Published November 21, 2014
Inverted teats (Mammillae invertitae) in gilts – Effect on piglet survival and growth rate
H. Chalkias,*1 E. Ekman,† N. Lundeheim,* L. Rydhmer,* and M. Jacobson‡
*Department of Animal Breeding and Genetics, Swedish University of Agricultural
Sciences, Box 7023, SE-750 07 Uppsala, Sweden; †Department of Biomedical Sciences
and Veterinary Public Health, Swedish University of Agricultural Sciences, Box 7028, SE-750 07 Uppsala, Sweden;
and ‡Department of Clinical Sciences, Swedish University of Agricultural Sciences, Box 7054, SE-750 07 Uppsala, Sweden
ABSTRACT: In the modern pig industry, the increasing number of piglets born per litter augments the
importance of the number of functional teats in the sow.
The aim of this study was to evaluate the function and
importance of inverted teats during nursing and to analyze structural and functional differences between the
mammary glands of inverted teats versus normal teats.
Nine farrowing gilts (8 purebred Swedish Yorkshire
gilts and 1 cross between Swedish Yorkshire and
Norwegian Landrace) and 94 piglets (59 piglets suckling normal teats, 32 piglets suckling protruded teats
[i.e., previously inverted], 2 piglets suckling inverted
teats, and 3 piglets suckling considerably smaller teats)
were included in the study. Teat fidelity (keeping the
same teat between the nursings) was registered, excluding the first 48 h postpartum. Piglet weight was recorded daily during the first week of life and thereafter once
a week until weaning at 4 wk of age. Weight and growth
rate were analyzed using repeated observation mixedmodel analysis of variance. The 2 piglets that suckled
the inverted teats were not able to emerge the teats and
they were euthanized 4 and 8 d after birth, respectively,
due to loss of BW. The average weight at weaning (28 d
of age) was 8.1 kg (range 3.2–13.8 kg). In the normal
teats (n = 53), the weight of the corresponding mammary gland tissue at necropsy was positively correlated
to the piglet average daily weight gain during wk 2 (r =
0.33, P < 0.05), 3 (r = 0.55, P < 0.001), and 4 (r =
0.47, P < 0.001). In the protruded teats (n = 32), the
weight of the corresponding mammary gland tissue was
positively correlated to the piglet average daily weight
gain during wk 2 (r = 0.63, P < 0.001) and 3 (r = 0.43,
P < 0.05). Among the piglets nursing normal teats, 82%
kept fidelity to its teat and the corresponding percent
for the protruded teats was 26%. In 7 of the 9 sows,
the weaning weight of the piglets suckling protruded
teats was numerically lower compared to the piglets
suckling normal teats, although the difference was not
statistically significant. Piglets nursing small teats had
lower weaning weight (4.8 kg) and the corresponding
mammary tissue also had lower weight (335 g). This
study indicates the importance of an adequate number
of well-developed and well-functioning teats in healthy
sows for the growth and development of the piglets
but also emphasizes the complexity of the interaction
between the nursing sow and her piglets.
Key words: functional teats, inverted teats, mammary gland, teat number, Yorkshire breed
© 2014 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2014.92:2587–2594
doi:10.2527/jas2013-7387
Introduction
Newborn piglets are born with low glucose levels
and poor stores of glycogen (Noblet et al., 1997). This
energy reserve is rapidly consumed and must be replenished to avoid hypoglycemia (Le Dividich and Noblet,
1981), hypothermia (Herpin et al., 2002), and weakness
1Corresponding
author: [email protected]
Received November 15, 2013.
Accepted March 5, 2014.
(Le Dividich and Noblet, 1983). The piglets are born
with an immature immune system (Dividich et al., 2005)
and the colostrum must be obtained before intestinal
closure occurs at 24 h postpartum (p.p.; Danielsen et al.,
2011). The piglets’ own synthesis of antibodies will not
start until about 1 wk of age (Rooke et al., 2003).
