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.

Lambs of the British breeds, such as Southdown, Shropshire, Oxford, Hampshire, Suffolk, and Dorset, generally reach puberty at an earlier age than do lambs of the Spanish breeds, such as Merino, Rambouillet, Columbia, and Corriedale. In general, most lambs reach puberty within 6 to 10 months of birth, providing that environmental determinants such as nutrient availability, health status, and photoperiodic environment are favorable. Prolific breeds, such as Romanov and Finnish Landrace, may reach puberty at earlier ages. Therefore, among the first considerations to be made in assessing a realistic breeding scheme is whether the ewe lambs in question are of sufficient age to reproduce. Once some reasonable determination has been made that the lambs meet or exceed the breed-defined age parameters to reproduce, focus can then be placed on environmental conditions that permit the lamb to express its minimum age at puberty.

There are many environmental determinants of a lamb’s age at puberty; however, the most frequently overlooked factor is the nutritional status of the developing lamb. The relationship between age and weight (or “condition”) of lambs at puberty is critical. Figure 88-1 illustrates this relationship for lambs growing at three different rates. The long-dash line illustrates the growth rate of lambs belonging to a producer who overfeeds his lambs in an effort to minimize their age at puberty. What this producer will find is that the “long-dash line” lambs do not reach puberty until they also satisfy the minimum age requirements. The cost of this mismanagement or unnecessary investment in excess feed is evident in the excess weight or condition carried by the “long-dash line” lambs as they start to cycle. Furthermore, evidence now exists that animals carrying excess fat have reduced fertility.

Fig. 88-1 Relationship between age and weight of lambs at puberty that are growing at three different rates.

In stark contrast to the overconditioned lambs represented by the long-dash line are the lambs managed by a producer who inadequately feeds his lambs (represented by the short-dash line), maintaining the presumption that the “short-dash line” lambs will begin to reproduce when they are of sufficient age. Clearly, in this situation the “short-dash line” lambs will surpass their genetically defined minimum age at puberty owing to insufficient metabolic reserves to initiate the reproductive process. In the ideal situation (represented by the solid line), a producer should feed lambs to a target weight and age to conserve feed without sacrificing reproductive performance. The minimum age requirement is dictated by the breed and the minimum weight requirement is generally 68% of their projected adult body weight. Producers who choose to breed ewe lambs at the minimum limits of their age and weight should manage the lambs so that they satisfy both minimum requirements at least 2 to 3 weeks prior to the time it is desired to have them cycling.

When ewe lambs start to cycle, the next logical question is when should they first be bred. Much research has provided evidence to support the argument that with few exceptions ewes should be selected, managed, and bred to produce offspring by 1 year of age. The benefits are reduced preproduction costs, shortened generation intervals (thus permitting more rapid genetic progress), and finally, increased lifetime productivity. The risks are slowed growth rates of the dam, low initial conception rates, and potential birthing or mothering problems as the result of the young dams giving birth to multiple or oversized offspring. The key to making the correct decision lies in the ability to plan and manage this process. For example, the earlier lambs are bred in the breeding season, the greater the probability those young dams will conceive single offspring, thus reducing the incidence of birthing difficulties associated with multiple births and reducing the risk of insufficient milk for multiple offspring. Furthermore, lambs could be bred to breeds of rams noted for their lambing ease to minimize potential lambing difficulty. Perhaps most the valuable information gained in efforts to breed ewe lambs at a young age is the identification and thus ability to select ewe lambs with the ability to reproduce.

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.

Also important in selecting replacement ewe lambs is their date of birth (Fig. 88-2). The reason for this selection variable is because in order for ewe lambs to reach puberty, they must first be exposed at an early age to increasing day lengths (summer solstice = June 21) and later to decreasing day lengths (winter solstice = Dec. 21), the latter of which provides the stimulus for them tobegin to cycle. Offspring born prior to June have the greatest likelihood of reaching puberty and starting to cycle during the subsequent fall or winter if conditions are favorable, whereas offspring born after June are less likely to reach puberty during the first fall or winter but will wait until the following fall or winter to reach puberty at over 1 year of age.

Fig. 88-2 Relationship between date of lamb’s birth, length of daylight, and age at puberty.

Finally, it should be noted (as illustrated in Fig. 88-3) that for ewe lambs, the breeding season will typically begin 3 weeks after the adult ewes and cease 3 weeks prior to the adult ewes. The consequence of these restrictions is reduced opportunity to conceive at a time in the ewe’s life when conception rates are typically low to begin with—thus the concern over delayed age at puberty.

Fig. 88-3 Breeding activity in sheep in the Northern Hemisphere relative to length of day and month of the year. The solid line symbolizes ewes with longer breeding seasons than those represented by the short dashed line.

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.

Table 88-1 Estrus, Ovulation, and Ovulation Rate in Ewes in Idaho and Texas