Efecto del sistema de alimentación y del sexo sobre la ingestión, el rendimiento productivo y las características de la canal y de la carne de corderos de raza Assaf y del cruce Assaf X Merina

Abstract

The aim of the present study was to examine the effect of feeding system and sex on Assaf or Assaf x Merino lambs at different stages of growth on feed intake, animal performance and non-carcass (weight, components and chemical composition), carcass (weight, loss during chilling, yield, conformation, degree of fat cover, colour of subcutaneous fat, morphology, and primal cut, tissue and chemical composition) and meat (pH, colour, water-holding capacity, texture and chemical composition) characteristics. To achieve this objective, 3 experimental trials were conducted, using animals at different stages of growth. In the first trial, 36 new born lambs (18 males and 18 females) were used to study the effect of feeding system (natural suckling vs. artificial rearing) and sex on daily weight gain, body composition, and carcass and meat characteristics of milk-fed Assaf lambs. The lambs were divided into 6 experimental groups according to a 3 x 2 factorial design defined by 2 sexes (males vs. females) and 3 feeding systems (NS: natural suckling; ARad: artificial rearing with ad libitum milk intake twice a day; and AR-rest: artificial rearing with a milk intake amounting to 80% of the previous group's intake). Groups were equilibrated according the live weight at birth. All lambs (natural suckling and artificially reared) were slaughtered once they had reached 10 kg of live body weight (LBW). Average daily weight gain (ADG) was not affected by sex (P>0.10), but significant differences were observed according to feeding system (P<0.05). The highest values obtained corresponded to the NS group, and the lowest values to the AR-rest group (307, 253 and 150 g/day for groups NS, AR-ad and AR-rest, respectively). Resúmenes 224 Sex did not affect (P>0.10) non-carcass weight although wool weight and total internal and digestive fat deposits were higher in females (P<0.05). Females also presented a higher total fat content in non-carcass component (P<0.05). Feeding system did not significantly affect either the weight or chemical composition of non-carcass (P>0.10). However, significant differences were observed (P<0.05) in the weight of wool (145, 160 and 199 g for groups NS, AR-ad and AR-rest, respectively), red offal (432, 470 and 497 g), white offal (629, 620 and 560 g), and total internal fat (216, 202 and 187 g), with the AR-rest group presenting higher values than the NS group for the first two parameters, but lower values than the same group for the other two parameters. Sex had no effect (P>0.05) on carcass weight, chilling losses, dressing percentage or carcass conformation and fatness. In contrast, feeding system had a significant effect on all parameters above mentioned (P<0.05) except in dressing percentage. Both conformation and fatness score were higher in NS than AR-rest animals, while intermediate values were obtained for AR-ad lambs. In contrast, values for chilling losses were higher in the AR-rest group than in the AR-ad group, while the NS group presented intermediate values. Sex did not significantly affect carcass pH at 24 h post mortem nor the colorimetric parameters for subcutaneous fat (P>0.10). Feeding system did not affect carcass pH either (P>0.10), but a trend towards significance was observed for fat brightness (P<0.10), with AR-ad animals obtaining a higher value for this parameter than animals in the other two groups. No significant differences were observed in the weight of the different commercial carcass cuts by sex (P>0.10), although internal carcass length was significantly higher in females (40.3 vs. 39.1 cm). Neither did sex significantly affect the weight of the main tissue components obtained by dissection of the shoulder (muscle, fat and bone), although the ratio between muscle and fat weight was significantly lower in females (P<0.05). In contrast, feeding system did affect the weight and tissue composition of the shoulder (P>0.05), with a higher weight (297, 306 and 332 g for NS, AR-ad and AR-rest groups, respectively) and proportion of muscle (58.1, 61.5 and 62.0%) in AR-rest animals than in NS animals. However, content of subcutaneous fat (37, 28 and 30 g for NS, AR-ad and AR-rest groups, respectively) and intermuscular fat (36, 28 and 32 g) in the shoulder was higher in the NS group than in AR-rest animals. These differences in shoulder fat content Resúmenes 225 was also reflected in carcass fat content, which was significantly higher (P<0.05) in females (11 vs. 8 g/kg) and in NS animals (11, 10 and 8 g/kg for NS, AR-ad and AR-rest groups, respectively). Sex did not affect any of the recorded parameters indicative of meat quality (area, pH, colour, water-holding capacity, tenderness), with the exception of chemical composition, where meat from female lambs had a lower water content (744 vs. 766 g g/kg) and a