An interaction between feeding rate and season affects fertility of sows

Trials were conducted in two piggeries in the summer-autumn and winter-spring periods to determine the effects of rate of feeding and type of housing (group or individual stalls) on farrowing rate. Sows were fed a low (1.6–2.0 kg per sow per day), moderate (2.5 kg per sow per day) or high (> 3.6 kg per sow per day) rate for the first 4 weeks postmating and from then until farrowing fed at a moderate (2.5 or 3.2 kg per sow per day) rate. The low level of feeding in the summer-autumn was associated with increased numbers of delayed returns to oestrus and low farrowing rate (< 75%) typical of the infertility that occurs at this time of the year. This same rate of feeding in winter-spring was associated with few delayed returns to oestrus and a high farrowing rate (> 85%). Feeding sows at the high rate during the summer-autumn significantly improved the farrowing rate reducing the adverse effect of season. Litter size was not affected by feeding rate during early pregnancy. The seasonal effect on farrowing rate was not evident in individually stalled sows fed a moderate level during pregnancy. This study demonstrated effects on farrowing rate of interactions between rate of feeding and season and type of accommodation and season. Current practice restricting feed intake postmating, aimed at reducing embryonic mortality and increasing litter numbers, appeared to contribute substantially to the decrease in farrowing rate commonly observed in summer-autumn. The optimal level of feeding to minimise the effect of season on fertility has yet to be defined. Accommodation of sows in individual stalls, rather than in groups, removed the seasonal effect on fertility.