This study reports genetic parameters for yearling and adult wool and growth traits, and ewe reproductive performance. Data were sourced from an Uruguayan Merino flock involved in a long-term selection program focused on reduced fiber diameter (FD), and increased clean fleece weight (CFW) and live weight (LW). Pedigree and performance data from approximately 5,700 mixed-sex yearling lambs and 2,000 mixed-age ewes born between 1999 and 2019 were analyzed. The number of records ranged from 1,267 to 5,738 for yearling traits, and from 1,931 to 7,079 for ewe productive and reproductive performance. Data on yearling and adult wool traits, LW and body condition score (BCS), yearling eye muscle area (Y_EMA), and fat thickness (Y_FAT), and several reproduction traits were analyzed. The genetic relationships between FD and reproduction traits were not different from zero. Moderate unfavorable genetic correlations were found between adult CFW and ewe lifetime reproduction traits (−0.34 ± 0.08 and −0.33 ± 0.09 for the total number of lambs weaned and total lamb LW at weaning, respectively). There were moderate to strong positive genetic correlations between yearling LW and all reproduction traits other than ewe-rearing ability (−0.08 ± 0.11) and pregnancy rate (0.18 ± 0.08). The genetic correlations between Y_EMA and reproduction traits were positive and ranged from 0.15 to 0.49. Moderate unfavorable genetic correlations were observed between yearling FD and Y_FAT and between adult FD and BCS at mating (0.31 ± 0.12 and 0.23 ± 0.07, respectively). The genetic correlations between adult fleece weight and ewe BCS at different stages of the cycle were negative, but generally not different from zero. This study shows that selection for reduced FD is unlikely to have any effect on reproduction traits. Selection for increased yearling LW and Y_EMA will improve ewe reproductive performance. On the other hand, selection for increased adult CFW will reduce ewe reproductive performance, whereas selection for reduced FD will negatively impact body fat levels. Although unfavorable genetic relationships between wool traits and both FAT and ewe reproductive performance existed, simultaneous improvements in the traits would occur using appropriately designed indexes. Fiber diameter (FD), clean fleece weight (CFW), live weight (LW), and reproductive performance are important traits in Merino flocks. This study estimated the genetic parameters for a range of production traits and ewe reproductive performance. Data from approximately 5,700 mixed-sex yearling lambs and 2,000 mixed-age ewes born in a single Uruguayan Merino flock were analyzed. There were generally favorable (positive) genetic correlations between LW and reproduction traits. The genetic relationships between FD and reproduction traits were generally negligible. In addition, moderate unfavorable (negative) genetic correlations were found between adult CFW and ewe reproduction traits. This study indicates that selecting finer fleeces will yield little to no change in ewe reproduction traits, whereas heavier fleeces are related to reduced ewe reproductive performance. On the other hand, genetically heavier yearling ewes will display greater reproductive performance.

The aim of this study was to identify genomic regions and genes associated with the fiber diameter (FD), clean fleece weight (CFW), live weight (LW), body condition score (BCS), pregnancy rate (PR) and lambing potential (LP) of Uruguayan Merino sheep. Phenotypic records of approximately 2000 mixed-age ewes were obtained from a Merino nucleus flock. Genome-wide association studies were performed utilizing single-step Bayesian analysis. For wool traits, a total of 35 genomic windows surpassed the significance threshold (PVE ≥ 0.25%). The proportion of the total additive genetic variance explained by those windows was 4.85 and 9.06% for FD and CFW, respectively. There were 42 windows significantly associated with LWM, which collectively explained 43.2% of the additive genetic variance. For BCS, 22 relevant windows accounted for more than 40% of the additive genetic variance, whereas for the reproduction traits, 53 genomic windows (24 and 29 for PR and LP, respectively) reached the suggestive threshold of 0.25% of the PVE. Within the top 10 windows for each trait, we identified several genes showing potential associations with the wool (e.g., IGF-1, TGFB2R, PRKCA), live weight (e.g., CAST, LAP3, MED28, HERC6), body condition score (e.g., CDH10, TMC2, SIRPA, CPXM1) or reproduction traits (e.g., ADCY1, LEPR, GHR, LPAR2) of the mixed-age ewes.

This study reports the phenotypic trends for wool, growth, and reproductive traits in mixed-age ewes after twenty years of genetic selection. Data were obtained from the Merino nucleus flock in Uruguay between 1999 and 2020. Overall, the aim of this selection flock was to reduce the fiber diameter (FD) and increase both the clean fleece weight (CFW) and live weight (LW). Data on ewe wool traits, LW, body condition score (BCS), the total number of lambs weaned (TLW), and the total LW of lambs weaned (TWW) across all lambing opportunities (1–8 mating seasons) were analyzed. Between 1292 and 2063 ewes were measured, depending on the trait considered. Ewe FD decreased by approximately 3 µm (19–16 µm), whereas greasy fleece weight (GFW) increased by 0.2 kg. This improvement in wool traits was accompanied by increases in LW at mating (3 kg), decreases in BCS at mating (approximately by 1 unit), and a small positive change in TWW across years. This study demonstrated that increasing farmer income by selecting for finer wool, heavier fleeces, and heavier animals can be obtained without compromising ewe lifetime reproduction.

This study evaluated the phenotypic trends for wool and growth traits of the fine Merino genetic nucleus in Uruguay. Data were collected from one-year-old lambs over a twenty-year period (1999–2018). The overall aim of the selection flock was to reduce fiber diameter with concomitant increases in fleece and live weights. Traits analyzed included fiber diameter (FD), greasy fleece weight (GFW), coefficient of variation of FD (CVfd), staple length (SL), scoured yield (SY), live weight post-shearing (LW), eye muscle area (EMA) and fat thickness (FAT). Data from approximately 5300 one-year-old male and female lambs were analyzed. During the study period, FD decreased by approximately 3 µm, whereas GFW and LW increased by at least 0.5 and 3.0 kg, respectively. There were interactions between the sex of the individual and the year for all wool traits. Except for FAT, all other traits were affected by the dam age. This study indicates that the selection program applied in the fine Merino genetic nucleus over a twenty-year period resulted in reductions in FD and increases in GFW and LW. Therefore, the results indicate it is possible to produce ultrafine wool in semi-extensive grazing systems without compromising other economically relevant traits in one-year-old lambs.