الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
1. Introduction
The introduction dealt with the reasons for resorting to rabbits as a meat animal, whether from a productive perspective for the breeder or from a nutritional perspective for the consumer, and the importance of working to develop the efficiency of meat production from it.
The introduction explained the reasons for the importance of rabbits for the breeder, as they possess interesting physiological characteristics that could make it an ideal animal producing meat, it given a large letter size, had early reproductive maturity, and it has a short length of the generation interval. Rabbits are also monogastric herbivores, can adapt and produce under diverse ecosystems. from an economic point of view, rabbit farming is characterized by low initial cost, low land requirements, high capital return rate and rapid production cycle. The importance of rabbits as a nutritional value to the consumer is due to the fact that, compared to other animal species, rabbit’s meat contains high levels of protein with essential amino acids, high content of unsaturated fatty acids, low levels of cholesterol and sodium, and many vitamins especially vitamin E and B12, so its meat is highly recommended for pregnant women, the elderly, and teenagers who require higher than average nutritional care. Previous studies have shown a decrease in meat consumption as a result of its high prices, which require intensive work to search for breeding methods to increase the efficiency of meat production from rabbits.
The introduction ended by identifying the best breeding methods to work on increasing the meat production capacity from rabbits using selection indices using an estimate of genetic and phenotypic parameters for live performance traits and carcass characteristics.
2. Review of literature
2.1. This chapter dealt with a reference review of estimates of means and coefficients of variation for body weight characteristics and linear body measurements, in addition to carcass characteristics in previous results.
2.2. This section also reviewed estimates of heritability estimates for required traits related to the live performance and carcass trait of rabbits.
3.3. This chapter discussed the genetic and phenotypic correlations between live body weight traits, body linear measurements, and carcass traits in various references.
3.4. Finally, this chapter discusses the methods of applying selection indices in previous studies and the recommended indexes with its undesirable consequences on some carcass traits in rabbits, in which restricted selection index was recommended to prevent the expected deterioration of these characteristics.
Its notable that all previous studies lack the application of the double restricted selection indices to prevent the deterioration in two traits at the same time, which is what will be addressed in this study.
3. Materials and methods
- The animals used in this study were reared in a private rabbit farm, located in Qalyobia governorate (10 km from Cairo), during the period from 2021-2022, consisting of 458 growing NZW rabbits progeny of 25 bucks and 92 mature does, chosen randomly at weaning on 28 days of age.
- At weaning, rabbits were weighted (WW), ear-tagged and separated in rearing cages for fattening under natural environmental circumstances. They fed ad libitum a commercial diet containing 2800 kcal of digestible energy/kg up to slaughter age.
- At 90 days old of age, the animals were weighted (SW) and transferred to the Meat Laboratory at Faculty of Agriculture, Ain Shams University, for measuring the live body linear measurements, slaughtering, carcass processing, jointing and tissue dissection.
- The data used in the present study concerned with three live body weight traits: weaning weight (WW), slaughter weight (SW), and the daily weight gain between the above ages (DG), eight linear body measurements: body length (BL), head index (HI), chest width (CW), loin width (LW), Chest girth (CG), abdominal girth (AG), round girth (RG), and body depth (BD), and ten carcass traits: dressing percentage (DP), hind leg percent (HLP), foreleg percent (FLP), loin percent (LOP), thoracic cage percent (TCP), hind half to fore half ratio (HFR), boneless meat percent (BMP), side bone percent (SBP), meat to bone ration (MB), meat to fat ratio (MF).
- The data were analyzed using VCE-6 software package to estimate the genetic parameters and phenotypic parameters (heritability and genetic and phenotypic correlations).
- The net income of each growing rabbit was calculated by summing up the weights of the consumable parts (carcass, head, heart, kidneys and liver) multiplied by the price of kg rabbit meat, assuming constant variable expenditure per rabbit.
- The traits defining the aggregate genotype were determined by plugging in all measured traits as an explanatory variable for the net income in multiple regression model.
- The daily gain (DG), dressing percentage (DP) and boneless meat percent (BMP), had entitled for most of the net income variability (0.92 coefficient of determination), accordingly, they included in the aggregate genotype.
- The economic values of the true breeding value traits were 6.5, 2.0, and 1.2 for DG, DP, and BMP, respectively, which are the corresponding partial regression coefficients resulted from regressing the net income upon these traits.
- Different combinations of WW, SW, DG, BL, and RG were used to construct six unrestricted selection indices.
- The full index was restricted to either or both HLP and MF (i. e. constructing three indices) to circumvent any unintended deterioration expected in the above traits.
4. Results and discussion
- The highest phenotypic variability was recorded for the live body weight traits (CV= 16.95 to 25.27%), compared either body measurements (CV= 5.10 to 16.25%) and carcass traits (CV= 1.57 to 13.66%) with exception of meat to fat ratio (CV = 47.10%).
- The boneless meat percentage was the lowest variable trait among the aggregate genotype traits.
- The thoracic cage cut appears to be the most variable part of the carcass (13.66%), followed by fore leg% (10.15%).
- The live body weight traits appeared to be moderate heritable (h2= 0.33 to 0.41).
- The h2-estimates for body linear measurements were moderated (h2 = 0.20 to 0.39).
