ±0.96 0.017 0.568 0.860 Copper 0.05 ±0.003 0.06 ±0.004 0.06 ±0.002 0.06 ±0.002 0.785 0.187 0.172 Iron 0.95 ±0.07 1.14 ±0.070 1.08 ±0.019 0.96 ±0.028 0.182 0.774 0.017 Manganese 0.07 ±0.008 0.08 ±0.008 0.14 ±0.004 0.12 ±0.004 <0.001 0.449 0.187 Zinc 0.93 ±0.036 0.92 ±0.032 1.02 ±0.021 0.99 ±0.026 0.015 0.643 0.616 Data are means ± standard error of the mean. a,b : In rows, means with different superscript letters differ at p< 0.05 . H: HYLA. MS: Mecklenburger Schecke. M: male. F: female. Gen.: genotype. LTL:Longissimus thoracis et lumborum. Concerning a sex effect (Table 5), a higher level of Ca, Fe and Zn (p< 0.05) and lower level of Mg (p< 0.01) was found in the hind leg meat of females as compared to males. A significant effect of genotype x sex interaction on the content of Na (p< 0.001), Ca and Fe ( p< 0.01 ) in this meat was also found. In LTL meat, a higher content of K was found in males compared to females (p< 0.05). In addition, the rabbit genotype x sex interaction had a significant effect on the content of Fe (p< 0.05) in LTL. 4. Discussion 4.1. Slaughter Traits and Proximate Chemical Composition of Meat A higher SW of MS sired progeny at 108 days in the present study is similar to that found in 100-day-old New Zealand White rabbit males [ 26 ] and considerably higher than the 112-day-old Burgundy Fawn and Vienna Blue sired crossbred rabbits reared under organic production [ 17 ]. The value of SW in HYLA rabbits of the present study was significantly lower than in the same rabbit genotype fattened for 103 days [ 27 ]. Although MS sired progeny of the present study attained higher SW during the extended fattening which was also related to a heavier weight of hind leg and LTL meat, a higher yield of hind leg meat was found in fattened H rabbits than in MS sired rabbit progeny ( 13.5 vs. 13.1% of SW , respectively). This fact is demonstrated by the higher muscularity of hind legs in final commercial crossbred H rabbits. The hind leg meat yield of rabbit genotypes in the present study was similar to that reported for the Belgian Giant Grey and Termond White rabbits [ 28 ]. In the case of LTL muscles, their yield was also slightly higher in H rabbits than in MS sired rabbits (7.8 vs. 7.5% of SW, respectively) and a higher LTL yield was found in H females than in MS sired females (7.8 vs. 7.3%, resp.). In the present study, the lower meat yield of the assessed rabbit groups was found in MS sired female progeny. Additionally, slightly lower values for the LTL yield (6.9 to 7.1%) were found in 78-day-old rabbits [ 16 ], as compared to 108-old rabbits of the present study; however, it is well known that the Animals 2022,12, 1537 9 of 15 yield of hind leg meat increases with rabbit age [ 29 ]. Within the rabbit commercial breeding scheme, breeding for increased muscle volume in distinctive lines has begun recently. Due to the interest of consumers, the goal of this selection was focused mainly on increasing the volume of hind leg meat [ 30 , 31 ]. This is probably related to the findings of the present study in which a higher meat yield was found in final commercial crossbred H rabbits. In addition, it was found that a rabbit line selected for a higher thigh muscle volume also displayed some better production traits during fattening [32]. Regarding the proximate chemical composition of meat in the present study, the assessed traits were influenced mainly by a carcass part and the higher DM content of the LTL muscle was associated with the higher CP content and lower W/P ratio as compared to hind leg meat. This finding is in agreement with findings reported in three-month-old rabbits [ 29 , 33 ]. By contrast, Króliczewska et al. found the higher CP content in hind leg meat compared to LTL, furthermore the DM content was also higher in LTL meat of the five and a half-month-old New Zealand White rabbits [34]. The lower W/P ratio in LTL of the present study favours this rabbit meat for subsequent meat processing, since it contributes to a higher product yield due to lower loss of its own water and better ability to hold added water [ 33 ]. When compared to LTL, a higher intramuscular fat (IMF) level was found in hind legs in the present study, furthermore its content is inversely associated with the content of CP and water in rabbit meat [ 35 ]. This finding is in agreement with earlier reported results [ 29 , 33 ]; however, the IMF value in hind legs of the present study is still generally low, which confirms a leanness of rabbit meat [ 36 ]. Higher ash content was found in hind leg meat in the present study as compared to LTL. By contrast, the opposite trend for the ash level was found in the Pannon White rabbits [ 29 ]; their SW were similar to rabbits in the present study. The total mineral content in meat in the present study is similar to that found by Perna et al. and by Daszkiewicz and Gugolek [26,33]. A rabbit genotype in the present study influenced only the total mineral content in hind leg meat, its higher value was found in H rabbits. Metzger et al. [ 29 ] mention that rabbits with higher body weight (BW) at a given age may have a lower ash content in hind leg meat, which is in agreement with the finding of the present study in which MS sired females gained the higher SW and the significantly lowest total mineral content. In the present study, a rabbit’s gender affected only the IMF level in LTL; leaner meat was found in females. Similarly, Ortiz Hernández and Rubio Lozano reported lower IMF content in 70-day-old females of the New Zealand White breed compared to their males; nevertheless, they found the opposite trend in the Californian breed [ 37 ]. It is known that a higher growth intensity of rabbit results in a higher IMF content, if measured at the same age [ 29 ]. In the present study, rabbit SW considerably differed between genotypes assessed, which is a reason why the SW was used as a random effect in the statistical equation. Therefore, the slightly higher IMF content in hind leg meat of MS sired rabbits was not different from that found in H rabbits. However, the IMF content of LTL meat was higher in males of the present study, which also was observed (p> 0.05) by North et al. [ 38 ] in the LTL of meat rabbit genotypes. It seems that male rabbits may in some cases deposit more IMF into the LTL muscle with advancing age. Based on the IMF content of hind leg meat in the present study, MS sired females were most mature at the given slaughter age. 4.2. Amino Acid Profile In general, meat AA composition is influenced by different syntheses of AA as related to different biological stages of animals [ 8 ]. For consumers it is important to meet demands for EAA, as the intrinsic contents of EAA are usually used to assess a biological value of proteins [ 39 ]. A better nutritional value of proteins in LTL as compared to hind leg meat in rabbits of the present study was presented by a higher proportion of total AA (+2.9%) and particularly all EAA assessed (+11.4%). On the other hand, in hind leg meat, a higher proportion of Phe (+9.2%) and Gly (+7.9%) was found. In particular, the higher proportion of seven out of nine EAA assessed and of three NEAA was found in LTL meat. The order of representation of individual EAAs was almost identical in both evaluated rabbit carcass