Photoperiodicity in Suffolk sheep

Abstract

1. A gradually decreasing plane of light and increasing plane of darkness is not an essential factor for stimulating the onset of oestrus in sheep. 2. Sheep being short-day breeding animals, a standard and regularly maintained rhythm of short-light and long-dark will stimulate the onset of oestrus. 3. The terms short and long are used in the relative sense only, since their significance is solely a means of supplying the necessary contrast impulse to the pituitary gland. 4. A ratio of 1 part of light to 2 parts or more of dark is sufficient to supply the contrast effect. 5. Oestrous cycles induced by artificial light rhythms appear to be normal in all respects, con forming to the normal intervals between heat periods and associated with the ovulation of normal ova. 6. The anoestrous period varies in depth, and sheep which have just entered it may be brought back into oestrus very much faster than animals which are in deep anoestrum. 7. The milk yield of lactating ewes does not appear to be unduly depressed by the onset of oestrous periods induced by an artificial light-dark rhythm.