The increasing number of live-born piglets results
in decreased piglet weight at birth (Roehe, 1999), increased birth weight variability within litter (Wolf et al.,
2008), heterogenic maturity at birth (Herpin et al., 1993),
variation in growth rate, and increased sibling compe-
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Chalkias et al.
tition (Andersen et al., 2011). The nursing and suckling
behaviors of sows and piglets follow a complex pattern.
Therefore, it is important that the teat order is promptly established (Deen and Bilkei, 2004) and the small
milk cisterns and the short milk ejection (Fraser, 1980;
Verstegen, 1998) will imply that each piglet must have
readily access to a functional teat to achieve the amount
of milk necessary for nourishment and growth (Jensen et
al., 2001; Devillers et al., 2011). Variations in milk yield
depending on the position of the teat and the corresponding mammary glands (front, middle, or at the posterior
part of the udder) have been reported with higher weight
gain in piglets suckling the posterior teats (Nielsen et al.,
2001). Furthermore, the milk yield in multiparous sows
also depends on if the teat was nursed in the first lactation
(Farmer et al., 2012).
Failure of the fetal mammary pit to evert before birth
will result in an inverted teat. The mechanism behind
this failure is unknown but may be related to the connective tissue proliferation (Günther, 1984). The inverted
teats are often found near the umbilicus (Beilage et al.,
1996) and may emerge when stimulated (Nordby, 1934).
Inverted teats limit the pig rearing capacity of the sow and
may also increase the vulnerability to mastitis (Jonas et
al., 2008). The aim of this study was to evaluate the function and importance of inverted teats during nursing and
to analyze structural and functional differences among the
mammary glands of inverted teats versus normal teats.
Material and methods
The study was approved by the Ethics Committee
for Animal Experimentation, Uppsala, Sweden (C149/8
and C215/11).
Animals
A total of 9 farrowing gilts and 94 piglets was included
in the study. Eight gilts were purebred Swedish Yorkshire
gilts and 1 was a cross between Swedish Yorkshire and
Norwegian Landrace. At performance testing (at 100 kg
live weight), the 8 purebred gilts had between 1 and 8 inverted teats, 3 gilts also had a small teat (approximately
half the size of the normal teat), and 1 had an additional
extra teat. The Yorkshire gilts originated from 1 nucleus
herd, with well-known health status and within reasonable distance from the research facilities, and were preselected by skilled technicians from the breeding company
Nordic Genetics (www.nordicgenetics.se/). The crossbred
Yorkshire × Landrace gilt originated from a conventional
herd with a high prevalence of inverted teats in their recruitment gilts. This gilt was born by a sow from the nucleus herd and was selected based on the number of inverted
teats (8 inverted teats) at 100 kg live weight. All gilts were
inseminated with Hampshire semen. Three weeks before
expected farrowing, the gilts were transported to the research facilities at the Department of Clinical Sciences,
Swedish University of Agricultural Sciences, Uppsala.
Feeding and Accommodation
The gilts were housed in individual pens, 6.4 m2, with
concrete floor without fenders and a piglet area, 2 m2, provided with a heating lamp for the piglets. The pens were
bedded with wooded shavings and straw. The gilts were
fed a commercial diet (Lantmännen Piggfor Klara, Malmö,
Sweden) containing 12.7 MJ/kg ME and 0.7% of lysine,
supplemented with hay, twice daily according to the recommendations (Simonsson, 2006). Water was available
ad libitum. Teeth grinding was applied on all piglets within 12 h p.p. Iron was supplied individually by oral paste
or injection at 2 d of age and from 2 wk of age, as iron
flakes (Lantmännen Protect Järn&Torv, Malmö, Sweden)
given in the feed. Due to ethical considerations, creep feed
(Lantmännen Piggfor Trygge/Parvel, Malmö, Sweden)
containing 13.3 MJ/kg ME and 1.0% of lysine was provided in an automatic feeder from the second week.
