The effect of pregnancy order on various biochemical and hematological values in sows]

The following blood indices were determined in the blood of 34 pregnant sows of the Large White breed under standard feeding conditions: haemoglobin, haematocrit, leucocytes, and--in the blood serum, --total protein, glucose, urea, bilirubin, cholesterol, enzyme activity (AP, GOT, GPT, GGTP) and mineral concentrations (Ca, P--inorg., Mg, Na, K, Fe, Cu, Zn, Mn). The blood was sampled in the first and third pregnancy at an average live weight of 165.12 and 197.36 kg, at an average age of 318 and 630 days and at an about the same average length of pregnancy in the time of both samplings (59 days). In younger, still growing gilts (first pregnancy) a significantly (P less than 0.05) lower content of total protein, magnesium, iron and copper was revealed, as compared with adult sows. The content of glucose, calcium, potassium and manganese in the blood serum of the gilts was significantly (P less than 0.05) higher than in sows in their third pregnancy. The adult sows showed a significantly (P less than 0.05) lowered activity of alkaline phosphatase and gamma-glutamyl transpeptidase, as distinct from gilts.     

Catecholamine blood levels in sheep during superovulation in estrus]

The effects of a serum gonadotropin (SG) superovulation hormonal preparation were investigated on catecholamine levels (norepinephrine, dopamine and epinephrine) in the blood plasma of ewes with synchronized oestrus in the oestrus period. In this trial the blood plasma of eleven ewes of the Slovak Merino breed was analyzed to detect catecholamines in the oestrus period. Superovulation was induced by an i.m. administration of 1500 IU SG as soon as oestrus synchronization with Agelin vaginal tampons finished (20 mg chlorsuperlutin) which lasted for 10 days. Catecholamines were detected in the blood plasma before synchronization, on the day of Agelin vaginal tampons application, and in 48 and 72 hours after the hormone administration--on the days of the expected ovulation. Catecholamine concentrations in the blood plasma were determined by a radioenzymatic assay using a Catechola test (Praha). The results indicate that synchronization and hormonal stimulation influence plasma catecholamine levels. The norepinephrine (NE) concentration in the blood plasma of the control samples has the value of 8.31 +/- 0.732 pmol/ml. An insignificant increase in the NE levels (13.12 +/- 0.120 pmol/ml; Fig. 1) was recorded on day 1 of the trial, after the start of synchronization. On the day of the expected ovulation the NE concentrations rose to 17.12 +/- 1.289 pmol/ml (P less than 0.001) and they remained significantly increased (P less than 0.01) at the level of 12.89 +/- 1.020 pmol/ml on the following day of the experiment. The dopamine level (DA) in the plasma of a control sample is 6.42 +/- 0.350 pmol/ml (Fig. 2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between body fat and postweaning interval to estrus in primiparous sows

Twenty-two primiparous Yorkshire sows were used to determine whether a minimal threshold of body fat exists below which the return to estrus is delayed. A second objective was to examine the relationship between body fat and interval from weaning to estrus in restricted-fed sows. During lactation (28 d), sows received 7, 9, 11 or 13 Mcal of ME daily to produce a range of sow body fatness at weaning. Intake of all dietary essentials except ME was similar for all sows. Litter size was adjusted to 10 pigs for all sows by d 3 postpartum. Each day from weaning to estrus, sows received 110 kcal ME per kg metabolic body weight plus 1,359 kcal ME per sow. Body fat was estimated at weaning and at first postweaning estrus by deuterium oxide dilution. Last rib backfat depth was determined ultrasonically 24 h postpartum and at weaning. Irrespective of dietary ME intake, percentage body fat at weaning (R2 = .24; P less than .05) and first postweaning estrus (R2 = .03; P greater than .50) accounted for only a small portion of variation in interval from weaning to estrus. Likewise, loss of backfat depth during lactation was not an accurate predictor of interval from weaning to estrus (R2 = .24; P less than .05). The low coefficients of determination (less than .25) suggest that body fat is a minor controller of postweaning interval to estrus. In contrast, dietary ME intake during lactation accounted for the largest portion of the variation (R2; = .48; P less than .01) in postweaning interval to estrus. We conclude that timing of postweaning estrus in primiparous sows is not dependent on a minimal threshold of body fat. Furthermore, effects of lactational ME intake on the postweaning interval to estrus are more pronounced than the effects of body fat.     

