Direct and correlated responses to selection in two lines of rabbits selected for feed efficiency under ad libitum and restricted feeding: I. Production traits and gut microbiota characteristics.

To get insights into selection criteria for feed efficiency, 2 rabbit lines have been created: the ConsoResidual line was selected for residual feed intake (RFI) with ad libitum feeding and the ADGrestrict line was selected for ADG under restricted feeding (-20% of voluntary intake). The first objective of this study was to evaluate, after 9 generations of selection, the direct and correlated responses to selection on production traits in the 2 lines for traits recorded during growth. Second, applying the 2 feeding conditions used for selection to both selected lines plus the control unselected line (generation 0, G0) in a 2 × 3 factorial trial, the line performances were compared and the gut microbiota of the lines was characterized. The correlated responses in feed conversion ratio (FCR) were remarkably equivalent in both selected lines (-2.74 genetic σ) but correlated responses in other traits were notably different. In the ConsoResidual line, selection for decreased RFI resulted in a small negative correlated response in BW at 63 d old (BW63) and in a null response in ADG. In the ADGrestrict line, on the contrary, the correlated response in BW63 was substantial (+1.59 σ). The 2 selected lines had a FCR reduced by 0.2 point compared with the G0 line, and the same difference was found in both feeding regimens ( < 0.001). Indeed, selection on ADG would lead to heavier animals with no significant reduction of feed costs, whereas selection on RFI leads to lower feed costs and no increase of animal BW under ad libitum feeding. Altogether, our results do not suggest any genotype × environment interaction in the response to feeding regimens. The intestinal microbial communities from efficient rabbits differed from their unselected counterparts in terms of fermentation end products and microbial phylotypes, suggesting a central role of these microbes in the better feed efficiency of the rabbits.

Direct and correlated responses to selection in two lines of rabbits selected for feed efficiency under ad libitum and restricted feeding: II. Carcass and meat quality.

To get insights into selection criteria for feed efficiency, 2 rabbit lines have been created: the ConsoResidual line was selected for residual feed intake (RFI) with ad libitum feeding and the ADGrestrict line was selected for ADG under restricted feeding. The aim of the present study was to evaluate the impact on carcass and meat quality of the genetic selections. This comparison was performed using 2 different feeding strategies corresponding to the selection design. Carcass and meat quality traits were recorded for the 3 lines (ConsoResidual, ADGrestrict, and an unselected control [generation 0 {G0}]) in the 2 feeding systems (ad libitum and restricted) for 163 animals. Concerning the line effect, the BW at 63 d old was higher for the ADGrestrict line compared with the G0 and ConsoResidual lines ( < 0.0001). There was no line effect on the gastrointestinal tract. The rabbits did not exhibit a different carcass yield but showed different carcass traits. Indeed, the ConsoResidual rabbits had a higher hind leg yield ( < 0.0001) but no difference in the meat-to-bone ratio of the hind leg. On the contrary, the ADGrestrict line had a higher proportion of forelegs plus thoracic cage ( = 0.03). We also found lower perirenal ( < 0.0001) and scapular fat yields ( < 0.0001) in ConsoResidual rabbits. The ADGrestrict line had an intermediate perirenal fat yield compared with the other 2 lines. The G0 line always exhibited higher fat yields. Concerning meat quality, the ConsoResidual rabbits showed a lower ultimate pH ( < 0.0001) and higher water loss (drip and cooking loss; < 0.002) compared with the G0 and ADGrestrict rabbits. The feeding level had a strong effect on the gastrointestinal tract ( = 0.0004) and the carcass yield ( = 0.001). The latter was decreased in restricted rabbits. The effects of feeding strategy on meat quality were detrimental in the case of restricted feeding. Even if the ultimate pH was slightly higher in restricted rabbits ( = 0.0002), the carcass chilling loss ( = 0.0005) and the drip loss during 6 d ( < 0.0001) were higher. The selection on ADG with restricted access to feed seems to be less adversely correlated with carcass traits and meat quality traits but it leads to an increase in the live weight of the rabbits. Adversely, the selection on RFI, without affecting growth rate, would not modify age at slaughter because growth rate is not affected, but it might induce detrimental effects on meat quality.

Genetic parameters of rabbit semen traits and male fertilising ability.

