Genetic determinism for within-litter birth weight variation and its relationship with litter weight and litter size in the Ripollesa ewe breed

Abstract

Birth weight plays a central role in lamb survival and growth, and the knowledge of its genetic determinism has become essential in worldwide selection programmes. Within this context, within-litter birth weight variation (BWV) has been suggested as an attractive trait to homogenise litters in prolific species, although it has not been analysed in sheep. The objective of this study was to ascertain whether maternal additive genetic variance exists for BWV in Ripollesa ewes, and to study its genetic, permanent environmental and residual relationships with litter weight (LW) and litter size (LS) at birth. Data were recorded in the Ripollesa experimental flock of the Universitat Autò noma of Barcelona, between 1986 and 2005, and included 1 662 litters from 380 ewes, with 712 records of BWV and 1 530 records of LW. Traits were analysed with a multivariate animal model solved through Bayesian methodologies, and with a threshold characterisation of LS. Additionally, the effect of BWV on lamb survival was studied. Additive genetic variance was observed for BWV (h 2 ¼ 0.061), as well as for LW (h 2 ¼ 0.200) and LS (h 2 ¼ 0.141). Nevertheless, genetic correlations among those traits were not substantial (BWV and LW ¼ 0.151; BWV and LS ¼ 20.219; LW and LS ¼ 2 0.320) and suffered from a high degree of uncertainly, with the null correlation included within the highest posterior interval at 95%. Within-litter birth weight variation and LS showed a negative and large permanent environmental correlation (20.872), and LW and LS were negatively correlated due to residual (2 0.762) and permanent environmental (20.449) random sources of variation. Within-litter birth weight variation influenced lamb mortality during the first 7 days of life (P , 0.05), increasing and decreasing survivability in heavier and lighter littermates, respectively. Nevertheless, stillbirths and lambs died after the 1st week of life were not affected by BWV (P . 0.05). The low heritability found indicates that slow genetic progress may be expected from selecting for BWV. Close to zero genetic correlations suggest that this selection will probably not affect LS and LW, although some significant permanent and residual correlations must be taken into account. Further studies are needed to understand better the genetic architecture among these three reproductive traits.

Comparison of linear, skewed-linear, and proportional hazard models for the analysis of lambing interval in Ripollesa ewes1

“…The Ripollesa is a medium-sized sheep breed (rams, 75 to 90 kg of BW; ewes, 50 to 65 kg of BW; Casellas et al, 2007b) reared in Catalonia, the north-east region of Spain. This is a representative example of meat-type sheep managed under semiextensive Mediterranean conditions, the "pascual"-type lamb being its typical commercial product (22 to 24 kg of live BW at slaughter; Casellas et al, 2007a).…”

“…This is a representative example of meat-type sheep managed under semiextensive Mediterranean conditions, the "pascual"-type lamb being its typical commercial product (22 to 24 kg of live BW at slaughter; Casellas et al, 2007a). Ripollesa individuals have dark brown or black marks on the head and legs, whereas their wool is white with closed fl eece (fi ber diameter, 23 to 26 μm; Casellas et al, 2007b).…”

Carcass traits and meat fatty acid composition in Mediterranean light lambs

“…Although light lamb production represents the largest share of the lamb market in many Mediterranean countries (Sañudo et al 1998), little is known about carcass peculiarities and meat composition at early ages of lambs. Moreover, this kind of lamb production involves multiple sheep breeds, including native types such as the Ripollesa breed, with an approximate census of 30 000 ewes in the northeastern region of Spain (Casellas et al 2007). Within this context, this paper focuses on the characterization of carcass and meat composition traits for light lambs from sheep breeds with a worldwide distribution, i.e., the meat-type Lacaune, and a Spanish-native sheep breed, i.e., the Ripollesa.…”