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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
* Animal Reproduction and Biotechnology Laboratory, Colorado State University, Fort Collins 80523-1683 and
and
XY, Inc., Fort Collins, CO 80526
Abstract
The objectives of this study were to determine whether calves produced by sexed sperm differed from controls and to what extent the sex ratio of calves was altered by the sexing procedure. Data were collected from 1,169 calves produced from sperm sexed by flow cytometry/cell sorting after staining with Hoechst 33342, and 793 calves produced from control sperm during breeding trials between 1997 and 2001. Least squares ANOVA were completed using factors of treatment (sexed vs. control sperm), 19 management groups from 13 field trials, and calf sex. Responses analyzed include gestation length, birth weight, calving ease, calf vigor, weaning weight, abortion rate, and death rates (neonatal and through weaning). No significant difference was observed for any response due to treatment or treatment interactions (P > 0.10). Therefore, calves produced from sexed sperm grew and developed normally both pre- and postnatally. A neurological disorder was observed in four control calves and one sexed calf from one farm. No gross anatomical abnormalities were reported for any calves in the study. Differences were observed for all responses among management groups (P < 0.03 for abortions and P < 0.01 for all other responses). Heifer and bull calves differed (P < 0.001) in gestation length (278.4 and 279.6 d), birth weight (32.8 and 35.2 kg), calving ease (1.15 and 1.30), and weaning weight (233 and 247 kg). Gestation length did not affect characteristics of calves. The sex ratio at birth of calves from unsexed control sperm was 49.2% male. Sexing accuracy of X-sorted sperm was 87.8% female calves, and Y-sorted sperm produced 92.1% male calves. Flow cytometry/cell sorting can be used to preselect sex of calves safely with approximately 90% accuracy.
Key Words: Calves • Flow Cytometry • Normality • Sexing • Sex Ratio • Sperm
Introduction
Flow cytometry/cell sorting can be used to measure DNA content of sperm that have been stained with the fluorescent DNA binding dye, Hoechst 33342 (H33342), in order to separate populations of X- or Y-chromosome-bearing sperm (Johnson et al., 1999
). This process has produced offspring with significantly skewed sex ratios of the desired sex in several species (Seidel and Garner, 2002). Sex preselection is of interest to cattle producers. For example, X-sorted sperm can produce replacement heifers from more selected, genetically superior dams, whereas Y-sorted sperm can produce bull calves to be sold for slaughter from less valuable dams (Hohenboken, 1999; Seidel, 2002). The efficiency of progeny testing dairy bulls would be improved by using X sperm to produce the required heifers from the fewest dams (Hohenboken, 1999; Seidel, 2002).
The sexing process damages sperm in several ways. Motility, viability, and membrane integrity are compromised by high rates of dilution as protective molecules are removed (Catt et al., 1997; Maxwell and Johnson, 1997). Physical stress during sorting and handling of sperm leads to membrane damage, including premature acrosome reactions (Maxwell et al., 1998). Hoechst 33342 has caused chromosomal damage under some conditions (Libbus et al., 1987). High pressure within the system has been associated with decreased fertility (Seidel et al., 2003). No increase in calf abnormalities has been reported to date with sexed sperm relative to controls. There is a hint, however, that early embryonic mortality may be increased slightly with sexed sperm (Seidel et al., 1999).
Many offspring have been produced from sexed sperm, but no thorough study of the normality of offspring has been reported. Therefore, characteristics of calves were studied to determine whether there are any differences between populations of calves from sexed vs. unsexed sperm.
Materials and Methods
Sorting and Preparation of Sperm
Semen was collected from 30 bulls using an artificial vagina and sorted into X- and Y-chromosome populations using flow cytometry/cell sorting as described by Seidel and Garner (2002). Briefly, semen was stored undiluted at 20 to 23°C for up to 8 h before sorting, during which time, sequential aliquots of sperm were prepared for sorting every 90 min. Sperm were stained in a modified Tyrode’s medium supplemented with albumin, lactate, and pyruvate, to which H33342 (Sigma Chemical Co., St. Louis, MO) was added followed by addition of red food dye. Red food dye was used to identify dead sperm because it quenches fluorescence of the bound H33342 in dead sperm (Schenk et al., 1999), which can then be discarded. A Tris-based sheath fluid surrounded the sorting medium as sperm passed through the sorter (Schenk et al., 1999) at speeds up to 90 km/h. During this process, sperm were diluted an additional 10-fold.
