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ABSTRACT: To ensure sufficient numbers of pregnant females, particularly at hotter times of the year, hormonal induction of gilt oestrus may be necessary. However, the gilt oestrus and ovulation responses to gonadotrophin treatment have often proven unpredictable. The objective of this study was to examine possible reasons for this unpredictability. Prepubertal gilts (approximately 150 days of age, n = 63) were assigned to one of three treatments: injection of 300 IU hCG (n = 15); pre-treatment with 100 mg FSH in polyvinylpyrrolidinone administered as 2 x 50 mg injections 24 h apart, followed by 600 IU eCG at 24 h after the second FSH injection (n = 23); or FSH pre-treatment as above followed by 300 IU hCG at 24 h after the second FSH injection (n = 25). To facilitate oestrus detection, gilts were exposed to a mature boar for 15 min daily for 7 days. Blood samples were obtained on the day of eCG or hCG injection and again 10 days later and gilt ovulation responses determined based on elevated progesterone concentrations. The oestrus responses by 7 days were 6.7%, 17.5% and 64.0% for gilts treated with hCG, FSH + eCG and FSH + hCG, respectively (p < 0.001). The oestrous gilt receiving hCG alone and one oestrous FSH + hCG gilt did not ovulate, all other oestrous gilts ovulated. A further two anoestrous FSH + eCG-treated gilts ovulated. These data suggest that FSH pre-treatment facilitated the development of ovarian follicles to the point where they became responsive to hCG, but had little effect on the response to eCG.
Reproduction in Domestic Animals 11/2008; 44(3):432-4. · 1.36 Impact Factor
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ABSTRACT: To meet weekly breeding targets, it is occasionally necessary to inject exogenous gonadotrophins to induce oestrus in prepubertal gilts. However, the gilt oestrus response to equine chorionic gonadotrophin (eCG) either alone or in combination with human chorionic gonadotrophin (hCG) can be unpredictable. The objective of the present study was to examine possible reasons for this unpredictability. Prepubertal gilts (90 kg and 153 days of age, n = 109) received an injection of either 600 IU eCG or a combination of 400 IU eCG and 200 IU hCG (PG600), or were non-injected controls, and were then exposed to a mature boar for 15 min daily for 7 days for oestrus detection. At the time of injection, real-time ultrasound revealed that the gilt ovaries had primarily 1-2 mm follicles. Blood samples were obtained at time of hormone injection (day 0) and at days 3, 7 and 10 for assay of serum progesterone concentrations. The oestrus responses by 7 days were 15.5%, 73.3% and 0%, for eCG, PG600, and control gilts, respectively (p < 0.001). The oestrus response improved (p < 0.05) with increasing body weight. Based on circulating progesterone levels, all oestrous gilts ovulated except for four of the PG600 gilts. Failure to express oestrus in PG600 gilts was not associated with a premature rise in progesterone.
Reproduction in Domestic Animals 10/2008; 44(3):411-3. · 1.36 Impact Factor
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ABSTRACT: The objective of this study was to determine the effect of pre-treatment of prepubertal gilts with FSH on the estrus and ovulatory responses to eCG injection at two ages. A total of 149 prepubertal Hypor gilts were selected at 150 days (n=76) or 180 days (n=73) of age and assigned to injection of 400 IU eCG plus 200 IU hCG (PG600), 600IU eCG alone (Folligon), pre-treatment with 72 mg FSH (Folltropin) administered as 6 x 12 mg injections at 12 h intervals with 600 IU Folligon 12h after last FSH injection, or non-injected controls. To facilitate detection of estrus, gilts were exposed to a mature boar for 15 min daily for 7 days. To determine ovulatory responses, blood samples were obtained on the day of injection and 10 days later and assayed for progesterone content. Following treatment at 150 days, one control gilt (5.3%) was deemed estrus but ovulation did not occur. Compared to treatment with Folligon alone, PG600 injection tended (P=0.1) to increase the estrus response (52.6% compared with. 26.3%) and increased (P<0.01) the ovulatory response (89.5% compared with. 47.4%). The estrous response in gilts pretreated with Folltropin was intermediate (42.1%) but the ovulatory response (47.4%) was the same as for Folligon alone. Following treatment at 180 days, two control gilts (10.5%) were deemed estrus and ovulation did occur in these gilts. There was no difference between hormone-treated groups for estrus or ovulatory responses, although the ovulatory response of PG600-treated gilts tended (P=0.1) to be greater than for the Folligon-treated group (89.5% compared with 66.7%), with Folltropin-pretreated gilts being intermediate (76.5%). These data demonstrate that the estrus and ovulatory responses of gilts were greater for PG600 than for Folligon and that while responses to PG600 were not affected by gilt age, for the combined Folligon groups, estrous response (P<0.02) and ovulatory response (P<0.05) improved with increased gilt age.