higher fat content (29 vs. 16 g/kg). Feeding system only affected (P<0.05) water holding capacity (water losses under pressure); meat from AR-rest animals presented the highest value obtained for this parameter, while the lowest value corresponded to the NS group (10.0, 11.6 and 11.8%). In the second trial, 24 recently weaned (37 ± 1.9 days of age and 14.5 ± 0.08 kg of LBW) Assaf lambs (12 males and 12 females) were divided into four groups of equal weight, according to a 2 x 2 factorial design to study the effect of sex (male and female) and feeding system [conventional system (animals were fed long form barley straw and concentrate pellet offered ad libitum in separated feeding troughs)] vs. alternative free choice system [animals were fed whole-grain cereal and a protein supplement, offered ad libitum in different troughs) on feed intake, daily weight gain, body composition and carcass and meat characteristics. All lambs were slaughtered once when they had reached 25 kg of LBW. Lambs reared on the free choice system presented higher mean values of dry matter (861 vs. 765 g·animal-1 ·day-1 ; P<0.001) and protein (205 vs. 123 g·animal-1 ·day-1 ; P<0,001) intake than those reared on the conventional system. This difference in intake resulted in significant (P<0.05) differences in ADG (371 vs. 272 g/day) and feed to gain conversion rate (FCR, 2.40 vs. 2.83 g DM intake/g ADG), although only in the case of males. The differences in these parameters were associated with a greater deposition of fat both in the carcass (16.08 vs. 13.14 %; P<0.05) and in non-carcass (15.18 vs. 10.04%; P<0.05) in females. Sex did not significantly affect feed intake (P>0.10), but did affect ADG (238 vs. 321 g/day; P<0.05) and FCR (3.39 vs. 2.61 g food/g weight gain, P<0.05), which were lower and higher, respectively, in females than in males. Neither sex nor feeding system significantly affected carcass weight (12254 ± 466.7 g), non-carcass weight (9569 ± 438.1 g), dressing percentage (47.5 ± 1.01%) or true yield Resúmenes 226 (56.2 ± 0.83%) (P>0.10). However, animals raised on the free choice system presented a higher blood weight (1258 vs. 1161 g; P<0.05) and red offal weight (1500 vs. 1400 g; P<0.05). Males also presented a higher blood weight (1250 vs. 1170 g; P<0.05) and white offal weight (1802 vs. 1635 g; P<0.05) than females. The only carcass characteristics that differed according to feeding system were leg weight, buttock perimeter and colour of subcutaneous fat (P>0.05). Both buttock perimeter (55.2 vs. 56.3 cm; P<0.05) and leg weight (2017 vs. 2082 g; P<0.05) were lower in animals raised on the free choice feeding system. Subcutaneous fat of lambs reared on the free choice system presented higher values for the red index (4.68 vs. 3.18; P<0.05) and the yellow index (10.25 vs. 8.96; P<0.05), and therefore showed a lower hue (64.94 vs. 71.00; P<0.05) and higher chroma (11.36 vs. 9.55; P<0.05) values. Sex did not affect the colour of subcutaneous fat, but females carcasses presented more fatness and a greater buttock perimeter (56.4 vs. 55.2 cm; P<0.05) but lower values for thorax depth (24.2 vs. 24.8 cm; P<0.05) and leg length (34.2 vs. 35.3 cm; P>0.05). These differences were not associated with differences in the carcass cut weights, except that breast-flank (678 vs. 606 g; P<0.05) and tail (312 vs. 199 g; P<0.05) weighed more in females. Feeding system did not significantly affect any meat characteristics (P > 0.05), except water loss during cooking, which was lower in animals reared on the free choice system (11.9 vs. 14.7%; P<0.05). The only parameter affected by sex was chemical composition, with meat from females presenting a higher fat content (28.1 vs. 21.0 g/kg P<0.05). In the third trial, 24 newly weaned lambs (14.4 ± 0.09 kg of LBW) were divided into 4 groups, according to a 2 x 2 factorial design to study the effect of sex (male vs female) and genotype (pure Assaf vs Assaf x Merino) on feed intake, daily weight gain, body composition and carcass and meat characteristics. Lambs were using a conventional intensive feeding system (animals were fed long form barley straw and concentrate pellet offered ad libitum in separated feeding troughs) and they were slaughtered when they reached 20 kg of LBW. Genotype had no effect on total feed intake, but Assaf x Merino genotype animals presented a higher ADG (299 vs. 224 g/day; P<0.05) and a lower FCR (2.40 vs. 3.05 g DM intake/g ADG; P<0.05). Sex affected both parameters, with males presenting a higher ADG and a lower FCR. Resúmenes 227 Non-carcass from Assaf x Merino genotype animals weighed less (7887 vs. 8294 g; P<0.05), because of the lower weight of offal (3521 vs. 3901 g; P<0.05) and wool (338 vs. 501 g; P<0.05). Sex did not significantly affect non-carcass weight (P>0.10), but the wool from female animals weighed more (478 vs. 373 g; P<0.05). Both sex and genotype influenced chemical composition of non-carcass, with a lower fat content in Assaf x Merino animals (95.0 vs.120.0 g/kg; P<0.05) and in m