Induced estrus and breeding during lactation: Effects on sow and litter
performance
Roy N. Kirkwood DVM, PhD; Philip A. Thacker PhD
RNK: Alberta Pork Research Centre; Alberta Agriculture, Food
and Rural Development, 905 O.S. Longman Building,
6909-116 Street, Edmonton, Alberta, Canada T6H 4P2;
email: kirkwoo@agric.gov.ab.ca;
PAT: Department of Animal and Poultry Science, University of Saskatchewan.
Also available in Adobe Acrobat PDF format
Summary
Objective: To test the effect of pregnancy concurrent with lactation
on piglet growth and sow fertility.
Methods: At 28 days of lactation, sows were either weaned (controls)
or treated with 1000 IU pregnant mare serum gonadotropin (PMSG) and then
weaned at 42 days of lactation. Sows were bred at the first estrus after
weaning or PMSG.
Results: Pregnancy concurrent with lactation did not adversely
affect suckling pig performance. The proportion of sows that achieved estrus
7 days after weaning or PMSG treatment, the mean wean-to-estrus interval,
and subsequent litter size did not differ between treatments. However, farrowing
rate was lower (P<.01) for PMSG-treated sows.
Implications: Pregnancy can be achieved concurrent with lactation.
However, because of its likely adverse effect on sow fertility, it is not
advised for commercial application.
Keywords : sow, lactation, PMSG, estrus, pregnancy
Received: July 22, 1997
Accepted: March 9, 1998
One method for increasing sow prolificacy (i.e.,
pigs per sow per year) is to increase litters per year by reducing the interval
between farrowing and subsequent conception. Although this objective may
be realized by weaning the litter earlier, short lactations are often associated
with reduced sow reproductive performance.1 A potential alternative
to earlier weaning is to induce ovulation and conception concurrently with
lactation. Estrus and ovulation can be induced during lactation by temporarily
separating the sow and litter2 and by group-housing lactating
sows with boars.3 Unfortunately, the reproductive response to
these strategies are variable and unpredictable.
Administering gonadotropins during lactation may induce ovulation more
predictably. However, the response varies with the stage of lactation at
which the sow is treated. Cole, et al.,4 induced ovulation in
96% of sows that received pregnant mare serum gonadotropin (PMSG) from 40
days postpartum (PP) but few sows ovulated when injected prior to day 40
PP. Other investigators have confirmed that the stage of lactation affects
the ovulatory response to exogenous
gonadotropins.5-7
Previous experiments investigating the effects of pregnancy concurrent
with lactation have ignored the potential for an extended lactation. However,
if pregnancy can be successfully established during lactation without detriment
to sow prolificacy or litter performance, it may allow a longer lactation
without prolonging the interval between farr
realized, it could satisfy possible welfare concerns regarding short lactations,
while continuing to allow maximum sow performance. The present study was
undertaken to investigate the feasibility of inducing pregnancy concurrent
with lactation and its effects on the growth of the suckled litter and subsequent
sow reproductive performance.
Materials and methods
Fifty-four Yorkshire x Landrace sows of mixed parity were allocated equally
by parity, litter size, and litter weight at day 28 of lactation to one
of two treatment groups:
- a PMSG group that was induced to ovulate during lactation by an intramuscular
(IM) injection of 1000 IU PMSG (Equinex(TM), Ayerst Laboratories, Montreal,
Quebec, Canada). The litters of these sows were weaned at 42 days.
- a control group that received no injection and whose litters were weaned
on day 28.
Nutrition
During lactation, the sows were individually housed in 1.5 m x 2.1 m
(5 foot x 7 foot) farrowing crates mounted over a raised, perforated floor.
The sows were fed ad libitum a barley-wheat-soybean meal diet formulated
to provide 13.1 MJ DE per kg and 16% crude protein (CP). Water was available
ad libitum.
From 10-42 days of age, piglets were allowed access to a commercial creep
feed providing 20% crude protein. For the next 7 days, piglets were fed
a mixture (50:50) of creep feed and a commercial starter feed providing
18% crude protein. Thereafter, piglets received just the starter diet. All
piglets were weighed weekly from days 28-56 of age, inclusively.
Estrus and ovulation
To facilitate estrus detection, PMSG group sows were removed from their
farrowing crates and housed with a boar for 15 minutes per day from 3-7
days after injection. Control sows were housed adjacent to boars after weaning
at 28 days of lactation. All sows were bred by natural mating on the day
estrus was detected and again 24 hours later. Only sows detected in estrus
by 7 days after PMSG injection or weaning were included in the analysis.
A blood sample was obtained by jugular venipuncture from all induced
sows 12 days after PMSG injection. Serum from these samples was assayed
for concentrations of progesterone using a commercial kit (Intermedico,
Markham, Ontario, Canada), as described previously.8 Ovulation
was judged to have occurred if the progesterone concentration was at least
4.5 ng per mL.
