Abstract
Background:
Selenium (Se) and zinc (Zn) are essential trace minerals involved in antioxidant defense, immune function, rumen metabolism, and mineral transfer into milk. However, the additional benefit of organic Se and Zn enrichment within a feed supplement for early-lactation dairy cattle, particularly within the specific constraints of smallholder tropical production systems, remains inadequately characterized.

Aim:
This study evaluated the effects of a feed supplement enriched with organic Se and Zn on nutrient utilization efficiency, milk production, mineral partitioning in serum and milk, antioxidant status, and broader biochemical and hematological profiles of cows.

Methods:
Twenty-four Friesian Holstein crossbred cows (12.82±2.08 kg/day milk yield; 452.09±35.45 kg body weight; 37.65±15.41 days in milk) were allocated to three treatments: control (CON), feed supplement (FS; 1 kg/head/day), and feed supplement enriched with organic Se and Zn (FS-SZ; 0.45 mg Se/kg dry matter [DM] and 60 mg Zn/kg DM) for 49 days under tropical smallholder conditions.

Results:
Compared with CON, FS and FS-SZ increased DM, organic matter, and crude protein (CP) intake (P<0.05), while ether extract, acid detergent fiber, and neutral detergent fiber intake were unchanged (P>0.05). Compared with FS, FS-SZ improved CP digestibility (P<0.05) without affecting other digestibility parameters. Milk yield, feed efficiency, fat-corrected milk, and fat, protein, lactose, solid non-fat, and total solids yields increased in FS and FS-SZ compared with CON (P<0.05), but no additional production response was observed between FS and FS-SZ. Milk and serum Se and Zn concentrations were significantly higher in the FS-SZ group than in the FS and CON (P<0.05). FS-SZ increased serum glutathione peroxidase, superoxide dismutase, and total antioxidant capacity (P<0.05), reduced aspartate aminotransferase and alanine aminotransferase, and increased blood urea nitrogen (P<0.05). White blood cell count, neutrophil count, and mean corpuscular hemoglobin concentration were also elevated (P<0.05), while other biochemical and hematological parameters remained within normal ranges.

Conclusion:
Organic Se and Zn enrichment of a feed supplement enhanced mineral transfer into milk, improved antioxidant indicators suggesting enhanced antioxidant defense, and increased CP digestibility without adverse metabolic effects, although it did not further increase milk yield beyond conventional supplementation.

Key words: Blood biochemical profiles; Dietary supplementation; Hematological indices; Lactation performance; Organic selenium and zinc.