母猪批次化生产同期发情激素调控及应用

母猪批次化生产的基础是母猪同步化繁殖,其包括发情周期同步化和卵泡发育同步化.文章围绕母猪批次化生产中同期发情相关激素调控及应用进行简要介绍.     

Insertion and removal of intravaginal tampons during estrus synchronization in heifers]

In eight trials, 197 heifers were synchronized with gestagen chlorsuperlutin (CSL). The intravaginal tampons soaked with chlorsuperlutin were introduced in the vagina by means of an applier by a vaginal speculum. Silon threads were attached to the tampons, by which they were pulled out after treatment. The mentioned method of application removes the great physical exertion needed for manual application and increases labour productivity in tampon application by about 200%. The hazard of tampon contamination is reduced to the minimum. Of the two tampon types tested (ester and ether type) the ester type was found to be better for not needing sterilization if the disinfectant Aseptin is dropped upon it prior to application. The retention of the tampons was 94.9%. Eighty-four per cent of the retained tampons could be removed by pulling the silon threads. About 40 to 60% of CSL is excreted from the tampons during the treatment. If the initial content of CSL is increased, its excretion is raised as well.     

提高夏季断奶母猪发情率的几点措施

<正>广东省地处热带和亚热带季风气候区,每年夏季,平均气温为28～29℃,6～8月份最高气温达到36～38℃,且持续较久,给养猪生产带来极为不利的影响,表现为母猪发情率等生产指标明显下降。断奶母猪发情率,与其他月份相比往往下降20%～40%,甚至出现暗发情或发情状态很不稳定,造成配种后返情率很高,一定程度上困扰养猪生产。这可以从我场2008年全年一个单元内断奶母猪的发情率进行比较(见表1)。鉴于此,笔者从防暑降温工作落实、改善母猪膘情、改变查情时间等方面谈谈提高断奶母猪发情率的一些看法。     

利用烯丙孕素使授精未孕母猪重返预定的配种方案

为研究烯丙孕素、马绒毛膜促性腺激素(eCG)和人绒毛膜促性腺激素(hCG)或促性腺激素释放激素类似物(GnRH-A)D-Phe6-LHRH在促使诊断为未孕的高胎龄母猪同步发情同步排卵以使其重返预定的人工授精方案中的作用,特进行了一项现场研究。试验选用531头在人工授精后第21～35天期间经超声波检查确诊为未孕的母猪,并随机分入3个试验组:⑴试验Ⅰ组(EGⅠ):每头母猪按16mg/d的剂量连续15d口服烯丙孕素以阻止滤泡的生长,停服24h后肌肉注射1000IU马绒毛膜促性腺激素以刺激卵泡生长,78～80h后再肌肉注射500IU人绒毛膜促性腺激素以诱导排卵;⑵试验Ⅱ组(EGⅡ):除将烯丙孕素的剂量由16mg改为20mg和马绒毛膜促性腺激素剂量由1000IU改为800IU以外,其它与试验Ⅰ组相似;⑶试验Ⅲ组(EGⅢ):除用50μg促性腺激素释放激素类似物D-Phe6-LHRH替代500IU人绒毛膜促性腺激素来诱导排卵外,其它均与试验Ⅱ组相似。试验母猪分别在注射eCG/D-Phe6-LHRH后16h和40h各进行一次人工授精,并利用超声波扫描仪检查卵巢情况以确诊激素处理是否获得成功。记录母猪的受孕率、分娩率、总产仔数和活产仔数,并与同步发情首次配种的母猪比较。试验母猪在激素处理前卵巢结构各不相同,烯丙孕素的两种口服剂量都能使80%以上的母猪卵巢卵泡生长得到阻止,不过口服16mg/d的烯丙孕素会提高多囊卵巢变性的发生率。4%～18%的母猪经烯丙孕素处理后其卵巢中仍存在黄体,大多数母猪在2次人工授精之间或第2次人工授精之后排卵(P<0.05)。注射促性腺激素释放激素类似物(GnRH-A)D-Phe6-LHRH的母猪排卵往往早于注射人绒毛膜促性腺激素的母猪。被诊断为未怀孕的母猪受孕率和分娩率比初次交配母猪低25%(P<0.05)。在诊断为未孕的母猪中,注射促性腺激素释放激素类似物(GnRH-A)D-Phe6-LHRH的母猪受孕率较高(P<0.05),但在总产仔数和活产仔数上各试验组间无显著差异。总之,给诊断为未孕的母猪口服20mg烯丙孕素,然后肌注800IU马绒毛膜促性腺激素和50μg促性腺激素释放激素类似物D-Phe6-LHRH是诱使其同步发情和同步排卵的最佳方法,至于母猪口服烯丙孕素的时间是否一个较长的18d间隔还有待进一步的研究。     

