Possibilities of developing Favourable Body Fat Partition Via Selection Indexes-Application on Rabbits

Estimates of phenotypic and genetic parameters of body fat partition and other related traits were calculated using an experimental material of 152 New Zealand White rabbits of 15 bucks and 42 adult does. To construet nine selection indexes aiming at improving fat partition, estimates of phenotypic and genetic parameters for weight and linear dimensions of the live body were also obtained. The aggregate genotype included the percentage total body fat depositing subcutaneously and intermuscularly at the marketing age of 12 weeks. The füll index included the weaning body weight, marketing body weight, body length, loin width and heart girth. The füll index had the highest correlation with the true breeding value (rT| = 0.597). Comparable accuracy (rTi = 0.561) would be expected from the best-reduced index (I7 = 0.009 BW 0.988 HG) combining marketing body weight and heart girth. At each round of selection with intensity =1, such a reduced index is expected to result in developing rabbits with advantageous fat partition in terms of higher percentage total body fat depositing subcutaneously (+0.35 unit %), intermuscularly (+1.91 unit %) and intramuscularly (as reflected by 0.15 unit % increase in carcass muscle yield. As compared with its unrestricted form, the best reduced index reduced to zero change in total body fat (I7(TBF)) would result in drastic decline in percentage total fat depositing as kidney knob and Channel fat (+0.32 vs. -0.18 unit %) with little amelioration in percentage total fat aecumulating in the other depots. Selection on I7(TBF) instead of 17 would reduce improvement in marketing body weight (+0.52 vs. +0.33 gm), carcass yield (+0.10 vs. 0.04 unit %) and carcass boneless meat yield (+0.19 vs. 0.07 unit %) with increase of gain in carcass muscle yield (+0.15 vs. 0.32 unit %).


Introduction
Body fat partition influences the weight and acceptability of the carcass in that a gram of total body fat deposited as carcass fat (subcutaneously, intermuscularly or intramuscularly) is normally of much greater value than the same amount of total body fat aecumulated as non-carcass fat (heart, caul, mesenteric or kidney knob and Channel) depots.Types of body fat partition have been identified for dairy cattle (e.g.TRUSCOTT et al., 1976;BUTLER-HOGG and WOOD, 19.82), beef cattle (e.g.SHAHIN and BERG, 1985), beef x dairy crossbreds (e.g.KEMPSTER et al., 1976), water buffaloes (e.g.ABDALLAH et al., 1982), sheep (e.g.KEMPSTER, 1981;JONES, 1982); pigs (e.g.JONES et al., 1980) and broilers (e.g.GRIFFITHS et al., 1978;MERKLEY et al., 1980;SHAHIN et al., 1990).However, there is no available Information on the genetics of fat partitioning so as to modify it to the profit of meat producers.The aim of the present study was to investigate the possibilities of developing favourable body fat partition via selection indexes using dissection data on rabbits.With use of such a relatively lean species, it would be possible to develop selection indexes (S.I.) for animals with sub-optimal levels of fatness (unrestricted S.I.) as well as for those attaining optimal fatness (restricted S.I.).

