Effects of extended and seasonal calving systems on milk composition and heat coagulation time in a pasture-based dairy herd

<h2>Abstract</h2> Animal management and environmental conditions influence milk composition and processing properties. Seasonal calving systems are used in milk production to optimize costs and align milk supply with forage availability; however, the effects of concentrating calvings in fall and winter, compared with extended calving, on milk composition and heat coagulation time (HCT) have not been clearly defined. The objective of this study was to evaluate whether calving strategy affects milk composition and HCT in a pasture-based dairy system. Thirty New Zealand Holstein cows were assigned to 3 calving systems—extended (EXT: March – October, spanning fall-winter with calvings distributed across both seasons), winter-seasonal (WIN: June – August, concentrated in winter), and fall-seasonal (FALL: March – May, concentrated in fall)—with 10 cows per system, and monitored over one production year. Composite milk samples were collected in 4 seasonal windows (spring, summer, fall, and winter) and analyzed for composition (protein, casein, total solids, solids-not-fat, urea, citric acid, lactose, and titratable acidity) and heat stability, assessed both as HCT at native pH and as HCT–pH profiles after adjustment to target pH values from 6.5 to 7.0. No single compositional variable consistently explained the observed HCT patterns across calving systems and seasons. In pooled linear regression models, HCT was positively associated with grazed pasture in the diet and days in milk, and negatively associated with dry matter intake (DMI). These findings highlight the complexity of milk heat stability under pasture-based conditions and indicate that more detailed studies are needed to identify robust predictors of HCT.