Animal reproduction science 02/2008; 110(1-2):123-7. · 1.56 Impact Factor
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ABSTRACT: In swine, the use of frozen-thawed (FT) sperm for artificial insemination (AI) is limited because of poor sow fertility, possibly associated with a post-thaw capacitation-like status resulting in fewer fully viable sperm. Sow fertility to AI with FT sperm may improve with deeper deposition of sperm within the female tract, insemination very close to ovulation, or reversal of cryocapacitation by seminal plasma (SP). We performed two experiments to examine these suggestions. In experiment 1, 122 multiparous Yorkshire sows received 600 IU equine chorionic gonadotrophin at weaning and 5 mg pLH 80 h later to control time of ovulation. The predicted time of ovulation (PTO) was 38 h after pLH injection. Thereafter, sows were assigned on the basis of parity to a single AI of FT sperm at 2 h before PTO, or at 12 h before PTO, or FT sperm supplemented with 10% SP at 12 h before PTO. Control sows received fresh semen at 12 h before PTO. All semen doses were adjusted to 3 x 10(9) live cells and deposited into the cervix. Experiment 2 employed 99 multiparous crossbred sows and repeated the treatments of experiment 1 except that all FT inseminations were intrauterine. In both experiments, farrowing rates were lower (p < 0.01) following FT inseminations with no effect of time of insemination or of supplemental SP. In experiment 1, litter size was smaller following FT insemination (p < 0.05), but no effect on litter size was evident in experiment 2. Supplemental SP had no effect on litter size in either experiment. The lack of effect of either SP or timing of FT insemination on sow fertility suggests that the non-lethal sperm cryoinjury affecting fertility involves more than just cryocapacitation.
Reproduction in Domestic Animals 09/2007; 42(4):418-22. · 1.36 Impact Factor
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ABSTRACT: Frozen-thawed (FT) boar sperm have a reduced fertile life, due in part to a capacitation-like status induced by cooling. Reversal of this cryocapacitation in vitro by exposure to boar seminal plasma (SP) has been demonstrated. The objective of these studies was to determine the effect of SP on the ability of FT sperm to create an oviductal sperm reservoir following artificial insemination (AI). In Experiment one, 35 pre-pubertal gilts were injected (IM) with 400 IU eCG plus 200 IU hCG to induce oestrus. At detection of oestrus, gilts were inseminated with 3 x 10(9) live sperm, either fresh (FS; n = 13), FT (n = 10), or FT supplemented with 10% v/v SP (n = 12). Gilts were killed 8 h later, their reproductive tracts recovered and the uterotubal junctions (UTJs) flushed to recover sperm. Fewer (p < 0.01) sperm were recovered following FT, compared to FS, inseminations, and there was no evident effect of SP. In Experiment two, 30 pre-pubertal gilts received IM injections of 1000 IU eCG followed by 5 mg pLH 80 h later to control time of ovulation. Gilts were inseminated with 3 x 10(9) live FS sperm (n = 6), FT sperm (n = 15) or FT sperm plus 10% SP (n = 9) at 12 h before ovulation and then sacrificed 8 h later. The UTJs were dissected and flushed for sperm recovery. Fewest (p < 0.001) sperm were recovered following FT insemination and there was no evident effect of SP. These data demonstrate that the size of the sperm reservoir is markedly reduced in gilts inseminated with FT sperm. However, the lack of effect of SP suggested that either it did not reverse cryocapacitation or that such a reversal does not impact the in vivo ability to create a sperm reservoir.