Impact of amino acid nutrition during lactation on luteinizing hormone secretion and return to estrus in primiparous sows.

The impact of the dietary amino acid regimen of primiparous sows on LH secretion and weaning-to-estrus interval was evaluated. Thirty-six sows, nursing litters of 13 pigs, were allocated daily 6 kg of a corn-soybean meal diet containing a high (HP, 1.20% lysine) or low (LP, .34% lysine) protein content during a 23-d lactation. Dietary lysine concentration was achieved by altering the ratio of corn and soybean meal in the diet. Plasma LH, ACTH, and estradiol-17beta were evaluated at 15 min, hourly, and at 6-h intervals, respectively, during 6-h periods on d 0, 5, 10, 15, and 20 of lactation. Sows were checked daily for estrus from weaning to 45 d postweaning. Sows fed the LP and HP diets consumed 4.41 and 4.98 kg of feed daily during lactation. The LP sows weighed less (P < .05), had lighter (P < .05) litters at weaning, and had (P < .05) extended weaning-to-estrus intervals. Mean and baseline LH concentrations and LH pulses/6 h were lower (P < .01) in LP sows, and the differences between LP and HP sows were established by d 10 of lactation. Plasma estradiol and ACTH concentrations were not altered by diet. Mean LH concentrations on d 5 and 10 of lactation were correlated (r = -.54 and -.56, respectively, P < .01) with weaning-to-estrus interval. Also, mean LH concentrations on d 10 were correlated (P < .05) with the magnitude of dietary lysine deficiencies relative to demand for milk synthesis on d 0 to 5 and d 5 to 10 (r = -.39 and -.49, respectively). Inadequate dietary amino acid intake in sows during early lactation results in lower LH secretion by d 10 postpartum and is associated with increased weaning-to-estrus interval.     

Endocrine changes in sows exposed to elevated ambient temperature during lactation

Seven sows were placed into one of two environmental chambers at 22 C, 5 d prior to farrowing. On day 9 of lactation, one chamber was changed to 30 C (n = 4) and the other remained at 22 C (n = 3). On days 24 and 25, blood samples were collected every 15 min for 9 hr and 7 hr, respectively. On day 24, thyrotropin releasing hormone (TRH) and gonadotropin releasing hormone (GnRH) were injected iv at hour 8. On day 25 naloxone (NAL) was administered iv at hour 4 followed 2 hr later by iv injection of TRH and GnRH. Milk yield and litter weights were similar but backfat thickness (BF) was greater in 22 C sows (P less than .05) compared to 30 C sows. Luteinizing hormone (LH) pulse frequency was greater (P less than .003) and LH pulse amplitude was less (P less than .03) in 22 C sows. LH concentrations after GnRH were similar on day 24 but on day 25, LH concentrations after GnRH were greater (P less than .05) for 30 C sows. Prolactin (PRL) concentrations were similar on days 24 and 25 for both groups. However, PRL response to TRH was greater (P less than .05) on both days 24 and 25 in 30 C sows. Growth hormone (GH) concentrations, and the GH response to TRH, were greater (P less than .0001) in 30 C sows. Cortisol concentrations, and the response to NAL, were less (P less than .03) in 30 C sows. NAL failed to alter LH secretion but decreased (P less than .05) PRL secretion in both groups of sows. However, GH response to NAL was greater (P less than .05) in 30 C sows. Therefore, sows exposed to elevated ambient temperature during lactation exhibited altered endocrine function.     

