Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Animal Science

First Advisor

A.L. Slyter


One of the most intriguing aspects of reproduction is the reversible fertility governing seasonal breeding. Sheep are one of the many species that exhibit a circannual rhythm in reproduction, because they start estrous cycles in fall and winter months and become anestrus in spring and summer months. Seasonal changes appear to control this. Ewes seem to be more completely seasonal than are rams because only very limited ovarian activity, fewer follicles, and no ovulation occurs in ewes, although follicular size remains unchanged. Most rams will produce viable semen throughout all seasons, however, Lincoln and Short and Almeida and Lincoln reported that Soay rams showed cyclicity in testicular size and semen quality associated with season of the year or with changes in photoperiod. A major restrict ion to intensive, year round lamb production is the seasonal anestrus exhibited by ewes. Environmental conditions, genetics, and hormonal status are three of the factors that may cue the ewe's system to initiate or cease reproductive activity and are the three main factors that researchers have tried to manipulate in order to achieve out of season breeding. Many environmental factors have been implicated in causing the yearly cycle seen in sheep, including nutrition and temperature, but the factor seeming to exhibit the most control is photoperiod, i.e. a seasonal variation in the length of daylight versus darkness. Photoperiod, in turn, appears to affect hormonal secretion. A hypothesis for the differences seen in lag times during different parts of the year is that the ewe becomes photorefractory, and so late in the breeding season or early in the anestrous season she will respond less readily to short days. Speedy and Owen noted that they could not prolong the breeding season with light if it was applied after the shortest day of the year. It was determined by Robinson and Karsch that transition into the breeding season occurred when daylength was longer than the daylength at the onset of anestrus (11.5 h). From this they assumed that ewes become photorefractory. To test this hypothesis ewes were kept in 10 h light or in natural daylight; both groups reached anestrus at approximately the same time. These authors attributed photorefractoriness to a disruption of the post pineal processing of the photoperiod message.

Library of Congress Subject Headings

Ewes -- Breeding
Ewes -- Reproduction
Sheep -- Breeding
Sheep -- Reproduction



Number of Pages



South Dakota State University


No Copyright - United State