Effects of oestradiol benzoate injections, exposure to the boar and age on the attainment of puberty in the gilt

Thirty large white cross Landrace prepubertal gilts from five litters were allocated at random within litters to one of six treatment. All the gilts were given three daily intramuscular injections of 6 micrograms/kg liveweight of oestradiol benzoate but the first injection was given at either 100 days, 120 days, 140 days, 160 days, 180 days or 200 days old. From the time the first oestrogen injection was given, until the end of the experiment, the gilts were allowed daily contact with a boar for 20 minutes. The gilts were mated with a mature boar at the appearance of the second oestrus and they were slaughtered 20 days later. The numbers of gilts ovulating in the treatment groups were determined from plasma progesterone concentrations measured twice weekly throughout the experiment. The numbers of gilts ovulating within 10 days of the oestradiol benzoate injections were 3 of 5, 1 of 5, 0, 0, 3 of 5 and 1 of 5, respectively, in the groups first treated at 100 days, 120 days, 140 days, 160 days, 180 days and 200 days old. For those gilts which were successfully mated, no significant differences were observed between the treatment groups in the number of viable embryos per gilt.     

Effect of boar exposure at time of insemination on factors influencing fertility in gilts

The effect of boar exposure during artificial insemination (AI) on semen backflow, fertilization, and embryo quality was evaluated. Gilts (approximately 170 d) were induced into estrus with PG600, and ovulation was synchronized using hCG 72 h later. Estrus detection was initiated after PG600 and continued at 12-h intervals. At estrus, gilts were allotted to receive boar exposure (BE, n = 20) or no boar exposure (NBE, n = 20) during AI. Gilts receiving NBE were identified to be in estrus prior to AI and the boar was then removed for 1 h, whereas gilts in the BE group received 15 min of exposure during AI. Insemination occurred in crates at 12 and 24 h after onset of estrus with 3 x 10(9) sperm/80 mL. Backflow was collected continuously with samples taken at time 0, (during AI), and at 0.25, 0.5, 0.75, 1, 2, 4, and 8 h after first and second AI. The effect of treatment was evaluated for time of insemination (min), backflow (mL), and sperm in backflow samples. Oviducts were flushed 2 d after first AI to evaluate the effect oftreatment on fertilization rate, accessory sperm numbers on embryos (scored 1 to 5), and embryo quality. There was no effect of first or second AI; therefore, data were pooled. Average duration of AI was 3.7 +/- 0.2 min and was not influenced by BE (P < 0.10). However, during the initial stage of AI, BE reduced the volume of semen (18.6 vs 32.4 +/- 3 mL) and the number of sperm lost (0.8 vs 1.3 +/- 0.15 x 10(9) sperm) compared to NBE (P < 0.05). There was a treatment x time effect (P < 0.05) for volume of backflow. By 45 min, the BE gilts lost more volume (9.0 vs 3.6 mL) compared to the NBE group, but sperm loss did not differ. Between 1 and 8 h after AI, neither volume nor sperm loss was influenced by treatment. By 8 h, total leakage (65 vs 63 mL) and total sperm loss (1.6 x 10(9) vs 1.8 x 10(9) sperm) were not influenced by BE (P > 0.10). However, more accessory sperm (P < 0.01) were found on embryos for the NBE (> or = 11 sperm/embryo) compared to BE embryos (< or = 10 sperm/embryo). Despite this observation, percentages of fertilized embryos (99.5 +/- 0.5 %) and number of embryos (11.5 +/- 0.1) were not different (P > 0.10). In conclusion, AI in the presence of a mature boar did not affect total semen leakage, sperm loss, fertilized embryos, or embryo quality. The importance of boar exposure during insemination was evident from less leakage during insemination, but had no effect on fertility; this suggests that the elimination of boar exposure during AI may not be deleterious to reproductive performance.     

