Directory UMM :Data Elmu:jurnal:L:Livestock Production Science:Vol68.Issue1.Jan2001:

Livestock Production Science 68 (2001) 1–12
www.elsevier.com / locate / livprodsci

Review article

Endocrine changes and management factors affecting puberty in
gilts
A.C.O. Evans*, J.V. O’Doherty
Department of Animal Science and Production, Faculty of Agriculture, Conway Institute for Biomolecular and Biomedical Research,
University College Dublin, Belfield, Dublin 4, Ireland
Received 16 September 1999; received in revised form 17 February 2000; accepted 19 May 2000

Abstract
Puberty in gilts is the occurrence of first oestrus and the onset of reproductive capability (usually at between 200 and 220
days of age). It is the combined effect of genetic factors and management factors (including nutrition, boar exposure and
season) that contribute to the age at puberty. Consumer demands for leaner pork have lead to genetic selection for increased
lean tissue growth rate and reduced body fat. This has resulted in a delay in the age at puberty and a decrease in energy
stores for subsequent growth, pregnancy and lactation. Restriction of dietary protein during the prepubertal period is a way to
enhance body-fat reserves; however, this has detrimental effects on the age at puberty and ovulation rates, but can be
overcome by restoring dietary protein in the weeks before the induction of puberty. Luteinizing hormone (LH) is a key
hormone in regulating ovarian follicular growth and, hence, the age at which puberty occurs. Concentrations of LH in the

blood fluctuate during the prepubertal period in association with ovarian development. The nutritional modulation of
hormone secretion is not clear. Future management strategies to enhance both reproductive performance and to satisfy
consumer demands need to consider hypothalamo-pituitary maturation in association with body weight and bodycomposition parameters.  2001 Elsevier Science B.V. All rights reserved.
Keywords: Gilt; Puberty; Hormone; Nutrition; Management

1. Introduction
Reducing the number of non-productive sow days
is an important component of productivity in current
pig-production enterprises. It has been estimated that
between 15 and 25% of litters produced in pig units
are from gilts (Gordon, 1997). Since gilts have
smaller litters and longer weaning-to-service inter*Corresponding author. Tel.: 1 353-1-706-7771; fax: 1 353-1706-1103.
E-mail address: alex.evans@ucd.ie (A.C.O. Evans).

vals than sows, the percentage of gilts in a herd has a
marked effect on overall productivity.
The first management decision to be made about
animals entering the breeding herd is the timing of
first breeding. Breeding at a younger age is associated with fewer non-productive days and, consequently, with lower initial costs. However, when
deciding whether or not to bred gilts early, two

consequences must be considered: firstly, the effect
on subsequent reproductive performance and, secondly, the effect on longevity of these gilts in the
herd. It has been shown that, as the age at first

0301-6226 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S0301-6226( 00 )00202-5

2

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

farrowing increases, there is an increase in the
average number of piglets born alive and weaned per
litter, but that there is also a decrease in the average
number of litters at culling (Le Cozler et al., 1998).
In practical terms, this effect is only significant for
gilts that are successfully mated at greater than 270
days of age (Le Cozler et al., 1998). Extreme
leanness increases the age at puberty and has detrimental effects on lifetime reproductive performance
(Gaughan et al., 1995).

Puberty in gilts is the occurrence of first oestrus
and the onset of reproductive capability. Once puberty has been reached, gilts will normally exhibit
regular oestrous cycles of around 21 days duration.
Age at puberty is regulated by internal (breed, live
weight, backfat depth) and management factors
(nutrition, boar exposure, environmental factors),
both of which are mediated via the endocrine
reproductive axis. There is increasing awareness of
the possible conflict between that of increasingly
leaner pigs and of maintaining or increasing reproductive performance. The aim of this paper is to
review the hormonal changes that precede puberty in
gilts and the management options available to
producers with particular reference to modern,
leaner, highly growth-efficient gilts.

2. Endocrine changes

2.1. Hormone profiles and ovarian development
before puberty
In gilts, luteinizing hormone (LH) is the key

hormone that appears to control ovarian development
and, hence, the age at puberty in gilts (Fig. 1). In
European breeds, concentrations of LH in blood
decrease from birth to about 40 days of age (Colenbrander et al., 1977; Pelletier et al., 1981; Camous et
al., 1985). Between 80 and 120 days of age, there is
a transient rise in LH concentrations, which then
decline to a nadir by about 180 days of age (Pelletier
et al., 1981; Diekman et al., 1983; Camous et al.,
1985). From this nadir until first ovulation (puberty),
LH concentrations increase (Pelletier et al., 1981;
Prunier et al., 1993a), as is typical in most species.
This increase in LH secretion is characterised by an
increase in mean LH concentrations and LH pulse

Fig. 1. Changes in mean luteinizing hormone (LH) concentrations
(after Pelletier et al., 1981; Diekman et al., 1983; Camous et al.,
1985), oestrogen concentrations (after Lutz et al., 1984; Camous et
al., 1985) and numbers of follicles greater than 3 mm in diameter
(after Dyck and Swierstra, 1983; Grieger et al., 1986) in gilts
during sexual maturation.

frequency (Pelletier et al., 1981; Lutz et al., 1984;
Prunier et al., 1993a). This final increase is associated with final maturation of ovarian follicles (Beltranena et al., 1993) and culminates in a preovulatory
surge that successfully stimulates ovulation. Folliclestimulating hormone (FSH) concentrations are high
early in life and decrease after 70 to 125 days of age,
and it is clear that mean FSH concentrations do not
increase as puberty draws closer (Diekman et al.,
1983; Diekman and Trout, 1984; Camous et al.,
1985; Grieger et al., 1986; Prunier et al., 1993a).
Christenson et al. (1985) reviewed the maturation
of ovarian follicles in prepubertal gilts. Primordial
follicles account for most of the follicles in the
ovaries between birth and about 100 days of age
(Oxender et al., 1979). Antral follicles are rarely
seen in gilts before 60 days of age; however, they are
prevalent after about 100 days of age (Fig. 1; Casida,
1935; Oxender et al., 1979; Dyck and Swierstra,
1983; Grieger et al., 1986). Based on temporal
relationships, we suggest that the early transient
increase in LH secretion, between about 80 and 120

days of age, stimulates this initial increase in follicle
development (Camous et al., 1985; Prunier et al.,

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

1993b), as has been suggested for cattle (Evans et
al., 1994; Day and Anderson, 1998). In Meishan
gilts, ovarian development is more rapid than in
commercial breeds, with antral follicles first appearing between 30 and 60 days of age (Miyano et al.,
1990).
Oestradiol concentrations (Fig. 1) are low
throughout the majority of the prepubertal period but
increase prior to puberty (Esbenshade et al., 1982;
Lutz et al., 1984; Camous et al., 1985). Progesterone
concentrations only increase after puberty with the
formation of the first corpora lutea (Esbenshade et
al., 1982; Prunier et al., 1993a). Puberty usually
occurs between 200 and 220 days of age, although
considerable variation exists according to internal
(including genetic) and management factors.

