Abstract
Simple SummarySemen quality is an important indicator of reproductive health and fertility. With adverse temporal trends in human semen quality over the past 50 years paralleled in male animals, there is increasing concern for the causes and implications of perturbed male reproductive health. The evaluation of equine progressive motility (PM), a parameter closely associated with fertility, provides information on the fertilising capacity of equine ejaculate and current reproductive health of the equine stallion. Using systematic analysis, recent trends in equine PM were determined from 696 estimates from 280 individual studies. Temporal trends indicate equine PM has not significantly changed between the years 1990 and 2018. Significant breed, methodological, and geographical variations observed in equine PM may considerably influence actual and reported stallion fertility potential. Information on stallion PM meaningfully contributes to the wider literature on semen quality and provides avenues for future stallion fertility research. This systematic analysis presents the wider challenges associated with semen quality assessment, particularly within the equine sector, and provides recommendations to promote consistency across industry and research.Over the past five decades, there has been increasing evidence to indicate global declines in human semen quality. Parallel adverse trends measured in male animals indicate a potential environmental aetiology. This study evaluated the progressive motility (PM) of stallion ejaculate through a systematic review and meta-analysis. A total of 696 estimates of equine PM from 280 studies, which collected semen samples between the years 1990 and 2018, were collated for meta-analysis. The method of motility analysis, breed, season of collection, and geographical location were extracted. Simple linear regression determined temporal trends in stallion PM. Studies using microscopy estimated PM to be significantly greater compared to computer-automated methods (p ≤ 0.001). For Arabian breeds, PM was consistently higher than other breeds. Over time, there was a significant decline in PM for studies from Europe (n = 267) but a significant increase for studies from North America (n = 259). Temporal trends indicate the fertilising capacity of equine ejaculate has remained consistently high in the last three decades. That being so, variations observed suggest methodological, geographical, and individual stallion differences may significantly influence actual and reported stallion fertility potential.
Highlights
Semen quality is an important indicator of male reproductive health and fertility [1,2].Over the past five decades, there has been increasing concern over declining semen quality across species [3,4,5,6]
This study presents significant methodological, geographical, and breed variations in stallion progressive motility (PM), representing the wider challenges in the equine sector that need to be addressed to understand the considerable variation in semen quality among stallions
Variations in stallion PM have been reported to account for only 20% of the total variation in fertility, as such, PM below 40% is likely to compromise stallion fertility [30]
Summary
Over the past five decades, there has been increasing concern over declining semen quality across species [3,4,5,6]. The decline in human male fertility has remained controversial since the first meta-analysis reported a global reduction of 50% in mean sperm density [3]. Subsequent reanalysis and independent studies support global temporal declines in semen quality, suggesting increasing concern around the causes and implications of reduced male fertility [6,11,12,13,14,15]. A rigorous meta-analysis of global trends in human semen quality reported a 50–60% decline in sperm count in North America, Europe, Australia and
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