Examinations
Ten weeks before expected farrowing, the teats were
counted, palpated, and subjected to a thorough inspection
(3 of the sows had also been subjected to an extra inspection 1 wk before mating). After parturition, the piglets were
counted and marked individually. All excessive, functional
posterior teats were blocked with adhesive bandage during the whole nursing period (Fixomull Stretch; Smith &
Nephew, Hamburg, Germany), to ensure that the number of
available teats equaled to the number of piglets. If a piglet
died before the teat order was established, 1 additional posterior teat was blocked. If a piglet died after the teat order
was established, the teat used by that particular piglet was
blocked. For each piglet, the suckled teat was registered
as “inverted,” “protruded” (previously inverted), “small,”
or “normal” and the teat pair was registered and numbered
from 1 to 8, starting from the front. A “small” teat was of
approximately half the size of a normal teat but otherwise
with the same proportions. The udder underwent a thorough
daily examination and the adhesive bandage was inspected
and adjusted if necessary. For each piglet, the weight was
recorded daily during the first week and thereafter once a
week until weaning at 4 wk of age.
The suckling behavior was registered for 6 of the
sows by videotaping 2 to 6 times a day until teat order was
established. The fidelity to teat was registered, excluding
the first 48 h p.p. Fidelity was set as suckling the same
teat between all nursings, with allowance of 2 disloyalties.
Difference between if the piglet was searching a new teat
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Inverted teats in gilts.
or loosed its teat was not taken into consideration. The
side, left or right, on which the sow was laying during
nursing and whether the piglets were suckling the upper
teat row or the lower teat row were registered from the
video tape. All animals underwent a clinical examination
(body temperature, heart and respiratory rate, inspection
of visible mucosal membranes, inspection of the fur coat,
movements, body condition, and behavior) by a veterinarian and were recorded daily. All piglets that died during the study were subjected to necropsy. The gilts were
euthanized immediately after weaning and the mammary
gland for each teat was dissected and weighed.
Sampling
Following euthanization of the gilts, the skin covering the udder was removed, each individual mammary gland was dissected from adjacent tissue (fat and
muscle), and weight was recorded. In 8 of the gilts, the
length (mm) of the teats was measured. Samples were
collected from all inverted, protruded, and small teats,
and to each of these teats a normal, control teat was collected. In 6 sows, the sampled teats were longitudinally
transected and one-half was fixed in 10% phosphate
buffered formalin for histology, and in 3 sows, the entire
teat and adjacent mammary tissue were fixed.
Histological Examination
The formalin-fixed specimens were embedded in
paraffin, sectioned, and stained with hematoxylin and eosin. To visualize collagen and smooth muscle Masson’s
trichrome stain was used and Weigert’s elastin stain was
used to visualize elastic fibers. A total number of 26 teats
was examined histologically, 2 inverted, 10 protruded, 3
small, and 10 normal teats, from a total of 6 sows.
Statistical Analysis
For the analyses SAS software version 9.1 (SAS
Inst. Inc., Cary, NC) was used. Due to the low number
of still inverted teats and small teats, they were excluded
from the statistical analysis. The weight and growth rate
were compared between piglets that developed diseases
and healthy piglets as well as between healthy sows and
sows that developed disease. Piglets that died before
weaning were excluded from the statistical analyses.
The variation in weight and daily weight gain (DWG)
were statistically analyzed using the repeated observation mixed-model ANOVA. Fixed effects included in
the statistical model were sow, sex, teat type (normal/
protruded), and day (day after farrowing) for weight and
week (week after farrowing) for growth rate and also the
interactions day × teat type and week × teat type. Piglet
was included as a random effect nested within teat type.
Additionally, to decrease the within-litter difference
between piglets suckling normal and protruded teats,
pairwise analysis with TTEST was used. In the pairwise
analysis on weight, 23 within-litter pairs (46 piglets)
were included. In the pairs (protruded vs. normal teat)
the piglet with a normal teat was selected to achieve a
similar birth weight within pair.
The mammary gland weight was analyzed using
ANOVA (PROC GLM) with sow, teat type (normal vs.
protruded), and sex as fixed factors. The correlation between mammary gland weight and piglet weight gain
was estimated with Pearson’s correlation.
The chi-squared test was used to identify any differences in fidelity between protruded and normal teats.
