ABSTRACT

The objective of the present thesis was to study the effect of the selection for growth rate on the carcass and meat quality of line R, which is a line of rabbits selected for growth rate between the 4th and 9th week of age. This line is usually used in breeding schemes, where males from line R are mated with crossbreed females from two lines selected for litter size. When the line R was in the 7th generation of selection some embryos were recovered and vitrified. The selection of the line continued and the embryos were devitrified after some generations. The offspring of the rabbits obtained from the embryos formed the control group (Group C). This group was contemporary compared with the selected group (Group S), formed with rabbits from the present generation of selection. With groups S and C, two experiments were developed.
The first experiment consisted in the study of the effect of selection for growth rate on the relative growth of offal, organs, tissues, carcass linear measurements and weight of several parts of the carcass. Group C was contemporary compared with Group S, formed with animals from the 18th generation of selection. A total of 313 animals from both groups and sexes were slaughtered at 4, 9, 13, 20, and 40 weeks old. Organs, tissues and retail cuts were weighed and several carcass linear measurements were recorded. Huxley’s allometric equations relating the traits to liveweight by a parameter related to the scale (b) and an allometric coefficient (k) were fitted. Butterfield’s quadratic equations relating the degree of maturity of the traits and the degree of maturity of the liveweight by an allometric coefficient (q) were also fitted. Values obtained for k and q coefficients led to similar patterns of growth in most of the traits studied. Full gastrointestinal tract and organs such as liver, kidneys, and thoracic viscera were early maturing (k<1 or q>1) whereas the chilled and reference carcass where late maturing (k>1 or q<1). The retail cuts of the reference carcass were isometric (forelegs, k and q not different from 1) or late maturing (breast and ribs, loin, hind legs, and abdominal walls). Dissectible fat of the carcass and meat of the hind leg were late maturing and bone had an early development. Lumbar circumference was later maturing than the carcass length. No effect of selection for growth rate on b, k, and q values of any of the traits studied was found. Sex did not affect to the relative growth of most of the traits, but males had q values showing an earlier development of full gastrointestinal tract and later growth of kidneys than females.
When selecting for growth rate the adult weight increases. Therefore, rabbits achieved the slaughter weight, fixed by the market, at a lower degree of maturity than unselected rabbits. Carcass and meat quality could be affected not only by the selection for growth rate but also by the decrease of degree of maturity of the animal. In this way, the second experiment consisted in the study of the effect of selection on carcass composition and meat quality, comparing the Group C with a contemporary Group S, formed with rabbits from the 23th generation of selection. A total of 120 from both groups and sexes were slaughtered at 2000 g of liveweight, achieving the slaughter weight at ages 51 and 55 for groups S and C, respectively. The only relevant changes in carcass composition were an increase of kidneys, liver and dissectible fat percentages and a decrease of meat to bone ratio of the hind leg. In m. Longissimus, group S had lower yellowness of the carcass and higher redness and yellowness of the meat. The activity of the isocitrate dehidrogenase (ICDH) increased and ratio aldolase: ICDH decreased. In the hind leg, group S had higher values of polyunsaturated fatty acids, ratio polyunsaturated/saturated fatty acids and n-3 fatty acids. No changes on collagen content were found, but group S had higher collagen solubility. Shear force, shear firmness and area or total work needed to cut the sample were not different between groups, and hardness evaluated in the panel test was not relevantly changed. Most of the sensory properties studied did not differ relevantly between groups. Group S had less aniseed odour and more juiciness.