RETURN TO ESTRUS AFTER FIRST-INSEMINATION IN SOW HERDS (INCIDENCE, SEASONALITY, AND ASSOCIATION WITH REPRODUCTIVITY AND SOME BLOOD PARAMETERS)

As no systematic study has been done to get an accurate estimate of th e incidence of return to oestrus after first insemination in sows in t he Netherlands, the objectives of this investigation were: 1) to obtai n an estimate of the incidence of return to oestrus after insemination at the herd level; 2) to investigate the association between incidenc e of return to oestrus after first insemination and reproduction chara cteristics in order to get an impression of the economic importance of reproductive failure. These objectives were investigated by using the reproduction results of 240 swine breeding herds in the Southern Neth erlands in 1987. This information was obtained from CBK plus computeri zed herd management records. The average incidence rate of return to o estrus after first insemination at a herd level was 16.9 per 100 first inseminations. The occurrence of return to oestrus after first insemi nation was distinctly higher in the insemination months July and Augus t compared to the rest of the year. An increased incidence, with 10 re turns per 100 first inseminations corrected for confounders in a multi ple linear regression model, was associated with a decrease of approxi mately 0.3 live born piglets/sow/year. A prospective longitudinal stud y was started in 1988 and 1989 in 37 sow herds. Individual sows were m onitored from weaning to first insemination, to the occurrence of retu rn to oestrus, or not, after first insemination, and to farrowing. The investigation focused in particular on the relationship between retur n to oestrus after first insemination and seroconversion against porci ne parvovirus (PPV) and Leptospira interrogans serovar bratislava (L. bratislava). During a number of consecutive farm visits, blood samples were taken from sows at weaning and again 6 weeks later. The final da ta set analysed consisted of 161 animals that did not return to oestru s and 158 animals that did return to oestrus after first insemination. Seroconversion was defined as a 4-fold increase in titre between the two successive blood samplings: sera were investigated at dilutions of 1:100 to 1:3200 (L. bratislava) and 1:4 to 1:16384 (PPV). There was n o indication of an association between the occurrence of return to oes trus after first insemination and seroconversion against PPV or L. bra tislavn with the serodiagnostics used in this study. However, it shoul d be noted that L. bratislava infected sows can have a titre below the widely accepted minimum titre of 1:100 used in laboratories, and rece ntly from sows without a titre L. bratislava was isolated. It is sugge sted that the most appropriate way to assess the influence of infectio n with L. bratislava on reproductive failure in swine at this moment i s to use culture or other methods (e.g. PCR). Litter size and between- farrowing time of the last farrowing before the study period was not a ssociated with the occurrence of return to oestrus after first insemin ation in the study period. However, an increased weaning-to-first-inse mination interval, a high progesterone level at weaning, a low albumin concentration at weaning, and a decrease in albumin concentration bet ween the consecutive samplings and an increase in gamma globulin conce ntration between the two blood samplings increased the risk of occurre nce of a regular return to oestrus after first insemination. An increa sed weaning-to-first insemination interval, an increase in Leucocyte a nd gamma globulin concentrations and a reduction in alpha(1,2) globuli n concentration between the consecutive samplings increased the risk o f occurrence of an irregular return to oestrus after first inseminatio n.