2.2. Regulation of gonadotrophin secretion
Puberty occurs only when gilts reach a certain
stage of physiological maturation (i.e., when physiological development is adequate to support successful reproduction). While this involves physical
growth attributes, it is reliant on the culmination of a
series of maturational events in the endocrine system
that regulate hormone secretion, resulting in behavioural oestrus and ovulation.
The regulation of gonadotrophin secretion occurs
via changes in inhibitory (negative feedback) and / or
stimulatory actions at the hypothalamus and pituitary. The exact importance of these two contrasting
actions is unclear during sexual maturation in gilts.
During the early prepubertal period, it is likely that
the central mechanisms (i.e., above the pituitary) that
regulate reproduction are immature. During this time,
the pituitary is responsive to exogenous gonadotrophin releasing hormone (GnRH) stimulation
(Chakraborty et al., 1973; Foxcroft et al., 1975;
Pressing et al., 1992), indicating that the pituitary is
functional at a young age. Negative feedback by
ovarian steroids is weak, as shown by a lack of
increase in LH following ovariectomy (Elsaesser et

al., 1978). The early transient increase in LH concentrations may occur due to maturation of the
central control of gonadotrophin secretion causing an
increase in GnRH production by the hypothalamus,
which stimulates an increase in pituitary LH and
FSH secretion. The decrease from the transient rise

3

in LH concentrations may occur because of the
establishment of oestradiol negative feedback, which
is first seen at about 100 days of age (see Christenson
et al., 1985). FSH also appears to be regulated by
negative feedback from at least 84 days of age
(Prunier and Louveau, 1997).
In some species (rats and primates in particular),
the final maturational events leading to the increase
in LH concentrations and puberty appear to occur as
a result of positive stimulation to the hypothalamus
via activation of a central drive component to GnRH
neurones (Ojeda, 1991). Endogenous opioid peptides

regulate LH secretion in some farm animals (Haynes
et al., 1989); however, opioidergic systems do not
appear to regulate tonic LH secretion during the
peripubertal period in gilts (Barb et al., 1988;
Kuneke et al., 1993). None-the-less, opioidergic
systems may play a role in the preovulatory LH
surge in pubertal gilts (Asanovich et al., 1998). The
excitatory amino acids, glutamate and aspartate, are
important neurotransmitters in the central control of
gonadotrophin secretion in many species (Brann,
1995). However, like opioidergic systems, it is
unlikely that excitatory amino-acid stimulation of the
hypothalamus is the cause of increased gonadotrophin secretion before puberty in gilts (Barb et al.,
1992; Chang et al., 1993; Estienne et al., 1995;
Popwell et al., 1996). During the weeks preceding
puberty, the prevailing theory is that the sensitivity
to steroid negative feedback diminishes (gonadostat
hypothesis; see Day and Anderson, 1998). This
allows LH secretion to increase (Pelletier et al.,
1981; Prunier et al., 1993a) in the face of low or

increasing oestradiol concentrations (Esbenshade et
al., 1982; Lutz et al., 1984; Camous et al., 1985).
While there is evidence to support this theory
(Berardinelli et al., 1984; Lutz et al., 1984), others
have been unable to demonstrate a reduction in
oestradiol negative feedback prior to first ovulation
(Elsaesser et al., 1991). It has also been suggested
that increases (Pearce et al., 1988) or decreases
(Prunier et al., 1993a) in corticosteroid concentrations may increase LH secretion at about the time of
puberty.
Oestradiol has primarily negative feedback effects,
but positive feedback develops as sexual maturation
advances. Positive feedback of oestradiol stimulates
the surge release of LH that is necessary for ovula-

4

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

tion; this is absent at 6 days of age, weak at 60 days

of age (Kuneke et al., 1993) and evident at 160 days
of age (Elsaesser and Foxcroft, 1978; Foxcroft et al.,
1984).
In summary, fluctuations in LH concentrations are
closely related to maturation of the reproductive tract
and first ovulation in gilts. The control of LH
secretion is complex. The initial increase in LH
concentrations may be due to maturation of hypothalamic activity and / or factors regulating the hypothalamus, and the increase in the immediate peripubertal period appears to be due to a resetting of the
sensitivity to oestradiol negative feedback.

3. Influence of genetic factors on age at puberty

appears to be related to the genetic background of
the gilts and to other aspects of the environment in
which they are kept. Consequently, age is not
considered to be an accurate predictor of puberty in
gilts (Rozeboom et al., 1995).
Gilts must reach a minimum weight of about 75
kg before puberty can be attained (Hughes and Cole,
1975; Young et al., 1990); however, like age, great
variability exists. Recently, it was suggested that live
weight on its own does not account for the induction
of puberty, but contributes as one of the factors
leading to puberty (Gaughan et al., 1997). Body
weight is a measure of the combined components of
the different elements of body composition, and
consideration needs to be given to the fatness and
leanness of gilts (see Section 3.3).

3.1. Age and weight

3.2. Breed

Some authors suggested that age is the most
important factor determining puberty attainment
(Prunier et al., 1987; Newton and Mahan, 1992).
However, it is widely recognised that unstimulated
gilts may reach puberty from as early as 170 days to
as late as 260 days of age (Dyck, 1988; Eliasson et
al., 1991). Part of the variability of age at puberty

Large differences between breeds in the age at
puberty have been reported, with the average age
being greatest in Duroc gilts and lowest in Meishan
gilts (Table 1). Crossbred gilts reach puberty earlier
and have fewer non-productive days compared with
purebred animals (Bidanel et al., 1996). Besides
these differences between breeds, considerable vari-

Table 1
Mean age (6S.D., where available) at puberty of different breeds of gilts
Breed

Overall
mean age

Mean6S.D.

