Effect of time and depth of insemination on fertility of Bharat Merino sheep inseminated trans-cervical with frozen-thawed semen
© Kumar and Naqvi; licensee BioMed Central Ltd. 2014
Received: 5 May 2014
Accepted: 17 July 2014
Published: 24 July 2014
Artificial insemination (AI) can serve as a powerful tool to the sheep owners for making rapid genetic progress of their flock. The AI in sheep is mostly performed using fresh semen with two reasons i) lambing rate following trans-cervical AI with frozen semen is limited by the inability of frozen-thawed sperm to transit the cervix and ii) the need of circumventing the cervical barrier through laparoscope aided intrauterine AI. Therefore, AI with frozen-thawed semen is not as widespread in sheep as it is in other domestic species. However, to get maximum benefits through the use of AI, frozen-thawed semen is a prerequisite because instead of high fertility, the short shelf life of fresh semen coupled with a limitation on the number of insemination doses achievable per unit time restricts the widespread use of individual sires. Therefore, in order to enhance lambing rate, a total of 240 trans-cervical artificial inseminations with frozen-thawed semen were performed in Bharat Merino ewes during autumn season either once in the evening (G-I, 10 h after onset of estrus, n = 100) or twice (G-II, 14 h and 22 h after onset of estrus, n = 140) i.e. once in the morning and again in the evening.
The pregnancy rate (proportion of pregnant ewes confirmed by ultrasonography at day 40) and lambing rate (proportion of ewes lambed) were higher in G-II as compared to G-I (26.4 vs 20% and 19.3 vs 10%, respectively). The difference in lambing rates was statistically (P < 0.05) significant. The depth of insemination within cervico-uterine tract had no significant effect on pregnancy and lambing rates.
The results indicate that lambing rate in sheep following TCAI with frozen-thawed semen was significantly influenced by time of inseminations. Two inseminations after 14 and 22 h of onset of estrus enhanced the lambing rates of Bharat Merino sheep as compare to single insemination after 10 h of onset of estrus. The TCAI technique with frozen-thawed ram semen is promising and may serve as a valuable tool for genetic improvement of sheep breeds. Research efforts are going on worldwide to overcome the poor fertility following TCAI with frozen-thawed semen.
Artificial insemination (AI) can serve as a powerful tool to the sheep owners for making rapid genetic progress of their flock. The AI is expected to play a crucial role for large-scale production and multiplication of fecundity gene carrier germplasm via introgression of fecundity gene into the non-prolific sheep breeds. The AI in sheep is mostly performed using fresh semen, since fertility following AI with frozen semen is limited by the inability of frozen-thawed sperm to transit the cervix. Even when very large number of motile spermatozoa is deposited in the cervix, fertility is lower for frozen semen than for fresh semen . Therefore, AI with frozen-thawed semen is not as widespread in sheep as it is in other domestic species. However, to get maximum benefits through the use of AI, frozen-thawed semen is a prerequisite because instead of acceptable fertility, the short shelf life of fresh semen coupled with a limitation on the number of insemination doses achievable per unit time restricts the widespread use of individual sires.
In all domestic species AI with frozen-thawed semen gives acceptable pregnancy rates only when semen is deposited intrauterine. In sheep, the cervix is a formidable barrier to penetration for trans-cervical intrauterine deposition of frozen-thawed semen due to its caudally facing, eccentric series of four to five funnel-like rings [2–4]. At present satisfactory and reliable lambing results with frozen-thawed semen are only obtainable by circumventing the cervical barrier through laparoscope aided intrauterine AI [5–7]. But the cost, invasive nature and need for professional labor limits its wider use . The development of trans-cervical artificial insemination (TCAI) technique has got immense potential for using frozen-thawed semen because of its non-invasive nature and as an alternative technique for invasive laparoscope-aided intrauterine AI . The pregnancy and lambing rates achieved through TCAI with frozen semen is not still acceptable and there is need to improve lambing rates following TCAI. Besides breed, frozen-thawed semen quality, sperm numbers, ewe condition and time of insemination, the frequency of insemination to meet out the availability of sperm near ovulation for successful fertilization, and catheter penetration (site of semen deposition) are important factors which may influence success rate of TCAI.