An argon laser was used to excite the H33342 bound to the DNA of the sperm. Fluorescence detectors at a 90° angle to each other measured the fluorescence signals of the sperm to allow separation based on the difference in DNA content between the X- and Y-chromosomes—approximately 3.8% in cattle. The signal at 90° was used to determine which sperm were properly oriented for accurate evaluation, whereas the signal at 0° was used to determine DNA content (Seidel and Garner, 2002). Signals were then processed, and droplets were electrically charged depending on the presence of X- or Y-chromosome-bearing sperm. Charged droplets were deflected using charged plates (Seidel and Garner, 2002), and the sorted streams were collected into tubes with an egg yolk-Tris extender (Schenk et al., 1999; Seidel and Garner, 2002). Semen was concentrated by centrifugation and then extended and packaged in 0.25-mL straws for cryopreservation in liquid nitrogen vapor. In several of the early trials (Seidel et al., 1999; Schenk et al., 1999), sperm were not frozen before AI. Control semen was extended and cryopreserved from the same ejaculates as the sexed sperm, but was not subjected to staining or other procedures applied to the sexed sperm other than holding so that control and sexed sperm were frozen at the same time.
Insemination of Heifers and Cows
Groups of heifers and cows of beef breeds, mostly Angus and one group of Holstein heifers, were inseminated with sexed or control sperm during field trials from 1997 to 2001 (Table 1). Estrus was synchronized using one of three methods (Table 1) as described by Seidel et al. (1999). The first method was daily feeding of 0.5 mg of melengestrol acetate (MGA) for 14 d followed by 25 mg of PGF2
injected i.m. 17 to 19 d later. The second method used two doses of 25 mg of PGF2 injected i.m. 12 d apart. The third method used 50 or 100 µg of GnRH injected i.m. followed by 25 mg of PGF2 injected i.m. 7 d after GnRH treatment.
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). About two-thirds of the inseminations in each trial were with sexed sperm, and unsexed control sperm were used for the remaining inseminations. Inseminations were done as 13 different field trials in 13 herds with various experimental designs, mostly concerning numbers of sperm per inseminate and site of insemination (Seidel et al., 1999). However, after pregnancy diagnosis, three of the herds were divided into three groups, giving 19 management groups by calving time.
Pregnancy Diagnosis and Management
Transrectal ultrasonography was used to determine pregnancy status and fetal sex approximately 2 mo after insemination for all trials except Trial 13. After pregnancy diagnosis, heifers and cows were managed at the various farms with different levels of management through calving and weaning.
Collection of Calf Data
Forms requesting calving data were distributed to the respective farms during the calving season. The requested information included birth date, birth weight, calf sex, calving ease (1 = no assistance, 2 = some assistance, 3 = difficult pull, 4 = delivery by caesarian section), calf vigor (1 = healthy, nursed immediately, 2 = took some time to nurse, 3 = assistance to suckle, 4 = died shortly after birth, 5 = dead at birth; calves that died shortly after birth were scored as 4 even if they had reasonable vigor just after birth), and weaning weight. Observed abortions and deaths were recorded, as was any information on obvious abnormalities. Farm personnel were contacted to clarify any missing or unclear data. When necessary, visits were made to farms to complete datasets. Some farms did not collect all requested data, such as birth and weaning weights. Animals from those farms were not included in statistical analyses of those responses. Data were analyzed for 1,169 sexed and 793 control calves.
Data Editing
Data were edited to ensure, as much as possible, that all calves included in the analysis were in fact calves resulting from the sexed-semen experiment. Editing also ensured complete data for essentially all calves included in the analysis, and editing was done identically for calves resulting from sexed or control sperm. For calves with long gestation lengths (>290 d; i.e., born well after the expected calving date), breeding records were checked to determine whether pregnancy was from sexed AI or from possible rebreeding. Those rebred were excluded from the study. There were 61 calves excluded from the study because of excessively long gestation length; these calves were born in the absence of rebreeding records. Short gestation lengths (range 255 to 258 d) included six premature births (five of which died) and two dead fetuses aborted 188 d postinsemination. There were 79 additional abortions deduced when heifers were pregnant at 2 mo of gestation and then open after summer pasture. Loss of animals or records accounted for 29 animals being excluded from the study; further calving information was therefore not available for these animals. Twins (10 sets; six sexed and four control) were not included in the analysis due to differences in gestation length, birth weight, and weaning weight associated with twins compared with single-birth calves.