Reference

Duroc

235

Hampshire
Large White

207
205

Landrace

185

Meishan

97

195617
224620
259625
263
207628
173613
200620
206621
208620
211621
211620
215649
215
173625
185620
198635
8169
94615
115620

Haines et al., 1959
Christenson and Ford, 1979
Bryan and Hagen, 1991
Good et al., 1965
Christenson and Ford, 1979
Gaughan et al., 1997
Allrich et al., 1985
Sterning et al., 1998
Rydhmer et al., 1994
Christenson and Ford, 1979
Eliasson et al., 1991
Bidanel et al., 1996
Kerr and Cameron, 1997
Christenson and Ford, 1979
Allrich et al., 1985
Bidanel et al., 1996
Legault and Caritez, 1983
Pickard and Ashworth, 1995
Hunter et al., 1993

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

ation exists within breeds, as demonstrated by the
variability within studies (one standard deviation
about the mean age at puberty is about 20 days) and
between studies for a given breed (Table 1). These
differences may be explained by variations in genotypes within breeds and in management regimens.
Therefore, breed influences age at puberty, but,
among commercial breeds, management factors can
have a greater effect than breed on the onset of
reproductive capability.

3.3. Modern genotypes and body composition
The 1970s were a period of rapid change in the
pig industry, with consumers in many countries
demanding less fat and leaner meat. The industry
responded by selecting its breeding animals for low
backfat and the genotype of the animals changed.
Animals that are selected for lower backfat grow
bigger and, compared with animals of small mature
size, are leaner at the same body weight because they
have a lower proportion of their mature body size
and are physiologically younger (Gaughan et al.,
1997). As well as reduced backfat, voluntary food
intake of the selected animals has been reduced
(Webb, 1989). Based on measures of live weight,
backfat thickness and average daily gain, it has been
proposed that puberty occurs only after the attainment of a minimum level of leanness (Cunningham
et al., 1974), fatness (den Hartog and van Kempen,
1980) or the ratio of fat to lean (Kirkwood and
Aherne, 1985).
Genetic changes in body composition and appetites are associated with reduced reproductive performance in contemporary comparisons of lines of
pigs selected for different components of efficient
lean growth (Kerr and Cameron, 1995). Animals
selected for low daily food intake and / or a high
lean-to-food-conversion ratio show a delay in the age
at puberty (Kerr and Cameron, 1995). There is a
strong positive genetic correlation between leanness
at 90 kg body weight and subsequent age at puberty
(Eliasson et al., 1991; Rydhmer et al., 1994), indicating that leaner gilts reach puberty later than fatter
gilts. In another study, offspring of sows selected for
early puberty cycled at an earlier age, with more fat,
than offspring of sows selected for late puberty
(Hixon et al., 1987). In addition, leaner gilts show a

5

shorter oestrus and decreased reddening and swelling
of the vulva compared with fatter gilts (Rydhmer et
al., 1994). It has been suggested that puberty is
delayed in extremely fast-growing gilts because the
increase in growth rate is primarily due to a more
rapid accumulation of lean rather than fat tissue
(Beltranena et al., 1991). It has been determined that
a minimum fatness threshold of about 6 mm in
backfat thickness needs to be achieved for the
attainment of puberty (Beltranena et al., 1993).
However, it is considered that gilts need at least 13
mm of backfat before first mating (Yang et al.,
1989). Hence, body fat and muscle content are
associated with age at puberty, and there appears to
be genotype-specific minimum levels of body fat and
muscle required for the onset of puberty to occur
(Kerr and Cameron, 1997). More recently, the rate of
fat and / or protein deposition has been linked to
physiological maturity and reproductive performance
of gilts (Edwards, 1998), and this rate is suggested to
be a more important determinant for puberty than
age, weight or boar exposure (Gaughan et al., 1997).
In conclusion, to most efficiently meet customer
demands, selection for future genotypes of pigs
needs to concentrate on daily food intake, high lean
growth rate and consideration needs to be given to a
minimum level of fatness in order to avoid pubertal
retardation.

4. Influence of management factors on age at
puberty

4.1. Nutrition
Nutrition of gilts during rearing has effects on
subsequent reproduction, both in the short-term and
in the long-term (see Aherne and Kirkwood, 1985).
Anderson and Melampy (1972) reviewed the literature on the effects of energy intake on age and
weight at puberty in pigs. They noted that restricting
energy intake delayed puberty by an average of 16
days in nine experiments, but hastened puberty by an
average of 11 days in five other experiments. In later
studies, a reduction of feed intake during rearing
resulted in an increase in age and a decrease in body
weight at puberty (den Hartog and van Kempen,
1980; den Hartog and Noordewier, 1984; Le Cozler

6

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

et al., 1999a). Modern pig genotypes are extremely
sensitive to modest reductions in feed intake, which
can delay puberty by more than 3 weeks (van Lunen
and Aherne, 1987). The plane of nutrition used in
rearing gilts influences the age at which first oestrus
is shown (Dzuik, 1991; Prunier, 1991).
The improved genotype of modern sows codes for
increased lean growth and decreased body-fat content. This decrease in fat has partly resulted in gilts
that are less able to deal with the simultaneous
demands of growth, pregnancy and subsequent lactation. There are two possible approaches to overcome
this problem: firstly, increase the plane of nutrition
of gilts prior to puberty as a way of increasing fat
reserves or, secondly, use dietary protein restriction
to limit lean tissue growth and increase body-fat
reserves (Edwards, 1998). The first strategy is undesirable as it leads to an increase in mature body size
(O’Dowd et al., 1997), which increases management
problems associated with oversized animals and
leads to higher feed maintenance costs and a reduced
feed intake in the first lactation (Le Cozler et al.,
1999b). A 50% restriction of dietary lysine to
growing gilts is an effective way of restricting lean
tissue growth and increasing body lipid content (den
Hartog and Verstegen, 1990; Gill, 1997; Cia et al.,
1998; Fig. 2). However, this method reduces growth
rate, delays age at puberty and decreases ovulation
rate (Fig. 2). This negative effect of protein restriction on ovulation rate in gilts can be overcome by
protein flushing in the weeks prior to the induction of
puberty (Fig. 2). Such short-term changes in food
intake can modify reproductive function in the
absence of changes in body weight or body composition (Booth et al., 1994).
Decreasing the amount of food made available to
animals will reduce feed costs. While severe restrictions during the prepubertal period will have detrimental effects on reproductive performance (van
Lunen and Aherne, 1987; Dzuik, 1991; Prunier,
1991), moderate restrictions (75 to 90% of ad
libitum intakes) during rearing have recently been
shown not to affect reproductive performance
(Klindt et al., 1999; Le Cozler et al., 1999b).
However, restricted animals had a higher voluntary
food intake in first lactation and a higher culling rate
than non-restricted animals, suggesting that the econ-

