Relationship between age and body weight at farrowing over 6 parities in Large White × Landrace sow

At the beginning of the gestation, parity and back fat (BT) thickness are frequently used by farmers to choose among different feeding plans the most adapted one to feed each sow if individual feeding is possible. Otherwise, BT is used to allocate the sows to one of the available pens and to adapt the feed allowance at the group level. Usually a single target of BT at farrowing is retained at the herd scale, and each sow is expected to make up its BT for the difference between the target and its own initial BT. According to the factorial approach used to assess nutrient requirements, achieving an expected BT gain implies that enough energy is supplied above maintenance. Based on equation published by Dourmad et al., expected energy retention is obtained by the difference between the initial (calculated from measured BT and BW) and the final amount of energy (calculated from expected BT and BW). When no information is collected on BW during the gestation, the expected final BW is also used to assess daily BW and corresponding maintenance requirement and to take into account impacts of housing conditions (temperature, activity level). Then, adequacy of the energy supply depends not only on initial and final BT, but also on initial BW and final BW. Individual BW have been collected over successive parities in the IFIP facilities (n=6,288 from Large White × Landrace sows born since 2000). Data of 90 sows born between 2012 and 2015, studied over at least 6 parities and group-housed from the 28th to the 108th day of gestation, were used to characterize the relationship between age and BW after farrowing based on BW at the first farrowing (BW_P1) and BW gain afterwards, with BW at the 6th parity considered as the mature BW: BWi(Age), kg = 145.6 + 0.171 × Age_P1i+ 111.4 × (1 – exp(- 1.453/1000 × (Age-Age_P1i)1.084)), RMSEP=16 kg; with Age_P1i: the individual age at the first farrowing used to adjust the mean BW_P1. Based on this equation, the individual expected BW gain (both growth and recovering parts) can be calculated and, combined with BT gain, used to assess the corresponding energy requirement for precision feeding during gestation. Its calibration for other lines or farms will require specific measurements due to interactions with management.