Effect of selection for litter size and body weight on hormone-induced ovulation rate in mice.

Genetic differences in natural vs hormone-induced ovulation rates were compared in immature female mice from five lines that had undergone long-term single-trait and antagonistic index selection for litter size and(or) 6-wk BW. Lines used were control (K); high litter size (L+); high BW (W+); low litter size and high BW (L-W+); and high litter size and low BW (L+W-). Natural ovulation rate at a mean age of 34.3 d and hormone-induced (5 IU of pregnant mare's serum gonadotropin followed 2 d later by 5 IU of human chorionic gonadotropin) superovulation rate at a fixed age of 31 d were obtained. Total number of eggs ovulated was affected by line (P less than .001), treatment (P less than .001), and line x treatment interaction (P less than .001). Line differences were subsequently tested within treatment because of the significant line x treatment interaction. Line differences were important (P less than .001) for natural ovulation, hormone-induced ovulation, and response to hormones. Mean natural ovulation rates for K, L+, W+, L-W+, and L+W- were 14.1, 19.8, 15.1, 13.6, and 16.4, respectively. Selection changed ovulation rate by 40, 16, 7, and -4% in the L+, L+W-, W+ and L-W+ lines, respectively (P less than .01). Hormone-induced ovulation rates in K, L+, W+, L-W+, and L+W- were 32.3, 24.6, 19.6, 20.9, and 22.1, respectively. Exogenous hormones increased ovulation by 18.2, 4.8, 4.6, 7.3, and 5.7 ova for K, L+, W+, L-W+, and L+W-, respectively (P less than .001). Lines with lower natural ovulation rates had higher responses to superovulation. Increased ovulation rate due to treatment ranged from 24.3% in L+ to 129% in K. These results indicate significant differences among lines in ovarian response to exogenous hormones.     

Reproductive performance in a diallel cross among lines of mice selected for litter size and body weight

Genetic factors affecting female reproductive performance in lines of mice with a known history of selection were estimated from a 5 X 5 diallel cross. Lines were selected as follows: large litter size at birth (L+); large 6-wk body weight (W+); an index for large litter size and small 6-wk body weight (L+W-); the complementary index (L-W+) and randomly (K). Partitioning of direct and correlated responses for litter size, 6-wk body weight and related traits into average direct genetic (li) and average maternal genetic (mi) effects indicated that the magnitude of differences in li exceeded those in mi. Lines having positive responses in li were W+ greater than L+ greater than L-W+ for dam body weight, L+ greater than L+W- greater than W+ for litter size and L+ greater than (W+, L+W-) for litter birth weight, whereas L-W+ responded negatively for litter size. A positive association was found between mi for litter size and dam body weight, W+ and L-W+ being high and L+ and L+W- low for both traits. Female infertility and time from male exposure to parturition had relatively small correlated responses. Line rankings in general combining ability (gi) and net line effects were similar for the respective traits. Depending upon the line and trait involved, the relative contribution of average direct genetic and line direct heterotic (hi) effects to general combining ability [gi = (1/2) li + hi] varied. Line heterosis refers to average heterosis in crosses involving that line. Direct heterosis ( hij ) for each trait differed considerably among crosses. The three crosses showing the highest hij for litter size at birth, W+ X L-W+ (1.78), L+ X W+ (1.28) and L-W+ X L+W- (1.22), possibly had loci contributing directional dominance to litter size with frequencies of parental lines deviating in opposite directions relative to mean gene frequency. The correlation between absolute difference in parental line means and hij for litter size was not significant, suggesting that the magnitudes of absolute differences in parental means were not reliable predictors of divergence in gene frequency. Crossbred performance increased linearly with midparent values for litter size at birth (b = .88 +/- .09, R2 = .92) and dam parturition body weight (b = 1.13 +/- .04, R2 = .99), the latter trait showing an increase (P less than .01) in heterosis as midparent values increased.     