Effects of boar contact and housing conditions on estrus expression in weaned sows

Our objective was to study the effects of housing conditions and the amount of boar contact in a protocol for estrus detection on estrus detection rate, timing of onset of estrus, duration of estrus, and timing of ovulation. After weaning, 130 multiparous sows were assigned to three treatments: HI, in which 52 sows were housed individually in crates and received a high amount of boar contact during estrus detection; HG, in which 52 sows were housed in groups and received a high amount of boar contact; and NI, in which 26 sows were housed individually in crates and received a normal amount of boar contact. Estrus detection was performed every 8 h. For each treatment, the standing response to three levels of stimuli was recorded: a back pressure test (BPT) by a man (man-estrus), presence of a teaser boar (spontaneous-estrus), and BPT in the presence of a teaser boar (boar-estrus). In addition, for HI and HG, the standing response to a fourth level of stimuli was recorded: BPT in a detection-mating area, surrounded by four boar pens (DMA-estrus). To detect ovulation, ultrasonography was performed every 4 h during estrus. Of 117 sows that ovulated, 46% showed man-estrus, 56% spontaneous-estrus, 90% boar-estrus, and 97% DMA-estrus. Mean onset of man-estrus was 107 h (SD 26) after weaning, of spontaneous-estrus was 106 h (SD 22) after weaning, of boar-estrus was 99 h (SD 21) after weaning, and of DMA-estrus was 93 h (SD 22) after weaning. Duration of man-estrus was 22 h (SD 14), of spontaneous-estrus was 29 h (SD 16), of boar-estrus was 42 h (SD 20), and of DMA-estrus was 55 h (SD 18). The high amount of boar contact reduced the number of sows showing man-estrus (P < .05; 41% for HG and HI vs 68% for NI) and reduced duration of boar-estrus (P < .05; 43 h for HG and HI vs 52 h for NI). Duration of DMA-estrus for HG and HI was similar to duration of boar-estrus for NI. Onset of estrus and timing of ovulation were not affected by amount of boar contact. Group housing did not affect detection rate and duration of estrus, but it did postpone average onset of estrus by 10 h, paralleled by a postponement of ovulation. In conclusion, estrus expression is similar at the highest level of stimuli in different protocols for estrus detection. Including higher levels of stimuli in a protocol reduces estrus expression at lower levels of stimuli. This reduction indicates adaptation of sows to a given protocol for estrus detection. Group housing can delay ovulation and related behavioral estrus.     

