Project of The Latvian Council of Science (LCS) - LZP-2021/1-0489 project: “Development of an innovative approach to identify biological determinants involved in the between-animal variation in feed efficiency in sheep farming”. The aim of the project is to determine whether the feed efficiency status of Latvian meat sheep breeds could be predicted using a panel of genetic and molecular markers previously found to be associated with divergent FE status in a training population of lambs when fed the same diet. Novelty: to determine the parameters predicting the most productive result of lamb rearing, we set out to develop the cheapest and most effective method for determining markers of feed efficiency - based on molecular and genetic markers obtained from the blood of live lambs. About the project: The costs associated with lambing (buying or keeping sheep) and preparing or purchasing feed are the two most significant components of variable costs in sheep raising. Feed costs are high due to poor grain growing conditions in major producing countries, the use of feed grains in ethanol production, and increased competition for land in crop production compared to urban development. Feed efficiency in growing lambs (i.e., the animal’s ability to reach a market or adult body weight (BW) with the least feed intake) is a critical factor in the sheep industry. Improving FE reduces production costs. Improving FE by 5% can bring economic benefits up to four times higher than a 5% increase in average daily gain (ADG). Traditionally, meat breeding programs have focused on outputs due mainly to the routine availability of phenotypic data on outputs or correlated traits. Currently, no marker has successfully explained enough of the variability in feed efficiency that they were used as part of a routine improvement program. According to our data, no genetic parameters for performance and feed efficiency traits are available for sheep. The physiological determinants of feed efficiency or putative biomarkers used to analyse animal-to-animal variation in live lambs could be a cost-effective and rapid tool for genetic selection or management decisions. Myostatin (MSTN), a highly conserved member of the transforming growth factor-beta (TGF-b) superfamily, also known as growth/differentiation factor 8 (GDF8), the major regulator of myogenesis, functions as a negative regulator of muscle growth and development in mammals. Myostatin is a transforming growth factor (TGF)-β superfamily member and cannot be classified into the existing TGF-β subfamilies, such as inhibins or bone morphogenic proteins. Mutations MSTN produce a “double-muscle” phenotype, making it commercially invaluable for improving livestock meat production and providing high-quality human protein. However, mutations at different loci of the MSTN often produce a variety of different phenotypes. About the data of the project: LZP-2021/1-0489 project data on lamb samples of the year 2022, or the A22 group, which consists of 76 intensively fattened lambs from six breeds. The whole MSTN gene was sequenced using Illumina NGS technology (AmpliSeq). The MSTN gene has been fully sequenced in lambs of Latvia (in sheep research in Latvia) for the first time. Information about 23 loci (https://doi.org/10.5281/zenodo.8146724) compared with the last reference sequences ARS_UI_Ramb_v2.0. All variable SNPs in the MSTN gene were constructed by combining 24 distinct multi-loci genotypes. Reducing the number of loci to 16 by excluding those variables in only one sample or complete linkage disequilibrium did not decrease the number of genotypes. Among these 16 loci, 12 multi-locus genotypes were found in only one sample. To streamline the association analysis, the MSTN gene was divided into four regions: the promoter and exon 1, intron 1, intron 2, and the 3′UTR. This division helped reduce the number of multilocus genotypes. The database contains information on multiloci genotypes for four regions of the MSTN gene. The multi-locus genotypes are presented using standard IUB/IUPAC nucleic acid codes, representing the genotype of two alleles at a single locus with a single letter. The data is the joint property of the participants of the LCS project: the University of Latvia and the Latvian University of Life Sciences and Technologies