Postpartum performance in a diallel cross among lines of mice selected for litter size and body weight

Postpartum dam performance was studied in a complete diallel design involving five lines of mice. The selection criterion in each line was: large litter size at birth (L+); large 6-wk body weight (W+); an index for large litter size and small 6-wk body weight (L+W-); the complementary index (L-W+) and random (K). Females from the five lines and 20 reciprocal F1 crosses were mated to sires of a randomly selected control line (CC). Correlated responses in average direct genetic and average maternal genetic effects for dam body weight and litter size at parturition persisted throughout lactation, indicating important pleiotropic effects. Major correlated responses occurred for litter weight, feed intake and litter feed efficiency, primarily due to average direct genetic effects. Using general combining ability and net line effects as criteria for choosing among lines, L+ had a distinct advantage if the objective was to increase litter size in a crossing program. If the objective was to maximize litter weaning weight, then W+ would be favored for net line effects, while L+ and W+ would be about equivalent for general combining ability. None of the lines had an advantage for litter feed efficiency. Direct heterosis for dam weight at 12 and 21 d of lactation averaged 2.7 and 1.9%, while for litter size the respective averages were 7.4 and 7.3%. The W+ X L+W- cross exhibited overdominance for litter size. Direct heterosis was moderate for feed intake and litter weight, but was negligible for litter feed efficiency because of the mathematical relationship among the three traits. Maternal heterosis for preweaning progeny growth was suppressed because of heterosis for litter size in the dam. Grand-maternal effects on growth of the young were small and would not be an important consideration in choosing among these lines in a crossbreeding program.     

Genetic implications of a simulation model of litter size in swine based on ovulation rate, potential embryonic viability and uterine capacity: II. Simulated selection

Direct selection for ovulation rate, uterine capacity, litter size and embryo survival and selection for indexes of ovulation rate with each of the remaining traits were simulated for a swine population. The relationships among these traits were determined from a simulation model that assumed that litter size was always less than or equal to both ovulation rate and uterine capacity. Heritabilities of ovulation rate and uterine capacity were assumed to be .25 and .20, respectively, and uncorrelated genetically and phenotypically. No additional genetic variation was assumed. Responses to weak selection pressure were simulated by recurrent updating of phenotypic variances and covariances combined with the heritabilities of ovulation rate and uterine capacity. Two indexes of ovulation rate and uterine capacity each resulted in 37% greater increase in litter size than direct selection for litter size. Indexes of ovulation rate and either litter size or embryo survival increased litter size by 21% more than direct selection for litter size. Selection for ovulation rate, uterine capacity or embryo survival was 6, 35 and 79%, respectively, less effective than direct selection for litter size. Responses to intense selection pressure were determined by direct simulation of genotypes and phenotypes of individuals. The two indexes of ovulation rate and uterine capacity exceeded direct selection for litter size by 39 and 27%. The indexes of ovulation rate and either litter size or embryo survival exceeded direct selection for litter size by 19 and 13%, respectively. Intense selection for ovulation rate or uterine capacity decreased selection response by 26 and 67%, respectively, relative to direct selection for litter size. Intense selection for embryo survival decreased litter size slightly.     

Selection for components of reproduction in swine

The response per generation to 10 generations of mass selection for ovulation were 0.49 ova, ?1.6% in embryo survival and 0.06 piglets per litter at birth. Line differences (select-control) in generation 9 and 10 gilts and sows ranged from 3.4 to 5 ova. Control line gilts and sows had 5.4 to 10.6% higher embryo survival to days 30 and 70 of gestation than did select line females. One generation of random selection followed by four generations of litter size selection, selection for decreased age at puberty or relaxed ovulation rate selection in the high ovulation rate line has resulted in lines that differed from the control line in litter size at birth by 0.78 ± 0.22, 0.37 ± 0.39 and 0.84 ± 0.52 pigs per litter at first, second and third parity, respectively. These results were used to derive a selection index to increase litter size by selection for its components (ovulation rate, OR, and embryo survival, ES). A technique of selection based on laparotomy to increase the number of females tested with a given set of farrowing places is presented. Rate of response in LS from use of the selection index, I = 10.6 OR + 72.6 ES, in a population of 40 farrowing females and 15 males per generation, is expected to increase litter size 2.5 times faster than selection on LS due to higher selection intensity and optimum emphasis on the component traits.     