Fig. 2. Effects of protein restriction on backfat thickness ([a],
after Cia et al., 1998), age at puberty ([b], after Gill, 1997) and
ovulation rate at the second cycle ([c], after Crisol et al., 1997) in
gilts. Gilts were fed high protein ([a] and [c]: 0.9 g lysine / MJ
digestible energy; [b]: 1% lysine) or low protein ([a] and [c]: 0.3 g
lysine / MJ digestible energy; [b]: 0.5% lysine) diets from 118, 87
or 130 days ([a], [b] and [c], respectively). In [c], puberty was
induced by exogenous gonadotrophin treatment (PG600) at 180
days of age, and flushed gilts were fed the low diet from 130 days
and the high diet from 175 days of age. Bars within groups with
no common superscript are significantly different (P , 0.05).

omic gains of feed restriction during rearing may be
lost after the first two parities (Le Cozler et al.,
1999b).
The complicated link between nutrition and reproductive hormones has previously been reviewed
(Booth, 1990; Cox, 1997; Cosgrove, 1998). The
most likely hormonal links between nutrition and the
reproductive axis are insulin and insulin-like growth
factor I (IGF-I), which may act at either the hypothalamic–pituitary level or directly at the ovarian
level.
In conclusion, gilts of modern genotype are leaner
and this is associated with a delay in the age at
puberty compared with gilts of older genotypes.
Body fat can be increased by dietary protein restriction, but flushing with protein prior to puberty is
necessary to avoid the detrimental effects of protein
restriction on age at puberty and ovulation rate.

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

7

4.2. Boar effect
The timing of puberty in gilts is markedly influenced by the age at which they first come in
contact with a boar (reviewed by Hughes et al.,
1990). For maximum boar stimulation, it is essential
that gilts are reared out of sight and sound of boars
until they are about 165 days of age. At this time,
they are transported to a new location and are
regularly exposed to a sexually active boar. This has
the effect of bringing them into heat within 7 to 20
days (Hughes et al., 1990). For most effective boar
exposure, gilts and boars should to be in physical
contact for at least 30 min each day (Caton et al.,
1986; Hughes et al., 1990). Introducing and removing the boar is more effective than maintaining the
boar and gilts in constant fence-line exposure (Caton
et al., 1986). The pubertal response is further enhanced if boar contact occurs more than once a day
(Hughes et al., 1997; Hughes and Thorogood, 1999).
For best results, mature boars with high libido should
be used (Hughes, 1994; Fig. 3).
While tactile, auditory and visual cues from the
boar may be involved in the pubertal response,
pheromonal stimulation has an important role in
stimulating puberty in gilts (Signoret, 1970; Hughes
et al., 1990; Pearce and Paterson, 1992). When the
olfactory bulbs were removed from gilts, they were
unresponsive to boar stimulation (Kirkwood et al.,
1981). In addition, transportation of gilts increases
the response to boar stimulation, but transport alone
(not followed by boar stimulation) is insufficient to
stimulate early oestrous behaviour (Hughes et al.,
1997).
The procedures of transportation, relocation and
regular boar exposure are stressful to gilts, and stress
is likely to be an integral component of the boar
effect. Success of the boar effect relies on increasing
LH concentrations (Fig. 3) and it has been suggested
that cortisol mediates this stress effect (Pearce and
Hughes, 1987). In most gilts, boar exposure increases circulating concentrations of cortisol (Kingsbury and Rawlings, 1993; Turner et al., 1998; Fig. 3)
and cortisol infusion can increase LH concentrations
in prepubertal gilts (Pearce et al., 1988). However,
administering cortisol to concentrations seen in the
presence of a boar are ineffective in stimulating

Fig. 3. Effect of [a] boar libido on gilts reaching puberty, and of
the presence of a boar on circulating concentrations of [b] LH and
[c] cortisol. Gilts in [a] were exposed to boars with high or low
libidos (based on mating tests) for 20 min per day, starting at 160
days of age (from Hughes, 1994). Mean LH concentrations and
LH pulse frequencies in the gilts (n 5 12) in [b] were for the
periods of 6 h before (Before) and after (After) boar exposure at
135 days of age (after Kingsbury and Rawlings, 1993). Cortisol
concentrations in [c] are mean values from 15-min periods after a
back-pressure test for oestrus detection (no boar) or introduction to
a boar (with boar) on each of 11 days prior to oestrus (after Turner
et al., 1998). Bars within groups with no common superscript are
different (P , 0.05).

puberty (Pearce and Paterson, 1992), and treating
gilts with adrenal corticoids did not advance the age
at puberty (Esbenshade and Day, 1980). In addition,
some gilts also do not respond to boar exposure but
do show an increase in cortisol secretion (Kingsbury
and Rawlings, 1993). Hence, a pivotal role for
cortisol is in doubt and the exact endocrine mechanism(s) linking boar exposure with an increase in LH
secretion remains to be established. The increase in
LH concentrations in the immediate peripubertal
period may be due to a resetting of the sensitivity to
oestradiol negative feedback (Pelletier et al., 1981;
Berardinelli et al., 1984; Lutz et al., 1984). Based on
the above observations, we suggest that the mechanism of the boar effect may involve an as yet
unidentified central pathway(s) that impinges on the

8

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

system(s) regulating the sensitivity of LH to oestradiol negative feedback.
In summary, for optimal effect, gilts need to be
exposed to a mature boar with high libido for one or
more 30 min periods per day after 165 days of age.
Boar exposure increases LH concentrations but the
role of cortisol and the mechanism by which this
occurs is unclear.