Results
Gilts
All inverted teats had protruded at farrowing, except for 2 gilts where 1 teat in each of the gilts remained
inverted (Table 1). Most of the inverted teats had protruded already in early gestation and some were noted
as protruded already at the time of mating. No adverse
effects of the blocking of excessive teats with adhesive
bandage were noted. On average, 3.4 teats (range 2–5)
were blocked with bandage after parturition.
Piglets
In total 123 piglets were born whereof 118 were
liveborn and 5 were stillborn, and in 1 large litter (18
liveborn piglets), 4 small (0.6 kg BW) newborn piglets
were euthanized to ensure that the number of piglets
equaled the number of teats. Thirteen piglets died during the first 24 h p.p. and 3 piglets died 2 to 5 d p.p. due
to crushing. One piglet died due to an acute hypersensitivity reaction induced by treatment of lameness (Day
6), 1 piglet died due to weakness (Day 6), and 2 piglets
suckling inverted teats were euthanized (Day 4 and 8).
The number of piglets weaned varied between 9 and 12
piglets, with an average of 10.6 piglets per litter.
Health
Four sows developed fever on Day 0, 3, 7, and
13 p.p., respectively, and were treated with antibiotics. There was a significantly lower weaning weight
(P = 0.02) and DWG (P = 0.006) in piglets of treated
sows (0.332 kg; 36 g/d), but this difference was not influenced by the type of teat suckled. In total, 15 piglets
were treated for lameness, 2 piglets were trampled by
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Chalkias et al.
Table 1. Characteristics of the 9 studied sows
Sow1
1Y
2Y
3 LY
4Y
5Y
6Y
7Y
8Y
9Y
Teats at 100 kg Teats at farrowing No. live born piglets No. blocked teats No. piglets weaned
10 normal
10 normal
13
2
12
4 inverted
4 protruded
10 normal
10 normal
13
5
9
4 inverted
3 protruded
1 inverted
6 normal
6 normal
13
5
9
8 inverted
7 protruded
1 inverted
10 normal
10 normal
10
5
9
4 inverted
4 protruded
10 normal
10 normal
12
3
11
5 inverted
5 protruded
8 normal
8 normal
11
4
10
6 inverted
6 protruded
11 normal
11 normal
15
2
12
2 inverted
2 protruded
1 small
1 small
13 normal
13 normal
16
4
11
1 inverted
1 protruded
1 small
1 small
13 normal
13 normal
18
3
12
1 inverted
1 protruded
1 small
1 small
1Y = Yorkshire
Video
No
No. weighed mammary glands
8
No
8
No
10
Yes
9
Yes
11
Yes
10
Yes
12
Yes
11
Yes
11
sow; LY = crossbred sow (Landrace × Yorkshire).
the sow and therefore treated with antibiotics, and 1 piglet developed fever and an infection of unknown etiology in the eye. Furthermore, 13 piglets suffered from
congenital tremor.
The 2 piglets that suckled the inverted teats were not
able to emerge the teats by stimulation and were euthanized 4 and 8 d after birth, respectively, due to loss of
BW between the 2 last weighings (–150 and –80 g, respectively). As recorded by the video tape, both these
piglets were still fighting for other teats the day they
were euthanized and the health registration also revealed
a lot of facial lesions on the piglets and lesions on the
teats. At necropsy, empty intestines and sparse amount
of feed in the stomach were noted.
Videotape Recording
The teat order, when all piglets used the same teat
in between sucklings, was on average established Day 8.
One of the sows that still had an inverted teat at farrowing terminated all of the observed nursings by assuming
a sternal position: 15% before milk let-down, 15% before postejection massage, and 70% of the nursings after
a short postejection massage. After euthanization of the
2 piglets suckling the inverted teats, the sows seemed
less interrupted, accepted complete nursings and no further fighting among the piglets was noted on the video
tape. One sow had an extra teat in between teat pair 3
and 4. The videotape showed that the piglets tried to
nurse this teat the first 2 d p.p. Thereafter, the teat un-
derwent involution, and no further interaction was seen.