Material and Methods
Source of Data.Over two successive parities during the spring of 1997, forty-two adult New Zealand White does were inseminated with fresh semen collected from fifteen bucks of the same breed at the experimental rabbitry of the Faculty of Agriculture, Ain Shams University, Egypt.From these matings one hundred and fifty two weaned male progeny were used in the present analyses.Rearing of Animals.Following their birth, the experimental rabbits remained with their dams in breeding batteries tili weaning at four weeks of age.They were then ear tagged and transferred to fattening batteries where fed ad libitum commercial pelleted diet (SHEMEIS and ABDALLAH, 1998) containing 16.3 % crude protein, 14.0 % crude fiber, 2.5 % crude fat and providing 2670 K. cal.digestible energy/kg.diet through to marketing at twelve weeks of age.At marketing age, the animals were weighed and their body length, loin width and heart girth were measured (LUKEFAHR and OZIMBA, 1991).They were slaughtered and dressed with the heart, mesenteric, caul and kidney knob and Channel fats being removed and weighed.The carcasses were weighed and held at 2 °C for 24 hours before subcutaneous fat (SCF), intermuscular fat (IMF) and muscles of the right sides were dissected.The weights of SCF and IMF were multiplied by two and added (1) to weights of the non-carcass fat depots to give total body fat and (2) to twice the dissected side muscle weight to give the carcass boneless meat weight.Statistical Model of Analysis.The genetic and phenotypic parameters of the traits describing body weights and dimensions, total body fat partition and carcass attributes were estimated using the Least Squares and Maximum Likelihood program of HARVEY (1990) aecording to the following mixed model: Yijk^u + Si + Dij + Pk + eyu, where: Yij k | = The Observation of the l th rabbit of the k lh parity from the j th dam and the i ,h bück; u = The overall mean; Si = The random effect of the i th bück (i= 1,2,..., 15); Dy = The random effect of the j" 1 dam (j= 1, 2,... ,42) nested within the i lh bück; Pk = The fixed effect of the k" 1 parity (k=l, 2); and eijki = The random error assumed N.I.D. (0, a 2 e ).
The net income in the present study was defined as the revenue realised by the meat producer in terms of the added value to the carcass weight through the proportionate increase in deposition of total body fat as subcutaneous and intermuscular fats.
The true breeding value (T) was then defined as: T = aj g SCF + a 2 g IMF where: g SCF -The additive genetic value for subcutaneous fat measured as percentage of total body fat; g IMF = The additive genetic value for intermuscular fat measured as percentage of total body fat; and & x and a 2 = The relative economic weights for SCF and IMF as percentage of total body fat, respectively.
An economic value that equals to unity was assigned to both traits, since a unit % of total body fat depositing as either SCF or IMF would equally contribute to the net profit of meat producer.Sources of Information (weaning weight; marketing body weight, body length, loin width and heart girth) were used in different combinations to construct nine selection indexes (CUNNINGHAM et al., 1970) out of which two were developed by imposing restriction on total body fat weight.