4.3. Confinement
Overcrowding may decrease the percentage of
gilts showing oestrus (Cronin et al., 1983; Christenson, 1986). Gilts maintained with a space allowance of 1 m 2 per pig had significantly elevated free
corticosteroid concentrations and a lower proportion
displayed oestrus and were successfully mated compared with those pigs allowed 2 or 3 m 2 of space
(Hemsworth et al., 1986). This may be explained by
a delay in growth that has been observed in gilts
submitted to space restriction (Pearce and Paterson,
1993). However, others have crowded gilts together
to the extent that growth rate and feed efficiency
were reduced but without affecting the average age
at first oestrus (Ford and Teague, 1978). While
crowding may be a factor in controlling the age at
puberty, group size appears to be an additional
variable. Gilts housed in small groups (three or less)
reached puberty later than gilts housed in larger
groups (nine or more) at the same density (Christenson, 1986).

however, melatonin secretion in response to light:dark cycles is less clear in pigs. Some studies have
shown a nocturnal rise in melatonin secretion
(McConnell and Ellendorff, 1987; Paterson et al.,
1992a), whereas others have not (Minton et al.,
1989; Diekman et al., 1992). More recently, it has
been concluded that pigs have no clear circadian
rhythm of melatonin in peripheral blood (Bubenik et
al., 2000) but that there is a weak nocturnal rise in
serum melatonin concentrations (Green et al., 1996).
In addition, differences in melatonin secretion between light and dark periods may vary according to
light intensity and age (Green et al., 1999).
Lengthening the duration of the period of high
circulating melatonin concentrations by oral administration of melatonin in the afternoon to gilts kept
under long-days will overcome the seasonal inhibition of attainment of puberty (Paterson et al., 1992b).
Seasonal depression of the attainment of puberty in
gilts can also be partially or wholly overcome by
providing olfactory stimulation from boars (Hughes
et al., 1990; Paterson et al., 1991).
As in other species, links between long days, high
melatonin and increasing LH secretion are complex
in pigs (see Love et al., 1993). Since the inhibitory
influence of long-day regimes on gilt puberty attainment can be altered by the use of boar contact
(Paterson and Pearce, 1990; Paterson et al., 1991),
we speculate that photoperiodic regulation of the
attainment of puberty may be by a similar mechanism(s) as the boar effect (i.e., by altering the
sensitivity to oestradiol negative feedback by as yet
unidentified pathways).

4.4. Season
4.5. Manure gases
The attainment of sexual maturation in gilts is
subject to seasonal variation (Paterson et al., 1989;
Hughes et al., 1990), as puberty is delayed in
summer compared with winter (Paterson et al.,
1991). Heat stress may contribute to the delay of
puberty during summer (Flowers et al., 1989; Flowers and Day, 1990), but the long light duration
probably plays a more important role, since seasonal
variations can be reproduced under constant temperature by altering light patterns to mimic natural
photoperiods (Paterson and Pearce, 1990). In many
species (especially seasonal breeders), melatonin
secretion is elevated during periods of darkness;

Poor air quality delays the onset of puberty in
gilts. When gilts were exposed to aerial ammonia
concentrations of either , 5 or 20 ppm from 10 to
40 weeks of age, a greater proportion of the gilts
reared in the cleaner environment attained puberty by
26 weeks of age (Malayer et al., 1987). In that study,
average daily gain and feed efficiency were similar,
and LH and FSH secretory patterns were not different between groups. In another study, gilts reared in
an environment of 20 ppm ammonia attained puberty
later than gilts reared in an environment of , 10
ppm ammonia (Zimmerman et al., 1988). The mech-

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

anism whereby poor air quality delays the onset of
puberty in gilts is unknown, but it appears that
odorous gases, such as ammonia, may diminish the
ability of the gilts to perceive olfactory cues from
boars (Curtis, 1972).

5. Conclusion
Modern pig genotypes have been selected for
rapid, efficient, lean-tissue growth, in combination
with low body-fat content. This has been associated
with detrimental effects on pubertal development.
Using nutritional strategies, it is possible to alter
body composition and increase body-fat reserves to
improve the rate of sexual maturation. To fully
understand the importance of nutritional programmes
on reproduction, such strategies need to consider that
the onset of puberty in gilts is a function of hypothalamo-pituitary maturation rather than attainment
of specific body weight or composition parameters.
We suggest that there may be an as yet undescribed
link between a decrease in oestradiol negative feedback, that allows LH concentrations to increase in
the peripubertal period, and management regimens
used to stimulate puberty in gilts. Future research on
factors that regulate puberty in gilts needs to integrate understandings of the effects of dietary manipulations and management procedures on circulating hormone concentrations.

References
Aherne, F.X., Kirkwood, R.N., 1985. Nutrition and sow prolificacy. J. Reprod. Fertil., Suppl. 33, 169–183.
Allrich, R.D., Christenson, R.K., Ford, J.J., 1985. Age at puberty
and estrus activity of straightbred and reciprocal crossbred
gilts. Anim. Reprod. Sci. 8, 281–286.
Anderson, L.L., Melampy, R.M., 1972. Factors affecting ovulation
rate in the pig. In: Cole, D.J.A. (Ed.), Pig Production, Butterworths, London, pp. 329–366.
Asanovich, K.M., Johnson, B., Chang, W.J., Barb, C.R., Rampacek, G.B., Kraeling, R.R., 1998. Delay of estradiol-induced
surge secretion of LH in gilts by intracerebroventricular
injection of morphine. Dom. Anim. Endocrinol. 15, 45–53.
Barb, C.R., Derochers, G.M., Johnson, B., Utley, R.V., Chang,
W.J., Rampacek, G.B., Kraeling, R.R., 1992. N-Methyl-D,Laspartate stimulates growth hormone and prolactin but inhibits