Statistical analysis
Weaning-to-estrus interval, farrowing rate, and subsequent litter size
were all compared between treatment groups in this study. Treatment effects
on the percentage of sows bred within 7 days of PMSG injection or weaning,
and the percentage of these mated sows subsequently farrowing (farrowing
rate), were examined by Fisher's exact test using the Number Cruncher Statistical
System (NCSS 1987; N.J. Hintze, Kaysville, Utah). All other sow data were
examined by Student's t-test (NCSS 1987). Data for piglet weights
were subjected to a repeated-measures ANOVA, and differences at specific
weighings were examined by Student's t-test (NCSS 1987).
Results
Weaning-to-estrus interval
The proportion of sows bred by 7 days after weaning did not differ between
the control and the PMSG groups (Figure 1).
Of the 27 control sows, 24 (89%) were detected in estrus and mated by 7
days after weaning. Of the remaining control sows, two were in estrus at
13 days and one at 28 days postweaning. Of the 27 induced sows, 23 (85%)
were detected in estrus and mated within 7 days of PMSG injection. However,
two of the four nonresponding sows were confirmed as having ovulated, because
their serum progesterone concentrations were > 4.5 ng per mL.
Farrowing performance
Of the sows mated by 7 days after PMSG injection or weaning, fewer (P<.01)
of the induced (65.2%) than control (95.8%) sows farrowed. In the induced
sows that were mated but failed to farrow, th
estrus was variable. The observed intervals from initial mating to remating
were 18, 22, 22, 23, 24, 30, and 34 days. In those sows that did farrow,
there was no significant effect of treatment on subsequent litter size (Figure 1).
Piglet performance
The growth of the suckled litter to 56 days of age was not adversely
affected by concurrent pregnancy (Figure 2).
Indeed, mean weights at 35 days of age were greater (P<.01) for
the suckled than the weaned piglets. However, piglet weekly weight gains
from 35-49 days were greater (P<.05 to P<.01) for controls
than for those weaned at 42 days, resulting in similar mean weights at 56
days of age for piglets of both treatment groups (Figure
2).
Discussion
Ovulation
Previous investigators have demonstrated ovulatory responses similar
to those we observed in this study in sows given injections of PMSG and
human chorionic gonadotrophin (hCG) at a comparable stage of lactation.7
However, it is apparent from the present data that a single injection of
PMSG at day 28 of lactation, without a subsequent hCG injection, is an effective
means of inducing estrus and ovulation in sows.
Farrowing performance
Although ovulation has been induced from 7-15 days PP,5,8
subsequent pregnancy rates have been uniformly poor.5-7 Our observations
of lower farrowing rates for PMSG-treated than for control sows suggest
that sow fertility is reduced when females are mated after a hormonally
induced estrus during lactation. Although the cause of the reduction in
fertility is not known, the stage of lactation at which they are mated is
important.5-7
In contrast to our observations, previous investigators have observed
farrowing rates as high as 80% when sows were induced from 25 days of lactation.6,7
However, these authors did not include noninduced control groups and tended
to use very few sows. The pregnancy rates observed in the present study
suggest that at 28 days PP sows were insufficiently recovered from the suppressive
effects of lactation to exhibit acceptable sow reproductive performance.These
data suggest that continuing lactation during early gestation may not adversely
affect the in utero development of the new litter.
Mating to remating intervals
The variable mating-to-remating intervals we observed in PMSG-treated
sows may indicate initial conception with subsequent total embryo loss.
Alternatively, it is possible that accessory corpora lutea may have formed
a few days after mating due to the long half life of PMSG. Such a situation
has been observed in gilts subsequent to PMSG treatment and would account
for a prolonged estrous cycle.9 Regardless of the cause of the
variable length of the estrous cycles, the resultant loss of predictability
for nonconceiving sows presents an obstacle to the routine use of gonadotropins
for estrus induction during lactation.
Piglet growth
The lower weight gains we observed in suckling piglets from 35-42 days
of age may involve pen crowding, but are more likely due to weaned pigs
consuming a creep feed that was more nutrient dense and appropriately balanced
than sow's milk.
Implications
- Ovulation and subsequent pregnancy can be successfully established
in sows after gonadotropin treatment at 28 days of lactation.
- Pregnancy concurrent with lactation did not adversely affect the performance
of the suckled litter or the size of the subsequent litter.
- The lower farrowing rate of induced sows and the potential for unpredictable
returns to estrus indicate that this protocol is unlikely to be commercially
acceptable.
Acknowledgements
We wish to thank the staff of the Prairie Swine Research Centre for their
care and management of the experiment
study was provided by the Province of Saskatchewan Agriculture Development
Fund.
References
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