Influence of growth rate and exposure to bulls on age at puberty in beef heifers

Two experiments were conducted to test the following hypotheses: 1) exposure of beef heifers to sterile bulls increases the proportion of heifers attaining puberty by 14 mo of age and 2) rate of growth interacts with bull exposure to influence age at puberty in beef heifers. In Exp. I, heifers were assigned to one of two treatments: 1) heifers were exposed to bulls (BE; approximately 70-d period of exposure) or 2) heifers were isolated from bulls (NE) and served as controls. In Exp. II, heifers were assigned to either BE or NE treatments (175-d period of exposure to bulls) and were fed to gain at a moderate (MG; .6 kg/d) or high (HG; .8 kg/d) growth rate. Blood samples were collected twice weekly to determine concentrations of progesterone indicative of onset of corpus luteum function and puberty. In Exp. I a greater (P less than .05) proportion of heifers receiving the BE treatment than of heifers receiving the NE treatment initiated corpus luteum function by 14 mo of age. In Exp. II, there was a bull exposure x growth rate interaction (P less than .05). The effect of bull exposure was greater within the HG groups than within the MG groups. However, heifers fed to attain a moderate or high growth rate and exposed to bulls attained puberty at younger ages than heifers not exposed to bulls and fed to attain a moderate or high growth rate. Mean ages at puberty were 375, 422, 428, and 449 (pooled SEM = 8.6) d for heifers in the BE-HG, BE-MG, NE-HG, and NE-MG groups, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Analysis of influencing factors of boar claw lesion and lameness

This study aimed to investigate the factors affecting boar claw lesions and lameness. A total of 1299 boars were examined for claw lesions and lameness, including 788 boars reared in individual pens with solid concreted floor (IPS) and 511 boars raised in individual stalls with slatted floor (ISS). Flooring type showed significant impacts on all claw lesion types (P < 0.01). Except for swelling ankle, boar age had significant effects on all other claw lesion types (P < 0.01). In addition, only heel overgrowth and erosion, cracked wall horizontal, heel‐sole crack, dew claws, and toes were significantly related to boar breeds (P < 0.05). Furthermore, IPS lame boars had higher prevalence of lameness in the hind limb (P < 0.05), whereas in ISS lame boars, there were no significant differences in prevalence of lameness between the fore and hind limbs (P > 0.05). Boar lameness was moderately correlated with swelling ankle (Φ = 0.5571). In conclusion, claw lesions can be influenced by flooring type, boar age and breed, and could serve as a predictor for boar lameness.     

Effects of restricting energy during the gilt developmental period on growth and reproduction of lines differing in lean growth rate: responses in feed intake, growth, and age at puberty

The overall objective was to compare reproductive performance through 4 parities of gilts developed with ad libitum access to feed or with restriction of energy to 75% of ad libitum intake. Effects on growth and pubertal development are reported. The experiment was a 2 × 2 factorial with 661 gilts. One-half of the gilts (n = 330) were allowed ad libitum access to feed from weaning to breeding at 235 d of age (AL), and 331 littermates were developed with ad libitum access to feed to 123 d of age and then restricted to 75% of ad libitum intake to 235 d of age (Res). Diets for gilts on regimen AL were formulated to meet requirements for growth. All nutrients except energy and selenium were increased in the diet fed to gilts on regimen Res so that nutrient intake per unit of BW was expected to be similar to that of gilts on regimen AL. Sires of all gilts were from an industry maternal line. Dams were either an industry Large White-Landrace cross, or Nebraska selection Line 45, producing gilts denoted as LW/LR and L45X, respectively. Traits were recorded every 2 wk. Recording of feed intake and BW began at 53 d of age, and recording of backfat (BF) and LM area (LMA) began at 123 d of age. Estrus detection began at 140 d of age to determine age at puberty (AP). The G:F ratio from 123 to 235 d of age for gilts on the AL regimen was greater (0.269 vs. 0.257, P < 0.01) than for gilts on the Res regimen; the greatest difference occurred in the first 2-wk period following feed restriction. The LW/LR gilts were heavier, had less BF, and had greater LMA than L45X gilts, but interactions with feeding regimen and period of development existed. Feed restriction reduced BW, BF, LMA, and ratio of BF to BW, but had little effect on ratio of LMA to BW. More L45X gilts than LW/LR gilts (98 vs. 93%, P < 0.01) and more gilts developed on regimen AL than regimen Res (98 vs. 91%, P < 0.01) expressed estrus. Mean age at puberty was 178.6 d for LW/LR and 173.0 d for L45X gilts (P < 0.01) and 174.1 d for regimen AL and 177.5 d for regimen Res (P < 0.05). The Res regimen delayed pubertal development. Subsequently, it will be important to determine effects on reproduction through 4 parities.     