Effect of housing system and boar exposure on estrus expression in weaned sows

Reproductive efficiency depends on detection of estrus, which may be influenced by housing and boar exposure. This experiment investigated the effects of housing system and boar contact on measures of estrus in weaned sows. Mixed-parity sows were randomly assigned to be weaned into gestation crates away from boars (AWC, n = 45), into pens away from boars (AWP, n = 42), or into pens adjacent to a mature boar (ADJ, n = 46). Estrus detection was initiated at approximately 0700 (0 h) and again at 0.25-, 0.5-, 1-, 2-, 4-, and 8-h intervals beginning on d 4 and continuing through d 7 following weaning. Estrus detection involved observation of the standing response after application of nose-to-nose boar exposure, backpressure, and side rubbing. For the AWC sows, a mature boar was moved to the front of the crates for a 10-min period and then removed. Sows housed in AWP were moved approximately 15 m to an empty pen adjacent to a mature boar for a 10-min period, and then returned to their pen. Sows housed ADJ were not moved and estrus detection was performed in their home pen for a 10-min period. The proportion of sows expressing estrus within 7 d from weaning was lowest for ADJ (80%, 37/46) compared with AWP (98%, 41/42) and AWC (96%, 43/45; P < 0.05). There was an effect of interval from weaning to estrus on the percentage of sows expressing estrus, but there was no interaction with treatment. Sows in AWC and AWP (4.7 d) had decreased (P = 0.01) intervals from weaning to estrus compared with ADJ (5.2 d). The duration of estrus was also shorter (P < 0.001) for ADJ (45 h) compared with AWC (58 h) or AWP (62 h). There was a treatment x interval x day of estrus effect for the percentage of sows expressing estrus. After detection of the first standing response on the first day of estrus, only 62 to 82% of sows were detected standing over the next 2 h for all treatments. However, at 4 to 8 h, this increased to 85 to 98% for the AWC and AWP sows, but <73% of the ADJ sows were detected during this period. On the second day of estrus, estrus expression was not influenced by interval for the AWC and AWP sows and was between 90 to 100% during the 8-h period, whereas ADJ sow detection rates were between 68 to 88%. These data suggest that housing sows adjacent to boars negatively affects estrus expression and detection. In addition, refractory behavior occurs in approximately 30 to 40% of sows and is influenced by housing relative to the boar, day of estrus, and interval from last boar exposure.     

Effect of boar presence before and after weaning on estrus and ovulation in sows

Two hundred sixty-two sows were used to investigate the effects of boar exposure during the last week of nursing (BPRE) and after weaning (BPOST) on the return to estrus. Because approximately one-half of the sows were weaning their first litter, a third factor, primiparous vs multiparous (LITT), was considered in the statistical analysis. To evaluate the effect of treatment on ovarian activity, the sows were also blood-sampled twice weekly for 3 wk after weaning for the measurement of plasma progesterone concentrations as an index of ovulation. Boar exposure after weaning was the most important stimulus of early ovulation and estrus after weaning (P less than .001). A greater proportion of first-litter sows exhibited estrus later (P less than .02) and ovulated later (P less than .09) than did multiparous sows. First-litter sows were unaffected (P greater than .10) by boar exposure before weaning. Multiparous sows were sensitive to boar exposure during nursing. Maximal boar exposure for these sows (BPRE + BPOST) resulted in 95% of sows in estrus and ovulating within 20 d of weaning. No boar exposure resulted in 45% and 38% anestrus and anovulatory sows. Boar exposure, either before or after weaning, was effective in reducing the number of anestrus and anovulatory sows to between 15 to 30%. The effects of BPRE and BPOST on return to ovulation were additive and approximately equal.     

Biochemical and histological changes in the hypothalamus in hormone-stimulated sheep]

