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
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 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.
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.
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