Piglets that nursed normal teats kept the fidelity in 82%
of the video tape observations as compared to the piglets
that nursed protruded teats that kept the fidelity in 26%
of the cases (P < 0.0001).
Piglet Weight
Of 123 born piglets, 94 piglets (40 males and 54 females) were included in the statistical analyzes of weight
(59 piglets suckled normal teats, 32 piglets suckled protruded teats, and 3 piglets suckled small teats). There was
no significant difference in weight gain during the first 4
wk p.p. between treated piglets and healthy piglets and
therefore the treated piglets were also included in the analysis. The average live weight at birth was 1.4 kg (range
0.58–1.95 kg). The average weight at weaning was 8.1 kg
(range 3.2–13.8). Within the litters with 2 or more protruded (previously inverted) teats, the difference in weight
between piglets suckling normal and protruded teats varied (Fig. 1). In 7 of the 9 sows, the weaning weight of the
piglets suckling protruded teats (i.e., previously inverted
teats) was numerically lower as compared to the piglets
suckling normal teats, although the difference was not significant. A significant difference in growth rate of 27 g/d
(P = 0.04) was seen between the 2 groups during wk 1 but
not during the following weeks (2–4). The piglets (n = 3)
suckling small teats had a lower weight at weaning (average 4.8 kg) as compared to the piglets suckling normal or
protruded teats. In the pairwise statistical analysis, 23 pairs
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Inverted teats in gilts.
Table 2. The correlation between the individual mammary gland weight at necropsy and the daily weight
gain (DWG) during wk 1 through 4 in piglets suckling
protruded teats (i.e., previously inverted) and in piglets
suckling the corresponding normal teats
Daily
weight gain
DWG 1
DWG 2
DWG 3
DWG 4
Mammary weight
normal teats
–0.11
NS1
0.33
*
0.55
***
0.48
***
Mammary weight
protruded teats
0.34
NS
0.63
***
0.43
*
0.28
NS
1NS
Figure 1. The mean weight of piglets suckling protruded teats was compared to the mean weight of piglets suckling normal teats (given the relative
value 0 kg) within each litter. Litters with 2 or more protruded teats were
included. p.p. = postpartum; D0 = day of birth; D6 = 6 days p.p.; D14 = 14
days p.p.; D21 = 21 days p.p.; D28 = 28 days p.p.
(46 pigs) were included with an average of 2.8 pairs from
each sow (range 1 to 4 pairs). Significant differences in
DWG were seen during wk 1 (P = 0.008) and 2 (P = 0.04)
with 20 g higher average weight gain in the controls as
compared to the piglets suckling protruded teats, but no
difference was noted during wk 3 and 4.
Mammary Glands
The average weight of 88 mammary glands was
594 g (range 283 to 921). For glands of normal teats
the average weight was 597 g (range 326 to 912) and
for glands with protruded teats the average weight was
614 g (range 327 to 921). The average mammary gland
weight in the small teats, 335 g (range 283 to 377), was
lower than for normal and protruded teats. The amount
of mammary gland tissue at necropsy was positively correlated to the average DWG during wk 2 through 4 (r =
0.33, r = 0.55, and r = 0.48, P-value = 0.0192, P-value
< 0.0001, and P-value = 0.0003, n = 57; Table 2) in the
piglets suckling normal teats and during wk 2 through
3 in the piglets suckling protruded teats (r = 0.63 and
r = 0.43, P-value = 0.0001 and P-value = 0.014, n =
32; Table 2). The correlation between mammary gland
weight and teat pair number was negative and weak (r =
–0.06, n = 85, P = 0.05), showing a tendency to heavier
mammary gland weight at the front teat pairs. The average length of the normal teats was 3.9 (SD = 0.75) cm
and for protruded teats it was 3.6 (SD = 0.57) cm.
Histological Examination
The 2 inverted teats were shorter than the normal
teats and the protruded teats and broader than the small
teats. No obvious morphological differences in arrangement of the connective tissue or smooth muscles fas-
= not significant.
*P < 0.05; ***P < 0.001.
cicles or in the presence of vessels in the subcutaneous
tissue was observed between the groups. The elastin
content in the connective tissue was similar in normal
teats, protruded teats, inverted teats, and the small teats.
