[The effect of light intensity and the length of time under natural daylight or artificial lighting on the seasonal fertility rhythm of swine].

In 7 herds half of the sows of each service area have been illuminated, additionally to the natural daylight, with artificial light during 2 years (year 1: 100 1 x; year 2: 300 1 x). From 2965 services parameters of fertility of the natural and artificial light groups have been recorded and compared within and between herds as well as seasons. Seasonal variations of fertility, as reported by other authors, have been confirmed under natural and artificial light. Generally the results under additional artificial light were better in all seasons. The duration of daylight seems to be one of the main reasons for seasonal variations of fertility. During the period of April, May and June (increasing daylength from 13 h to 17 h, but moderate temperatures from 13 degrees C to 22 degrees C) fertility was most depressed, followed by the period of July, August and September (decreasing daylength from 17 h to 12 h, temperatures from 24 degrees C to 22 degrees C). The best fertility has been recorded during winter (October to March, less than 12 h daylength). In addition to increased light intensity a period of 12 h artificial lighting appears to be favourable in winter with less than 12 h natural daylength, but it is not effective during summer with daylengths longer than 12 h. In respect to these results reducing natural daylength to 12 h during summer maintaining sufficient light intensity might be useful.