Your aim should be to start boar contact to stimulate puberty in the gilts so that 85-90% of them are already cycling by the time mating starts at your chosen 'mating window' (usually 30-34 weeks of age). The vast majority of gilt matings in this window will occur at second heat. To do this effectively the rules are simple:
One of the reasons many producers fail here is because of the need to have boars in the same pen as the gilts - i.e. you have to supervise every exposure period if you don't want gilts bred at puberty.
A common problem we see with this system when put into practice is that cyclicity is often assumed, based on vulval changes, when in fact many gilts are non-cycling. In essence, as gilts approach puberty their ovaries start responding to stimuli such as boar exposure by growing follicles. These will start to put out the hormone oestrogen that causes the gilt to show some vulval reddening & swelling.
These signs of heat are often assumed to mean the gilt is now cycling, resulting in her transfer to the 'ready to breed' group. In this case when she is then mated she's likely to be on her pubertal heat, which will yield lower farrowing rates & smaller litters. The answer here is, of course, to not assume a gilt has started cycling until you have detected a standing heat.
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