Yearling and Adult Expressions of Reproduction in Maternal Sheep Breeds are Genetically Different Traits

Abstract

Reproductive data (N=19335 ewes recorded) from maternal breeds recorded in industry flocks were used to estimate genetic parameters for the number of lambs born (NLB) and weaned (NLW) per ewe joined, along with their component traits fertility (FERT), litter size (LSIZE) and lamb survival (LSURV). Data were analysed as different traits for ewes in different age groups (yearlings, two year olds, and 2+ year olds). Yearling performance was characterised by low FERT (54%), low LSIZE, reflecting an increased frequency of single births, and increased lamb losses relative to older ewes bred in the same flock-years. Heritability (h²) estimates were highest for yearling FERT (h²=0.16) and declined for this trait with ewe age group (h²~0.07). In contrast, heritabilities and variance increased with ewe age for LSIZE (h²: 0.05 to 0.11). Genetic correlations (rg) between yearling and later records within traits were significantly <1 (range 0.10 to 0.54). The exception was LSIZE where the genetic correlation between ewe age groups was consistently high (rg: 0.85 to 1.0). Trait values affected by fertility outcomes (FERT, NLB and NLW) had significant service sire effects, whereas service sire effects were insignificant for LSIZE and LSURV. Service sire recording was incomplete more frequently for infertile ewes. Yearling reproductive performance should be treated as genetically different to adult expressions of the same traits for genetic evaluation purposes, and the different genetic architecture of component traits towards NLB and NLW can then be appropriately accommodated.