During lactation, certainly up to 4 weeks post farrowing, part of the hormonal control mechanism (principally prolactin) normally acts to prevent the sow coming back into heat - unlike the picture seen in other farmed species.
Following weaning, the surge in energy that occurs as milk production stops trips off a different hormonal pathway centred around insulin and insulin-like growth factor that stimulates heat. The normal aim would be to re-serve the sow 3-7 days after weaning - a period usually regarded as the peak fertile time. Occasionally, sows will come on heat before 3 days - some even standing on the day of weaning. Service at this time tends to lead to sub-optimum results. Conversely, some sows will not show heat for 2-3 weeks after weaning and there is good reason to believe in many of these cases the sow has actually cycled before weaning 3 weeks before they are seen. Problems can occur in both indoor and outdoor herds and, whilst the underlying physiological reasons are the same, the practical events may be quite different.
In general, managemental control will be greater in the indoor farrowed sow, although the environment of the farrowing house does not lend itself to sows demonstrating that they are in heat, particularly if signs are not actually looked for. If sows show a delay in post weaning oestrous, specific observation of late suckling sows is needed, monitoring behaviour and feed intake - lactating sows on heat stop eating in the same way as weaned sows do! In general within a 4 week weaning system, serving a sow on heat during lactation should be avoided.
Circumstances that may lead to lactation oestrous in the indoor sow include:-
1) Death of a high proportion of the litter beyond 7 days post farrowing.
2) Heavy feeding - particularly where multiple feeds per day are given.
3) Split weaning to reduce the work load on the sow - if more than 2 pigs (usually the biggest ones) are removed 7 days early, the drop in demand for milk can provide an energy surge and trigger heat 3-5 days later. Note: routine weaning of piglets below 21 days is forbidden by legislation and then only where certain housing and hygiene conditions are met. Without those conditions pigs should be 28days old at weaning routinely.
4) Multi-suckling in the second half of lactation where a sow is not "pulling her weight" and piglets show a preference to suckle a specific sow in the group.
5) Prolonged lactation - particularly if a sow due for weaning is used for fostering a much younger (i.e. less demanding) litter.
6) Agalactia (loss of milk) for any reason provided appetite is maintained.
7) Provision of milk supplements for piglets such that demand from the sow is reduced.
8) Any other events which lead to drop in demand for milk and hence a surge in energy balance in the sow. However, a diseased sow that is off food and dries up will not tend to come on heat as the energy is used to fight the disease.
The same principles apply in that any activities which lead to a reduction in demand/supply of milk, will provide an energy surge in the sow and can trigger oestrous, particularly if ad lib feeding is practised.
However, the lower level of control of the sow outdoors can make this more of a problem, particularly as stimulation to come on heat may be greater due to proximity or contact with other animals.
1) Mis-mothering including litter desertion and "doubling up" of sows in arcs. Individual farrowing paddocks can reduce the risk of the latter but desertion can still occur particularly in hot weather when the sow leaves the arc to cool off. Conversely "doubling up" which can occur in gilts and sows, is more likely in cold weather.
2) Death of all or part of the litter due to disease, starvation, predation etc. particularly if it occurs later in lactation.
3) Part/split weaning or escapees to other arcs (Fender management is critical here).
4) Close proximity of boars in the service area, particularly if the farrowing paddocks are close and downwind of the boars.
5) Provision of shade and wallows in farrowing areas encouraging sows in warm weather to desert litters.
6) Excessive bedding use in hot weather and arc orientation again encouraging litter desertion.
7) Access to extraneous hormones in vegetation - e.g. oestrogenic chemicals in clover, particularly in the spring.
8) Interruption of water supply (particularly in winter) leading to drying up of the udder.
9) In a group paddock system, a single sow subject to an event which stimulates her to come on heat may act as stimulus for other sows to cycle, even if they have not suffered the same event (e.g. loss of litter). This probably acts in the same way that gilts or sows post weaning in groups seem to trigger heat in each other. In individual farrowing paddocks, problems tend to be less common.
In any system, if sows are showing delay in onset of oestrous and are indeed showing heat prior to or at weaning, then the likelihood is that the primary cause will be related to energy balance - a feature of feed supply and milk demand - encouraged by stimulation from boars and other sows on heat. A sow that cycles outside the window of 3-7days post weaning is likely to be sub fertile and will disrupt normal service programmes.
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