Reproduction in Domestic Animals 04/2007; 42(2):149-52. · 1.36 Impact Factor
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ABSTRACT: Insemination of sows with frozen-thawed spermatozoa results in lower fertility, in part due to spermatozoa having undergone a capacitation-like reaction. The present study employed chlortetracycline (CTC) staining analysis to investigate the effect of adding 20% (v/v) boar seminal plasma (SP) to boar spermatozoa on the temporal progress of capacitation and the acrosome reaction in spermatozoa cooled to 5 degrees C or incubated at 39 degrees C. Based on CTC staining patterns, seminal plasma appeared to reverse capacitation in spermatozoa that had undergone capacitation while incubated at 39 degrees C in a capacitation-supporting medium from 59.7 to 36.6% capacitated (P<0.001). Similarly, the addition of SP to boar spermatozoa cooled to 5 degrees C resulted in both the prevention of the capacitation-like reaction, and the reversal of an established capacitation-like reaction from 63.3 to 34.2% capacitated (P<0.001). These observations indicated that some constituent(s) of boar SP both prevent spermatozoa from undergoing capacitation as well as reverse capacitation in spermatozoa that have already undergone the process.
Animal Reproduction Science 07/2005; 87(1-2):121-32. · 1.75 Impact Factor
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ABSTRACT: The objective of this study was to determine the effect of subclinical Mycobacterium paratuberculosis infection on mature equivalent milk, protein, and fat production in a sample of Michigan dairy herds with a history of cows positive for M. paratuberculosis diagnosed by fecal culture. A prospective two-group cohort study was conducted. Participating herds were tested, and productivity and reproduction records were monitored for 18 mo. All cows aged 24 mo and greater were tested for M. paratuberculosis infection using the ELISA and radiometric fecal culture (RFC) techniques. Using both tests in parallel, the overall sample apparent prevalence for M. paratuberculosis infection was 41.8%. Adjusting for diagnostic sensitivity and specificity resulted in a calculated sample true prevalence of 59.9%. Subclinical paratuberculosis test-positive status had no statistically significant effect on mature equivalent milk, fat, or protein production. The results of this study concur with the findings of other studies, reporting that the magnitude and direction of the association between subclinical paratuberculosis infection and milk production depends upon the parity of the animal, stage of disease, and the stage in lactation being monitored. Assessment of the impact of subclinical paratuberculosis on milk production must consider the average parity of the sample population. In herds that have an average parity of 2 or less, subclinical paratuberculosis infection may have little impact on milk production.
Journal of Dairy Science 11/2001; 84(10):2188-94. · 2.56 Impact Factor
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ABSTRACT: A prospective cohort study design was used to evaluate the impact of subclinical Mycobacterium paratuberculosis infection on days open in a sample of Michigan dairy herds with a history of cows positive for M. paratuberculosis diagnosed by fecal culture. Participating herds were tested and productivity and reproduction records were monitored for 18 months. All cows > or = 24 months old were tested for M. paratuberculosis infection using the ELISA and radiometric fecal-culture (RFC) techniques. Test-negative cows were re-tested at the conclusion of the monitoring period. Multivariable regression models were used. Using both tests in parallel, the overall sample apparent prevalence for M. paratuberculosis infection was 41.8% (223/533 animals tested). Adjusting for diagnostic sensitivity and specificity, this resulted in a calculated sample true prevalence of 59.9%. ELISA-positive cows (on average) had a 28-day increase in days open when compared to ELISA-negative cows (p=0.02). The diagnostic method used to define a case altered the apparent association between paratuberculosis test status and days open. Fecal culture was a less-effective diagnostic tool for use in herds with a high prevalence of infected animals. The increase in days open in the ELISA-positive cows was an indication that perhaps reduced estrus expression or an increased post-partum anestrous period occurred in the subclinically infected ELISA-positive animals. This might have been due to a negative energy balance associated with M. paratuberculosis infection.
Preventive Veterinary Medicine 09/2000; 46(3):171-81. · 2.05 Impact Factor
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ABSTRACT: Semen samples from 100 beef breed bulls were evaluated for sperm morphology (phased contrast microscopy), seminal white blood cells, and the presence of potential reproductive pathogens. Eligibility required visualization of the glans penis throughout semen collection. Based on clinical spermiograms, bulls were grouped into normal, marginal, or unsatisfactory morphology classifications. The 3 experimental groups were similar in age and scrotal circumference and differed significantly in the percentage of primary sperm abnormalities. Most semen samples (94%) contained one or more potential reproductive pathogens (Hemophilus somnus. Mycoplasma bovigenitalium, Arcanobacterium pyogenes and Ureaplasma diversum). No significant relationship could be demonstrated between primary abnormalities and the assigned culture score. Our experimental results suggest that clinicians should interpret clinical semen culture results with great care. No significant relationship could be demonstrated between primary abnormalities and assigned white blood cell (WBC) score, although, only 1% of the samples was scored >5 WBC per high power field. The use of seminal WBC score may be valid adjunct to routine semen evaluation when that threshold is the basis for clinical decisions.
