Relationship of seminal traits and insemination time to fertilization rateand embryo quality

The nature of subfertility due to the male or inseminate is as complex as t hat of the female. Fertilization failure or failure in embryogenesis are bo th documented to be of seminal origin. Males also differ in the numbers of sperm required to reach their maximum fertilization rate. Males requiring m ore sperm would be considered to have compensable seminal deficiencies. The se include a number of known (viability and morphology) and unknown factors (functional or molecular traits) precluding sperm access to the ovum or ab ility to engage the ovum sufficiently to initiate fertilization and the blo ck to polyspermy. Differences in fertility among males or inseminates indep endent of sperm dosage are considered uncompensable. These deficiencies wou ld be associated with fertilizing sperm that are incompetent to maintain th e fertilization process or subsequent embryogenesis once initiated, with mo st failures occurring prior to maternal recognition of pregnancy. Such sper m would preempt fertilization by competent sperm. Chromatin aberrations in morphologically normal or near normal spermatozoa from abnormal semen sampl es appear to be the best candidates for the uncompensable deficiency. Howev er, recognition of uncompensable or incompetent fertilizing sperm has not b een achieved. Six-day-old non-surgically recovered bovine ova/embryos have been used to evaluate compensable and uncompensable seminal deficiencies as well as to test reproductive strategies. These ova/embryos provide informa tion on fertilization status and embryo quality as well as quantitative and qualitative data regarding associated accessory sperm. Thus, they permit t he separation of reproductive failure by fertilization from that by embryon ic development. Accessory sperm number is positively associated with both f ertilization rate and embryonic quality. Early insemination results in low fertilization rates (low accessory sperm number), but good embryo quality, whereas, late insemination results in high fertilization rates (high access ory sperm number), but poor embryo quality. Additional studies will be nece ssary to substantiate this model; however, if true, future research designe d to improve results to artificial insemination should be tested by breedin g early in estrus where sperm viability is most limiting and embryo quality is best. (C) 2000 Elsevier Science B.V. All rights reserved.