Photoperiod influences age at puberty of heifers

Two experiments were conducted to determine if exposure of prepubertal heifers to supplemental lighting hastens the onset of puberty. In Exp. 1, 16 heifers were paired according to birth date (April 21 to July 4) and assigned randomly to exposure to either 18 h light/d (L) or natural photoperiods (N) from 22 wk of age until puberty. Twenty-two heifers in Exp. 2, born between February 27 and March 31 and between May 3 and May 17, 1981, were exposed to L or N from 24 wk of age until March 23, 1982. In Exp. 2, animals were bred at all estrous periods until conception. Age at first ovulation and first estrus were less (P less than .01 for Exp. 1 and P less than .10 for Exp. 2) for L than N heifers. Average ages at first estrus were 318 (L) and 367 d (N) for Exp. 1 and 367 (L) and 394 d (N) for Exp. 2. Age at conception in Exp. 2 was similar for L (380 d) and N (396 d) groups. There were no significant differences between L and N heifers in changes in body weight for either experiment. There was a photoperiod X age interaction (P less than .06) for ovarian volume in Exp. 1 because the rate of ovarian growth was greater for L than N heifers. Concentrations of LH were not affected by photoperiod in Exp. 1 and not measured in Exp. 2. There were no significant changes in LH concentrations between 22 and 34 wk of age. When expressed relative to first ovulation, LH levels were highest at 7 and 2 wk before first ovulation. Concentrations of prolactin in Exp. 1 were not significantly affected by photoperiod. It was concluded that supplemental lighting after 22 or 24 wk of age reduced ages at first ovulation and first estrus in heifers born from February to July. These effects of photoperiod were accompanied by changes in ovarian development.     

Effects of exercise during the growing-finishing period on performance, age at puberty and conception rate of gilts

Littermate gilts were used to study the effects of exercise during the growing-finishing period on performance, age at puberty and conception rate. In trial 1, 12 gilts walked 15 min daily on a treadmill 6 d/wk (Monday through Saturday) until they attained body weights of about 100 kg, while 12 littermate gilts were not forced to exercise. Initial weight and age averaged 24.7 kg and 74 d, respectively. For trial 2, 12 gilts walked 30 min daily 6 d/wk (Monday through Saturday) on a treadmill, while 12 littermate gilts were not exercised. Initial weight and age of these gilts averaged 26.8 kg and 76 d, respectively. The treadmill was operated at 0 slope and at a speed of 2.6 km/h. Within trial, there were no differences (P greater than .05) in weight gain, average daily gain, feed consumed, feed to gain ratio, average backfat depth of loin eye area between gilts exercised and those not exercised. In both trials, there were no differences (P greater than .05) in age at puberty between exercised and nonexercised gilts. In trial 2, 10 gilts were selected in each of the exercised and nonexercised groups and were hand-mated to the same boar on the second estrous cycle. Eight gilts from the exercised group conceived, while nine from the nonexercised group conceived. Four gilts from the nonexercised groups had severe locomotor problems while only one gilt from the exercised groups had difficulty walking.     

Influence of growth rate and onset of boar contact on puberty attainment of replacement gilts raised in Thailand