Theriogenology 05/1999; 51(6):1197-206. · 1.96 Impact Factor
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ABSTRACT: Transrectal palpation of an 18-mo-old, anestrous Holstein nulligravida revealed an enlarged left ovary. Ultrasonographically, the mass resembled known equine granulosa cell tumor (GCT) images. Slowly, virilism developed. Fifty-five days after initial diagnosis, unilateral ovariectomy was performed The presurgical serum concentration of testosterone was equal to that of a mature bull. During the 3 mo period following surgery, estrus expression resumed, the serum testosterone concentration returned to baseline, and the heifer was diagnosed pregnant following AI. A low grade, malignant GCT was the final histopathologically diagnosis. Clinical diagnosis of GCT should involve ultrasonography and steroid hormone assay. An ultrasound image, similar to the one shown, may be characteristic of GCT in the bovine.
Theriogenology 08/1997; 48(1):11-7. · 1.96 Impact Factor
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ABSTRACT: Lameness has contributed to reproductive inefficiency and increased the risk of culling in dairy cows. We developed a 5-point lameness scoring system that assessed gait and placed a novel emphasis on back posture. Our objective was to determine if this system predicted future reproductive performance and the risk of culling. The study was conducted at a commercial dairy farm with a history of declining reproductive efficiency and an increasing prevalence of lameness. A total of 66 primipara and pluripara calved, received an initial lameness score, and completed their 60-d voluntary waiting period. The overall prevalence of lameness (mean lameness score >2) was 65.2%. Scoring continued at 4-wk intervals and ceased with conception or culling. The percentage of cows confirmed pregnant and culled was 77.3 and 22.7, respectively. For each reproductive endpoint, a 2 x 2 table was constructed with lameness score >2 as the positive risk factor and either performance greater than the endpoint mean or being culled as the positive disease or condition. Positive and negative predictive values, relative risk, Chisquare statistic and regression analysis were used to evaluate the data. The positive predictive values for days to first service, days open, breeding herd days, services per pregnancy and being culled were 58, 68, 65, 39 and 35%, respectively. Similarly, the negative predictive values were 79, 96, 100, 96 and 100%, respectively. Except for one reproductive endpoint, the total number of services, all linear regressions were significant at P < 0.01. Having a lameness score >2 predicted that a cow would have extended intervals from calving to first service and to conception, spend or be assigned to (explained herein) more total days in the breeding herd, require more services per pregnancy and be 8.4 times more likely to be culled. We believe that this lameness scoring system effectively identifies lame cows. Observation of the arched-back posture in a standing cow (> or =LS 3) should trigger corrective interventions.
Theriogenology 04/1997; 47(6):1179-87. · 1.96 Impact Factor
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ABSTRACT: The Society for Theriogenology recently adopted a minimum standard of 70% normal spermatozoa for the bull breeding soundness examination (BSE). We conducted this study to determine if spermiograms derived by brightfield microscopy of eosin-nigrosin stained semen smears (Society method) overestimated the proportion of normal spermatozoa. Comparison of the above method was made with that of phase contrast microscopy (Phase method). We then evaluated our ability to discern head abnormalities by comparing brightfield microscopy of Feulgen-stained sperm DNA (Feulgen method) with those of the Society and Phase methods. Spermiograms were determined for each of the 181 beef bulls using all 3 methods. Only bulls that were being routinely tested prior to the 1993 breeding season were included. The mean percentage of normal spermatozoa surpassed the minimum standard with the Society (72.8%) but not the Phase (52.5%) method, which identified more distal cytoplasmic droplets that adhered to cells without distal midpiece reflexes. The Phase method also identified more total primary and fewer primary head abnormalities. We conclude that the Phase method is not a suitable substitute for the Society method when applying the minimum standard during a routine BSE. The Feulgen method identified more head abnormalities, especially in the pattern of DNA, than the other methods, however, when compared to the minimum standard that improvement was not clinically important. Both the Phase and Feulgen methods are better than the Society method for monitoring changes in abnormal spermiograms over time.
