Selection for fertility in Australian Merino sheep

Abstract

Phenotypic and genetic parameters for fertility in sheep, in terms of number of lambs born and number weaned, have been estimated in a flock of medium Peppin Merinos. Repeatability of fertility traits over all ages is low, but there is an age effect, the record at 3 years of age having a higher value than those at 2 or 4 years. The regression of subsequent performance on a difference of 1 lamb at the initial lambing was higher for the difference between 1 and 2 lambs than between 0 and 1 lamb, which indicated that selection for twins is likely to raise fertility in the current flock more rapidly than selection against barrenness. The estimate of heritability for 2 years of age was negligible for each fertility trait, but the estimates for the 3-year-old record were higher, the value for lambs born being over 0.3. Mass selection for number of lambs born at 3 years of age would thus be expected to lead to appreciable genetic progress, while there would be at least some progress in number of lambs weaned. Phenotypic and genetic correlations were also estimated between fertility at different ages and 10 sheep and wool traits measured at 15–16 months of age. Phenotypically, fertility is positively correlated with body weight and negatively with skin wrinkle score. Genetically, it is suggested from the estimates that fertility is positively correlated with body weight and staple length and negatively correlated with fibre diameter, clean scoured yield, and wrinkle score. No phenotypic or genetic correlation was found between greasy or clean wool weight and either measure of fertility. The application of the findings to breeding for higher fertility is discussed. In an appendix, a maximum likelihood method of estimating heritability for all-or-none traits in half-sib data is presented.

Relationship of ewe reproduction with subjectively assessed wool and conformation traits in the Elsenburg Merino flock

“…The comparable genetic correlation between NLB and WFS reported by Young et al (1963) was negative at -0.12. However, Young et al (1963) concluded that there is no indication of genetic and phenotypic antagonism between face cover and fertility in their study, possibly because of the absence of animals with heavily covered faces. Phenotypic and environmental correlations of ewe reproduction traits with subjective wool traits varied in sign and were mostly low in magnitude and negligible.…”

“…In addition, sheep that have yellow wool are likely to wean more lambs. The genetic relationships between ewe reproduction traits over three lambing opportunities varied in sign (Table 6) and were significant between NLB3 and WFS (0.23 ± 0.11) and between TWW3 and FCS (-0.33 ± 0.16 Previous work on Australian Merino sheep (Young et al, 1963) reported both negative and positive relationships between NLB and WFS (in their study these authors term the trait 'face cover score', but their definition corresponds to WFS in the present study) measured at different ages. The comparable genetic correlation between NLB and WFS reported by Young et al (1963) was negative at -0.12.…”

“…The genetic relationships between ewe reproduction traits over three lambing opportunities varied in sign (Table 6) and were significant between NLB3 and WFS (0.23 ± 0.11) and between TWW3 and FCS (-0.33 ± 0.16 Previous work on Australian Merino sheep (Young et al, 1963) reported both negative and positive relationships between NLB and WFS (in their study these authors term the trait 'face cover score', but their definition corresponds to WFS in the present study) measured at different ages. The comparable genetic correlation between NLB and WFS reported by Young et al (1963) was negative at -0.12. However, Young et al (1963) concluded that there is no indication of genetic and phenotypic antagonism between face cover and fertility in their study, possibly because of the absence of animals with heavily covered faces.…”

Hill country sheep production

“…Many workers (Terrill & Stoehr 1942;Coop & Hayman 1962;Young et al 1963;Purser 1965;Lax & Brown 1968;Kennedy & Kennedy 1968;Edey 1970) have found that live weight and number of lambs weaned are positively associated. For example, an increase of 8 more lambs per 100 ewes joined per lQlb (4.5 kg) increase in body weight at 15-16 months of age was reported by Lax & Brown (1968), which is similar to the relationship with pre-mating weights found by Coop & Clark (1966).…”

Composite trait selection to improve reproduction and ewe productivity: a review

“…Young et al 1963;Bradford 1985). However, selection for a single component of a composite trait does not always result in an overall improvement of a complex trait, such as reproduction or ewe productivity (Snowder 2002).…”