Ovulation rate and embryo survival during gestation and the effects of ovariectomy on maternal nurturing ability

Correlated responses to selection for increased litter size were studied in mice. The selected line (L+) was compared with an unselected control line (K') in two experiments. The first experiment provided a profile of correlated changes in female reproductive traits at d 0, 6, 14 and 18 of gestation. Experiment two examined the effects of ovariectomy on d 18 of gestation, sham surgery and no surgery on litter size and maternal performance. Females of the L+ line had increased (P less than .001) body weight, ovulation rate and uterine length at d 0 of gestation compared with K' females, but uterine weight and ovarian weight did not differ. Positive correlated responses (P less than .001) in uterine weight and length at d 6 and 14 of gestation were associated with a larger number of viable fetuses. Space per fetus was reduced (P less than .001) in the uterus of L+ females, but a lower fetal mortality was still maintained in L+ throughout gestation. Prenatal survival was about 10% higher (P less than .06) in L+, the major difference (P less than .01) occurring before implantation. A second experiment was conducted to determine the effect of ovariectomy on d 18 on litter size and maternal performance. In experiment two, no significant line X treatment interactions were found for maternal performance, indicating that both lines responded similarly to ovariectomy. Line L+ showed a positive correlated response in maternal performance. Ovariectomized females had a reduced (P less than .05) number born alive compared with sham-operated females, but the nonsurgically treated females were intermediate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inheritance of litter size and its components in crosses between the D'man and Sardi breeds of sheep

Data on ovulation rate, litter size and embryo survival of 364 Sardi (S), D'man (D), S x DS, DS x S, S x D, D X S (F1), F2, D x DS and DS x D ewes mated for first and second lambing to F1 rams were analyzed. Breed group, birth group and season had significant effects on ovulation rate and litter size but not on embryo survival. D'man ewes had the highest ovulation rate (2.79) and litter size (2.00), with an essentially linear increase in each of these variables with percentage of D'man inheritance in the ewe (b = .017 +/- .001 CL and .009 +/- .001 lambs born). Embryo survival was influenced only by the number of ova shed. D'man direct genetic effects were higher (P less than .01) than those of Sardi for ovulation rate (+1.78) and litter size (+1.08) but did not differ for embryo survival (-.07). Maternal effects differed little for any of the three traits. Individual heterosis estimates were negative and significant for ovulation rate but not significant for litter size and embryo survival. Maternal heterosis and epistatic recombination effects were small and not significant for any trait.     

Integration of ovulation rate, potential embryonic viability and uterine capacity into a model of litter size in swine

A mathematical model of litter size in swine was developed from ovulation rate, potential embryonic viability and uterine capacity. The model assumed that ovulation rate was reduced to potentially viable embryos by factors innate to the ovum and embryo. Potentially viable embryos then could be further reduced to uterine capacity, the maximum number of fetuses that a female can carry to term. Consequently, litter size can be no greater than either ovulation rate or uterine capacity. Means and variances of ovulation rate and potential embryonic viability used in the model were based on experimental results. The mean and variance of uterine capacity were varied until the simulated mean and variance of litter size were equal to experimental results. Simulated results of relationships among ovulation rate, embryo survival and litter size were similar to observed experimental relationships. Heritabilities of simulated litter size and embryo survival were similar to literature values when the heritability of ovulation rate was set at .25 and the heritability of uterine capacity was set at either .15 or .20. Litter size was simulated for 25 combinations of average ovulation rate and uterine capacity to develop equations relating mean ovulation rate and uterine capacity to litter size, embryo survival and correlations among them. Results suggest that changing either ovulation rate or uterine capacity independently will not result in large changes in litter size. Consequently, the model suggests that a single gene, hormonal manipulation or nutritional change will not result in large increases in litter size and that combinations of factors will be needed to increase litter size.     

Direct responses to selection for increased litter size, decreased age at puberty, or random selection following selection for ovulation rate in swine

Nine generations of selection for high ovulation rate were followed by two generations of random selection and then eight generations of selection for increased litter size at birth, decreased age at puberty, or continued random selection in the high ovulation rate line. A control line was maintained with random selection. Line means were regressed on generation number and on cumulative selection differentials to estimate responses to selection and realized heritabilities. Genetic parameters also were estimated by mixed-model procedures, and genetic trends were estimated with an animal model. Response to selection for ovulation rate was about 3.7 eggs. Response in litter size to selection for ovulation rate was .089 +/- .058 pigs per generation. Average differences between the high ovulation rate and control lines over generations 10 to 20 were 2.86 corpora lutea and .74 pigs (P less than .05). The regression estimate of total response to selection for litter size was 1.06 pigs per litter (P less than .01), and the realized heritability was .15 +/- .05. When the animal model was used, the estimate of response was .48 pigs per litter. Total response in litter size to selection for ovulation rate and then litter size was estimated to be 1.8 and 1.4 pigs by the two methods. Total response to selection for decreased age at puberty was estimated to be -15.7 d (P less than .01) when data were analyzed by regression (realized heritability of .25 +/- .05) and -17.1 d using the animal model. No changes in litter size occurred in the line selected for decreased age at puberty. Analyses by regression methods and mixed-model procedures gave similar estimates of responses and very similar estimates of heritabilities.     