Data Analysis
After compilation and editing of calving information, data were analyzed within each trial group (herd), as well as in a combined analysis of calves from all trials to determine whether any differences existed between the sexed and control populations. Factorial, least squares ANOVA with Type III sums of squares were performed using the GLM procedures of the Statistical Analysis System (SAS Inst., Inc., Cary, NC). Data expressed as a percentage were transformed using the arc sine transformation. Factors used in the analyses include farm (management group), sex of calf, sexed vs. control semen, and all first-order interactions; gestation length was included as a covariate in some cases. Management group was considered to be a random effect and the other factors were fixed effects. Responses analyzed include gestation length, birth weight, calving ease, calf vigor, weaning weight, abortion rates, and death rates neonatally and through weaning. Adjusted weaning weights were not analyzed because they were not available for a majority of the calves. Results of analyses for each of the 19 individual management groups are not presented because there rarely were statistically significant effects due to the relatively low numbers of animals within each herd.
In addition to the aforementioned analyses, data from individual trial groups were analyzed to determine possible effects of bull, site of insemination, and sperm doses. However, none of the factors in these analyses had any significant (P > 0.10) effect on characteristics of calves or pregnancies, so related data are not presented.
The accuracy of sperm sexing and fetal sex diagnosis by ultrasound were calculated as the percentage of correct sorted sex out of the total sexed. Sex ratios for each trial were calculated as the percentage of males out of the total number of calves for sexed, control, and combined populations.
Results and Discussion
Gestation Length
The ANOVA for gestation length is shown in Table 2. The ANOVA for other responses were similar in format to gestation length, and therefore are not presented. Differences in gestation length were observed only for management group and calf sex (P < 0.001; Table 2). Least squares mean gestation lengths for sexed and control calves were nearly identical (P = 0.80; Table 3). Therefore, the sexing process did not influence gestation length.
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). Sex of calf has previously been shown to influence gestation, with bulls routinely having longer gestation lengths than heifers (Azzam and Nielsen, 1987; Reynolds et al., 1990).
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). Groups were managed in different environmental conditions, including different climates, diets (availability of pasture, etc.), and level of management. Management group differences also included animal differences, such as breed and age of dam (heifer vs. cow) as well as breed of bull. Age of dam (Bourdon and Brinks, 1982), parity of dam (Foote, 1981; Azzam and Nielsen, 1987), and breed of bull (Reynolds et al., 1990) have all been reported to influence gestation length.
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and 5; P < 0.001). Sexed and control treatments showed no difference (P = 0.48; Table 3). No interactions were observed among the factors (P > 0.10).
Numerous factors influencing birth weight, such as age and breed of dam and sire effects and environmental factors (e.g., weather and feed availability), likely contributed to the management group differences. Age of dam influences birth weight, with younger dams producing lighter calves (Reynolds et al., 1990; Rege and Famula, 1993; Holland and Odde, 1992). The least squares means in Table 5 show higher birth weights in calves born to multiparous cows (Groups 3, 5, and 10) compared with primiparous heifers. Dam weight also influences birth weight, with heavier dams producing heavier calves (Holland and Odde, 1992). There is also great variability in birth weight among breeds (Holland and Odde, 1992). Large-breed sires also usually produce calves of higher birth weight (Reynolds et al., 1990).
Lower birth weights were observed at several ranches where there were concerns about feed availability. Locoweed was prominent at one location (Group 7C), whereas another group (8C) had a feed problem suspected of causing abortion in several heifers. Birth weights in Wyoming appeared to be higher than in Colorado, likely due to differences in genetic selection. Season influences birth weight as well, as spring calves usually are heavier than fall calves (Rege and Famula, 1993). Most calves in this study were born in the spring. Trial 6 took place over 1.5 yr, with calves born in different seasons, but birth weights were not available at this location.
Bull calves were heavier than heifer calves at birth (P < 0.001; Table 4). Previous studies also demonstrated higher birth weights for bull calves (Gregory et al., 1991; Holland and Odde, 1992). This higher birth weight in bulls may be due to longer gestation length and/or presence of androgens in the male fetus (Holland and Odde, 1992).
