MANAGEMENT FACTORS AFFECTING SHEEP FERTILITY
An Arab horse breeder in the early 13th century carried out the first insemination reported,
by trapping stallion semen in wool placed in the vagina of a mare and transferring this to
the vagina of another mare (Heape, 1898). Later, in 1780, an Italian priest and physiologist
named Lazzaro Spallanzani performed artificial insemination with dog semen, and
revolutionised the way scientists thought. Since then, scientist and farmers have striven to
improve this technology, motivated by the benefits that could be achieved. Sheep is one
of the species subsequently linked to this technology and in which many questions still
remain to be resolved to improve fertility. However, the potential impact of this technique
on the genetic progress of sheep is high and further studies are needed to improve its
Artificial insemination programs in sheep are linked to the genetic selection
schemes of the breeds, but it has not been successfully integrated with reproductive
technology on farms as happen in sows or cows. The technical difficulty and weak fertility,
ranged between 15 to 60 % for pregnancy rate, limits its application.
Female associated factors
Management factors associated with artificial insemination in the ewe can modify fertility.
In reproductive planning, intervals between lambings, season, age of ewe, heat stress,
nutrition state or breed are some of the factors which have a great effect on fertility results.
David et al. (2008), using a joint model combining two main traits, one relative to female and
the other relative to the male, reported that the main variation factors of AI success were
relative to non-sex-specific effects and to female effect, suggesting that choosing females to
inseminate might slightly improve the AI results.
Seasonal variations are described as a limiting factor in sheep reproduction. In natural
conditions, seasonality, which is mediated by photoperiod, modifies hormonal balance and
causes seasonal reproductive variations in sheep (Karsch, et al. 1984; Yeates, 1949), giving
rise to a decrease in reproductive activity during long days (anoestrous season).
Photoperiodic information is translated into neuroendocrine changes through variations in
melatonin secretion from the pineal gland (Bittman, et al., 1983). Melatonin, secreted in
pineal gland, triggers variations in the secretion of luteinising hormone-releasing hormone
(GnRH), luteinising hormone (LH) and follicle stimulating hormone (FSH) (Arendt, et al.,
1983, Karsch, et al., 1984). In any case, seasonal changes in reproductive activity are clearly
defined in sheep breeds from high latitudes (>40º)(Pelletier, et al., 1987), where the
differences in daylight duration between short days and long days are more notable.
As in natural mating, season affects fertility after AI, although hormonal treatment is used
to synchronise and induce oestrus. Windsor (1995) reported low cervical AI fertility rates in
non-breeding season in Merino ewes, a shallow seasonal breed. According to this, Anel et al.
(2005) found a season effect on the AI fertility in Churra ewes, which was more important in
cervical than laparoscopic artificial insemination. In cervical AI, semen is deposited in the
external portion of the cervix and the sperm transport is affected by cervical mucus quality.
Theses authors suggest that photoperiod could alter progestagens and so cervical mucus
characteristics, making it scarcer and more viscous. In consequence, sperm transport in the
cervix can be interfered with. It is important to note that seasonality affects the ram
reproductive parameters in the same way and changes in seminal quality during anoestrous
season may decrease the fertility results after AI.
It has been reported than in tropical and sub-tropical areas the local sheep show restricted
sexual activity in the summer months (Marai et al., 2004). Marai et al., 2007 reviewed how
exposure to high ambient temperature causes impairment of reproductive functions in
sheep. The heat effect is aggravated when heat stress is accompanied with high ambient
humidity (Marai et al., 2000, 2004, 2006, 2007). Heat stress evokes a series of drastic changes
in animal biological functions, which include a decrease in feed intake efficiency and use,
disturbances in the metabolism of water, protein, energy and mineral balances, enzymatic
reactions, hormonal secretions and blood metabolites. (Shelton, 2000; Marai et al., 2006). Male associated factors
It has been reported that variation in fertility of ram ejaculates exists independently of the sperm quality (Choudhry,
et al., 1995; Paulenz, et al., 2002). Variations in the fertility of rams have been reported after
cervical inseminations with fresh semen (Anel, et al., 2005; Paulenz, et al., 2002), with frozen
semen (Colas, 1979; Windsor, 1997; Soderquist, et al., 1999; Paulenz, et al., 2005, 2007) and
after laparoscopic inseminations with frozen semen (Eppleston et al., 1986; Maxwell, 1986;
Eppleston, et al., 1991; Eppleston & Maxwell, 1995). In a large scale epidemiological study,
Anel et al. (2005) observed that the male factor significantly influenced fertility. Despite the
restrictions in the choice of ejaculates, the authors found important differences in fertility
among rams. Salamon and Maxwell (1995) proposed that ram differences in fertility could be both genetic and
environmental, whereas ejaculate differences are probably due to nutrition, management
and previous frequency of ejaculation.
Whereas differences in fertility have been demonstrated among fertile males in different
species, the causes of these differences remain unclear (Ostermeier, et al., 2001). Saacke et al.
(1988, 1994) have suggested in bulls that factors associated with semen quality which affect
fertility can be classified as either compensable or non-compensable. It was suggested that
the effects of compensable factors on fertility might be sensitive to the number of sperm
inseminated, whereas those of non-compensable factors were not. As the number of sperm
inseminated increases, fertility increases until a plateau is reached (den Daas, 1992). At this
point, compensable factors no longer have an effect on fertility. Commercial insemination of
ovine in Mediterranean Countries provides at least the plateau number of sperm in an
insemination dose. It is thus the non-compensable factors that contribute most to the fertility
level of a ram. A non-compensable defect in sperm would be one in which a sperm reaches
the fertilisation site and initiates the egg activation process, but fails to sustain zygotic,
embryonic, or foetal development (Ostermeier, et al., 2001). Evidence of such defects in
sperm has been described in bulls with fertility differences (Eid, et al., 1994). Likely
candidates for non-compensable factors would be incorrectly assembled chromatin or
damaged DNA within the sperm nucleus. It seems logical to assume that the transfer of a
complete and intact DNA molecule from sperm to ovum is crucial to obtain fertilisation
with certain prospects of success. It is well-known that the presence of defects in the genetic
material, such as anomalies in chromatin condensation related with the sperm maturation
process, the integrity of the DNA molecule associated with the presence of breaks both of
single and double DNA strands, or the presence of chromosomal anomalies, are closely
associated with infertility (Aravindan, et al., 1997).
Pilar Santolaria, Inmaculada Palacin and Jesús Yániz
Instituto Universitario de Ciencias Ambientales y Departamento de Producción Animal y
Ciencia de los Alimentos. Escuela Politécnica Superior, Huesca
Universidad de Zaragoza