Milk yield (MY) is highly heritable and an economically important trait in dairy livestock species. To increase power to detect candidate genomic regions for this trait, we carried out a meta-analysis of genome-wide association studies (GWAS). In the present study, we identified 19 studies in PubMed for the meta-analysis. After review of the studies, 16 studies passed the filters for meta-analysis, and the number of chromosomes, detected markers and their positions, number of animals, and p-values were extracted from these studies and recorded. The final data set based on 16 GWAS studies had 353,698 cows and 3950 markers and was analyzed using METAL software. Our findings revealed 1712 significant (p-value < 2.5 × 10⁻⁶) genomic loci related to MY, with markers associated with MY found on all autosomes and sex chromosomes and the majority of them found on chromosome 14. Furthermore, gene ontology (GO) annotation was used to explore biological functions of the genes associated with MY; therefore, different regions of this chromosome may be suitable as genomic regions for further research into gene expression.

The present study aimed to investigate the causal relationships between clinical mastitis and some reproductive traits, including success at first insemination (SFI), the number of inseminations to pregnancy (INS), the interval from calving to first service (CTFS), first and last service interval (IFL), and open days (OD) in first-parity Holstein cows. For this purpose, the records of 58 281 first parity Holstein cows were analysed. These data sets were collected from 17 large dairy herds from 2008 to 2017. Recursive Mixed Models (RMMs) were applied and compared with the estimations under Standard Mixed Models. Then, one trivariate and three bivariate Gaussian-threshold models were used for the analyses. Recursive models were applied, considering that clinical mastitis can influence fertility traits. Mastitis is considered a covariate for the reproductive traits to determine their causal relationship. The results of this study indicated that causal effects of mastitis on SFI (on the observed scale, %), CTFS, IFL, OD, and INS were -5.7%, 3.3 days, 12.27 days, seven days, and 0.26 services, respectively. The estimated structural coefficients of the recursive models in the first parity imply that mastitis significantly lengthened the fertility interval and decreased the conception rate. In addition, genetic, residual, and phenotypic correlations between mastitis and the reproductive traits under both models were statistically significant. Results of genetic correlations between mastitis and fertility traits suggest that more incidence of mastitis during lactation is related to the delays in the heat show and pregnancy rate after insemination. In summary, considering the causal effects under RMMs may be advantageous to comprehend complicated relationships between complex traits better.

This study evaluated the effect of rumen-protected methionine (RPM) and guanidinoacetic acid on performance, growth, hematological indices, and immune-related gene expression in suckling Holstein calves. Forty-eight Holstein calves (with an average weight of 39 ± 1 kg) were randomly divided into four experimental groups (n = 12). Each group randomly received one of four diets: (1) control, (2) a basic diet with 0.73% RPM, (3) a basic diet with 1000 mg of GAA acid, and 4) a basic diet with 0.73% PRM and 100 mg of GAA. Body weight (BW) was higher in the RPM group in comparison to the GAA group (P < 0.05). At day 56, glucose and cholesterol concentrations in calves receiving GAA, RPM, and mixed feeding were lower than in the controls (P < 0.05). The results demonstrated that the GAA and Mix groups experienced significant increases in IFN-γ gene expression (P < 0.05). In conclusion, RPM supplementation increased body weight compared to GAA, while GAA and RPM effectively reduced glucose and cholesterol levels at day 56. GAA also lowered cholesterol at weaning age compared to RPM. Significant increases in IFN-γ gene expression were observed in GAA and Mix groups.