Focal ulcerations of the stratified squamous epithelium
covering the teat, with infiltration of inflammatory cells
in the adjacent tissue, were present in 5 teats (2 normal,
2 protruded, and 1 inverted). Furthermore, a pyogranulomatous inflammation in the dermis was present in 1
protruded teat and in 1 of the small teats an inflamed
lactiferous duct was observed with infiltration of lymphocytes, neutrophils, and eosinophils.
Discussion
This study indicates the importance of an adequate number of well-developed and well-functioning teats in sows for
the growth and development of the piglets. Furthermore, the
results emphasize the complexity of the interaction between
the nursing sow and her piglets. To our knowledge, this is
the first study that investigates the relationship between piglet growth and the presence of inverted teats.
Since pigs with inverted teats are usually culled before
mating, there is no information on the occurrence or frequency of inverted or protruded teats in sows. According
to the literature, the inverted teat can sometimes protrude
at farrowing (Nordby, 1934) but in the present study, most
of the inverted teats protruded already in early gestation.
However, at 100 kg live weight, it was not possible to
predict whether the teats would protrude before farrowing
or not. If the teat is still inverted at farrowing, it is of no
value to the suckling piglets. Furthermore, the small teats
clearly had less corresponding mammary tissue and the
piglet growth rate was also lower. One of the sows with an
inverted teat at farrowing terminated all her nursings during early lactation. This might indicate that the sow was
disturbed by the fighting and the shrill screaming, since
Wallenbeck et al. (2008) demonstrated that sows usually
only terminate 30% of the nursings in early lactation.
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Chalkias et al.
There was no significant difference in growth rate
between the entire control group and the piglets suckling protruded teats. However, in the pairwise analysis
of piglets with similar birth weight, differences were
noted during wk 1 and 2. It is possible, that the older
piglets compensated for a poor access to milk, either by
additional suckling attempts in between nursings or by
eating creep feed. Since we lack 24-h video recordings,
these differences were not possible to further analyze.
The considerable variation in birth weight (Milligan et
al., 2001) and thereby the high SD in piglet weight further argue for a careful interpretation of the results.
In the present study, the fidelity to the teat was higher among piglets nursing normal teats and thereby at the
anterior and posterior parts of the udder, as also shown
by Fraser and Thomsson (1986). In line with Skok and
Škorjanc (2013), the teat fidelity in the present study
was established at the last in the middle part of the udder.
This could indicate that the piglets were less satisfied
with the teats near the umbilicus or that these teats may
have been harder to access. Fights occur among the piglets to access the functional teats and to keep teat fidelity
and with increased litter size the sibling competition and
piglet mortality also increases (Andersen et al., 2011).
De Passillé et al. (1988) further showed that piglets that
were frequently fighting during nursing were less prone
to keep the teat fidelity. Interestingly, lesions were also
more common in the small and protruded teats than in
the normal teats. Observations made on the sow with
the extra teat as well as of 1 sow with an inverted teat
showed that the piglets spent unavailing time and energy
trying to suckle these teats. However, in most studies on
nursing behavior, the quality of the teats and the corresponding mammary glands have not been validated, thus
disregarding the fact that some nursing behavior may be
related to the quality of the teats and mammary glands.
The difference in the mammary gland weight between normal and protruded teats at slaughter was not
significant. Furthermore, the length of the teat was similar and no morphological differences between protruded teats and normal teats were observed by histology.