This study aimed to estimate genetic parameters for rabbit semen production, semen characteristics and fertilising ability following artificial insemination. It involved five successive batches of 30-36 bucks each, 22 weeks of semen collection, and 11 weeks of semen recording per batch. Semen analyses were based on 2312 ejaculates. A total of 2019 inseminations were performed on 674 females with semen from 236 ejaculates from 128 bucks. Heritability estimates of semen traits ranged from 0.05 to 0.18. At approximately 0.05-0.06 for pH, volume and mass motility, they were higher for concentration (0.10) and the total number of sperms per ejaculate (0.12), and even higher for motility traits based on computer-assisted semen analysis. The percentage of motile sperms had the highest heritability (0.18) and appeared to be a good candidate criterion to select for both sperm number and motility. The heritability estimates were close to zero for all three criteria of fertilising ability: fertility (F), prolificacy (live births, LB) and their product (LB per insemination). A permanent environmental effect of the male seemed to be higher for LB (0.04) than for F (0.01). The rabbit does accounted for approximately 10% of the variance of the three criteria. With respect to the female, the male contribution was negligible for fertility and in a ratio of 4-10 for the number of live births. In our experimental conditions, prolificacy would thus be more highly influenced by the buck than fertility.

Relationships between rabbit semen characteristics and fertilising ability after insemination.

This study aimed to analyse the relationship between rabbit semen characteristics and semen fertilising ability after insemination, which is generally found to be weak. Our hypothesis was that using high semen dilutions (1 : 19), non-oestrus-stimulated does, and homospermic inseminations would make it easier to predict semen fertilising ability. Semen characteristics were evaluated on 275 ejaculates of 128 INRA1001 bucks, distributed into five successive batches. A total of 1970 inseminations were performed. The continuous semen variables were subdivided into three classes of similar size to account for any non-linear relationship between semen characteristics and fertilising ability. Mass motility was divided into two classes according to the presence or absence of waves under microscope observation. Libido, the presence or absence of gel, volume, percentage of progressive sperms, curvilinear velocity, beat frequency of the flagellum, and straightness and linearity of sperm movement did not affect fertility, prolificacy or productivity. It was confirmed that mass motility, estimated by visual observation under the microscope, significantly influenced fertility as well as the percentage of motile and of rapid sperms, and the amplitude of lateral head displacement, estimated by a computer-assisted semen analysis system. To a lesser extent, the percentage of motile cells and of rapid cells significantly influenced prolificacy. Consequently, mass motility and the percentage of motile cells significantly influenced rabbit doe productivity (+1 live births/AI when the semen showed at least a beginning of wave movement, or when the percentage of motile cells was >84%). Interestingly, a gain of 1.5 rabbits was observed when the percentage of rapid cells changed from 64% to 79%, whereas productivity significantly dropped beyond 83% of rapid cells, reflecting a non-linear relationship.

Some factors that influence semen characteristics in rabbits.

The aim of this study, based on a five-year-long experiment, was to analyse some of the factors that influence rabbit sperm production. A total of 174 bucks between 23 and 44 weeks of age from five successive groups were used for semen collection one day per week, two times, at a 15 min interval (ejaculates of rank 1 and 2), over a period of 21 weeks. Immediately after collection, pH, mass motility, volume and concentration were measured using classical methods, and a set of motility parameters were recorded by a computer-assisted semen analysis system. Between groups, the number of motile sperm per ejaculate, considered as a synthetic criterion combining both qualitative and quantitative aspects of semen characteristics, varied from simple to double (from 150 to 326×10(6)), reflecting the strong influence of uncontrolled environmental factors. Adult (37-43 weeks old) expressed a higher number of motile sperm/ejaculate than younger bucks (300 vs. 205×10(6)). In autumn the number of motile sperm/ejaculate was higher than in summer (287 vs. 188×10(6)). Sperm production was higher on average for the first ejaculate compared to the second one (270 vs. 167×10(6)). For several semen characteristics, the effect of the collector was significant but without any repercussion on sperm production. Bucks born to nulliparous or primiparous does had higher performances. This study highlights the high variability of rabbit semen characteristics and the multitude of factors involved, either controlled or uncontrolled.

Evolution of anti-eCG antibodies in response to eCG doses and number of injections. Relationship with productivity of rabbit does.

The aim of this experiment was to study the kinetics of anti-eCG (equine chorionic gonadotrophin) antibodies in relation to eCG dose (8 or 25 IU) and number of injections (n = 11) in comparison with a control group (no injection), and to relate antibody production to sexual receptivity and productivity of rabbit does. In all, 124 lactating primiparous rabbit does were inseminated every 35 days for a year. Just before eCG injection (48 h before insemination), blood samples were collected from all the does to assay anti-eCG antibodies. The anti-eCG antibody binding rate, regardless of the injected dose, shows that none of the does developed detectable anti-eCG antibodies before the 7th injection. The level of detectable anti-eCG antibodies began to show an increase at the 7th injection and was significant only for the 25 IU dose at the 11th injection. At the end of the experiment, 15% and 39% of does treated with 8 and 25 IU, respectively, developed immunity to eCG (binding rate >6%: higher binding rate of the control group). Consequently, the immune response depends on the eCG dose and on the number of injections. Moreover, productivity of does estimated from the number of weaned rabbits produced per insemination is not influenced by the level of eCG antibodies (7.0 and 6.9 for binding rate <6% and binding rate 6%, respectively). Only 19 inseminations (n = 6 and n = 13 for 8 and 25 IU, respectively) were made on hyperimmune does. Consequently, the immune response to eCG seems to be marginal for rabbit does. Moreover, under the described experimental conditions, reproductive performances of hyperimmune does were not affected.