Results and Discussion
Phenotypic Variation.Coefficients of phenotypic Variation for the variables studied are given in Table 1.Much larger amount of phenotypic Variation was observed for the percentage total body fat depositing viscerally (30.2 to 38.7 %) and subcutaneously (30.4 %) than for that occurring intermuscularly (13.8 %).In the works of KEMPSTER et al.(1976) and SHAHIN and BERG (1985) on beef cattle, the phenotypic Variation for the proportionate total body fat accumulating around the kidneys was much higher than for that deposited between muscles.Slaughter traits showed low and comparable amounts of phenotypic Variation (4.3 % for carcass yield; 3.2 % for carcass muscle yield and 3.1 % for carcass boneless meat yield).Phenotypic Variation in body weight decline from weaning to marketing (difference of 9.8 %).At marketing, the Variation in the weight of the body (11.8 %) was considerably much lower than in the weight of its total fat (50.3 %).
Heritabilities.Heritability estimates (h 2 ) based on paternal half sib variance components for the traits considered in the study are also given in Table 1.Generally, body fat partition and slaughter traits were less heritable than live body measurements.
Heritability had low estimates (0.131 to 0.150) for intermuscular fat, mesenteric fat and caul fat, medium estimates for subcutaneous fat (0.266) and heart fat (0.329) and a high estimate (0.787) for kidney knob and Channel fat.(1) Fat depot weight relative to total body fat weight.
(2) Hot carcass weight relative to marketing body weight.
(3) Twice side boneless meat weight relative to twice side weight.
(4) Twice side muscle weight relative to twice side weight.
As for body dimensions, the heart girth was the most heritable trait (0.773) followed by loin width (0.593) then body length (0.145).
Correlations.The genetic and phenotypic correlations among the traits considered in the study are given in Table 2. Selection against excessively fat bodies would greatly decrease the percentage total body fat depositing as kidney knob and Channel fat (r G = +0.79).This would be associated with increase in the percentage total body fat accumulating around the heart (r G = -0.92)and intestines (r G = -0.37)and subcutaneously (r G = -0.75).The genetic correlation between total body fat weight and its percentage occurring intermuscularly is negligible (-0.01), which indicates that in this material body fat weight in no way describes the genetic Variation in its proportion depositing between muscles.Genetic improvement of the percentage total body fat depositing as kidney knob and Channel fat would seem to go along with improvement in carcass boneless meat yield (r G = +0.77)and carcass yield (r G = +0.33).Genetic correlations showed concomitant reduction in shares in total body fat would be expected at more drastic levels viscerally (around the heart, 0.91; and intestines, 0.92) than in the carcass (intermuscularly, -0.10; and subcutaneously, -0.30) Marketing body weight and dimensions are genetically correlated positively with total body fat weight (r G = 0.68 to 0.98) and its proportion depositing as kidney knob and Channel fat (r G = 0.82 to 0.94), and genetically correlated negatively with percentage total body fat accumulating around the heart (r G = -0.69 to -0.91) and intestines (r G = -0.18 to -0.82).This relation expresses early maturity of large sized male rabbits of this breed.
In spite of the foregoing relationships, marketing body length and loin width are less useful than marketing body weight and its traditional estimator, the heart girth, to evaluate genetic differences in total body fat partition due to their lower h values.
Indexes.The estimates of genetic and phenotypic (co)variances and the relative economic values obtained in the present study were used to construct nine selection indexes .Table 3 gives the weighing factors, Standard deviation, accuracy of selection for each index together with the relative efficiency in relation to the füll index.
Weighing factors were positive for marketing body weight and negative for marketing body dimensions due to their respectively high positive and negative genetic correlations with intermuscular fat.The maximum accuracy of selection (r T1 = 0.597) was obtained using the füll index (Ii).Whereas selection based on body weights (I2 and I 3 ) or body dimensions (I 4 , I 5 and l^) is expected to be of little effect in predicting the true breeding value considered in the present study (r T j = 0.368 to 0.395 and 0.271 to 0.321, respectively), selection on the best reduced index involving marketing body weight and heart girth (I7) is expected to be 96 % as efficient as the füll index.Restricting the expected genetic change in total body fat weight to zero through use ofIi (TB F) and I7(TBF) would cause only one percent reduction in the accuracy of selection.Table 4 gives results of the expected outcome for individual traits through use of the füll index and the best reduced index in their original (I; and I 7 ) and restricted forms (II(TBF) and I 7 (TBF) ) using intensity of selection = 1.0.Selection based on the füll index is expected to develop rabbits having higher body fat weight (+4.9 gm) and favourable body fat partition (+2.40 unit % increase in percentage total body fat depositing as carcass fat).This genetic gain in the total merit is expected to be associated with increases of 78 gm in marketing body weight, 0.33 unit % in carcass yield, 0.08 unit % in carcass boneless meat yield and a reduction of 0.60 unit % in carcass muscle yield.It is, however, possible through use of the restricted füll index (II(TBF)) to obtain comparable improvement in body fat partition with lower reduction in carcass muscle yield (-0.24 unit %) at the cost of genetic change in carcass boneless meat yield from gain (+0.08 unit %) to loss (-0.09 unit %) and reduction in genetic improvement of 30 gm in marketing body weight and 0.07 unit % in carcass yield.Selection on the best reduced index (I 7 ) is expected to develop rabbits with advantageous fat partition in terms of higher percentage total body fat depositing subcutaneously (+0.35 unit %), intermuscularly (+1.91 unit %) and intramuscularly (as reflected by +0.15 unit % increase in carcass muscle yield).As compared with its unrestricted form, the best reduced index without changing total body fatness (I 7 ( T BF)) would result in drastic decline in percentage total body fat depositing as kidney knob and Channel fat (+0.37 vs. -0.18unit %) with slight amelioration in that accumulating in the other depots.Selection on I 7 (TBF) instead of I 7 would reduce improvement in marketing body weight (+52 vs. 33 gm), carcass yield (0.10 vs. 0.04 unit %), carcass boneless meat yield (+0.19 vs. 0.07 unit %) and would increase gain in carcass muscle yield (+0.15 vs. 0.32 unit%).
It could be concluded that in case of sub-optimal level of body fatness the use of body weight (BW) and heart girth (HG) taken at marketing as sources of information in the selection index I 7 = 0.009 BW-0.988HG; (r TI = 0.561) would be recommended to optimize selection for the given aggregate genotype, and its restricted form: I7(TBF) = 0.009 BW -1.058 HG; (r TI = 0.558) would be advised in case of populations reaching optimal body fatness.

Table 3
Weighing Factors, Standard Deviation, Accuracy of Selection and Relative Efficiency Estimated for Each Index Without and with Restriction on Total Body Fat Weight (TBF) (Wichtungsfaktoren, Standardabweichung, Genauigkeit und Effizienz verglichener Indexvarianten)

Table 2 i
Total body fat partition