- The heritability estimates of round girth and body length, the two linear measures used as source of information, were 0.20 and 0.35, respectively.
- Heritability estimated for carcass traits ranged from moderate to high (h2 = 0.25 to 0.72), the exception from this was BMP trait (h2 =0.16).
- Among the carcass parts, the highest heritability estimate was recorded for the fore leg cut% (h2 = 0.72), while the lowest was for thoracic cage % (h2 = 0.38).
- Higher correlation was observed in present study between WW and SW (rg= 0.72), which means that the rabbit weaned with heavy weight are expected to reach the slaughter age at heavy weight.
- The longer bodied rabbits will tend to have wider chest (rg= 0.57), wider loin (rg= 0.46) and boarded at round (rg= 0.48).
- There are positive genetic inter-relationships noticed in present study among CW, LW, CC, and RC which serve as indicators for meatiness attributes (rg= 0.35 to 0.60).
- Genetically, the rabbits having high BMP are expected to have higher DP (rg= 0.89), heaver loin (rg= 0.46) and higher MB ratio (0.42).
- loin% is positively correlated (rg=0.44) with thoracic cage % and negatively correlated with hand leg % (rg= -0.89).
- The meatiness indicating measures (CW, LW, CC, and RC) measured from the current sample of rabbits are more related to slaughter weight (rg= 0.41 to 0.60) than either weaning weight (rg= 0.33 to 0.54) or daily gain (rg= 0.20 to 0.31).
- The genetic and phenotypic correlations between slaughter weight from one side and HLP, TCP, SBP and MF ratio from the other side were found to be negative (rg= -0.03 to -0.35, rp= -0.08 to -0.41).
- The length of the body and the width of loin, which are easily measured, could be used as indicators for DP (rg= 0.35 and 0.34, respectively) and BMP (rg= 0.40 and 0.42, respectively), which requiring more effort to record.
- Carcass cuts percentages occurring at hind leg and thoracic cage are negatively correlated with the body length (rg=-0.14 and -0.11, respectively).
- The highest accuracy of selection (rTI = 0.71) was given by the unrestricted full index (I1) which makes use of all sources of information.
- Omitting weaning weight from the full index (I2) didn’t result on noticeable change of the full index’s accuracy of selection (rTI = 0.71).
- The two reduced indexes (I3, I4) were found to have comparable accuracy of selection (rTI = 0.70, 0.68, respectively).
- The index relies upon DG only as information source (I5), appears to be the most efficient single trait index (RTI = 0.63).
- Index I3 was found to be more efficient to be applied than the full index (I1) for improving the aggregate genotype traits, because it accomplishes comparable accuracy (rTI = 0.70 vs 0.71), and the incorporation of lower number of traits as information source guarantee economic advantage.
- Applying the selection based on the full index (I1), was expected to yield the highest positive response in the aggregate genotype traits.
- selection based on most efficient constructed indices (I1 to I5) is expected to improve DG by 3.88 to 4.31 g/day, DP by 0.01 to 0.11 unit and BMP by 0.01 to 0.04 unit, per selection round. Moreover, longer rabbits (0.07 to 0.22Cm) with a larger thigh circumference (0.07 to 0.15cm), and heavier at slaughter age (30 to 90gm), would be expected.
- selection based on the best reduced indices would expect to give lighter weaning weight (-10.0 to -20gm).
- Enhancement in meatiness indicating morphometric traits (chest width, loin width, breast depth and girth at both chest and abdomen) was expected as well, form selection based on best reduced indices, which range from 0.013cm for chest width to 0.252cm for abdominal girth.
- As correlated response with the improvement in the aggregate genotype, Slight favorable genetic changes were experienced in fore leg percentage (0.0 to 0.02 unit %), meat to bone ratio (0.03 to 0.08 unit), and hind to fore half ratio (0.002 to 0.004 unit).
- The expected correlated genetic changes in hind leg percentage (0.00 to -0.02 unit %) and meat to fat ratio (-0.34 to -0.75 unit percentage) follow unfavorable trajectory, both economically and biologically.
- Restricting the full index (I1) to produce zero genetic changes in HLP or MF ratio and in both simultaneously, are expected to result in a reduction of the selection accuracy (rTI)by 0.01, 0.02 and 0.03, respectively.
- Applying the restriction indices would reduce the rate of improvement in the aggregate genotype traits compared to the unrestricted index. Low reduction was expected in the DG (1.16% to 2.55%), while DP (45.45% to 54.55%) and BMP (50% to 75%) expected to experience sever reduction.
Conclusion:
- In the rabbits populations where the levels of HLP and MF ratio do not constitute imminent concern to the breeder, the use of the best reduced index:
I3 = 31.11 SW + 2.83 DG + 3.62 BL (RTI = 0.70)
Would be recommended to improve the aggregate genotype traits.
In crucial level of both of HLP and MF ratio, applying the following double restricted form of the full index:
I1(HLP, M:B) = 18.39 WW – 51.37 SW + 2.52 DG + 0.84 BL + 4.00 RG (RTI = 0.68)
Would be recommended to improve the aggregate genotype traits without any genetic changes in both of HLP an MF ratio simultaneously.