9

luteinizing hormone secretion in the pig. Dom. Anim. Endocrinol. 9, 225–232.
Barb, C.R., Rampacek, G.B., Kraeling, R.R., Estienne, M.J.,
Taras, E., Estienne, C.E., Whisnant, C.S., 1988. Absence of
brain opioid peptide modulation of luteinizing hormone secretion in the prepubertal gilt. Biol. Reprod. 39, 603–609.
Beltranena, E., Aherne, F.X., Foxcroft, G.R., 1993. Innate variability in sexual development irrespective of body fatness in
gilts. J. Anim. Sci. 71, 471–480.
Beltranena, E., Aherne, F.X., Foxcroft, G.R., Kirkwood, R.N.,
1991. Effects of pre- and postpubertal feeding on production
traits at first and second estrus in gilts. J. Anim. Sci. 69,
886–893.
Berardinelli, J.G., Ford, J.J., Christenson, R.K., Anderson, L.L.,
1984. Luteinizing hormone secretion in ovariectomized gilts:
effects of age, reproductive state and estrogen replacement. J.
Anim. Sci. 58, 165–173.
Bidanel, J.P., Gruand, J., Legault, C., 1996. Genetic variability of
age and weight at puberty, ovulation rate and embryo survival
in gilts and relations with production traits. Genet. Sel. Evol.
28, 103–115.
Booth, P.J., 1990. Metabolic influences on hypothalamic–pituitary–ovarian function in the pig. J. Reprod. Fertil., Suppl. 40,
89–100.
Booth, P.J., Craigon, J., Foxcroft, G.R., 1994. Nutritional manipulation of growth and metabolic and reproductive status in
prepubertal gilts. J. Anim. Sci. 72, 2415–2424.
Brann, D.W., 1995. Glutamate: a major excitatory transmitter in
neuroendocrine regulation. Neuroendocrinology 61, 213–225.
Bryan, K.A., Hagen, D.R., 1991. Effects of nutrient intake and
sexual age of the dam at mating on fetal development in swine.
Growth Dev. Aging 55, 27–33.
Bubenik, G.A., Pang, S.F., Cockshut, J.R., Smith, P.S., Grovum,
L.W., Friendship, R.M., Hacker, R.R., 2000. Circadian variation of portal, arterial and venous blood levels of melatonin in
pigs and its relationship to food intake and sleep. J. Pineal Res.
28, 9–15.
Camous, S., Prunier, A., Pelletier, J., 1985. Plasma prolactin, LH,
FSH and estrogen excretion patterns in gilts during sexual
development. J. Anim. Sci. 60, 1308–1317.
Casida, L.E., 1935. Prepubertal development of the pig ovary and
its relation to stimulation with gonadotropic hormones. Anat.
Rec. 61, 389–396.
Caton, J.S., Jesse, G.W., Day, B.N., Ellersieck, M.R., 1986. The
effect of duration of boar exposure on the frequency of gilts
reaching first estrus. J. Anim. Sci. 62, 1210–1214.
Chakraborty, P.K., Reeves, J.J., Arimura, A., Schally, A.V., 1973.
Serum LH levels in prepubertal female pigs chronically treated
with synthetic luteinizing hormone-releasing hormone / follicle
stimulating hormone-releasing hormone (LH-RH / FSH-RH).
Endocrinology 92, 55–61.
Chang, W.J., Barb, C.R., Kraeling, R.R., Rampacek, G.B.,
Asanovich, K.M., 1993. N-Methyl-D,L-aspartate modulation of
pituitary hormone secretion in the pig: role of opioid peptides.
Dom. Anim. Endocrinol. 10, 305–313.
Christenson, R.K., 1986. Swine management to increase gilt
reproductive efficiency. J. Anim. Sci. 63, 1280–1287.

10

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

Christenson, R.K., Ford, J.J., 1979. Puberty and estrus in confinement-reared gilts. J. Anim. Sci. 49, 743–751.
Christenson, R.K., Ford, J.J., Redmer, D.A., 1985. Maturation of
ovarian follicles in the prepubertal gilt. J. Reprod. Fertil.,
Suppl. 33, 21–36.
Cia, M.C., Edwards, S.A., Glasgow, V.L., Shanks, M., Fraser, H.,
1998. Modification of body composition by altering the dietary
lysine to energy ratio during rearing and the effect of reproductive performance of gilts. Anim. Sci. 66, 457–463.
Colenbrander, B., Kruip, T.A., Dieleman, S.J., Wensing, C.J.,
1977. Changes in serum LH concentrations during normal and
abnormal sexual development in the pig. Biol. Reprod. 17,
506–513.
Cosgrove, J.R., 1998. Nutrition–endocrine interactions in the
female pig. In: Wiseman, J., Varley, M.A., Chadwick, J.P.
(Eds.), Progress in Pig Science. Nottingham University Press,
Nottingham, pp. 343–360.
Cox, N.M., 1997. Control of follicular development and ovulation
rate in pigs. J. Reprod. Fertil., Suppl. 52, 31–46.
Crisol, M., Edwards, S.A., English P.R., 1997. Short and long
term effects of protein nutrition on reproduction in gilts. In:
Proceedings of the British Society of Animal Science, p. 12.
Cronin, G.M., Hemsworth, P.H., Winfield, C.G., Muller, B.,
Chamley, W.A., 1983. The incidence of, and factors associated
with, failure to mate by 245 days of age in the gilt. Anim.
Reprod. Sci. 5, 199–205.
Cunningham, P.J., Naber, C.H., Zimmerman, D.R., Peo, Jr. E.R.,
1974. Influence of nutritional regime on age at puberty in gilts.
J. Anim. Sci. 39, 63–67.
Curtis, S.E., 1972. Air environment and animal performance. J.
Anim. Sci. 35, 628–634.
Day, M.L., Anderson, L.H., 1998. Current concepts on the control
of puberty in cattle. J. Anim. Sci. 76 (Suppl. 3), 1–15.
den Hartog, L.A., Noordewier, G.J., 1984. The effect of energy
intake on age at puberty in gilts. Neth. J. Agric. Sci. 32,
263–280.
den Hartog, L.A., van Kempen, G.L.M., 1980. Relation between
nutrition and fertility in pigs. Neth. J. Agric. Sci. 28, 211–227.
den Hartog, L.A., Verstegen, M.W.A., 1990. Nutrition of gilts
during rearing. Pig News and Information 11, 523–526.
Diekman, M.A., Brandt, K.E., Green, M.L., Clapper, J.A.,
Malayer, J.R., 1992. Lack of a nocturnal rise of serum
melatonin in prepubertal gilts. Dom. Anim. Endocrinol. 9,
161–167.
Diekman, M.A., Trout, W.E., 1984. Serum profiles of progesterone, LH, FSH, and prolactin immediately preceding induced
puberty in gilts. J. Anim. Sci. 58, 1262–1270.
Diekman, M.A., Trout, W.E., Anderson, L.L., 1983. Serum
profiles of LH, FSH and prolactin from 10 weeks of age until
puberty in gilts. J. Anim. Sci. 56, 139–145.
Dyck, G.W., 1988. Factors affecting sexual maturation, puberty
and reproductive efficiency in the gilt. Can. J. Anim. Sci. 68,
1–13.
Dyck, G.W., Swierstra, E.E., 1983. Growth of the reproductive
tract of the gilt from birth to puberty. Can. J. Anim. Sci. 63,
81–87.
Dzuik, P.J., 1991. Reproduction in domestic animals. In: Cups,

