Vet Column 10-4-10
Fort Collins, Colo.
Intensive management systems for sheep flocks often includes programs to achieve more than one lambing per ewe each year. Because sheep are seasonal breeders, accelerated lambing approaches are generally more effective for breeds showing less seasonality in their reproduction. The main factor in modulating seasonal reproduction in sheep is the alternation between long days and short days.
Variations in previous studies involving photoperiod effects on sheep reproduction include the use of artificial schedules of long days and short days to induce sexual activity in the nonbreeding season. Other studies have looked at applying continuous alternating periods of long days and short days to induce heat in ewes at various times of the year. Results from these studies showed variations or declines in the fertility rate with advancing reproductive cycles.
Investigators from Canada and France have recently published results from a study intended to assess the reproductive performance of sheep subjected to an annual photoperiod program based on alternating four-month sequences of long days and short days continuously in an accelerated lambing system. They also looked at the consistency of results over three reproductive cycles and effects of season.
This study used 248 Rideau Arcott ewes, a very prolific breed developed in Canada, and 12 rams (six Rideau Arcott, two Texel, two Suffolk, and two Dorset). At the beginning of the experiment, the average age of ewes was 14 to 26 months-of-age. All animals were exposed to natural variations in day length prior to the start of the experiment. The study was conducted near Quebec City, Canada.
All sheep were kept in total confinement. In one section of the barn (insulated, windowless), animals were exposed to a fixed long day sequence of 16 hours of light per day using incandescent bulbs. In a second section of the barn (insulated, windowless), the light exposure was set to provide short day sequence of eight hours of light per day. Control ewes were maintained under natural day length, and housed in an insulated barn near the building used for photoperiodic treatments. Control ewes were treated with intravaginal sponges in the out-of-season breeding period.
All ewes were managed to achieve three lambings in a two year period with mating planned every eight months. Ram:ewe ratios of 1:15 and 1:20 were used in this study. Pregnancy status was determined 70 to 75 days after ram introduction and assessed by abdominal ultrasound.
Results of this study indicate that use of continuous, alternating four months periods of long days and short days was effective. The time from ram introduction and conception for the groups exposed to artificial photoperiod was 9.4 days. For the 12 breeding periods studied (8 out-of-season and 4 in-season), fertility rate of the ewes treated with photoperiod, mated at various times of the year, was 91.6 percent.
The number of lambs born per ewe remained constant across reproductive cycles and was greater in photoperiodic treated groups, averaging 2.81 lambs per ewe for photoperiod groups and 2.27 for control ewes. For out-of-season breedings, photoperiod treated ewes averaged 91.1 percent fertility rate compared with control ewes of 76.3 percent fertility rate.
Ewes managed under the photoperiod regimen produced 1.38 lambings per year and 69 percent lambed three times in two years. Overall, the ewes in the photoperiodic treatment produced 3.78 lambs per ewe annually.