Threshold model analysis of lamb survivability in Romney sheep

Abstract

of 80.8%. Lambs were born between 1997 and2000. Fixed effects fitted in the model included sex and contemporary group, defined as the interaction of age of dam, flock, and birth date class. Median heritabilities were 0.106 ± 0.010 for direct, 0.082 ± 0.005 for maternal, and a median proportion of phenotypic variation 0.098 ± 0.003 for permanent environmental effects. The direct-maternal correlation was -0.75 ± 0.03. Estimated breeding values on the underlying scale were back-transformed to the phenotypic probability scale. For direct lamb survivability, the highest performing sire's lambs had a 14.8% greater chance of survival than the lowest performing sire's lambs for flocks with an average lamb survivability of 80%. Although heritabilities for both components of lamb survivability are low, selection for this trait can be accomplished and is of economic importance for the sheep breeder.Keywords lamb survival; sheep; estimated breeding values; heritability; threshold model Abstract Lamb survivability is a major determinant of the reproductive efficiency of the flock. The objective of this study was to estimate variance components for direct, maternal, and permanent environmental components of lamb survivability. Threshold model procedures were used to estimate variance components from 25 362 lamb survival observations representing 136 sires from two Romney flocks in New Zealand with an average lamb survival A06028;

Genetic parameters and trends for lamb survival following long-term divergent selection for number of lambs weaned in the Elsenburg Merino flock

“…Since maternal genetic effects were not fitted, the current results also excluded the direct-maternal genetic correlation (r am ) as a component of variance affecting lamb survival. This is in contradiction to the negative and large r am estimate of -0.60 previously reported by Cloete et al (2009), an estimate that was similar to the r am of -0.75 also derived from threshold analysis by Welsh et al (2006). The issue surrounding r am is pertinent if found to have a biological significance, since it counter-intuitively implies that ewes with a high genetic tendency for survival would tend to be poor mothers.…”

“…Similarly, Matos et al (2000) reported that the h 2 of survival from birth to weaning from a threshold model amounted to about three-fold the magnitude of the value derived by linear models in the same study. However, a moderate h 2 estimate of 0.11 for Tsv has also been derived from threshold models (Welsh et al 2006). Compared with the present results, high h 2 estimates of 0.18-0.33 have also been reported for postnatal survivability derived from sire models (Sawalha et al 2007).…”

“…Maternal effects are generally expected to have both genetic and environmental components (Bradford 1972). Similarly, Muller et al (2020) found no evidence of a significant m 2 component in Tsv of Dormer lambs, but this contradicted a substantial body of evidence that has found m 2 to be influential to survival, albeit generally low, with a range between 0.02 and 0.08 (Lopez-Villalobos and Garrick 1999;Morris et al 2000;Welsh et al 2006;Brien et al 2009;Hatcher et al 2010;Everett-Hincks et al 2014;Vanderick et al 2015) and a mean of 0.05 AE 0.01 (Safari et al 2005). Also, m 2 is equivalent to rearing ability when survival is evaluated as a trait of the ewe, and several studies have also reported the genetic component of ewe rearing ability as low, but significant with h 2 between 0.01 and 0.08 (Safari et al 2005;Afolayan et al 2008;Hebart et al 2010;Bunter et al 2016), which provides further evidence to support maternal genetic effects.…”

Triplet lambs and their dams – a review of current knowledge and management systems

“…The study of Welsh et al (2006) suggested that examining both individual birth weight and within litter birth weight variation would be a worthwhile approach to improve understanding. Since then a number of studies have examined the impact of these traits on triplet survival.…”

“…A large number of factors contribute to lamb survival and therefore unsurprisingly heritability and repeatability estimates for lamb and litter survival are low (Everett-Hincks et al 2005;Safari, Atkins, Fogarty et al 2005Brien et al 2010;Hinch and Brien 2014, Everett-Hincks and Cullen 2009, Ferreira et al 2015Hebart and Brien 2018) indicating slow genetic progress only. Welsh et al (2006) suggested that even slow genetic progress is worth targeting as a long-term solution. Hebart and Brien (2018) indicated that considering lamb survival separately for each birth rank would be more effective than treating it as a single trait across rank.…”