Producción de leche y sólidos de vacas manejadas en sistemas pastoriles con distinta época de parto

This study analyzes the effect of calving season on milk production and composition in cows managed in pasture-based systems in Uruguay. The experiment was conducted at the INIA La Estanzuela Experimental Station with 60 New Zealand–origin Holstein cows, randomly assigned to three treatments of 20 cows each: autumn-concentrated calving (OTO), winter-concentrated calving (INV), and an extended calving period (EXT) covering both seasons. Productive, nutritional, and climatic variables were recorded using automated measurement technologies, milk compositional analysis, and forage management evaluation. The hypothesis proposed that individual milk and solids production would be similar in cows calving in winter and autumn, and higher than that of cows with an extended calving period in autumn and winter. Methodologically, a completely randomized design was employed, with appropriate statistical analyses to compare treatments. Results showed that individual milk production did not differ significantly among treatments, although the INV lactation curve exhibited a sharper decline after peak production. Milk composition in OTO revealed higher fat and protein concentrations (4% and 6% higher, respectively) compared to INV and EXT. Average body condition was 0.12 units higher in OTO and EXT relative to INV, which could be related to differences in forage supply and climatic impact. The study also revealed differences in forage availability and supplement use among treatments. INV benefited from a higher share of fresh pasture in the diet (57%) during mid-lactation, whereas OTO and EXT required greater contributions from silage (33% and 16%, respectively) and concentrates (32.8% and 31.6%). Climatic conditions influenced productive responses, particularly in EXT, which experienced more days of elevated heat stress. In the discussion, the results are interpreted in light of previous studies, analyzing the implications of calving season on productive efficiency and forage resource management in pasture-based systems. This study provides valuable insights for planning calving seasons in pasture-based dairy systems, highlighting that autumn-concentrated calving may improve milk composition without affecting individual production, and emphasizing the importance of further research to optimize these systems under Uruguayan conditions.