Reproductive Physiology of Sheep
C. W. Alliston Animal Sciences department
The reproductive performance of sheep is influenced by a number of factors which act upon the ewe or the ram or both. The factors that influence the ability to reproduce and the number of offspring include:
Hered i ty
The number of offspring varies widely. For example, the Merino in certain areas usually gives birth to only one lamb whereas the Finnish Landrace may average 3 to 4 lambs. This trait is usually most directly associated with the number of ova ovulated by the female.
Puberty
The onset of reproductive activity also varies considerably. Published studies have concluded that the lifetime production is greatest if first breeding is at approximately 18 months of age.
Age of Ewe
Although chronological age is not the sole criterion, ewes of 3 to 5 years of age have been demonstrated to maintain reproduction of a flock at the maximum level. However, ewes should be culled more on the basis of physical vigor, condition of teeth, condition of udder, production record, and so forth, than solely upon chronological age.
Nutri tion
Efficiency of reproduction is adversely affected by either too high or too low a plane of nutrition. Research reported from Purdue has shown that feeding during gestation at the rate of 80 per cent of the energy level recommended
by the National Research Council (NRC) was satisfactory during the last 105 days of gestation. Seven pounds of low moisture alfalfa silage at a cost of 5.5 cents per day met this requirement. During lactation, addition of 0.75 pound of rolled shelled corn successfully supplied 80 per cent of NRC recommendations at a daily cost of 7.6 cents per ration. Bone meal and trace mineralized salt were added to the ration at the rate of 0.04 pound each.
The term "flushing11 is generally used to refer to the practice of increasing the level of nutrition 3 to 4 weeks before breeding to increase the ovulation rate. Although the majority of information indicates that an increase does occur with flushing, a specific recommendation is difficult. Perhaps an increase of 150 to 175 per cent of maintenance requirements is a reasonable level of increase. This might be accomplished either by fresh pasture or by increased feed levels. The greatest increase in ovulation rate has been noted just prior to onset of the breeding season with little or no increase in response to flushing during the peak of the breeding season. Perhaps it should be emphasized that the increased level of nutrition required for "flushing" should not be continued throughout gestation.
Lambing Interval
The 5-month gestation period of the ewe has been the basis of various efforts to accelerate lambing programs to greater than one lamb crop per year. Of such management systems, the most practical appears to be an eight-month lambing interval with lambs born in January, September and May. At Purdue, Rambouillet ewes
Cooperative Extension Service, PURDUE UNIVERSITY, Lafayette, Indiana