The effects were investigated of a hormonal preparation of serum gonadotrophin (SC) on neurosecretion, PAS-positive mucopolysaccharides and on histological changes in the third cerebral ventricle; at the same time variations were determined of catecholamine concentrations of ependyma surface in the hypothalamic regions that control directly reproductive functions in ewes of the Merino and Wallachian breeds in the oestrus period. Ewe oestrus was synchronized by insertion of Ageline (Spofa) sponges into the vagina. A dose of 2000 IU of serum gonadotrophin (Bioveta, Ivanovice in Haná) was administered to stimulate the ewes (n = 30). Twelve ewes of the Wallachian breed, six control and six test ones, were used for radioenzymatic detection of catecholamines (Catechola test, Prague). The brains were segmented immediately after bleeding and samples were taken from the eminentia medialis, area preoptica and corpus mamillare pursuant to the stereotactic configuration of the sheep hypothalamus (Welento et al., 1968). The brains of ten ewes of the Merino breed, four control and six test ones, and of eight ewes of the Wallachian breed (four control and four test ones) were used for histological treatment. For the purposes of a histological study, the brains were treated with current histological methods. The amount of neurosecretory material was assessed by light microscopy (Nakahara, 1963). Samples for the study of ependyma of the lower part of the third cerebral ventricle were examined in a scanning electron microscope. The obtained results demonstrate a statistically significant decrease by 36.4% (P less than 0.05) in epinephrin concentrations in the area preoptica of hypothalamus after SG administration (Fig. 3), in comparison with the control group. A decrease in norepinephrin concentration in the corpus mamillare (Fig. 4) was statistically significant (P less than 0.001). The dose of 2000 IU SG influenced most markedly catecholamine concentrations in the eminentia medialis (Fig. 5), where a statistically significant decrease in epinephrin and norepinephrin concentration was observed (P less than 0.01). Dopamine concentrations also decreased significantly (P less than 0.05) in the eminentia medialis (Fig. 5). Following the hormonal stimulation, a significant increase in dopamine and epinephrin concentrations was observed in the hypophysis of sheep (Fig. 6; P less than 0.05) and in the epiphysis there was a significant decrease in epinephrin and dopamine concentration (P less than 0.01).(ABSTRACT TRUNCATED AT 400 WORDS)     

Responses to delayed estrus after weaning in sows using oral progestagen treatment

Oral progestagen treatment extends the weaning-to-estrus interval (WEI) in weaned sows. Particularly in lower parity sows, this allows recovery from lactational catabolism and improves sow productivity. However, the optimal duration of progestagen treatment in contemporary dam-line sows is unclear. Therefore, sows (n = 749) weaned over consecutive 3-wk periods in June and July and classified as parity 2 and 3 (P2-3); 4, 5, and 6 (P4-6); or parity 7 or higher (P7+) were organized into 2 breeding groups using 1 of 3 strategies: 1) oral progestagen for 2 d before and 12 d after weaning (M14; n = 249); 2) oral progestagen for 2 d before and 5 d after weaning (M7; n = 250); or 3) no progestagen treatment (M0; n = 250). Progestagen (altrenogest) was administered directly into the sow's mouth at a dosage of 6.8 mL (15 mg of altrenogest) daily. Sows were bred using artificial insemination at first detection of estrus after weaning (M0) or altrenogest withdrawal, and every 24 h thereafter, until they no longer exhibited the standing reflex. The WEI for M0 sows was 5.1 +/- 0.1 d. Estrus was recorded sooner (P < 0.001) after withdrawing treatment in M14 than in M7 sows (6.9 +/- 0.1 vs. 7.4 +/- 0.1 d, respectively). More (P < 0.001) M14 sows (88.6 +/- 2.5%) were bred within 10 d of altrenogest withdrawal than M7 (72.8 +/- 2.8%) sows, or within 10 d of weaning in M0 sows (78.8 +/- 2.6%). Reproductive tracts were recovered after slaughter at d 30 or 50 of gestation. For P2-3 sows, ovulation rate (least squares mean +/- 95% confidence interval) in M7 (23.1 +/- 1.0) was greater (P < 0.001) than in M14 (20.7 +/- 1.0) or M0 (19.7 +/- 1.0) sows; no differences were detected in P4-6 and P7+ sows. At d 30, M7 and M14 sows had more (P < 0.01) embryos (16.4 +/- 0.6 and 15.8 +/- 0.4, respectively) than M0 (13.9 +/- 0.5) sows. At d 50 of gestation, number of fetuses in M14 sows (13.6 +/- 0.4) was greater (P < 0.001) than in M0 (11.8 +/- 0.4) and M7 (12.2 +/- 0.3) sows. Use of oral progestagen to delay the return to postweaning estrus for greater than 18 d appears to have potential for improving weaned sow productivity. Given the incidence of high ovulation rates and associated evidence of intrauterine crowding of embryos around d 30 of gestation, the changing dynamics of prenatal loss resulting from longer periods of progestagen treatment may represent an additional production advantage.     