Progestagen supplementation during early pregnancy does not improve embryo survival in pigs

Progesterone supplementation during early pregnancy may increase embryo survival in pigs. The current study evaluated whether oral supplementation with an analogue of progesterone, altrenogest (ALT), affects embryo survival. A first experiment evaluated the effect of a daily 20-mg dosage of ALT during days 1-4 or 2-4 after onset of oestrus on embryo survival at day 42 of pregnancy. A control group (CTR1) was not treated. The time of ovulation was estimated by transrectal ultrasound at 12-h intervals. Altrenogest treatment significantly reduced pregnancy rate when start of treatment was before or at ovulation: 25% (5/20) compared to later start of treatment [85% (28/33)] and non-treated CTR1 [100% (23/23)]. Altrenogest treatment also reduced (p < 0.05) number of foetuses, from 14.6 ± 2.6 in CTR1 to 12.5 ± 2.5 when ALT started 1-1.5 days from ovulation and 10.7 ± 2.9 when ALT started 0-0.5 days from ovulation. In a second experiment, sows with a weaning-to-oestrous interval (WOI) of 6, 7 or 8-14 days were given ALT [either 20 mg (ALT20; n = 49) or 10 mg (ALT10; n = 48)] at day 4 and day 6 after onset of oestrus or were not treated (CTR2; n = 49), and farrowing rate and litter size were evaluated. Weaning-to-oestrous interval did not affect farrowing rate or litter size. ALT did not affect farrowing rate (86% vs 90% in CTR2), but ALT20 tended to have a lower litter size compared with CTR2 (11.7 ± 4.1 vs 13.3 ± 3.1; p = 0.07) and ALT10 was intermediate (12.3 ± 2.9). In conclusion, altrenogest supplementation too soon after ovulation reduces fertilization rate and embryo survival rate and altrenogest supplementation at 4-6 days of pregnancy reduces litter size. As a consequence, altrenogest supplementation during early pregnancy may reduce both farrowing rate and litter size and cannot be applied at this stage in practice as a remedy against low litter size.     

Influence of energy intake on growth and utilisation of dietary protein and energy in germ-free and conventional chicks

The effect of metabolisable energy (ME) intake on the growth and utilisation of dietary protein and energy in germ-free (GF) and conventional (CV) chicks was investigated in two experiments. In experiment 1 a high energy diet (HED, 14.8 kJ ME/g) and a marginally-adequate energy diet (AED, 11.7 kJ ME/g) were fed to the GF and CV chicks at 240 g/2 birds/10 d. In experiment 2 a diet with 13.7 kJ ME/g was fed at 118 g (low level, LL) or 128 g (high level, HL)/bird/10 d. Body weight gain, protein retention and protein retention rate were similar in GF and CV chicks on both AED and HED in the first experiment, but in the second were higher in GF than in CV chicks. The increased ME intake of the CV chicks in experiment 2 may be too small to compensate for the increased requirement. ME intake was significantly higher in the CV chicks than in the GF chicks, whereas energy retention was similar in both groups.     

OVULATION RATE AT FIRST MATING AND REPRODUCTIVE PERFORMANCE OF GILTS

SUMMARY Laparoscopy was used to estimate ovulation rate at first mating in 460 Large White/Landrace gilts. For 385 gilts which farrowed litter size was recorded and the relationships between age and mating, maternal litter size, ovulation rate and reproductive performance were examined. The mean ovulation rate of the gilts which farrowed was 10.9 ± 0.14 corpora lutea and the mean first litter size was 8.0 ± 0.12 piglets born with 7.5 ≥.± 0.13 born alive. Ovulation rate was related to first litter size (r = 0.29, P < 0.001) but embryo loss was the major factor determining litter size, accounting for about 58% of the variation. None of the variable examined at the time of mating was sufficiently correlated with litter size to be useful as selection criteria for improving reproductive performance.     