Calving Ease
Calving ease scores were recorded for only 1,247 calves because two farms (i.e., farms 7C and 13) did not collect these data and were not included in the analysis. Differences were observed among management groups and for calf sex (P < 0.001). In agreement with other studies, bull calves resulted in more dystocia than heifers (Gregory et al., 1991; Bellows et al., 1996). The larger size of bull calves at birth is probably responsible for their higher mean calving ease scores compared with heifers (Table 4). Least squares mean calving ease scores (Table 3) were similar for sexed and control calves (P = 0.87).
Mean calving ease scores for management groups ranged from 0.99 to 1.70 (Table 5). Differences observed in management groups might be associated with dam, bull, and calf factors that influence dystocia. Trials involving cows (Groups 3, 5, and 10) show lower mean calving ease scores than many of the trials involving heifers (1, 2, 4, 6, and 8). This is in agreement with other findings that younger dams have higher average calving ease scores than older dams (Reynolds et al., 1990).
Calf Vigor
Calf vigor scores were collected in nine management groups (Table 5), from 725 calves. There were significant differences among management groups (P < 0.01), likely due in part to the subjective nature of such scoring and partly due to differences in cow vs. heifer dams. However, there were no significant differences in calf vigor scores due to sexed vs. control semen, nor due to sex of calf or interactions (P > 0.10; Tables 3 and 4).
Weaning Weight
Weaning weights were not collected at all farms; calves for which no weaning weight was recorded were not analyzed. Differences were observed between management groups and calf sex (P < 0.001), whereas no difference was observed between treatments of sexed and control sperm (P = 0.24).
The larger size of bull calves at birth contributes to their greater weaning weight. These differences are apparent in the least squares means observed for weaning weights in bull and heifer calves (Table 4). Bull calves often grow faster than heifer calves, but this can be influenced by environment. Reynolds et al. (1990) showed that during some years, females grew faster than males, depending on environmental conditions. An interaction between management group and sex of calf was observed for weaning weight in this study (P < 0.00). This may be caused by different genetics between management groups.
Management group likely affected growth of calves after birth due to different environmental conditions and age of calves at weaning. Differences in rainfall and feed availability between different years have been associated with differences in calf growth (Reynolds et al., 1990). Genetics of individual animals will also affect growth rates in different groups (Reynolds et al., 1990). Least squares mean weaning weights (Table 5) ranged from 219.2 to 272.0 kg among management groups.
Calf Losses
Abortions.
Abortion rates between 2 mo of pregnancy and term from 1,389 pregnancies were compared for treatment and management group. The arc sine transformation was used for ANOVA of percentages to determine statistical significance, but least squares means of untransformed data are presented. Abortion rates could not be determined for Trial 13 because 2-mo pregnancy data were unavailable. Abortion rates also were analyzed by fetal sex as determined by ultrasound for trials in which fetuses were sexed (1 through 10). An additional ANOVA was done on the subset of these in which overall fetal sexing accuracy (Table 6) was greater than 90% (Trials 1, 5, 7, 8, and 9).
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are for all trials in which fetal sex was determined. Mean abortion rates were 4.0 and 2.4% for male and female fetuses (P = 0.52) in the subset of trials with 90% or greater ultrasound sexing accuracy. Conceptus losses between 1 and 2 mo gestation have been similar between sexed and control populations (Seidel et al., 1999).
Abortion rates can be influenced by environment, such as presence of locoweed, as shown by the variation among management groups. Least squares means ranged from 0.0 to 12.7% (Table 5). Bulls and other factors associated with AI may also influence abortion rates.
Neonatal Deaths.
Neonatal deaths included calves born dead or that died within 24 h of birth. There was a difference in neonatal deaths (P < 0.001) among management groups, but no effect of calf sex, treatment, or interactions thereof (P > 0.10).
Major factors influencing survival at birth include illness, dystocia, and intensity of management. These factors likely explain the differences in neonatal death rates between management groups, which ranged from 0.0 to 17% (Table 5). Calves requiring assistance due to dystocia have higher rates of death than those not requiring assistance (Reynolds et al., 1990). It is possible that some deaths were due to overlooked abnormalities because calves were not studied intensively (e.g., by autopsying dead calves). It is unlikely that sexing sperm caused such abnormalities because death rates were not increased due to sexing sperm.
Total Preweaning Deaths.