P.T. (Ed.), Reproduction in the Pig. Academic Press, New
York, pp. 471–489.
Edwards, S.A., 1998. Nutrition of the rearing gilt and sow. In:
Wiseman, J., Varley, M.A., Chadwick, J.P. (Eds.), Progress in
Pig Science. Nottingham University Press, Nottingham, pp.
361–382.
Eliasson, L., Rydhmer, S., Einarsson, S., Andersson, K., 1991.
Relationship between puberty and production traits in the gilt.
1. Age at puberty. Anim. Reprod. Sci. 25, 143–154.
Elsaesser, F., Foxcroft, G.R., 1978. Maturational changes in the
characteristics of oestrogen-induced surges of luteinizing hormone in immature domestic gilts. J. Endocrinol. 78, 455–456.
Elsaesser, F., Parvizi, N., Ellendorff, F., 1978. Steroid feedback on
luteinizing hormone secretion during sexual maturation in the
pig. J. Endocrinol. 78, 329–342.
Elsaesser, F., Parvizi, N., Schmitz, U., 1991. Inhibitory feedback
action of oestradiol on tonic secretion of luteinizing hormone in
pre- and postpubertal gilts. Anim. Reprod. Sci. 25, 155–168.
Esbenshade, K.L., Day, B.N., 1980. The effects of triamcinolone
acetonide on plasma glucocorticoid levels and occurrence of
estrus in gilts. J. Anim. Sci. 51, 153–157.
Esbenshade, K.L., Paterson, A.M., Cantley, T.C., Day, B.N.,
1982. Changes in plasma hormone concentrations associated
with the onset of puberty in the gilt. J. Anim. Sci. 54, 320–324.
Estienne, M.J., Harter-Dennis, J.M., Barb, C.R., Hartsock, T.G.,
1995. Luteinizing hormone and growth hormone concentrations
in serum of prepubertal gilts treated with N-methyl-D,L-aspartate. Dom. Anim. Endocrinol. 12, 207–213.
Evans, A.C.O., Adams, G.P., Rawlings, N.C., 1994. Follicular and
hormonal development in prepubertal heifers from 2 to 36
weeks of age. J. Reprod. Fertil. 102, 463–470.
Flowers, B., Cantley, T.C., Martin, M.J., Day, B.N., 1989. Effect
of elevated ambient temperatures on puberty in gilts. J. Anim.
Sci. 67, 779–784.
Flowers, B., Day, B.N., 1990. Alterations in gonadotropin secretion and ovarian function in prepubertal gilts by elevated
environmental temperature. Biol. Reprod. 42, 465–471.
Ford, J.J., Teague, H.S., 1978. Effect of floor space restriction on
age at puberty in gilts and on performance of barrows and gilts.
J. Anim. Sci. 47, 828–832.
Foxcroft, G.R., Elsaesser, F., Stickney, K., Haynes, N.B., Back,
H.L., 1984. Ovarian oestrogen-dependent maturation of the
LH / FSH surge mechanism during prepubertal development in
the gilt. J. Endocrinol. 101, 371–380.
Foxcroft, G.R., Pomerantz, D.K., Nalbandov, A.V., 1975. Effects
of estradiol-17b on LH-RH / FSH-RH-induced, and spontaneous LH release in prepubertal female pigs. Endocrinology
96, 551–557.
Gaughan, J.B., Cameron, R.D., Dryden, G.M., Young, B.A., 1997.
Effect of body composition at selection on reproductive
development in large white gilts. J. Anim. Sci. 75, 1764–1772.
Gaughan, J.B., Cameron, R.D.A., Dryden, G.M., Josey, M.L.,
1995. Effect of selection for leanness on overall reproductive
performance in Large White sows. Anim. Sci. 61, 561–564.
Gill, P., 1997. Profit through progress in pig research at MLC’s
Stotfold pig development unit. In: Profit Form Progress.
Proceedings of the Third Bi-Annual Symposium of the Northern Ireland Pig Conference, pp. 32–54.