Responses in ovulation rate, embryonal survival, and litter traits in swine to 14 generations of selection to increase litter size

Eleven generations of selection for increased index of ovulation rate and embryonal survival rate, followed by three generations of selection for litter size, were practiced. Laparotomy was used to count corpora lutea and fetuses at 50 d of gestation. High-indexing gilts, approximately 30%, were farrowed. Sons of dams in the upper 10% of the distribution were selected. Selection from Generations 12 to 14 was for increased number of fully formed pigs; replacements were from the largest 25% of the litters. A randomly selected control line was maintained. Responses at Generation 11 were approximately 7.4 ova and 3.8 fetuses at 50 d of gestation (P < .01) and 2.3 fully formed pigs (P < .01) and 1.1 live pigs at birth (P < .05). Responses at Generation 14 were three fully formed pigs (P < .01) and 1.4 live pigs (P < .05) per litter. Number of pigs weaned declined (P < .05) in the index line. Total litter weight weaned did not change significantly. Ovulation rate and number of fetuses had positive genetic correlations with number of stillborn pigs per litter. Significantly greater rate of inbreeding and increased litter size at 50 d of gestation in the select line may have contributed to greater fetal losses in late gestation, greater number of stillborn pigs, and lighter pigs at birth, leading to lower preweaning viability. Heritabilities of traits were between 8 and 25%. Genetic improvement programs should emphasize live-born pigs and perhaps weight of live-born pigs because of undesirable genetic relationships of ovulation rate and number of fetuses with numbers of stillborn and mummified pigs and because birth weight decreased as litter size increased.     

Increased litter size in Rambouillet sheep: I. Estimation of genetic parameters.

The variance and covariance components needed to estimate heritabilities of and genetic correlations among litter size, ovulation rate, scrotal circumference, and BW in a flock of Rambouillet sheep were estimated using REML via an expectation-maximization type algorithm. The heritability estimates from univariate analyses were .14, .21, .25, .36, and .15 for litter size, ovulation rate, scrotal circumference, 180-d BW of females, and 180-d BW of males, respectively, and average heritability estimates from bivariate analyses were .19, .20, .20, .34, and .10 for litter size, ovulation rate, scrotal circumference, 180-d BW of females, and 180-d BW of males, respectively. The genetic correlation between litter size and ovulation rate was near unity. Body weight in ewes had a moderate genetic correlation with both litter size (.22) and ovulation rate (.20) and a low residual correlation with both litter size (.03) and ovulation rate (.09). The genetic correlation between BW in rams and scrotal circumference was 0, whereas the residual correlation was .71. The genetic correlations of scrotal circumference with litter size and ovulation rate were -.25 and +.20, respectively.     

Effect of soluble and insoluble dietary fiber on embryo survival and sow performance

Two experiments were conducted to evaluate the effects of soluble (SF) and insoluble (ISF) dietary fiber during gestation on embryo survival and sow performance. In Exp. 1, 43 gilts were assigned randomly to 1 of 4 experimental diets: a corn-soybean meal control (C; 1.16% SF, 9.98% ISF); a 30% oat bran high in SF (HS; 3.02% SF, 10.06% ISF); a 12% wheat straw diet high in ISF (HIS; 1.08% SF, 18.09% ISF); and a 21% soybean hull diet (HS + HIS; 2.46% SF, 24.55% ISF). Gilts were fed the experimental diets based on their initial BW to meet their daily nutrient requirements. At estrus, gilts were inseminated artificially 3 times using pooled semen. Reproductive tracts were harvested 32 d postmating (range = 28 to 35 d). Statistical analysis of data included the effects of diet with days of gestation as a covariate. There were no differences in ovulation rate among gilts fed the experimental diets (avg. = 14.1). Number of live embryos was less for HIS and HS + HIS gilts compared with C and HS (9.9 and 9.1 vs. 11.9 and 10.6, respectively; P < 0.05). Total embryo survival rate (P < 0.05) was less for gilts fed HS + HIS compared with those fed the C and HS diets. These results suggest that high dietary ISF might decrease the total embryo survival rate without affecting ovulation rate. In Exp. 2, 716 sows were used in 3 concurrent trials. In trial 1, diets included a corn-soybean meal control (C; 0.43% SF, 10.50% ISF; n = 122) or a 31% oat bran diet (HS; 1.93% SF, 8.87% ISF; n = 124). In trial 2, diets included a C (n = 97) or a 13% wheat straw diet (HIS; 1.10% SF, 17.67% ISF; n = 119), and in trial 3 sows were fed a C (n = 123) or a 21% soybean hull diet (HS + HIS; 1.50% SF, 17.77% ISF; n = 131). All diets were offered to sows beginning 2 d postmating. All sows had ad libitum access to a standard lactation diet. Statistical analysis included the effects of diet, parity group, genetic line, and season as well as their interactions. The inclusion of SF and ISF in gestation diets did not affect litter size. Sows fed the HS + HIS diet had a greater ADFI and lost less BW during lactation (P < 0.01) than sows fed C. Under the conditions of this study, feeding gestating sows increased levels of SF and ISF from d 2 after breeding to d 109 of gestation did not increase litter size.     

