I am sure you have seen the interesting report on age at first service by JSR Genetics revealing that age at first service – in terms of numbers born alive and returns to service – takes precedence over bodyweight and fat P2 level. They claim that 240 days old shows the best response and is most cost effective, with bodyweight and fat cover far less important within quite wide boundaries.
For the past ten years best opinion has been not to grow the gilt too rapidly between final selection, say at 100 kg at 180 days old, and first service at 225/230 days at 125/130 kg. This six week period was said to be a sufficiently-long induction period to strengthen her immune competence, which is still a desirable factor today.
240 days extends this by ten to 14 days, which from the immunocompetence viewpoint is probably beneficial as it allows more time for a 14-day challenge period and then a 28 to 35 day fortification period which the specialist pig vets advise. I strongly suspect that this two-stage induction protocol helps boost immunity to PRRS, as those farmers who have adopted it seem less plagued by this troublesome disease.
My suggested weight for age table based on 240 days is given below.
Table 1. Gilts: Suggested typical weights for age for modern high lean gain European breeds. Gilt growth rate = Aim to achieve 100 kg in 170-180 days, gilt growth rate at 550 g/day, rising to 600 g/day towards puberty.*
||25th or 26th week
||6.5 months + old
||7 months old
||8 months old
* Consult your seedstock supplier for actual targets
But what about cost?
Of course this will cost more, especially when a gilt developer diet is used, as it should be for these latest hyperprolific gilts.
Higher numbers born
But when you take into account higher numbers born alive and fewer returns at first litter as well as less of a second-litter fallaway and a longer SPL (Sow Productive Life) then the extra cost is repaid fivefold (Gadd, in press). A gilt developer diet is quite different to the conventional grower diet many producers are still using to grow-on their replacement gilts.
A good pig nutritionist can design one on a daily intake basis to achieve from selection at 100 kg to the 240 day first-service date, in cooperation with the seedstock house you have chosen. Important as genotypes do differ between lines.
Thus my weight suggestions in Table 1 might vary (usually downwards) a little according to the 'brand' of gilt you have chosen and the gilt developer diet and daily feed scale designed for them.
Sounds complex, but with the right advice it is simple enough as it looks as if we need to worry less, within the bounds of common sense of course, about weight and fat cover in future.