The time of insemination is related to the time of ovulation. Ovulation occurs around the end of estrus. Insemination should be performed at a time sufficiently before ovulation so that by the time of ovulation a large population of sperm is established in the ampulla, the site of fertilization. In general, with twice daily inspections for heat detection, if animal showing estrus in the morning it should be inseminated in the evening (10 h after onset of estrus), and if animal showing estrus in the evening it should be inseminated in the morning (14 h after onset of estrus). However, time of insemination needs to be adjusted according to whether the semen is fresh or frozen‑thawed, since the former should have a fertilizing life in the female reproductive tract well in excess of 24 h, while for the latter the fertilizing life may not be more than 12 h. Therefore, with frozen-thawed semen two inseminations 8 h apart may play role in providing large population of live sperm (owing to the short life span of frozen-thawed sperm) at the site of fertilization by the time of ovulation. Keeping this in view a fertility trial was conducted in Bharat Merino ewes to determine the influence of time of insemination (10 h Vs 14 & 22 h after onset of estrus) and depth of insemination within cervico-uterine tract on pregnancy and lambing rates following TCAI with frozen-thawed ram semen.
Results and discussion
Effect of timing and depth of insemination within cervico-uterine tract on pregnancy and lambing rates of Bharat Merino ewes inseminated with frozen-thawed semen
Timing of insemination after onset of estrus
Depth of insemination within cervico-uterine tract
Mid (3rd to last cervical fold)
Partial (os to 2ndcervical fold)
Pregnancy rate (%)
Lambing rate (%)
Pregnancy rate (%)
Lambing rate (%)
Pregnancy rate (%)
Lambing rate (%)
Pregnancy rate (%)
Lambing rate (%)
14 & 22 h
The success rate attained in this study in terms of pregnancy and lambing rates following TCAI of ewes with frozen-thawed semen for one oestrus cycle is at par with other reports [10–16]. The variation in the results may be due to the differences in breed, season, feeding, management, freezing and thawing procedures, insemination dose (number of spermatozoa per straw), equipment used for TCAI, level of penetration of catheter into the cervix, time and frequency of insemination in relation to ovulation and onset of estrus whether natural or synchronized. Moreover, the process of manipulating an insemination catheter through the cervix has been linked to reductions in pregnancy and lambing rates. It has been suggested that cervical trauma and vaginal/cervical stimulation caused by the catheterization may activate pathways that are associated with uterine immune defenses and that interrupt pregnancy at its early stages [17, 18, 14]. It is believed that frozen-thawed sperm are less motile and lack stamina to transverse the highly viscous cervical mucus, because of ultrastructural, biochemical and functional changes undergone by a large sperm population but phagocytosis of the sperm by leukocytes is also considered a cause of the reduced fertility . Ultrastructural changes mainly affect sperm membranes because during the frozen-thawed process there is a redistribution of lipids that alters lipid-lipid and lipid-protein relations, which are necessary for the normal function of sperm membranes .
The higher lambing rate of the ewes inseminated 14 and 22 h after onset of estrus as compare to ewes inseminated 10 h after onset of estrus indicates that the time and/or frequency of insemination of frozen-thawed semen is a critical determinant of fertility, due primarily to the significantly reduced lifespan of frozen-thawed semen in the female reproductive tract. However, Salamon  reported no difference between the mean lambing results for frequency of inseminations i.e. single (44.4%, 59/133) and double insemination (46.2%, 61/132) with frozen semen. The effect of double insemination depends upon the time when the first insemination is performed in relation to ovulation. So, the effect of the double insemination is more reliable when the first insemination is carried out at the beginning of estrus than when it takes place in the middle or at the end of estrus. The beneficial effect of two insemination at 14 and 22 h after onset of estrus in the present study may be because insemination is artificial, natural sexual behavior is absent and the male and female reproductive events have to be carefully synchronized to minimize the gamete waiting time before fertilization occurs. Moreover, two insemination may prevent from the possibility that insemination is practiced too early in relation with ovulation as seems in the G-I where insemination was done at 10 h after onset of estrus.