Flushing and altrenogest affect litter traits in gilts

Gilts (n = 267) were allotted to flushing (1.55 kg/d additional grain sorghum), altrenogest (15 mg.gilt-1.d-1) and control treatments in a 2 x 2 factorial arrangement. Altrenogest was fed for 14 d. Flushing began on d 9 of the altrenogest treatment and continued until first observed estrus; 209 gilts (78%) were detected in estrus. The interval from the last day of altrenogest feeding to estrus was shorter (P less than .05) with the altrenogest + flushing treatment (6.6 +/- .2 d) than with flushing alone (7.6 + .3 d). Ovulation rates (no. of corpora lutea) were higher (P less than .05) in all flushed gilts (14.5 +/- .4 vs 13.4 +/- .4), whether or not they received altrenogest. Flushing also increased the total number of pigs farrowed (.9 pigs/litter; P = .06) and total litter weight (1.43 kg/litter; P = .01), independent of altrenogest treatment. Number of pigs born alive and weight of live pigs were higher for gilts treated with altrenogest + flushing and inseminated at their pubertal estrus than for gilts in all other treatment combinations. In contrast, gilts receiving only altrenogest had greater live litter weight and more live pigs born when inseminated at a postpubertal estrus than when inseminated at pubertal estrus. We conclude that flushing increased litter size and litter weight, particularly for gilts that were inseminated at their pubertal estrus. Increased litter size resulted from increased ovulation rates, which, in nonflushed gilts, limited litter size at first farrowing.     

Reproductive performance of chimeric mice produced from genetic lines that differ in litter size

A population of chimeras was made by aggregating 8- and 16-cell embryos from two mouse strains: a randomly bred line (C) and a selected line characterized by large litters (JU), with litter sizes of 7.7 and 13.5, respectively. The two genotypes were developmentally "balanced", as judged by the high frequency (90%) of chimeras with an intermediate or high degree of coat-color chimerism, a chimeric sex ratio of 2.2:1 males:females, and a high percentage of chimeras (31% of males, 71% of females) with germ cells of both strains. Litter size characteristics, including ovulation rate, implantation rate, rates of pre- and postimplantation embryo survival and number born were studied in the female chimeras and compared with the performance of both parent lines and to the genetic cross of the two lines. Values for JU females exceeded those for C females for all parameters studied except postimplantation embryo survival, which was the same for both lines in second litters and was lower for JU's third litters. For most traits, means for genetic crossbreds and chimeras were similar, regardless of whether the means were at or above the midparent average. In contrast, for ovulation rate and body weight, genetic crossbreds and chimeras clearly differed, with chimeric females being similar to the JU line and genetic crossbred females exhibiting additive inheritance. Because of phenotypic differences between experimental chimeras and crossbreds produced from the same two lines, chimeras may provide a useful model for studying the physiologic basis for expression of genetic differences in quantitative traits.     

