CHAPTER 88 Sheep Breeding Strategies
Sheep are among the most versatile of the livestock species managed today in adaptability to diverse production environments and purveyors of various products throughout the world. Certainly the origin of this diversity evolves from the 400 to 800 different breeds of sheep that exist worldwide. As an example, although most sheep breeds birth 1 to 3 lambs per lambing, unique breeds such as the Booroola Merino exists functionally as litter-bearing ruminants, giving birth to 6 or more offspring per lambing. Compounding this diversity among breeds is the fact that sheep are seasonal short-day breeders, and adherence to or deviation from the short-day breeding activity will vary with the geographic latitude of their origin and subsequent management. Therefore, as advantageous as this diversity may be, it also imposes the liability that one set of recommendations cannot fit all paradigms. The objective with this chapter therefore is to describe the major variables that influence breeding activity and subsequent lambing performance in a manner that allows the informed individual to plan or diagnose schemes for managing breeding strategies that optimize ewe production performance.
CONSIDERATIONS IN SELECTING A BREEDING STRATEGY
Breeding Ewe Lambs
From a producer’s perspective, a major dilemma exists in the decision of which ewe lambs should be kept as replacement females to enter the flock and which ewe lambs should be sold to capture market lamb prices while that “window of opportunity” still exists. Certainly the animal’s genetic merit, production potential, and structural integrity are important variables to consider, but perhaps not as critical, but still significant, are the answers to the following two questions: (1) Will the selected ewe lambs ever achieve reproductive competence—(given that 10% to 20% of ewe lambs may possess some anomaly that prohibits the ability to ever reproduce)? (2) If the ewe lambs are capable of reproducing, when will they exhibit that capability (research has noted that ewe lambs that reach puberty at an early age are more reproductively efficient throughout their lifetime than are ewe lambs that reach puberty at an older age)? The factors that regulate the age at which lambs reach puberty can be categorized as genetic, environmental, and the interactive effects of genetics and environment.

Fig. 88-1 Relationship between age and weight of lambs at puberty that are growing at three different rates.
Nonetheless, once the young dams lamb, it is critically important to feed them adequately to ensure their con tinued growth and development while lactating and beyond. Failure to adequately feed dams that lamb at a young age is one of the most common mistakes made by producers.
Breeding Adult Ewes
A proportion of all ewes will cycle year round (also represented in Fig. 88-3 and Table 88-1) and typically for ewes living closer to the Equator (e.g., Idaho versus Texas), the proportion of ewes cycling year round increases. In contrast, it should be noted that relocation of breeds of sheep that are adapted to the equatorial latitude to locales more distant from the equator will result in a lower proportion of ewes cycling year round. As an example, efforts to establish a flock of ewes that breed year round in New York simply by importing ewes from the tropics will not be effective.
Finally, among ewes that cycle throughout the year, ovulation rate will vary with season of the year (see Table 88-1). More specifically, ewes that are bred during the summer months (e.g., June) produce fewer lambs (ovulation rate = 1) than the same ewes bred during the fall or winter months (ovulation rate = 1.7–1.8). This latter point is especially important to consider for producers who manage a program to breed ewes “out of season” (during the summer months) in an effort to produce more lambs per year.
EFFECT OF ENVIRONMENTAL TEMPERATURE
Table 88-2 Effect of Temperature on Variables of Reproductive Importance
HOUSING FOR RAMS AND EWES (AUGUST) | ||
---|---|---|
Variable | Outside | Air-Conditioned Room |
% eggs fertilized | 26% | 64% |
% embryonic death | 49% | 22% |
% ewes lambing | 13% | 50% |
Dutt and Simpson, 1957.
EFFECT OF EWE AGE AND BREED
Breed and age also affect the breeding activity of ewes (Table 88-3). Some breeds of sheep, such as Rambouillet, Merino, and Dorset, have longer breeding seasons than others, such as Southdown and Cheviot. Fortunately, long breeding seasons are genetically dominant; therefore, it is possible to increase the length of the breeding season through selective breeding. Within a breed, as an animal’s age increases, the length of the breeding season and fertility also increases (see Table 88-3). Ewe lambs that are starting to cycle for the first time generally tend to start cycling about 3 weeks after the adults and stop cycling about 3 weeks before the adults (see Fig. 88-3). As age increases, reproductive efficiency also increases when measured by number of surviving offspring (due, in part, to learned mothering ability) and number of offspring produced (due, in part, to an increase in ovulation rate as ewe age increases).
EFFECT OF STRESS
When considering the stress of suckling, ewes that suckle lambs take longer to return to estrus than ewes from which lambs have been weaned at an early age (Fig. 88-4). The recipe for predicting the outcome of stress management approaches may not always be as anticipated, depending on the source of the stress. Consequently, selective removal of specific stressful stimuli can produce responses that are either ineffective, permissive, or facilitating (e.g., ram introduction, discussed subsequently) to reproductive performance. The ability to predict the outcome of selective stressful stimulus requires the ability to collectively assess the total impact of that stress on the balance that exists between the animal’s inherent drive to reproduce and the demands that keep her from reproducing. For example, although early weaning of lambs may appear to provide a simple solution to accelerate the rebreeding of ewes, the technique is ineffective when used as the only stimulus to induce cyclic activity in ewes lambing during the spring or summer months. The differential response between groups of ewes lambing in the fall versus the summer can be attributed to the powerful inhibitory effects of long day lengths. In contrast, by creating an environment in which several inhibitory stressors are negated, using combined approaches, such as lamb removal and flushing, may be sufficient to initiate cyclic activity in ewes even in the presence of other inhibitory influences such as long day lengths.