In the present study the depth of cervical penetration i.e. site of semen deposition did not influence statistically (p < 0.05) pregnancy and lambing rates, however, these are slightly more in case of uterine deposition as compared with cervical deposition. Taking into consideration the anatomy of the sheep cervix, a highly complex fibrous structure with many folds that obliterate the lumen, it is understandable. In partial cervical penetration, a part of semen or the whole quantity can flow back into the vagina where the acidic pH is unfavorable condition for the sperm. In mid cervical penetration, a large number of sperm practically come to a dead end in the rings of the cervix and cannot get into the body of the uterus. In full cervical penetration, semen enters directly into the body of the uterus. The depth of cervical penetration is affected by the anatomy of cervical lumen. Cervices with a less convoluted lumen are more penetrable . The eccentric folds are the ones most difficult to pass and they influence the design of the tip of the AI needle . Generally, the second fold is more misaligned than the first and third, which originates a narrow cervical lumen [3, 24]. The depth from the os to the first 2 folds of the cervix has been shown to be the major factor limiting the cervical penetrability [25, 26]. Several authors have reported that low fertility obtained when semen is applied via the cervix, is associated with low penetration. The high fertility rate on intrauterine deposition of semen has also been reported by several workers [10, 12, 27–30]. However, recently it has also been reported that the degree of catheter penetration into the cervix had no correlation with fertility of ewes inseminated with frozen-thawed semen [16, 31].
The linear‒array real‒time ultrasound scanning by the transabdominal route is a reliable method for (early) pregnancy diagnosis in sheep with accuracy of more than 90% [32, 33]. The difference in pregnancy and lambing rates in the present study may be due to fetal mortality or unobserved abortions that took place after day 40 of pregnancy. Dixon et al. reported that approximately 19.9% of the ewes experienced late embryonic loss, fetal loss, or both; and 21.2% of the embryos or fetuses were lost from d 25 to term. The proportions of fetuses lost were associated with breed type, concentrations of progesterone, estradiol and vascular endothelial growth factor. Cryoinjuries of spermatozoa associated with freezing and thawing, and manipulations associated with TCAI technique can all contribute to fetal mortality, probably due to inadequate vascular growth in the endometrium and perhaps conceptus. Langford et al. and Olesen  also reported increased embryonic mortality between day 18 and term when frozen‒thawed semen was used. The contribution of false positive results to the differences between pregnancy and lambing rates may not be ignored.
The results of this study indicate that lambing rate in sheep following TCAI with frozen-thawed semen was significantly influenced by time of inseminations. Two inseminations after 14 and 22 h of onset of estrus enhanced the lambing rates of Bharat Merino sheep as compared to single insemination after 10 h of onset of estrus. The TCAI technique with frozen-thawed ram semen is promising and may serve as a valuable tool for genetic improvement of sheep breeds. Research efforts are going on worldwide to overcome the poor fertility following TCAI with frozen-thawed semen. Improvement in the freezing protocol and the use of exogenous cervical dilatators in sheep have been investigated but a much better understanding of sheep cervical physiology and the mechanism of natural cervical dilatation at estrus is required to facilitate TCAI for sheep with frozen-thawed semen to achieve acceptable fertility.
The study was conducted with the approval of Institute Animal Ethics Committee (IAEC), which is in the semi-arid tropical area of the India at 75°-28′E longitude, 26°-26′N latitude and at an altitude of 320 m above mean sea level. The climate of this place is typically hot with yearly minimum and maximum temperature of 4° and 46°C, respectively. The annual rainfall ranges from 400 to 700 mm with an erratic distribution throughout the year.
The semen of individual rams was frozen well in advance and kept cryopreserved till the day of insemination. On the day of freezing, ejaculates were obtained from donor rams in quick succession by artificial vagina after mounting on the restrained estrus ewe secured in the service crate. The semen samples were evaluated for volume, consistency, wave motion (0–5 scale), concentration (photometrically) and percentage of motile spermatozoa (0-100%). Ejaculates having thick consistency, rapid wave motion, 90% initial motility and more than 3 × 109 spermatozoa per ml were immediately diluted with a TEST-yolk-glycerol extender  at 25°C to a final concentration of 1 × 109 spermatozoa per ml. The extended semen samples from individual ram were frozen in a Planer R-204 programmable cell freezer under controlled cooling and freezing rates . Briefly, diluted samples were aspirated into 0.5 ml size French plastic straws (IMV Technologies, L’ Aigle, France), sealed with polyvinyl alcohol powder, submerged in water kept in a rectangular glass tray at 25°C and after drying straws were loaded vertically in the programmable cell freezer pre-cooled to 25°C. Controlled-rate cooling of straws was initiated in the cell freezer at the linear rate of 0.15°C per min from 25 ° to 5°C followed by a holding time of 2 h at 5°C and freezing from 5° to -125°C at the rate of 25°C per min and then plunged into liquid nitrogen for storage until required. Thawing of individual straws was done at 60°C for 10 second in a water bath just prior to TCAI. Post-thaw attributes of spermatozoa were objectively assessed by a computer-assisted semen analysis technique (Hamilton-Thorn HTM-S version 7.2 Y motility analyzer) straw per straw. Single straw (0.25 ml French straw having 250 million sperm) of frozen-thawed semen with more than 50% post-thaw motility was immediately used for TCAI of one ewe.
