Date Published: January 7, 2008
Publisher: BioMed Central
Author(s): Jane M Morrell, Anders Johannisson, Anne-Marie Dalin, Linda Hammar, Thomas Sandebert, Heriberto Rodriguez-Martinez.
Artificial insemination is not as widely used in horses as in other domestic species, such as dairy cattle and pigs, partly because of the wide variation in sperm quality between stallion ejaculates and partly due to decreased fertility following the use of cooled transported spermatozoa. Furthermore, predictive tests for sperm fertilising ability are lacking. The objective of the present study was to assess sperm morphology and chromatin integrity in ejaculates obtained from 11 warmblood breeding stallions in Sweden, and to evaluate the relationship of these parameters to pregnancy rates to investigate the possibility of using these tests predictively.
Aliquots from fortyone ejaculates, obtained as part of the normal semen collection schedule at the Swedish National Stud, were used for morphological analysis by light microscopy, whereas thirtyseven were used for chromatin analysis (SCSA) by flow cytometry. The outcome of inseminations using these ejaculates was made available later in the same year.
Ranges for the different parameters were as follows; normal morphology, 27–79.5%; DNA-fragmentation index (DFI), 4.8–19.0%; standard deviation of DNA fragmentation index (SD_DFI) 41.5–98.9, and mean of DNA fragmentation index (mean_DFI), 267.7–319.5. There was considerable variation among stallions, which was statistically significant for all these parameters except for mean_DFI (P < 0.001, P < 0.01, P < 0.001 and P < 0.2 respectively). There was a negative relationship between normal morphology and DFI (P < 0.05), between normal morphology and SD_DFI (P < 0.001), and between normal morphology and mean_DFI (P < 0.05). For specific defects, there was a direct relationship between the incidence of pear-shaped sperm heads and DFI (P < 0.05), and also nuclear pouches and DFI (P < 0.001), indicating that either morphological analysis or chromatin analysis was able to identify abnormalities in spermiogenesis that could compromise DNA-integrity. A positive relationship was found between normal morphology and pregnancy rate following insemination (r = 0.789; P < 0.01) and a negative relationship existed between DFI and pregnancy rate (r = -0.63; P < 0.05). Sperm motility, assessed subjectively, was not related to conception rate. Either or both of the parameters, sperm morphology and sperm chromatin integrity, seem to be useful in predicting the fertilising ability of stallion ejaculates, particularly in determining cases of sub-fertility.
The number of artificial inseminations (AI) carried out in mares has increased considerably over recent years but, at around one million inseminations worldwide annually, use of this technique lags far behind that seen in other domestic animals, particularly those species used for food production. More than 100 million inseminations in cattle and more than 250 million inseminations in turkeys are carried out annually. Some of the limiting factors in the use of AI for mares are believed to be as follows: (i) highly variable quality between stallions and between ejaculates ; (ii) a decline in conception rate associated with the increased use of cooled transported spermatozoa rather than AI with fresh spermatozoa immediately after semen collection; (iii) lack of suitable on-site tests for sperm quality assessment . Variation in stallion semen quality has been attributed to the selection of breeding sires based only on competitive performance .
The characteristics of the 41 ejaculates made available for this study are shown in Table 1. There was considerable variation among stallions in terms of sperm concentration and motility in the ejaculates: however, only sperm concentration between stallions differed significantly (P = 0.03). Differences in sperm concentration among ejaculates for each stallion were not significant.
Values for concentration and sperm motility were within the normal range for this species . Considerable variation in these parameters between stallions has also been observed by others [1,19], although in this study there was no difference between ejaculates from each stallion collected over a short time period (15 days) at the height of the breeding season. Seasonal variation has been reported previously among stallion ejaculates .
JMM participated in the design of the study, supervised and participated in data collection at Flyinge AB, was responsible for analysis and interpretation of the data and drafted the manuscript. AJ carried out the chromatin analysis. A-MD assisted in coordination of the study and supervised the morphology analysis. LH assisted in data collection. TS supervised semen collection. HR-M participated in the design and coordination of the study and helped to draft the manuscript. All authors have read and approved the final manuscript.