Rapeseed oil was infused continuously (1.0 to 1.1 kg/d) into the duodenum of Holstein x Friesian multiparous cows during the first 3 wk of lactation (oil treatment, 6 cows versus 6 controls, early lactation trail) or after 100 d of lactation (midlactation trial, 9 cows in a crossover design). In the midlactation trial, plasma glucose, 3-hydroxybutyrate, and free glycerol were not affected by oil infusion. Postprandial plasma NEFA were higher in oil-infused than in control cows; plasma triglycerides, phospholipids, and cholesterol were significantly increased in oil treatment. Plasma insulin was lower and somatotropin higher in oil-infused cows, whereas insulin-like growth factor-I and triiodothyronine were not affected. During the early lactation trial, there were few significant effects of oil infusion on measured plasma metabolites and hormones. Preprandial glucose and NEFA were lower in oil treatment during wk 2, preprandial phospholipids were higher in wk 1 and 3, and free cholesterl was higher in wk 1. Responses of plasma glucose and NEFA to insulin challenge were not clearly affected by oil treatment during either trial. Differences in plasma triglycerides between jugular and mammary veins were higher in oil treatment in both trials. Oil infusion did not affect jugular-mammary differences of other metabolites except for a slight increase in cholesterol esters in midlactation. The relationships between jugular-mammary differences and jugular plasma concentrations showed that up to 24, 27, 54, and 17% of jugular glucose, NEFA, triglycerides, and 3-hydroxybutyrate apparently were taken up by the mammary gland.