This study aimed to investigate the influence of growth rate and onset of boar contact on age at first observed estrus of the replacement gilts raised in Thailand. In total, 766 gilts were measured for body weight and backfat thickness prior to insemination. Body weight was further calculated for growth rate. Estrus detection was performed twice a day by back pressure test with an existence of mature boars with high libido. The first date of boar exposure and that of first observed estrus were individually recorded. Due to growth rate, they were classified into three groups: high (>700 g/day), moderate (600–700 g/day), and low (<600 g/day). According to onset of boar contact, the gilts were grouped into two categories: early (<150 days) and late (≥150 days). The results revealed that the gilts expressed first observed estrus, averagely, at age 205.1?±?34.1 days, had a growth rate of 615.5?±?57.6 g/day, and first contact with boars at 160.7?±?19.9 days of age. The gilts with low growth rate expressed first estrus later than those with moderate (208.6?±?2.0 vs 198.0?±?3.2 days, P?=?0.033) and high growth rate (208.6?±?2.0 vs 193.9?±?6.7 days, P?=?0.005) groups. Together with the influence of boar exposure, the gilts contacted boar earlier with high growth rate showed first estrus at age 180.3?±?10.1 days, whereas those with later boar contact with low growth rate showed first estrus at age 197.9?±?3.2 days. In summary, the replacement gilts should have high growth rate and contact boar early to attain puberty faster and possess decent subsequent reproductive performance.     

Genome-wide association study for age at puberty in young Nelore bulls

Selection for bulls that would reach puberty early reduces the generation interval and increases fertility and herd productivity. Despite its economic importance, there are few QTL associated with age at puberty described in the literature. In this study, a weighted single-step genome-wide association study was performed to detect genomic regions and putative candidate genes related to age at puberty in young Nelore bulls. Several protein-coding genes related to spermatogenesis functions were identified within the genomic regions that explain more than 0.5% of the additive genetic variance for age at puberty in Nelore bulls, such as ADAM11, BRCA1, CSNK2A, CREBBP, MEIOC, NDRG2, NECTIN3, PARP2, PARP9, PRSS21, RAD51C, RNASE4, SLX4, SPA17, TEX14, TIMP2 and TRIP13 gene. Enrichment analysis by DAVID also revealed several GO terms related to spermatogenesis such as DNA replication (GO:0006260), male meiosis I (GO:0007141), double-strand break repair (GO:0006302), base excision repair (GO:0006284), apoptotic process (GO:0006915), cell–cell adhesion (GO: 0098609) and focal adhesion (GO:0005925). The heritability for age at puberty shows that this trait can be improved based on traditional EBV selection. Adding genomic information to the system helps to elucidate genes and molecular mechanisms controlling the sexual precocity and could help to predict sexual precocity in Nelore bulls with greater accuracy at younger age, which would speed up the breeding programme for this breed.     

Some factors influencing the exposure of grazing cattle to trichostrongyle infection

Fluctuations in the contamination of herbage with trichostrongylid nematode larvae are determined by a number of factors, the most important being climate and management. Practical measures to prevent calves from ingesting large numbers of larvae are based on detailed knowledge of the herbage contamination pattern and its epidemiological consequences. Intensification of animal husbandry methods including increased stocking rates and application of slurry to pasture may increase the hazards of gastrointestinal helminthiasis.

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Kurzfassung Die Fluktuation in der Weidekontamination durch Trichostrongylid-Nematodenlarven wird durch zahlreiche Faktoren bestimmt, deren wichtigste Klima und Bewirtschaftung sind. Praktische Kontrollmassnahmen zur Verhinderung einer hohen Larvenaufnahme durch die Kälber beruhen auf genauen Kentnissen der Weidelandkontaminationsschemata und ihrer epidemiologischen Folgen. Durch Intensivierung der Tierhaltungsmethoden, einschliesslich grösserer Stückzahlen und Düngen des Weidelands mit Jauche können die Gefahren einer gastrointestinalen Helminthiasis zunehmen.

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Resume La contamination des herbages par les larves de nématodes du genre des trichostrongyloides varie en fonction d'un nombre de facteurs, dont les plus importants sont les conditions climatiques et le mode d'exploitation. Les mesures pratiques de lutte visant à empêcher les veaux d'absorber une trop grande quantité de larves sont basées sur une étude détaillée des modalités de la contamination des herbages et de ses conséquences épidémiologiques. Le recours à des méthodes d'élevage intensif, avec l'accroissement du taux de charge et le traitement des pâturages à l'aide de certaines boues que cela implique, peut accroître le risque d'helminthiases gastrointestinales.