Theriogenology 04/1996; 45(4):757-64. · 1.96 Impact Factor
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ABSTRACT: Ultrasound-guided fetal fluid sampling was performed on 13 pregnant goats at Days 59 to 65 of gestation to establish safe techniques for accurate sampling and to determine the feasibility of prenatal diagnosis of beta-mannosidosis. Fluids were analyzed for electrolyte and creatinine content to assess accuracy of sampling. Values correlated well with previously reported concentrations for caprine and ovine fetal fluids at the same gestational stage. The single abortion which occurred following ultrasound-guided sampling was correlated with placentome penetration and aspiration of bloody fluids. Thin layer chromatography of amniotic and allantoic fluids was performed to detect oligosaccharides that accumulate in beta-mannosidosis. Abnormal accumulated oligosaccharides were identified in the allantoic but not amniotic fluid from a beta-mannosidase-deficient 62-d-old fetus. Thus, allantocentesis was shown to be an optimal, safe procedure for providing information at this gestational stage to diagnose caprine beta-mannosidosis.
Theriogenology 10/1995; 44(4):517-27. · 1.96 Impact Factor
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Journal of Inherited Metabolic Disease 08/1995; 18(5):647-648. · 3.58 Impact Factor
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ABSTRACT: This study constitutes the review of 44 dairy herd investigations that were initiated because of complaints relating to decreased reproductive efficiency. Each investigation was conducted at the request of the veterinary practitioner who provided the routine reproductive examinations and consultations at the farm. Thus the types of problems identified were those not resolved by routine veterinary care and management practices. A total of 4.5, 27.3 and 31.8% of the farms, respectively, failed to keep reproductive records, failed to maintain accurate records on events such as breeding dates, or failed to evaluate available summary reports with the local veterinary practitioner. Of the 44 farms, 50.0, 38.6, 54.5 and 11.4%, respectively, reported problems related to estrus detection rate, number of days to first service (for reasons other than estrus detection), conception rate and early embryonic death. Within each of these reproductive parameters specific problems were defined and discussed. Reproductive inefficiency was found to be most commonly associated with ineffective estrus detection and decreased conception rate. Most importantly, farm managers and employees frequently misunderstood the relationship between the accuracy of estrus detection and the conception rate (61.4% of the farms). We therefore identified factors which have an impact on dairy herd reproductive efficiency to suggest topics for training programs for producers and practicing veterinarians as well as for elective courses for veterinary students in the area of theriogenology.
Theriogenology 02/1995; 43(2):373-80. · 1.96 Impact Factor
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Journal of Inherited Metabolic Disease 02/1995; 18(5):647-8. · 3.58 Impact Factor
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ABSTRACT: Forty-one Holstein cows were injected with 0, 5, or 14 mg/d of bST for the last 46 +/- 6 d before parturition. Compared with data for controls, the 5- and 14-mg doses of bST increased apparent protein synthesis about 16% before parturition. Exogenous bST before parturition increased apparent protein degradation 30% during wk 1 after parturition. During wk 1 of lactation, 14 mg of bST also increased milk protein yield 33%. No treatment differences were present in concentration of serum NEFA, body condition score, or thickness of subcutaneous fat. Therefore, administration of bST before parturition did not alter metabolism of subcutaneous fat. Prepartum treatment with 5 and 14 mg of bST increased and maintained serum somatotropin at 6.5 and 22.7 ng/ml, respectively, compared with 1.6 ng/ml in controls. Concentrations of serum IGF-I were initially increased but were not maintained as parturition approached. On d -23, IGF binding protein 3 was increased 65% but was not different among groups by d -7. For groups administered the 5 and 14 mg/d of bST, IGF binding protein 2 was decreased 40%. Administration of bST before parturition increased protein reserves and stimulated milk protein yield for 1 wk but did not alter metabolism of subcutaneous fat. Furthermore, energy balance appeared to be a major regulator of concentrations of IGF binding protein 3 and responsiveness of IGF-I to exogenous somatotropin before parturition.
