Crossbreeding, experimental groups and feeding
This investigation was carried out according to The Care and Use of Agricultural Animals in Research and Teaching guidelines at the Fars Agricultural and Natural Resources research and Education Center, Shiraz, Iran.
A total of 180 Grey Shirazi ewes belonging to the Fars Agricultural and Natural Resources research and Education Center, Shiraz, Iran, were used for this study. Two separate place breeding was created. In first place, breeding between Ghezel rams (n = 6, live weight of 88 ± 3 kg, age average 2.5 ± 0.5 years old) with Grey Shirazi ewes (n = 90, live weight of 45 ± 5 kg, age average 2.5 ± 0.5 years old) and in second place breeding between Grey Shirazi rams (n = 6, live weight of 75 ± 3 kg, age average 2.5 ± 0.5 years old) with Grey Shirazi ewes (n = 90 live weight of 45 ± 5 kg, age average 2.5 ± 0.5 years old) was done. The ratio of ram to ewe was 1:15 in each flock and allowed ability to fresh water and mineral blocks. The concentrate composition was 40% barely grain, 30% corn grain, 27.5% wheat bran, 0.5% calcium carbonate, 1% salt and 1% vitamin mineral premix. After mating, the rams separated and ewes in each flock were kept separately. After lambing, ewes and theirs lambs were numbered and lambs were kept with mothers until weaning. Average birth weight of lambs in purebred and crossbred flocks were 4.5 ± 0.3 kg and 5.0 ± 0.2 kg respectively. Afterwards the lambs in each group were kept in the similar conditions until selection time for present experiment. For the main study, 48 weaned lambs (live weight of 28.50 ± 0.5 kg and 115 ± 5 days of age) (24 male and 24 female lambs) were taken randomly from the each flock and were penned individually in separately cages. Lambs were divided into 4 groups (treatments) (12 lambs per each group). The GM group (12 purebred male lambs of Grey Shirazi ewe × Grey Shirazi ram), GF group (12 purebred female lambs of Grey Shirazi ewe × Grey Shirazi ram), CM group (12 crossbred male lambs of Grey Shirazi ewe × Ghezel ram) and CF group (12 crossbred female lambs of Grey Shirazi ewe × Ghezel ram). Animals were permitted 14 days adaptation. At the beginning of the adaptation diet, all the lambs were cured for external and internal parasites and immunized against enterotoxaemia. The animals were fed to meet requirements according to NRC (2007). The formulated diet contained 13.49% CP and 2.40 Mcal ME/kg DM for ADG of 250 g/d and the percent of forage to concentrate ratio was 47:53 % of DM. The lambs were nourished two times every day (08:15 and 17:15 h) ad libitum to ensure 5% orts and with allowed ability to fresh water and mineral–vitamin bricks. In the all days before morning feed meal, surplus feed of the earlier day was gathered and the orts were measured.
Growth performance
Lambs weight was measured each 15-d intervals during 90 d of the experiment. Body weight was measured for each separate lamb prior to morning feed meal. The ADG was determined for each lamb from day 1 to day 90. Feed conversion ratio (FCR) was calculated as the ratio between DMI and ADG.
Carcass traits
At the finish of the trial days, lambs were weighed after 16 h fasting. Lambs were slaughtered separately by cutting the arteries and the jugular vein in the neck region and were allowed to whole bleeding (Farid 1991). Then the lamb's skins were separated and weights of skin, head and feet were recorded. The internal organs such as rumen, kidneys, lungs, spleen, heart and liver were separated and weighed. The internal fat content was completely separated and weighed. Gut digestive contents were removed and empty body weight was determined by detracting these contents from body weight. The hot carcasses of each lambs were weighed and carcasses were kept at refrigerator (temperature of 4 °C for 24 h). Then weight of cold carcasses was measured. The cold carcasses halved cautiously (Farid 1991). Commercial Iranian cuts performed for halving. The cuts on the left side of each carcass were stored at -20 °C. The commercial cuts were neck, brisket, thigh, hands, flank, loin, fat tail. For these sections subcutaneous fat in excess of 5 mm was removed, deboned and the weight of each section was recorded. Fat trim and meat tissue from each wholesale cut were combined, ground twice, thoroughly mixed, and samples of approximately 100 g were taken. The samples were packed and frozen at -20 °C for subsequent chemical analysis.
Carcasses were cut between rib 12 and 13 and width and depth of rib eye area (Longissimus dorsi; LD area) were measured on the left cut carcass (Esenbuga et al. 2009). The back-fat thickness was measured in 2 points above section of the LD. The LD area was measured by acetate paper and planimeter. The maximum depth and width of LD was measured by digital coulis. Tight, hand, neck, loin, brisket, flank, fat tail, total fat half-carcass without fat tail, total fat half-carcass with fat tail, leans and bones were separated and weighed (Farid 1991).
Laboratory analyses
The samples of consumed ration and orts were dried by oven at 60 °C for 48 h and were ground in size of 1 mm. Dry matter of samples was measured using oven drying at 105 °C for 16 h (AOAC 1990; method 930.15). The amount of Ash was measured using burning of samples at 550 °C for 12 h. Organic matter content was calculated as difference between sample weight and ash content (AOAC 1990; method 942.05). The NDFom was determined by method of Van Soest et al. (1991). ADFom content was determined according to AOAC (1990; method 973.18). Lignin was measured by solubilization of cellulose with sulfuric acid (Robertson and Van Soest 1981). Nitrogen content was measured by the Kjeldahl device (AOAC 1990; method 954.01).
For meat chemical analysis, moisture, ash, ether extract (EE) and crude protein (CP) were determined according to the AOAC (1990) methods.
Statistical analyses
Data were analyzed using the general linear model with a completely randomized design considering genotype, sex and genotype × sex interaction as variables. Data was analyzed by the SAS (2001) software and using the initial weight as a covariate. The means treatments were compared by Tukey's test with 5% probability level.
Yijk= µ+ Gi + Sj + GSij + 1(Cov)m + eijk
Where Yijkl is the overall observation, μ the overall mean, Gi the ith effect of genotype, Sj, the jth effect of sex, GSij the interaction of genotype and sex, 1(Cov)m the covariate effect and eijkl the standard error term.