Total preweaning death rates include neonatal deaths plus deaths of calves that died more than 24 h after birth through the time of weaning. Differences were observed among management groups (P < 0.001), but no difference was observed for treatment or calf sex (P > 0.10; Tables 3 and 4).
Factors causing preweaning death include sickness and environmental differences, as seen in the different management groups. Environment greatly influences death rates between birth and weaning (Reynolds et al., 1990). Total preweaning death rates ranged from 0 to 18% for the management groups (Table 5).
Sex Ratios and Accuracy of Sexing
Of the 1,169 calves born from sexed sperm, 954 resulted from insemination with X-sorted sperm, and 215 were from Y-sorted sperm. Sex was not recorded for 11 calves that were dead at birth. Fetal ultrasound sex could be assigned to five of these calves; for the remaining six, fetal sex was undetermined. There were 838 heifer calves born from X-sorted sperm, resulting in 87.8% accuracy. The Y-sorted sperm produced 198 bull calves with 92.1% accuracy.
The 793 control calves resulted from unsorted sperm. Fetal ultrasound sex was used for three of six control calves dead at birth for which sex was not recorded. The sex ratio was 49.2% males; this is not different from the expected sex ratio for non-sexed offspring.
For Trial 13, there was no pregnancy diagnosis at 2 mo of gestation as in the other trials. Gestation length was used to determine which calves resulted from AI, and which resulted from rebreeding at a later estrus. Calves born within 290 d of insemination were assumed to be part of the study. The sex ratio of calves beyond this cut off (data not used) was 47.1% males, which is convincing evidence that the cut off used was appropriate and that nearly all calves included in the analysis were part of the experiment as their sex ratio was 11.9% males.
The purpose of sexing sperm is to produce more offspring of the desired sex. The majority of trials used X-sorted sperm to produce more heifers. Only Y-sorted sperm were used in Trial 5, whereas Trials 7, 10, 11, and 12 concerned both X- and Y-sorted sperm; the remaining trials used only X-sorted sperm. Table 6 shows sex ratios of calves born for each trial.
Ultrasound diagnosis of fetal sex was attempted in Trials 1 through 10, with a total of 945 calves designated as male or female. Technicians were not told whether sexed or control semen was used in each mating. Of the 320 fetuses determined to be male, 283 resulted in bull calves (89.3%). Of the 622 fetuses diagnosed as females, 545 (88.2%) were heifer calves at birth. Four calves from each of the fetal sex-diagnosed groups did not have sex recorded at birth. Two sets of male twins and one set of female twins were identified by ultrasound; all produced one calf of the diagnosed sex. Twin diagnosis was either incorrect or one of the fetuses was aborted. Those fetuses not diagnosed for fetal sex produced 161 bulls and 198 heifers, with five dead calves without recorded sex at birth. Overall fetal sexing accuracy by ultrasound was 88.3% (Table 6).
The lowest overall accuracies were observed for Trials 2 and 3. Technicians in these earlier trials were not as experienced as those in the later trials. Trials 4, 6, and 10 also show less than 90% overall accuracy. For Trial 4, there was low accuracy of sexing males, whereas in Trial 10, accuracy was low in sexing female fetuses. Fetal sexing accuracy approaching 100% is possible with properly trained personnel and appropriate facilities and equipment (Table 6).
Implications
No difference between calves produced from sexed or control sperm was detected for any of the characteristics studied. In addition, no gross anatomical abnormalities occurred. A neurological disorder that was observed in five calves from one location likely was not caused by the sexing process because it was observed in both sexed and control calves. Some offspring produced during these trials were bred with sexed sperm to produce consecutive generations with sexed sperm, further supporting that there is little or no damage to offspring derived from sexed sperm. A more thorough study of offspring (e.g., autopsying dead calves) would allow for detection of abnormalities that might have been overlooked. Management group and calf sex caused differences in characteristics, as expected and reported in the literature. This study indicates that sexing sperm by flow cytometry can be approximately 90% accurate. Producers can be confident that sexed sperm will not result in increased abnormalities or affect calf characteristics.
Footnotes
1 Sperm were sorted by personnel at XY, Inc. Numerous persons assisted with estrus detection, artificial insemination, and pregnancy and fetal sex diagnoses. We especially acknowledge personnel at the farms and ranches who allowed us to use their cattle and assisted with data collection. This research was supported financially by XY, Inc., Fort Collins, CO, and the Agric. Exp. Stn. at Colorado State University. 