Effects of anoestrus and bromocryptine treatment on the expression of prolactin and LH receptors in the rabbit ovary during lactation.

The aim of the present study was to correlate the number of prolactin and LH receptors in the ovary with the changes in sexual behaviour that occur within a few days following parturition in rabbits. Multiparous New Zealand white rabbits at days 0, 3 and 10 of lactation were tested for their receptivity upon presentation to a male. Rabbits were classed as either receptive or nonreceptive at each stage of lactation; half of the animals in each class were treated with bromocryptine to examine the effects of prolactin deprivation. Ovarian receptors for LH and prolactin, as well as the concentration of their corresponding mRNA, were measured at each stage of lactation in every group. Results indicate that receptive behaviour is correlated with significantly more follicles on the rabbit ovary (diameter > 1 mm; P < 0.05) and an increase in the concentration of LH receptor mRNA (P < 0.001) and prolactin receptors (P < 0.05). In addition, on day 4 of lactation, there were significantly fewer follicles in nonreceptive rabbits (P < 0.05). LH receptor content remained constant on days 1 and 4 of lactation but increased on day 11 (P < 0.05). Bromocryptine treatment had no effect on the number of follicles or on the amount of LH receptor mRNA in does, but it significantly increased LH receptors (P < 0.01), and the concentration of prolactin receptor mRNA (P < 0.001), particularly on day 11 of lactation (P < 0.05), and prolactin receptor content (P < 0.001). Receptive rabbit ovaries therefore display more follicles that can respond to an LH surge via newly transcribed LH receptors than do nonreceptive.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acrosome defects and sperm chromatin decondensation on fertility and litter size in the rabbit. Preliminary electron-microscopic study.

Sections of ejaculated spermatozoa from rabbits with known fertility were observed by transmission electron microscopy. Acrosome defects, odd head shapes, the presence of spermatids and the decondensation of sperm nuclei were recorded. The fertility negatively correlated with the number of acrosomal defects, while litter size was negatively related to nuclear decompaction. Sperm nuclear decondensation was present in all ejaculates. Its frequency was correlated with that of spermatids in ejaculates.

[Reproduction of French Angora rabbits: ovulation in the female, semen production in the male]. / La reproduction du lapin Angora de souche française: ovulation chez la femelle, production de semence chez le mâle.

Ovulation rate and semen quality in French Angora rabbits were investigated to determine the potential of improving breeding practices in this breed. The proportion of does ovulating and their ovulation rate were studied in 40 females, as well as sexual behaviour and semen quality in 8 males. The experiments took place in autumn 1987 and were repeated in winter. Three groups of does were injected with 25, 50 IU hCG or 0.8 microgram GnRH (groups 1, 2 and 3, respectively). Another group was mated and served as a control (group 4). Hormonal treatments improved the proportion of does which ovulated (95, 90, 74 and 28% in groups 1-4, respectively; P less than 0.05) but did not change their ovulation rate (10.9 +/- 0.7, 10.7 +/- 0.7, 11.3 +/- 0.8 and 8.9 +/- 1.3 corpora lutea per ovulating female, groups 1-4, respectively; m +/- SEM, NS). In males, two ejaculates were collected twice weekly for 3.5 weeks from 8 bucks. Ejaculates obtained in March were better than those collected in November (volume: 0.33 vs 0.23 ml, P less than 0.01; raw motility: 5.2 vs 3.9, P less than 0.01; individual motility: 2.7 vs 2.1, P less than 0.05; number of living spermatozoa per ejaculate: 71 vs 28 x 10(6), P less than 0.01, respectively). These results suggest that artificial insemination may be utilized to improve reproductive performance in French Angora rabbits.

[Measurement of ovulation rate in rabbits by celioscopy]. / Mesure du taux d'ovulation par coelioscopie chez la lapine.

With the aim of determining the efficiency of a simple and non destructive method for measuring the ovulation rate, 20 doe rabbits were subjected to coelioscopy and slaughtered on day 14 of gestation. Among the 40 ovaries examined, 45% of corpora lutea counts were accurate and 35% approximate (+/- 1), i.e. a total of 80% acceptable counts. This percentage represented 96% when the number of corpora lutea per ovary did not exceed 8. As compared with a few control does, coelioscopy did not seem to have a significant unfavourable effect on embryonic survival rate on day 14 of gestation, but this result has to be checked. It may be concluded that this method can be used in further experiments.