Influence of follicular ablation during lactation on postweaning interval to estrus, ovulation rate, and endocrine function in sows

Duroc sows farrowed second litters in March and lactated 35 +/- 2 days. At 36 hr before weaning, electrocautery of follicles greater than or equal to 3 mm in diameter (n = 8) or sham surgery (n = 5) was performed to test the hypothesis that ablation of medium-sized follicles would prolong the duration of postweaning anestrus. Number of follicles and diameters at surgery were: 1.3 +/- .6 (greater than 5 mm diameter), 26 +/- 1 (3 to 5 mm) and greater than 20 (less than 3 mm). Blood samples were collected at 15 min intervals for 3 hr beginning at -12, 0, 12, 60 and 96 hr from weaning. Interval to estrus was 3.4 +/- .2 days in seven of eight cauterized sows and 3.6 +/- .6 days for sham-surgery sows. The remaining cauterized sow was anestrus at slaughter, 40 days after weaning. Number of corpora lutea and pregnancy rate were 15.8 +/- .6 and 92%, respectively, and were similar between sham-surgery and cauterized sows. Concentration of follicle stimulating hormone (FSH) at 12 hr before weaning was greater in sows subjected to electrocautery than for sham-surgery sows, but FSH values were similar at other sampling times. Concentrations of estradiol were similar at all times for both treatment groups. Luteinizing hormone (LH) was higher (P less than .05) at 60 hr in cauterized sows because of the onset of the preovulatory LH surge in one sow. We conclude that destruction of medium-sized ovarian follicles before weaning did not influence postweaning reproductive performance.     

Factors influencing estrus and ovulation in weaned sows as determined by transrectal ultrasound.

Characterization of factors influencing estrus and ovulation in sows may facilitate development of procedures for improving reproductive performance. The experiment was conducted in confinement during 1997 to 1999 using 174 Large White x Landrace sows. After weaning, sows were checked for estrus twice daily. In the 1st yr, transrectal ultrasound was performed once daily and in the 2nd yr twice daily at estrus and on every day until ovulation. The effects of lactation length (< or = 16 d, 17 to 24 d, 25 to 31 d or > or = 32 d), parity (1, 2, or > or = 3), season (winter, spring, summer, or fall) and weaning-to-estrus interval (3, 4, 5, or 6 to 8 d) and their interactions on estrual and ovulatory responses were studied. There was no effect of frequency of ultrasound on any response variable, so data across years were pooled. Percentage of sows expressing estrus within 8 d of weaning was influenced by lactation length (P < 0.001), with sows lactating < or = 16 d (35.2%) less likely to express estrus than sows lactating > or = 17 d (94%). A parity x season interaction was observed (P < 0.001) for estrus, with the lowest expression in parity 1 (73.0%) and parity 2 sows in fall (67.2%), compared with > or = parity 3 sows (98.1%). No explanatory variable had a significant effect on weaning-to-estrus interval (4.4 d) or on follicle size at estrus (8.1 mm). Ovulation hour after onset of estrus was affected by weaning-to-estrus interval (P < 0.01), with sows returning in 3 d ovulating at 46.2 h and between 6 and 8 d at 30.2 h. For sows that expressed estrus within 8 d of weaning, the percentage of sows ovulating was influenced by lactation length (P < 0.001) and weaning-to-estrus interval (P < 0.001). Sows that lactated < or = 16 d were less likely to ovulate (78.0%) than those lactating > or = 17 d (> 92%). Sows that returned to estrus in 3 d were also less likely to ovulate (79.5%) than sows returning > or = 4 d after weaning (> 92%). A parity x season interaction was also observed on ovulation (P < 0.001), with parity 1 and 2 sows less likely to ovulate after expressing estrus in fall and spring compared with parity 3 and greater sows. The data suggest lactation length, early return to estrus, and parity by season effects are associated with risk of failure to express estrus and ovulate.