In a preliminary experiment carried out in Essex sows, three treatments were compared with a control group of lactating sows. The treatments were: (1) intermittent separation of the sow from the young, (2) injection of pregnant mare serum gonadotrophin (pmsg), and (3) a combination of treatments 1 and 2.
None of the control sows exhibited oestrus during lactation. Of the oestrus-induction methods used, only the combination of separation and pmsg injection induced oestrus consistently.
In an extension of the preliminary experiment, the combined treatment was tested in Essex, Large White and Large White × Landrace sows. Of forty sows treated overall, thirty-three exhibited oestrus during lactation and twenty-four carried pregnancies to term as a result of mating at the induced oestrus. The mean interval between parturitions was reduced by approximately 25 days in sows becoming pregnant during lactation. There appeared to be no detrimental effects of 2 or 3 weeks of concurrent lactation on pregnancies established during lactation.
The results are discussed in relation to the possible modes of action of separation and pmsg injection in the lactating sow.