2 Correspondence—phone: 970-491-5287; fax: 970-491-3557; e-mail: gseidel{at}colostate.edu.
Received for publication August 22, 2003. Accepted for publication December 11, 2003.
Literature Cited
Azzam, S. M., and M. K. Nielsen. 1987. Genetic parameters for gestation length, birth date, and first breeding date in beef cattle. J. Anim. Sci. 64:348–356.
Bellows, R. A., P. C. Genho, S. A. Moore, and C. C. Chase, Jr. 1996. Factors affecting dystocia in Brahman-cross heifers in subtropical southeastern United States. J. Anim. Sci. 74:1451–1456.[Abstract]
Bourdon, R. M., and J. S. Brinks. 1982. Genetic, environmental, and phenotypic relationships among gestation length, birth weight, growth traits and age at first calving in beef cattle. J. Anim. Sci. 55:543–553.
Catt, S. L., L. K. O’Brien, W. M. C. Maxwell, and G. Evans. 1997. Assessment of ram and boar spermatozoa during cell-sorting by flow cytometry. Reprod. Domest. Anim. 32:251–258.
Foote, R. H. 1981. Factors affecting gestation length in dairy cattle. Theriogenology 15:553–559.
Gregory, K. E., L. V. Cundiff, and R. M. Koch. 1991. Breed effects and heterosis in advanced generations of composite populations for birth weight, birth date, dystocia, and survival as traits of dam in beef cattle. J. Anim. Sci. 69:3574–3589.[Abstract]
Hohenboken, W. D. 1999. Applications of sexed semen in cattle production. Theriogenology 52:1421–1433.[Medline]
Holland, M. D., and K. G. Odde. 1992. Factors affecting calf birth weight: A review. Theriogenology 38:769–798.
Johnson, L. A., G. R. Welch, and W. Rens. 1999. The Beltsville Sperm Sexing Technology: High-speed sperm sorting gives improved sperm output for in vitro fertilization and AI. J. Anim. Sci. 77 (Suppl. 2):213–220.
Libbus, B. L., S. D. Perreault, L. A. Johnson, and D. Pinkel. 1987. Incidence of chromosome aberrations in mammalian sperm stained with Hoechst 33342 and UV-laser irradiated during flow sorting. Mutation Res. 182:265–274.
Maxwell, W. M. C., and L. A. Johnson. 1997. Chlortetracycline analysis of boar spermatozoa after incubation, flow cytometric sorting, cooling, or cryopreservation. Mol. Reprod. Dev. 46:408–418.[Medline]
Maxwell, W. M. C., C. R. Long, L. A. Johnson, J. R. Dobrinsky, and G. R. Welch. 1998. The relationship between membrane status and fertility of boar spermatozoa after flow cytometric sorting in the presence or absence of seminal plasma. Reprod. Fertil. Dev. 10:433–440.[Medline]
Rege, J. E. O., and T. R. Famula. 1993. Factors affecting calving date and its relationship with production traits of Hereford dams. Anim. Prod. 57:385–395.
Reynolds, W. L., J. J. Urick, and B. W. Knapp. 1990. Biological type effects on gestation length, calving traits, and calf growth rate. J. Anim. Sci. 68:630–639.[Abstract]
Schenk, J. L., T. K. Suh, D. G. Cran, and G. E. Seidel, Jr. 1999. Cryopreservation of flow-sorted bovine spermatozoa. Theriogenology 52:1375–1391.[Medline]
Seidel, G. E., Jr. 2002. Sexing sperm for beef and dairy cattle breeding. Pages 281–286 in Factors Affecting Calf Crop: Biotechnology of Reproduction. M. J. Fields, R. S. Sand, and J. V. Yelich, ed. CRC Press, Boca Raton, FL.
Seidel, G. E., Jr., Z. Brink, and J. L. Schenk. 2003. Use of heterospermic insemination with fetal sex as the genetic marker to study fertility of sexed sperm. Theriogenology 59:515. (Abstr.)
Seidel, G. E., Jr., and D. L. Garner. 2002. Current status of sexing mammalian spermatozoa. Reproduction 124:733–743.[Abstract]
Seidel, G. E., Jr., J. L. Schenk, L. A. Herickhoff, S. P. Doyle, Z. Brink, R. D. Green, and D. G. Cran. 1999. Insemination of heifers with sexed sperm. Theriogenology 52:1407–1420.[Medline]
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