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Riassunto La fluttuazione dei livelli di contaminazione dei pascoli erbosi da parte di larve di tricostrongili è dovuta a fattori diversi, fra i quali particolarmente importanti sono il clima ed il tipo di conduzione. Nella pratica, le misure di controllo destinate a proteggere i vitelli da una massiva assunzione di larve si basano sull'analisi particolareggiata dell'andamento del processo di contaminazione dei pascoli erbosi e delle relative conseguenze sul piano epidemiologico. L'introduzione di metodi di allevamento piu intensivi, compresa una maggiore concentrazione dei capi e la fertirrigazione dei pascoli, può aumentare i rischi di insorgenza delle elmintiasi gastrointestinali.

     

Effect of selection for growth traits on age and weight at puberty in bovine females

This 20-yr study, using 584 beef heifers born in the spring of 1984 or 1985, was conducted to determine whether selection for growth traits affected age and weight at puberty. Heifers were from three Hereford lines selected for weaning weight, final weight, and final weight plus muscling score, a control line and a line of Angus cattle. In addition, heifers from reciprocal crosses among the selection lines, control and Angus groups were evaluated. Intact bulls and androgenized heifers fitted with marking harnesses were used to aid in detection of estrus for heifers born in 1984 and 1985, respectively. The time of puberty was identified by the first behavioral estrus. During both years, heifers were weighed at 56-d intervals; these weights were used to calculate weight at puberty. Mean age at puberty was determined using survival analysis; percentages of heifers that were pubertal by the end of the study were transformed to logits for analysis. Heifers born in 1985 were heavier (P less than .05) at puberty than those born in 1984. Heifers in lines selected for growth traits were younger at puberty (P less than .05; 20 d) than were heifers in the control line because of combined direct and maternal genetic effects. Heifers from the final weight, control (P less than .05) and weaning weight lines (P less than .10) weighed less at puberty than Angus heifers; selection line x Angus crossbred heifers were heavier at puberty (P less than .05) than their respective pureline or purebred contemporaries due to heterotic effects. Selection for weaning weight, final weight or final weight plus muscling score did not have a detrimental effect on age at puberty in heifers.     

Genetic relationships of body composition, serum leptin, and age at puberty in gilts

Leptin produced by adipocytes acts through leptin receptors in the hypothalamus to control appetite and food intake and thus communicates information about degree of fatness. It is thought that a degree of body fat is required for initiation of puberty and maintenance of reproductive function in mammals. The objective of this study was to determine whether polymorphisms in the leptin (LEP), leptin receptor (LEPR), paired box 5 (PAX5), aldo-keto reductase (AKR), and pro-opiomelanocortin (POMC) genes were associated with age, leptin concentration, backfat as an indicator of body condition, or BW at puberty in 3 lines of gilts and to characterize genetic relationships among these traits. The first 2 lines, born in 2001, were formed by crossing maternal White Cross (Yorkshire x Maternal Landrace) gilts to Duroc (n = 210) or (lean) Landrace (n = 207) boars. The remaining line (n = 507), born in 2002, was formed by crossing progeny of the Duroc- and Landrace-sired lines. At first estrus, age, BW (BWP), and backfat (BFP) at puberty were recorded and blood was collected for leptin assays. Nine SNP were detected in candidate genes/regions: 1 in LEP, 3 in LEPR, 1 in PAX5, 2 in AKR, and 2 in POMC. Animals were genotyped for each of the SNP; genotypes were validated using GenoProb. The association model included fixed effects of farrowing group, covariates of SNP genotypic probabilities (from GenoProb), and random additive polygenic effects to account for genetic similarities between animals not explained by SNP. Variance components for polygenic effects and error were estimated using MTDFREML. Leptin concentrations were logarithmically transformed for data analysis. All 4 traits were moderately to highly heritable (0.38 to 0.48). Age and leptin at puberty had a significant (P < 0.01) genetic correlation at -0.63 +/- 0.097, and the genetic correlation between BWP and age at puberty was 0.65 +/- 0.083 (P < 0.01). Significant additive associations (a; P < 0.05) were detected at PAX5 for age at puberty (a = 3.2 d) and for BFP (a = 0.61 mm). One SNP in LEPR was associated with leptin concentration (a = 0.31 log units; P < 0.05). The associations from PAX5 correspond to a QTL peak for age at puberty detected on SSC1. Although not necessarily the causative mutation, this result implies that a QTL that can decrease age at puberty without increasing BFP and BWP at puberty may exist in this region in commercial pigs.     