Estrus was detected in ewes twice daily (07:00 and 19:00) by parading adult rams, fitted with an apron. Ewes were inseminated for one cycle either once, only in the morning (G-I, single insemination, n = 100) or twice, once in the morning and again in the evening (G-II, double inseminations, n = 140). Ewes exhibiting estrus in the morning along with ewes in estrus on the previous evening were brought daily to the semenology laboratory for insemination. Ewes exhibited estrus on previous evening were kept in G-II and inseminated twice 8 h apart i.e. between 09:00 to 10:00 as well as 17:00 to 18:00 (14 h and 22 h after onset of estrus). Ewes exhibited estrus same day in the morning were kept in G-I and inseminated once between 17:00 to 18:00 (10 h after onset of estrus). The inseminations were made using individual ram semen allotted to the particular ewe as per the breeding plan (improvement by selective breeding) of Bharat Merino sheep.
The equipment and cradle used for TCAI were fabricated as described earlier by Naqvi et al.. The ewes were restrained in the cradle in dorsal decumbency with the hind quarter elevated so that the vulva was positioned at an angle of 80 to 90° from the ground. The insemination was performed throughout the study by the same inseminator using the procedure of Naqvi et al.. In brief, the speculum with plunger lubricated with small amount of medical gel was introduced into the vagina and external os was located after removal of the plunger with the aid of the light. The needle was gently introduced into cervix and manipulated through the cervical folds into the deepest location possible. A maximum time limit of 5 minutes was set to achieve full penetration of the cervix. If full penetration had not been achieved semen was deposited in the cervix. The depth of cervical penetration was scored as: (i) os to second fold: partial, (ii) third to last fold: mid and (iii) needle inserted to its full length: full (uterine deposition).
The inseminated ewes were closely observed for return in estrus over two-cycle period and until lambing. At 40 days of gestation, all the inseminated ewes were examined for the presence of fetuses by using a real time Desktop Veterinary Ultrasound Scanner system equipped with convex array 5.0 MHz/40R/60D transducer (Model SA-600 V, MEDISON Company, Limited. Korea). Data on pregnancy rate (proportion of pregnant ewes confirmed by ultrasonography) and lambing rate (proportion of ewes lambed) were recorded. The data were statistically analyzed by chi square test as per Snedecor and Cochran  to determine the effect of time of inseminations and depth of insemination within cervico-uterine tract on pregnancy and lambing rates.
The authors are thankful to the Director of the Institute for providing all the research facilities and Mr Munir Ahmed and Mr N.L. Gouttam for rendering technical assistance during this study.
- Maxwell WMC, Hewitt LJ: A comparison of vaginal, cervical and intrauterine insemination of sheep. J Agricul Sci. 1986, 106: 191-193.View ArticleGoogle Scholar
- Halbert GW, Dobson H, Walton JS, Buckrell BC: The structure of the cervical canal of the ewes. Theriogenology. 1990, 33: 977-992.View ArticlePubMedGoogle Scholar
- Naqvi SMK, Pandey GK, Guatam KK, Joshi A, Geethalaxmi V, Mittal JP: Evaluation of gross anatomical features of cervix of tropical sheep using cervical silicone moulds. Anim Reprod Sci. 2005, 85: 337-344.View ArticlePubMedGoogle Scholar
- Anel L, Alvarez M, Martinez-Pastor F, Garcia-Macias V, Anel E, Paz P: Improvement strategies in ovine artificial insemination. Reprod Domestic Anim. 2006, 41: 30-42.View ArticleGoogle Scholar
- Salamon S, Maxwell WMC: Frozen storage of ram semen. II. Causes of low fertility after cervical insemination and methods of improvement. Anim Reprod Sci. 1995, 46: 89-96.Google Scholar
