Effects of Myrtus (Myrtus communis L.) Extract Supplementation in the Diet on Metabolic, Immune, and Performance Parameters of Dairy Cows During the Transition Period

This study evaluated the effects of dietary <i>Myrtus communis</i> L. extract on the production performance, immune functions, and metabolic and health status of Holstein cows throughout the transition period. Forty cows were assigned to either a control group (n = 20) or a treatment group (n = 20) receiving <i>Myrtus communis</i> L. extract at 1.5 mL/kg body weight per day. Milk yield and feed intake were recorded daily, whereas blood samples were collected weekly. Reproductive performance and health status were monitored. <i>Myrtus communis</i> L. supplementation increased dry matter intake during prepartum and early and mid-lactation and elevated prepartum glucose levels. Treated cows produced more milk in early (44.56 vs. 40.72 kg/day) (<i>p</i> < 0.0001), and mid-to-late lactation (42.57 vs. 37.19 kg/day) (<i>p</i> < 0.0001). NEFA and BHBA concentrations were lower in the treatment group (<i>p</i> = 0.015 and 0.019, respectively), whereas cortisol and ALT were higher around calving (<i>p</i> < 0.001). Despite similar prepartum inflammatory profiles, treated cows demonstrated higher TNF-α concentrations at calving (<i>p</i> = 0.006). The days open and insemination number per pregnancy did not differ; however, the number of ovarian cysts were less in treated cows. Overall, <i>Myrtus communis</i> L. extract offered at a dose of 1.5 mL/kg body weight during the transition period improved feed intake, enhanced milk production, and reduced metabolic stress, suggesting its potential as a natural additive to support energy balance and reproductive performance in cows. However, it increased the blood cortisol and ALT concentrations, which requires further investigation.