Pig Health - Gilt Infertility
Mark White BVSc LLB DPM MRCVS
In a normal year, an established pig breeding herd will
replace 40% or more of the sows with gilts.
This means that at any given time, gilts will make up
15%+ of the servings and, thus, any problems with gilt
fertility will have a significant impact on herd
production. This effect will exacerbate the lower litter
size that would normally be expected from gilts
compared to the sow herd.
The following is a resumé of some of the more
common faults and problems that occur with gilts that
lead to low production and poor fertility:1) Inadequate health monitoring of stock, integration
and acclimatisation. There are a whole range of
diseases to which incoming gilts may be naïve. If the
gilt meets a disease challenge in the immediate period
prior to or around service, it is likely to disrupt breeding
capacity potentially for life. Serious pneumonia around
puberty can stop gilts cycling, in some cases for life. Parvovirus and Erysipelas can be specific reproductive
diseases and are seen in gilts that are inadequately
vaccinated. PRRS (Blue Ear Disease) can have a
similar effect.
Producers should liaise closely with their veterinary
surgeons to ensure that incoming stock are of known
health status, that it is compatible with the recipient
farm, that vaccinations are appropriate and correctly
administered and that suitable acclimatisation
procedures are followed. Isolation on entry is essential
to protect the farm from initially unknown infectious
agents that gilts may be carrying. Bear in mind that all
diseases have an incubation period and it is always
possible that gilts will be incubating disease on arrival
which at that time is unknown on the parent farm.
It should also be borne in mind that many enzootic
infections have highly varying serotypes and whilst the
headline health status of the supplying farm may match
the recipient it is always possible that incoming stock
will carry a different serotype eg of E coli, Rotavirus,
Haemophilus parasuis etc that will upset the
equilibrium of the farm.
Furthermore, introduction of new animals can lead to
multiplication of organisms in those gilts - when they
meet the host farms bug population - which then acts
to destabilise the health balance of the recipient farm.
This challenge may or may not adversely affect the
gilts themselves but is likely to upset the whole health
dynamic of the farm. Thus, matching and introducing
replacement stock has a lot more considerations than
simply "is the supply farm positive or negative for
disease X"
2) Age and weight at service. As the trend over the
last 30 years has been for leaner and leaner slaughter
pigs, the mature size of the adult has increased. The
gilt at any given age and weight is now less mature
than it was and this can have effects on fertility and
longevity. F1 hybrid gilts should be at least 7 months
old and weigh a minimum of 125kg amd preferably
near 135kg at service. This will normally coincide with
the third and fourth heat period. Current thinking
suggests that weight is more important than age per se
and that the second observed heat is the earliest at
which a gilt should be served
3) Where natural service is practised, gilts tend to be
served with young boars, which themselves may have
lower fertility levels. This problem can be overcome by
cross serving - particularly using older (though not too
large) boars or with AI, although some stockmen have
considerable difficulty "locking in" a catheter in gilts. Only competent and experienced stockmen should
undertake AI in gilts and the process can be assisted
by ensuring that the gilt has had a natural service - be
that by a normal working boar or a vasectomised boar prior to insemination i.e. use AI as the top up service.
The vasectomised boar provides a number of benefits
including
-
Stimulating gilts to come on heat
-
Synchronising gilts to come on heat
-
Acting as a primer by actually serving a gilt with non
- fertile semen
-
Not allowing pregnancy to arise from
non-supervised service that would occur with entire
teaser boars.
4) Timing of Service. Weaned sows will come on heat
at a predictable time. Maiden gilts do not. It is a good
idea to observe and record all heats in gilts from arrival
to provide an indication as to when to expect a service
heat. Batches of gilts should be checked for heat at
least once per day with a boar. Serving late in the heat
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period risks introducing ascending infection and
is a common cause of low litter size. The use of
hormonal control techniques to synchronise heat
in batches of gilts and to improve predictability
of timing of heat periods is now widespread
particularly in farms applying batch serving
and farrowing programmes. Administration of
progestagens orally for an 18 day period
prevents the gilt coming on heat but then allows a
predictable heat 3-5days after withdrawal. It is
vital when using such techniques that each gilt
receives the correct daily dose (over and
under-dosing will both lead to compromise of the
natural hormonal physiology) and that each gilt
was actually cycling when treatment is given.
5) Discharges. Occasionally, maiden gilts are seen
to discharge when on heat. It is important to
differentiate a normal oestrous discharge - clear or
slightly milky mucous - from a purulent pathological
discharge. There is little known about the
development of the latter but, in most cases, may
be the result of unsupervised service (even in a
bacon house) or environmental contamination in
poor conditions. Rarely, the problem will be seen in
groups of gilts in clean conditions where no boar
contact has occurred. Such cases should be fully
investigated with laboratory tests; recent reports
from the US has implicated Erysipelas as a cause
of such problems. There is also the possible role of
Leptospira australis group infection or possibly
Chlamydophila infection although these agents are
more likely to be associated with pregnancy failure
with a discharge 2-3 weeks or more after service.
Extensive diagnostic work is required when such
pictures are presented.
where present in a farm, gilts should be
vaccinated with a live vaccine prior to
introduction to the main herd. The vaccination
programme will be part of that applied to the
whole herd and will be determined and advised
by the veterinary surgeon who will tailor the plan
to the specific herd requirement.
-
Porcine Circovirus Type 2 (PCV2)- the cause of
wasting and immune compromise in young
growing pigs has been shown to cause
reproductive failure (abortions, irregular returns
and mummification) particularly in gilts. The
majority of pigs produced will have been
vaccinated as babies on their farm of origin but
the impact of such doses is likely to have faded
by the time gilts are due to breed. It is now
common practice to give gilts a booster dose
prior to breeding. Again the veterinary surgeon
will advise.
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6) Specific reproductive viruses. Three specific
viruses are now enzootically established in many
UK pig breeding farms and it is important that gilts
are fully immune to these prior to service. A fourth Aujeszky's Disease - was eradicated from GB in the
mid 1980's (and more recently from NI) and does
not concern us here.
-
Porcine Parvovirus (PPV) - the most important
cause of the SMEDI complex and particularly a
risk to gilts. Commercial vaccines have been
available for 30 years and are highly efficacious
when given at least 2 weeks prior to service to
allow immunity to develop.
-
Porcine Reproductive and Respiratory
Syndrome (PRRS)- a widespread disease and
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