- Naqvi SMK, Joshi A, Mathur AK, Bag S, Mittal JP: Intrauterine artificial insemination of Malpura ewes in natural estrus with frozen ram semen. Ind J Anim Sci. 1997, 67: 982-983.Google Scholar
- Fukui Y, Kohno H, Togari T, Hiwasa M, Okabe K: Fertility after artificial insemination using a soybean-based semen extender in sheep. J Reprod Develop. 2008, 54: 286-289.View ArticleGoogle Scholar
- Aisen EG, Alvarez HL, Venutrino A, Garde JJ: Effect of trehalose and EDTA on cryoprotective action of ram semen diluents. Theriogenology. 2000, 53: 1053-1061.View ArticlePubMedGoogle Scholar
- Buckrell BC, Buschbeck C, Gartlet CJ, Kroetsch T, McCutcheon W, Martin J, Penner WK, Walton JS: A breeding trial using a transcervical technique for artificial insemination in sheep. Proc 12th Int Cong Anim Reprod. 1992, 3: 1531-1533.Google Scholar
- Windsor DP, Szell AZ, Buschbeck C, Edward AY, Milton JTB, Buckrell BC: Transcervical artificial insemination of Australian Merino ewes with frozen-thawed semen. Theriogenology. 1994, 42: 147-157.View ArticlePubMedGoogle Scholar
- Buckrell BC, Buschbeck C, Gartley CJ, Kroetsch T, McCutcheon W, Martin J, Penner WK, Walton JS: Further development of a transcervical technique for artificial insemination in sheep using previously frozen semen. Theriogenology. 1994, 42: 601-611.View ArticlePubMedGoogle Scholar
- Naqvi SMK, Joshi A, Bag S, Pareek SR, Mittal JP: Cervical penetration and transcervical AI of tropical sheep (Malpura) at natural estrus using frozen-thawed semen. Technical Note. Small Rum Res. 1998, 29: 329-333.View ArticleGoogle Scholar
- Cappai P, Sanna SR, Branca A, Fraghi A, Bomboi G: Comparison of laparoscopic and transcervical insemination with frozen semen in Sarda dairy ewes. Anim Sci. 1998, 66: 369-373.View ArticleGoogle Scholar
- Wulster-Radcliffe MC, Wang S, Lewis GS: Transcervical artificial insemination in sheep: effect of new transcervical artificial insemination instrument and traversing the cervix on pregnancy and lambing rates. Theriogenology. 2004, 62: 990-1002.View ArticlePubMedGoogle Scholar
- Gunduz MC, Turna O, Cirit U, Ucmak M, Tek C, Sabuncu A, Bacinoglu S: Lambing rates and litter size following carazolol administration prior to insemination in Kivircik ewes. Anim Reprod Sci. 2010, 118: 32-36.View ArticlePubMedGoogle Scholar
- Alvarez M, Chamorro CA, Kaabi M, Anel-Lopez L, Boixo JC, Anel E, Anel L, de Paz P: Design and “in vivo” evaluation of two adapted catheters for intrauterine transcervical insemination in sheep. Anim Reprod Sci. 2012, 131: 153-159.View ArticlePubMedGoogle Scholar
- Campbell JW, Harvey TG, McDonald MF, Sparksman RI: Transcervical insemination in sheep: an anatomical and histological evaluation. Theriogenology. 1996, 45: 1535-1544.View ArticleGoogle Scholar
- Wulster-Radcliffe MC, Lewis GS: Development of a new transcervical artificial insemination method for sheep: effects of a new transcervical artificial insemination catheter and traversing the cervix on semen quality and fertility. Theriogenology. 2002, 58: 1361-1371.View ArticlePubMedGoogle Scholar
- Salamon S, Maxwell WMC: Storage of ram semen. Anim Reprod Sci. 2000, 62: 77-111.View ArticlePubMedGoogle Scholar
- Park JE, Graham JK: Effects of cryopreservation procedures on sperm membranes. Theriogenology. 1992, 38: 209-222.View ArticleGoogle Scholar
- Salamon S: Fertility following deposition of equal numbers of frozen-thawed ram spermatozoa by single and double insemination. Aust J Agri Res. 1977, 28: 477-479.View ArticleGoogle Scholar
- Kershaw C, Khalid M, McGowan M, Ingram K, Leethongdee S, Wax G, Scaramuzzi R: The anatomy of the sheep cervix and its influence on the transcervical passage of an inseminating pipette into the uterine lumen. Theriogenology. 2005, 64: 1225-1235.View ArticlePubMedGoogle Scholar