Journal of Dairy Science 08/1994; 77(7):1835-47. · 2.56 Impact Factor
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ABSTRACT: Eight of 19 calves born to bovine viral diarrhea virus (BVDV)-negative and -immunocompetent dams were determined to be infected with a noncytopathic strain of BVDV. Six of the 8 calves had diarrhea and 2 had no clinical signs of disease. In 3 euthanatized calves, lesions consistent with mucosal disease were found throughout the gastrointestinal tract, and the virus was isolated from the spleen, lymph nodes, and small intestine. In 5 calves, BVDV was isolated from mononuclear cells in blood samples obtained 21 days apart, indicating persistent infection. The virus was not isolated from sera obtained from 2 calves, with chronic nonclinical infections, that had neutralizing antibody titers > or = 1:512 against bovine viral diarrhea-Singer virus and titers > or = 1:256 against the persistent BVDV. Twenty-one days after vaccination with a vaccine that contained inactivated noncytopathic and cytopathic biotypes of BVDV, 4 of 5 persistently infected calves had neutralizing antibody titers < or = 1:4 against the bovine viral diarrhea-Singer virus and their persistent virus. Prior to vaccination, 2 of 11 virus-negative calves had neutralizing antibody titers < or = 1:128 against the bovine viral diarrhea-Singer virus, and after vaccination, only 1 virus-negative calf had a titer < or = 1:512. At 149 days after revaccination and prior to weaning, 4 virus-negative calves had neutralizing antibody titers < or = 1:512 (range, 1:16 to 1:384). Under the specific conditions in this herd, we were not able to detect a beneficial effect of vaccination.(ABSTRACT TRUNCATED AT 250 WORDS)
Journal of the American Veterinary Medical Association 06/1993; 202(11):1849-54. · 1.79 Impact Factor
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ABSTRACT: Studies were designed to evaluate 2 methods for transvaginal-transuterine collection of bovine fetal fluids. The first technique (direct) required simultaneous transrectal palpation and retraction of the gravid uterus and direct, intravaginal manipulation of a needle and vacuum tube assembly. The direct technique was only suitable for use in multiparous animals and was attempted when fetal age ranged from Day 55 to Day 75. The second technique (indirect) may be used in primiparous cows, because aspiration was accomplished through a plastic infusion pipet, altered by attachment of a needle to its tip. When this technique was used, fetal age ranged from Day 50 to Day 65. The direct technique provided more control over needle placement and resulted in a higher success rate for aspiration of fetal fluid following single needle penetration (77 versus 50%). Both techniques were associated with rates of abortion (3/13 for the direct and 4/10 for the indirect) that were judged to preclude prospective use in diagnostic strategies for first trimester fetal wastage. Within the controlled study, the diagnostic quality of the aspirate was determined. It was concluded that the altered pipet technique provided aspirates that were of diagnostic, noncontaminated quality. Field use of fetal fluid aspiration following discovery of nonviable pregnancies by B-mode ultrasonography is discussed.
Theriogenology 11/1992; 38(4):581-7. · 1.96 Impact Factor
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ABSTRACT: A severe outbreak of dairy herd pregnancy wastage was investigated. At the beginning of the outbreak, a total of 121 lactating cattle were pregnant and considered to be at risk. Overall, 33.1% of the population at risk aborted, while 25.6% gave birth to calves that either died during the early neonatal period or demonstrated signs compatible with congenital defects (abnormal births). A laboratory diagnosis of bovine viral diarrhea virus (BVDV) infection was made in two surviving neonatal calves with symptoms of cerebellar hypoplasia and blindness. An on-farm investigation was conducted to determine if the abortions and abnormal births were associated with BVDV infection. The rate of abortions versus abnormal births was biphasic when graphed by the date of occurrence. The cases of abortion occurred early in the outbreak and were followed by the neonatal losses. Within the population at risk, the mean values for gestational age at the beginning of the outbreak were different between the subpopulations described by gestational outcome. The outcome of each pregnancy that existed at the beginning of the outbreak was determined. Classifications included normal birth (birth of a normal calf), abnormal birth (a neonatal loss of the type described above), abortion, and continued gestation (normal, uncompleted pregnancy). The average gestational age at the time of the index case (the first cases of pregnancy wastage) for these four pregnancy outcome classfications was 142.0, 106.2, 86.7 and 31.3 days, respectively. Reasons for assuming that this outbreak was related to BVDV are discussed.
Theriogenology 11/1991; 36(4):597-606. · 1.96 Impact Factor