Studies in sow reproduction: 4. The effect of level of feeding in lactation and during the interval from weaning to remating on the subsequent reproductive performance of the early-weaned sow

An investigation of the effects of level of nutrition, both in lactation and from weaning to remating, on subsequent litter size and associated reproductive characteristics in the early-weaned sow is reported. Subjects were 75 sows in 5 groups. In 4 of the groups the sows were weaned after a 10-day lactation period. Group 5 was weaned following a 42-day gestation. The control group was fed up to 6.3 kg/day during lactation and 2.7 kg/day from weaning to remating. The 4 early-weaned groups were each fed differently. In lactation and during the inverval from weaning to remating they were fed either 2 or 4 kg/day. The group receiving only 2 kg/day during each period lost more weight than the others (p less than .05). Weight loss in lactation was significantly (p less than .001) affected by feeding. Sows weaned after a 10-day lactation period farrowed 2.7 piglets/litter less in the next parity than sows weaned after a 6-week lactation period. Weight losses during lactation were not related to subsequent litter size. Level of nutrition from weaning to remating in these tests had no influence on subsequent litter size. The early-weaned sow, even with large fluctuations in weight change over the period from parturition to remating, did not alter their ceiling for litter size. It seems unlikely that ovulation rate is the major factor limiting litter size in the early-weaned sow. Results suggest that embryo mortality following ovulation and coitus is increased in the early-weaned sow and that this effect then manifests itself as a ceiling to subsequent litter size.     

Selection for ovulation rate in rabbits

An experiment of selection for ovulation rate was carried out. Animals were derived from a synthetic line first selected 12 generations for litter size, then 10 generations for uterine capacity. Selection was relaxed for 6 generations. Selection was based on the phenotypic value of ovulation rate with a selection pressure on does of 30%. Males were selected from litters of does with the highest ovulation rate. Males were selected within sire families in order to reduce inbreeding. Ovulation rate was measured in the second gestation by a laparoscopy, 12 days after mating. Each generation had about 80 females and 20 males. Results of three generations of selection were analyzed using Bayesian methods. Marginal posterior distributions of all unknowns were estimated by Gibbs sampling. Heritabilities of ovulation rate (OR), number of implanted embryos (IE), litter size (LS), embryo survival (ES), fetal survival (FS), and prenatal survival (PS) were 0.44, 0.32, 0.11, 0.26, 0.35, and 0.14, respectively. Genetic correlation between OR and LS was 0.56, indicating that selection for ovulation rate can augment litter size. Response to selection for OR was 1.80 ova. Correlated responses in IE and LS were 1.44 and 0.49, respectively. Selection for ovulation rate may be an alternative to improve litter size.     

Selection for ovulation rate in rabbits: genetic parameters, direct response, and correlated response on litter size

The aim of this work was to evaluate the response to 10 generations of selection for ovulation rate. Selection was based on the phenotypic value of ovulation rate, estimated at d 12 of the second gestation by laparoscopy. Selection pressure was approximately 30%. Line size was approximately 20 males and 80 females per generation. Traits recorded were ovulation rate at the second gestation, estimated by laparoscopy as the number of corpora lutea in both ovaries; ovulation rate at the last gestation, estimated postmortem; ovulation rate, analyzed as a single trait including ovulation rate at the second gestation and ovulation rate at the last gestation; right and left ovulation rates; ovulatory difference, estimated as the difference between the right and left ovulation rates; litter size, estimated as the total number of kits born and the number of kits born alive, both recorded at each parity. Totals of 1,477 and 3,031 records from 900 females were used to analyze ovulation rate and litter size, respectively, whereas 1,471 records were used to analyze ovulatory difference, right ovulation rate, and left ovulation rate. Data were analyzed using Bayesian methodology. Heritabilities of ovulation rate, litter size, number of kits born alive, right ovulation rate, left ovulation rate, and ovulatory difference were 0.16, 0.09, 0.08, 0.09, 0.04 and 0.03, respectively. Phenotypic correlations of ovulation rate with litter size, number of kits born alive, and ovulatory difference were 0.09, 0.01, and 0.14, respectively. Genetic correlations of ovulation rate with litter size and with number of kits born alive were estimated with low accuracy, and there was not much evidence for the sign of the correlation. The genetic correlation between ovulation rate and ovulatory difference was positive (P = 0.91). In 10 generations of selection, ovulation rate increased in 1.32 oocytes, with most of the response taking place in the right ovary (1.06 oocytes), but there was no correlated response on litter size (-0.15 kits). In summary, the direct response to selection for ovulation rate was relevant, but it did not modify litter size because of an increase in prenatal mortality.