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12
Good, L., Warnick, A.C., Wallace, H.D., 1965. Effect of dietary
energy level upon reproduction and the relation of endometrial
phosphatase activity to embryo survival in gilts. J. Anim. Sci.
24, 959–963.
Gordon, I., 1997. Breeding pigs at younger ages. In: Controlled
Reproduction in Pigs. CAB International, New York, pp. 218–
239.
Green, M.L., Clapper, J.A., Andres, C.J., Diekman, M.A., 1996.
Serum concentrations of melatonin in prepubertal gilts exposed
to either constant or stepwise biweekly alteration in scotophase.
Dom. Anim. Endocrinol. 13, 307–323.
Green, M.L., Clapper, J.A., Diekman, M., 1999. Serum concentrations of melatonin during scotophase and photophase in
3, 4, 5 and 6 months old gilts and barrows. Anim. Reprod. Sci.
57, 99–110.
Grieger, D.M., Brandt, K.E., Diekman, M.A., 1986. Follicular
fluid concentrations of estradiol-17b and progesterone and
secretory patterns of LH and FSH in prepubertal gilts reared in
confinement or outdoor lots. J. Anim. Sci. 62, 751–758.
Haines, C.E., Warnick, A.C., Wallace, H.D., 1959. The effect of
two levels of energy intake on reproductive phenomena in
Duroc Jersey gilts. J. Anim. Sci. 18, 347–354.
Haynes, N.B., Lamming, G.E., Yang, K.-P., Brooks, A.N., Finnie,
A.D., 1989. Endogenous opioid peptides and farm animal
reproduction. Oxf. Rev. Reprod. Biol. 11, 111–145.
Hemsworth, P.H., Barnett, J.L., Hansen, C., Winfield, C.G., 1986.
Effects of social environment on welfare status and sexual
behaviour of female pigs. II. Effects of space allowance. Appl.
Anim. Behav. Sci. 16, 259–267.
Hixon, A.L., Mabry, J.W., Benyshek, L.L., Weaver, W.M., Marke,
M.A., 1987. Estimates of genetic parameters for sexual and
compositional maturity in gilts. J. Anim. Sci. 64, 977–982.
Hughes, P.E., 1994. The influence of boar libido on the efficacy of
the boar effect. Anim. Reprod. Sci. 35, 111–118.
Hughes, P.E., Cole, D.J.A., 1975. Reproduction in the gilt. 1. The
influence of age and weight at puberty and on the ovulation
rate and embryo survival in the gilt. Anim. Prod. 21, 183–189.
Hughes, P.E., Pearce, G.P., Paterson, A.M., 1990. Mechanisms
mediating the stimulatory effects of the boar on gilt reproduction. J. Reprod. Fertil., Suppl. 40, 323–341.
Hughes, P.E., Philip, G., Siswadi, R., 1997. The effects of contact
frequency and transport on the efficacy of the boar effect.
Anim. Reprod. Sci. 46, 159–165.
Hughes, P.E., Thorogood, K.L., 1999. A note on the effects of
contact frequency and time of day of boar exposure on the
efficacy of the boar effect. Anim. Reprod. Sci. 57, 121–124.
Hunter, M.G., Biggs, C., Foxcroft, G.R., McNeilly, A.S., Tilton,
J.E., 1993. Comparisons of endocrinology and behavioural
events during the periovulatory period in Meishan and LargeWhite hybrid gilts. J. Reprod. Fertil. 97, 475–480.
Kerr, J.C., Cameron, N.D., 1995. Reproductive performance of
pigs selected for components of efficient lean growth. Anim.
Sci. 60, 281–290.
Kerr, J.C., Cameron, N.D., 1997. Variation in age at puberty in
pigs with divergent selection for components of efficient lean
growth rate. In: Proceedings of the British Society of Animal
Science, p. 30.
Kingsbury, D.L., Rawlings, N.C., 1993. Effect of exposure to a

11

boar on circulating concentrations of LH, FSH, cortisol and
oestradiol in prepubertal gilts. J. Reprod. Fertil. 98, 245–250.
Kirkwood, R.N., Aherne, F.X., 1985. Energy intake, body composition and reproductive performance of the gilt. J. Anim. Sci.
60, 1518–1529.
Kirkwood, R.N., Forbes, J.M., Hughes, P.E., 1981. Influence of
boar contact on attainment of puberty in gilts after removal of
the olfactory bulbs. J. Reprod. Fertil. 61, 193–196.
Klindt, J., Yen, J.T., Christenson, R.K., 1999. Effect of prepubertal
feeding regimen on reproductive development of gilts. J. Anim.
Sci. 77, 1968–1976.
Kuneke, G., Parvizi, N., Elsaesser, F., 1993. Effect of naloxone
and pulsatile luteinizing-hormone-releasing hormone infusions
on oestradiol-induced luteinizing hormone surges in immature
gilts. J. Reprod. Fertil. 97, 395–401.
Le Cozler, Y., Dagorn, J., Lindberg, J.E., Aumaitre, A., Dourmad,
J.Y., 1998. Effect of age at first farrowing and herd management on long-term productivity of sows. Livest. Prod. Sci. 53,
135–142.
Le Cozler, Y., Ringmar-Cederberg, E., Johansen, S., Dourmad,
J.Y., Neil, M., Stern, S., 1999a. Effect of feeding level during
rearing and mating strategy on performance of Sweedish
Yorkshire sows. 1. Growth, puberty and conception rate. Anim.
Sci. 68, 355–363.
Le Cozler, Y., Ringmar-Cederberg, E., Rydhmer, L., Lundeheim,
N., Dourmad, J.Y., Neil, M., 1999b. Effect of feeding level
during rearing and mating strategy on performance of Sweedish
Yorkshire sows. 2. Reproductive performance, food intake,
backfat changes and culling rate during the first two parities.
Anim. Sci. 68, 365–377.
Legault, C., Caritez, J.C., 1983. L’experimentation sur le proc
chinois en France. I. Performances de reproduction en race
pure et en croissement. Genet. Sel. Evol. 15, 225–240.
Love, R.J., Evans, G., Klupiec, C., 1993. Seasonal effects on
fertility in gilts and sows. J. Reprod. Fertil., Suppl. 48, 191–
206.
Lutz, J.B., Rampacek, G.B., Kraeling, R.R., Pinkert, C.A., 1984.
Serum luteinizing hormone and estrogen profiles before puberty in the gilt. J. Anim. Sci. 58, 686–691.
Malayer, J.R., Kelly, D.T., Diekman, M.A., Brandt, K.E., Sutton,
A.L., Long, G.G., Jones, D.D., 1987. Influence of manure gases
on puberty in gilts. J. Anim. Sci. 64, 1476–1483.
McConnell, S.J., Ellendorff, F., 1987. Absence of nocturnal
plasma melatonin surge under long and short artificial photoperiods in the domestic sow. J. Pineal Res. 4, 201–210.
Minton, J.E., Davis, D.L., Stevenson, J.S., 1989. Contribution of
the photoperiod to circadian variations in serum cortisol and
melatonin in boars. Dom. Anim. Endocrinol. 6, 177–181.
Miyano, T., Akamatsu, J., Kato, S., Najo, I., Kanda, S., 1990.
Ovarian development in Meishan pigs. Theriogenology 33,
769–775.
Newton, E.A., Mahan, D.C., 1992. Effect of feed intake during
late development on pubertal onset and resulting body composition in crossbred gilts. J. Anim. Sci. 70, 3774–3780.
O’Dowd, S., Hoste, S., Mercer, J.T., Flower, V.R., Edwards, S.A.,
1997. Nutrition modification of body composition and the
consequences for reproductive performance and longevity in
genetically lean sows. Livest. Prod. Sci. 52, 155–165.

12

A.C.O. Evans, J.V. O’ Doherty / Livestock Production Science 68 (2001) 1 – 12

Ojeda, S.R., 1991. The mystery of mammalian puberty; how much
do we know? Persp. Biol. Med. 34, 365–383.
Oxender, W.D., Colenbrander, B., van de Wiel, D.F., Wensing,
C.J., 1979.