Genetic and nongenetic factors associated with the fate of maiden ewe lambs: slaughtered without ever lambing versus retained for breeding

peer-reviewedThe decision on which ewe lamb to retain versus which to sell is likely to vary by producer based on personal preference. What is not known, however, is if any commonality exists among producers in the characteristics of ewe lambs that influence their eventual fate. The objective of the present study was to determine what genetic and nongenetic factors associate with the fate of maiden ewe lambs. The fate of each ewe lamb born in the present study was defined as either subsequently: 1) having lambed in the flock, or 2) was slaughtered without any recorded lambing event. A total of 9,705 ewe lamb records from 41 crossbred flocks were used. The logit of the odds of the ewe lamb being retained for lambing was modeled using logistic regression. Variance components were then estimated for the binary trait representing the fate of the ewe lamb using animal linear and threshold mixed models. The genetic correlations between fate of the ewe lamb and preweaning, weaning, or postweaning liveweight were also estimated. From the edited data set, 45% of ewe lambs born entered the mature flock as ewes. Ewe lambs reared as singles, with greater levels of heterosis but lower levels of recombination loss, born to dams that lambed for the first time as hoggets, with greater breed proportion of the Belclare, Suffolk, Texel, and Llyen breeds were more likely (P < 0.001) to eventually lamb in the flock than be slaughtered without ever lambing. Irrespective of the age of the animal when weighed, heavier ewe lambs were more likely to eventually lamb (P < 0.001). The genetic SD and direct heritability of fate of the ewe lamb estimated in the univariate linear model was 26.58 percentage units and 0.31 (SE = 0.03), respectively; the heritability was 0.30 when estimated using the threshold model. The corresponding direct heritability of fate of the ewe lamb estimated in the bivariate analyses with liveweight ranged from 0.29 (SE = 0.03; preweaning weight) to 0.35 (SE = 0.04; postweaning weight). The genetic correlations estimated between fate of the ewe lamb and the liveweight traits were weak to moderate but strengthened as the age of the ewe lamb at weighing increased. Results from this study provide an understanding of the factors producers consider when selecting females for retention versus slaughter which may form useful parameters in the development of a decision support tool to identify suitable ewe lambs for retention

Phenotypic factors associated with lamb live weight and carcass composition measurements in an Irish multi-breed sheep population

peer-reviewedUnderstanding the phenotypic factors that affect lamb live weight and carcass composition is imperative to generating accurate genetic evaluations and further enables implementation of functional management strategies. This study investigated phenotypic factors affecting live weight across the growing season and traits associated with carcass composition in lambs from a multibreed sheep population. Four live weight traits and two carcass composition traits were considered for analysis namely; birth, preweaning, weaning, and postweaning weight, and ultrasound muscle depth and fat depth. A total of 427,927 records from 159,492 lambs collected from 775 flocks between the years 2016 and 2019, inclusive were available from the Irish national sheep database. Factors associated with live weight and carcass composition were determined using linear mixed models. The heaviest birth, preweaning, and weaning weights were associated with single born lambs (P 0.01). Breed class affected lamb live weight and carcass composition with terminal lambs weighing heaviest and having greater muscle depth than all other breed classes investigated (P 90% and ≤100%) resulted in heavier lambs at weaning compared with lambs with lower levels of heterosis coefficients (P < 0.001). A heterosis coefficient class <10% resulted in lambs with greater muscle depth while recombination loss of <10% increased ultrasound fat depth (P < 0.001). Results from this study highlight the impact of multiple animal level factors on lamb live weight and carcass composition which will enable more accurate bio-economic models and genetic evaluations going forward.Irish Department of Agriculture, Food and Marine Research Stimulus Fun

Runs of homozygosity (ROH) hotspots across all breeds, as defined as the top 1% of SNPs that occurred in a ROH.

<p>The number of SNPs within these hotspots are listed, as well as the average recombination rate (cM/Mb) within each hotspot and the putative candidate genes under selection.</p

The correlation between runs of homozygosity (ROH) based inbreeding coefficients and inbreeding coefficients estimated from pedigree (F_PED), the genomic relationship matrix (F_GRM) and the observed versus expected homozygotes (F_HOM).

<p>Three different ROH inbreeding measures were used which corresponded <b>to</b> the minimum length of the ROH used in the estimation (F_ROH1Mb, F_ROH5Mb, F_ROH10Mb).</p

Estimated effective population size across generations for each breed.

<p>Estimated effective population size across generations for each breed.</p

Admixture analysis of six commercial sheep breeds.

<p>The number of clusters was set to k = 5.</p

Proportion of autosome covered in runs of homozygosity (ROH) per animal.

<p>The black line indicates the median ROH sum per individual within each breed.</p

Genomic regions detected to be under divergent selection across all breeds.

<p>A) The frequency of a single nucleotide polymorphism (SNP) in a run of homozygosity (ROH) B) global F_ST values across all breeds where the blue line indicates SNPs that exhibited great differentiation and the red line indicates SNPs that exhibited very great differentiation C) a haplotype-based hapFLK test where the red line indicates the significance level threshold of 0.0001. SNPs highlighted in green are those identified within putative selection signatures.</p

Putative selective sweeps identified in the hapFLK-based analysis.

<p>Putative selective sweeps identified in the hapFLK-based analysis.</p