Influence of calving season and seasonal diet composition on butter color, firmness, and fatty acid profile in grazing systems

The quality of butter in pasture-based dairy systems is largely determined by seasonal shifts in diet composition, particularly the balance between fresh pasture and silages. However, it remains unclear whether calving system management exerts an independent effect on butter characteristics once feeding conditions and lactation stage are considered. This study evaluated the physicochemical, colorimetric, and textural properties of model butter produced from milk from cows managed under 3 calving season systems (autumn, winter, and extended) over 2 years. The objective was to determine whether seasonal diet composition and milk fat profile were the primary drivers of model butter quality. Milk from 30 New Zealand Holstein-Friesian cows (n = 10 per calving group) was sampled at 7 time points over a 2-year period, and model butter was produced at laboratory scale. Model butter composition, color (L*, b*), firmness at 10°C and 20°C, and fatty acid profile were analyzed. No differences were detected among calving-season treatments for any of the variables evaluated. In contrast, strong associations were observed between model butter attributes and diet composition. Yellowness (b*) increased with a higher proportion of pasture in the diet (Dactylis glomerata (cocksfoot), Medicago sativa (alfalfa), and Festuca arundinacea (tall fescue)) (r = 0.80) and with higher concentrations of α-linolenic acid in butter fat (r = 0.94), whereas it decreased with increasing whole-plant corn silage intake (r = -0.82) and with higher palmitic acid (C16:0) concentrations in butter fat (r = -0.66). Lightness (L*) was positively associated with C16:0 (r = 0.71) and negatively associated with CLA (r = -0.61) and pasture proportion (r = -0.49). Firmness at 20°C increased with C16:0 (r = 0.71) and with the C16:0/C18:1 spreadability index (r = 0.60). Overall, these results indicate that butter quality in this pasture-based system is determined primarily by seasonally driven feeding conditions rather than by calving-season management.