Effect of photoperiod on reproductive activity and hair in mares.

Abstract: The effects of photoperiod on reproductive activity and hair changes in pony mares were studied in 2 experiments. In experiment I, the effect of a fixed daily photoperiod on the onset of the breeding season was studied in 36 mares from Nov 13, 1973, to June 13, 1974. The 4 treatment groups were as follows: daily photoperiod equivalent to the normal day length (control group); constant light 24 hours a day with no dark (L24:D0 group); 16-hour daily photoperiod with 8 hours of dark (L16:D8 group); and 9-hour daily photoperiod with 15 hours of dark (L9:D15 group). The intervals from beginning of experiment to 1st ovulation of breeding season, to shedding of hair in tufts, and to appearance of a smooth coat were shorter (P less than 0.05) for L16:D8 group (107.1 +/- 11.1, 56.0 +/- 0, and 145.8 +/- 4.0 days, respectively) than for control, L24:D0, and L9:D15 groups and were shorter (P less than 0.05) for L24:D0 group (less than 156.1 +/- 12.2, 99.5 +/- 9.5, and 173.9 +/- 9.9 days, respectively) than for control group (192.1 +/- 3.3, 134.9 +/- 8.9, and 205.0 +/- 0 days, respectively) or L9:D15 group (less than 200.3 +/- 5,8, 150.6 +/- 12.9, and 201.7 +/- 3.3 days, respectively). These intervals were not significantly different between the control group and the L9:D15 group, but fewer (P less than 0.05) mares in the L9:D15 group had at least 1 ovulation by termination of the project. In experiment II, the effect of photoperiod on onset of anestrus was studied in 3 groups of 7 mares each. Mares in group A, as part of a previous experiment, were induced to enter the breeding season earlier than normal by a gradual increase in daily photoperiod beginning on Oct 13, 1972. From Feb 16, 1973, to June 22, 1973, group A mares were maintained at a fixed daily photoperiod of 15 hours 23 minutes. Mares in group B, as part of a previous experiment, were kept under environmental conditions simulating normal conditions in southern Wisconsin. On June 22, 1973 (beginning of the present experiment), the following treatments began: groups A and B were exposed to natural day length. In addition, 7 mares (group C) were allotted from a band of mares that had been exposed to natural day length and were exposed to 15-hour 23-minute daily photoperiod from the beginning of the present experiment (June 22, 1973) to the end (June 22, 1974). The interval to onset of anestrus was longer (P less than 0.05) for group C mares (234.6 +/- 35 days) than for group B mares (133.6 +/- 16.5 days). Significant difference did not exist between group A (144.0 +/- 45.9 days) and group B. A fixed daily photoperiod of 16 or 24 hours induced early onset of the breeding season and early shedding of hair, with development of a smooth coat. A photoperiod of 9 hours retarded the onset of the breeding season. Mares induced to begin the breeding season earlier than normal did not become anestrous earlier than normal. Mares kept on a long daily photoperiod in the fall became anestrous later than normal.