Effects of lipid supplementation on milk fatty acid profile of lactating cows under semi-confined summer conditions in southern Brazil.

Heat stress is a major challenge in tropical and subtropical dairy systems; however, limited evidence exists on how dietary lipid supplementation influences cow behavior, physiology, and milk quality under semi-confined summer conditions. This study evaluated the effects of different lipid sources (protected fat, flaxseed, and a mixture) on milk yield, composition, fatty acid profile, and behavioral and physiological responses of Holstein cows. Twelve cows were allocated to three 4 × 4 Latin squares, with each period lasting 15 days (10 days adaptation, 5 days data collection). Treatments consisted of a basal diet supplemented with 278 g protected fat, 790 g flaxseed, or a combination of 139 g protected fat and 395 g flaxseed, compared with a control. Behavioral observations, physiological parameters, and milk samples were collected. Results showed that daily thermal load indicated moderate thermal challenge, but behavioral and physiological responses remained within normal ranges. Milk yield and protein content were unaffected by lipid supplementation. Flaxseed significantly increased milk fat percentage and improved the fatty acid profile, with higher concentrations of unsaturated fatty acids, including omega-3 and CLA, compared with protected fat. Somatic cell count did not differ among treatments, indicating no adverse effect on udder health. Elevated milk urea nitrogen values suggested imbalances between protein and carbohydrate supply. Overall, flaxseed supplementation enhanced milk nutritional quality while maintaining production stability under moderate heat stress. These findings suggest that flaxseed may represent a practical nutritional strategy to improve milk value and support sustainable dairy management in warm climates.