Experimentally, a proliferation of capillaries and fibroblasts in the subcutaneous tissue has been described following distraction of the inverted teats, but the changes
reverted after 4 wk (Zhou et al., 2012). No such alterations were observed in the present study. It may still be
possible that the teat has successively protruded during
the suckling period, thereby affecting the suckling of
these teats during the first 2 wk. In agreement with previous studies (Kim et al., 2000; Nielsen et al., 2001), a
positive correlation was found between mammary gland
weight and piglet DWG. Milk production is at a maximum in wk 3 through 4 (Hurley, 2001; Hansen et al.,
2012) but already in the second week p.p. it will become
a limiting factor for piglet growth especially in large litters (Auldist et al., 1998), although the milk yield may
increase by a frequent nursing (Auldist et al., 2000). The
low but significant correlation between mammary gland
weight and the position of the teat pair is in agreement
Nielsen et al. (2001). According to Orihuela and Solano
(1995), the presence of heavier glands at the anterior part
of the udder may also depend on the piglet birth weight,
since heavier piglets may massage the udder more vigorously, achieve a greater blood flow, and thereby increase
the oxytocin release (Fraser, 1984; King et al., 1997).
Poor udder health is an important cause of the sow removal in commercial herds (Engblom et al., 2007), thus
emphasizing the importance of the sow’s lactating performance. Furthermore, our results clearly demonstrated
the importance of the health status of the sows especially
during lactation.
During the period 1992 through 2012, the mean total number of teats increased from 14.4 to 14.6 in the
Swedish Yorkshire sow. Parallel with the selection for increased litter size, 14 functional teats has been the threshold for further selection since the 1980s. This threshold
selection might have increased the number of functional
teats but not the total number. Furthermore, a genetic correlation between the litter size and the number of teats has
not been proven and thus an increase of the litter size will
not automatically increase the number of teats (Zhang et
al., 2000; Lundeheim et al., 2013). Number of nonfunctional teats in gilts at 100 kg live weight is a heritable trait
(Chalkias et al., 2013) but figures on the heritability of
inverted teats in the Swedish Yorkshire is lacking. Long
et al. (2010) estimated the heritability of the number of inverted teats in the Norwegian Landrace to 0.3 in gilts and
slightly lower in boars. Therefore, it is adequate to emphasize the importance of this trait in the breeding herds
and consequently to cull the animals with inverted teats,
to avoid the risk of accumulation in the population.
Inverted teat is a complex trait. The phenotype may
occur in various teat pairs, at varying frequencies, and to
different degrees. The development of the mammary and
teat tissue in pigs occurs at 3 main stages, regulated by
hormones and growth factors: embryonic, pubertal, and
adult (Mikkola and Millar, 2006; Watson and Khaled,
2008). At 3 mo of age, the mammary gland and the teat
have an increased phase of development and at mating,
the mammary gland is small but with a well-developed
ductal system. The breeding organizations register the
teat status in the gilts at an approximate weigh of 100 kg
at about 5 mo of age. The last major development takes
place during the last month of gestation. In the present
study, some of the inverted teats had protruded already at
the time of mating, and most had protruded in early gestation. The mammary growth is regulated by hormones
such as estrogen, both indirectly through the act of other
Inverted teats in gilts.
hormones and directly on the mammary epithelial cells
(Zhang et al., 2002; Robinson, 2007). In addition, diets
based on, for example, soybeans contain estrogenic compounds and may also contribute to the mammary gland
development and may in addition also affect the fetus.
It is possible that the protrusion of the teat is depending
on the development of the mesenchyme during fetal life
and, later, on the development of the tissue in the single
mammary gland (Thomsen et al., 2006; Latendresse et al.,
2009). The teat will thus not protrude until the gland tissue has achieved a certain level of development and a less
developed gland might also result in a lower weight gain
in the piglet suckling that particular gland.
Conclusion
This study clearly shows the importance of an adequate number of well-developed and well-functioning
teats in sows for the growth and development of the piglets but also emphasizes the complexity of the interaction
between the nursing sow and its piglets. The registration
of teats at 5 mo of age will not predict the function at nursing, but it is important to emphasize the importance of this
trait in the breeding and exclude animals with inverted
teats to avoid the risk of accumulation in the population.
Furthermore, more knowledge about the teat development
and the mechanisms behind the development of inverted
teats, including the genetic background, is necessary.
Acknowledgments
This study was funded by the Swedish Farmers’ Foundation for
Agricultural Research, Foundation of Swedish Pig Research, and
Axel Adler’s Foundation. The authors thank the personnel at research
clinic and are grateful for the access to the herds.
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