Analysis of factors affecting reproductive performance (parameters of reproduction and natality, SP interval) - year, season, parity, weather characteristics at A.I., sow's father, mated boar - was the principal objective of the study. Anderson and Melampy (1972) found that the number of piglets born increased between the first and fourth litters; as related to age, a decline of litter size was evident after about 4.5 years. Britt et al. (1983) noted that although sows of all parities were affected, postweaning anoestrus was more prevalent in primiparous sows during the summer and autumn seasons. It is known that seasonal variations' in response to exogenous estrogens may occur in pigs (Cox et al., 1987) and this may be a reason for the differing seasonal responses noted in weaned sows. Daylength is another important factor related to the seasonal factor. Kermabon et al. (1995) examined effects of daylength on gonadotropin profiles in lactating sows. Weather conditions (environmental temperature) affect ova fertilization. Huhn et al. (1995) analysed results of 25 215 first inseminations carried out over a S-year period. The highest number of piglets born per 100 inseminations occurred for maximum daily temperatures of 10-15 degrees C; the lowest for temperatures higher than 30 degrees C. The following reproductive parameters (related to litter size) were controlled and analysed in the breeding and commercial herd of ZOD Zichlinek in the period 1990-1997: born piglets in total, mortality (stillborn piglets), piglets born alive. Data characterizing 4 881 sows - Landrace (L) x White purebred (BU) crosses - were compiled and classified. The following factors were studied: year, season, weather characteristics at A.I., morning temperature at A.I., lunar phase, sow's father, mated boar, parity. The following values characterizing litter size were found: born piglets in total: 10.20 +/- 2.76; nu = 27.05 - Table 2; piglets born alive: 8.83 +/- 2.43; nu = 27.49 - Table 2. As for the mentioned litter size parameters, differences between individual effects related to year, season, temperature at A.I., sow's father, and boar were highly significant (P < 0.01) and significant (P < 0.05) in two cases. Differences between effects related to weather characteristics, lunar phase at A.I. and parity were nonsignificant (P > 0.05) - Table 1. The highest total number of born piglets and the highest proportion of piglets born alive were recorded in the period September-April, the lowest in the period May-August. November and December are the optimum months for farrowing. Sow's father and mated boar factors had significant (P < 0.05) or highly significant (P < 0.01) effects on litter size parameters. The same herd and the same population of crossbred (successfully inseminated) sows (n = 2 246) was used for studying effects of year, season, parity, weather characteristics at A.I., morning temperature at A.I., lunar phase at A.I. on SP interval (farrowing-conception interval). Mean SP interval 43.37 +/- 18.95 days (coefficient of variation 45.78%) was found (Table 6). SP was shorter in the period March-August (36.83-41.32 days) than SP in the period September-February (42.21-56.05 days) - Table 7. Highly significant differences (P < 0.01) between effects were found in the case of temperature at A.I. Any tendency indicating significant effects of temperature (temperature range) on SP was not found. The longest SP was associated with the first farrowing (67.24 days), a gradual decrease within 38.98 days (8th farrowing) was noted - Table. 8.