- Halbert GW, Dobson H, Walton JS, Buckrell BC: A technique for transcervical intrauterine insemination of ewes. Theriogenology. 1990, 33: 993-1010.View ArticlePubMedGoogle Scholar
- Kaabi M, Alvarez M, Anel E, Chamorro CA, Boixo JC, de Paz P, Anel L: Influence of breed and age on morphometry and depth of inseminating catheter penetration in the ewe cervix: a postmortem study. Theriogenology. 2006, 66: 1876-1883.View ArticlePubMedGoogle Scholar
- Eppleston J, Maxwell WMC: Recent attempts to improve the fertility of frozen ram semen inseminated into the cervix. Wool Tech Sheep Breed. 1993, 41: 291-302.Google Scholar
- Eppleston J, Salamon S, Moore NW, Evans G: The depth of cervical insemination and site of intrauterine insemination and their relationship to the fertility of frozen-thawed ram semen. Anim Reprod Sci. 1994, 36: 211-225.View ArticleGoogle Scholar
- Paulenz H, Soderquist L, Adnoy T, Nordsroga A, Berg KA: Effect of vaginal and cervical deposition of semen on the fertility of sheep inseminated with frozen-thawed semen. Vet Rec. 2005, 156: 372-375.View ArticlePubMedGoogle Scholar
- Szabados T, Gergatz E, Vitinger E, Zi T, Gyoker E: Lambing rate as a function of artificial insemination depth in ewe lambs, primiparous and multiparous ewes. Acta Agraria Kaposvariensis. 2005, 9: 41-49.Google Scholar
- Leethongdee S: Development of trans-cervical artificial Insemination in sheep with special reference to anatomy of cervix. Suranaree J Sci Technol. 2010, 17: 57-69.Google Scholar
- Taqueda GS, Azevedo HC, Santos EM, Matos JE, Bittencourt RF, Bicudo SD: Influence of anatomical and technical aspects on fertility rate based on sheep transcervical Artificial insemination performance. ARS Veterinaria. 2011, 27: 127-133.Google Scholar
- Richardson L, Hanrahan JP, Donovan A, Martí JI, Fair S, Evans AC, Lonergan P: Effect of site of deposition on the fertility of sheep inseminated with frozen-thawed semen. Anim Reprod Sci. 2012, 131: 160-164.View ArticlePubMedGoogle Scholar
- Kahn W, Achtzehn J, Kahn B, Richter A, Schulz J, Wolf M: Sonography of pregnancy in sheep. II Accuracy of transrectal and transcutaneous pregnancy diagnosis. Dtsch Tierarztl Wochenschr. 1993, 100: 29-31.PubMedGoogle Scholar
- Yotov S: Diagnostics of early pregnancy in Stara Zagora dairy sheep breed. Bulg J Vet Med. 2005, 8: 41-45.Google Scholar
- Dixon AB, Knights M, Winkler JL, Marsh DJ, Pate JL, Wilson ME, Dailey RA, Seidel G, Inskeep EK: Patterns of late embryonic and fetal mortality and association with several factors in sheep. J Anim Sci. 2007, 85: 1274-1284.View ArticlePubMedGoogle Scholar
- Langford GA, Marcus GJ, Hackett IJ, Ainsworth L, Wolynetz MS, Peters HF: A comparison of fresh and frozen semen in the insemination of confined sheep. Canad J Anim Sci. 1979, 59: 685-691.View ArticleGoogle Scholar
- Olesen I: Effects of cervical insemination with frozen semen on fertility and litter size of Norwegian sheep. Livestock Prod Sci. 1993, 37: 169-184.View ArticleGoogle Scholar
- Singh VK, Arora AL, Mehta BS, Mishra AK, Kumar S, Prince LLL: Bharat Merino. New Strains of Sheep Evolved. 2006, 21-Avikanagar (India): CSWRIGoogle Scholar
- Schmehl MK, Anderson SP, Vazques IA, Graham EF: The effect of dialysis of extended ram semen prior to freezing on post-thaw survival and fertility. Cryobiology. 1986, 23: 406-414.View ArticlePubMedGoogle Scholar
- Joshi A, Kumar D, Naqvi SMK, Maurya VP: Effect of controlled-rate cooling and freezing on motion characteristics and acrosomal integrity of cryopreserved ram spermatozoa. Cell Pres Tech. 2008, 6: 277-284.View ArticleGoogle Scholar
- Snedecor GW, Cochran WG: Statistical Methods. 1989, Ames, Iowa: Iowa State University Press, 8Google Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.