Effect of age and physical or fence-line boar exposure on estrus and ovulation response in prepubertal gilts administered PG600

Boar exposure has been used for estrus induction of prepubertal gilts, but has limited effect on estrus synchronization within 7 d of introduction. In contrast, PG600 (400 IU of PMSG and 200 IU of hCG; Intervet, Millsboro, DE) is effective for induction of synchronized estrus, but the response is often variable. It is unknown whether boar exposure before PG600 administration might improve the efficiency of estrus induction of prepubertal gilts. In Exp. 1, physical or fence-line boar contact for 19 d was evaluated for inducing puberty in gilts before administration of i.m. PG600. Exp. 2 investigated whether 4-d boar exposure and gilt age influenced response to PG600. In Exp. 1, 150-d-old prepubertal gilts were randomly allotted to receive fence-line (n = 27, FBE) or physical (n = 29, PBE) boar exposure. Gilts were provided exposure to a mature boar for 30 min daily. All gilts received PG600 at 169 d of age. Estrous detection continued for 20 d after injection. In Exp. 2, prepubertal gilts were allotted by age group (160 or 180 d) to receive no boar exposure (NBE) or 4 d of fence-line boar exposure (BE) for 30 min daily before receiving PG600 either i.m. or s.c. Following PG600 administration, detection for estrus occurred twice-daily using fence-line boar exposure for 7 d. Results of Exp. 1 indicated no differences between FBE and PBE on estrus (77%), age at puberty (170 d), interval from PG600 to estrus (4 d), gilts ovulating (67%), or ovulation rate (12 corpora lutea, CL). Results from Exp. 2 indicated no effect of age group on estrus (55%) and days from PG600 to estrus (4 d). A greater (P < 0.05) proportion of BE gilts expressed estrus (65 vs. 47%), had a shorter (P < 0.05) interval from PG600 to estrus (3.6 vs. 4.3 d), and had decreased (P < 0.05) age at estrus (174 vs. 189 d) compared with NBE. Ovulation rate was greater (P < 0.05) in the BE group for the 180-d-old gilts (12.7 vs. 11.9 CL) compared with the NBE group. However, age group had no effect on ovulation (77%) or ovulation rate (12 CL). Collectively, these results indicate that physical boar contact may not be necessary when used in conjunction with PG600 to induce early puberty. The administration of PG600 to 180-d-old gilts in conjunction with 4 d prior fence-line boar exposure may improve induction of estrus, ovulation, and decrease age at puberty.     

Growth and sexual characteristics of suckling beef calves as influenced by age at castration and growth implants

A 2-yr study using crossbred male calves (n = 228) evaluated castrating at birth or at 4 mo of age and use of anabolic growth implants (none, zeranol or estradiol-17 beta). Angus, Hereford and Brahman crossbred calves produced in seven cow-calf units that varied by breed component, stocking rate and calving season were allotted to treatment at birth in a 2 x 3 factorial arrangement. Growth implants decreased (P less than .01) testicular weight and expression (P less than .05) of male secondary sex characteristics in suckling bull calves at 4 mo. Implanted calves were shorter (P less than .05) at the hips and had shorter front legs (P less than .01) at both 4 mo and at weaning. Cannon bone circumference at weaning was increased (P less than .05) by growth implants. Age at castration did not affect (P greater than .05) calf performance or body characteristics. Calves given growth implants had higher (P less than .05) rates of gain from birth to 4 mo than did nonimplanted calves (.75 vs .71 kg/d). Implanted calves were heavier (P less than .01) at weaning and had higher (P less than .01) ADG from 4 mo to weaning than did nonimplanted calves. These data indicate no benefit from delaying age at castration, but implanting increased weaning weight an average of 8.2 kg.     

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.