Flax hull, a co-product obtained from flax processing, is a rich source of n-3 fatty acids (FA) but there is little information on digestion of flax hull based diets and nutritive value of flax hull for dairy production. Flax oil is rich in α-linolenic acid (LNA) and rumen bypass of flax oil contributes to increase n-3 FA proportions in milk. Therefore, the main objective of the experiment was to determine the effects of abomasal infusion of increasing amounts of flax oil on apparent digestibility, dry matter (DM) intake, milk production, milk composition, and milk FA profile with emphasis on the proportion of LNA when cows were supplemented or not with another source of LNA such as flax hull. Six multiparous Holstein cows averaging 650±36 kg body weight and 95±20 d in milk were assigned to a 6×6 Latin square design (21-d experimental periods) with a 2×3 factorial arrangement of treatments. Treatments were: 1) control, neither flax hull nor flax oil (CON), 2) diet containing (DM basis) 15·9% flaxseed hull (FHU); 3) CON with abomasal infusion of 250 g/d flax oil; 4) CON with abomasal infusion of 500 g/d flax oil; 5) FHU with abomasal infusion of 250 g/d flax oil; 6) FHU with abomasal infusion of 500 g/d flax oil. Infusion of flax oil in the abomasum resulted in a more pronounce decrease in DM intake for cows fed the CON diets than for those fed the FHU diets. Abomasal infusion of flax oil had little effect on digestibility and FHU supplementation increased digestibility of DM and crude protein. Milk yield was not changed by abomasal infusion of flax oil where it was decreased with FHU supplementation. Cows fed FHU had higher proportions of 18:0, cis9-18:1, trans dienes, trans monoenes and total trans in milk fat than those fed CON. Proportion of LNA was similar in milk fat of cows infused with 250 and 500 g/d flax oil in the abomasum. Independently of the basal diet, abomasal infusion of flax oil resulted in the lowest n-6:n-3 FA ratio in milk fat, suggesting that the most important factor for modification of milk FA profile was the amount of n-3 FA bypassing the rumen and not the amount of flax hull fed to dairy cows. Moreover, these data suggest that there is no advantage to supply more than 250 g/d of flax oil in the abomasum to increase the proportion of LNA in milk fat.

Four multiparous Holstein cows, each equipped with a duodenal cannula and an ultrasonic mammary blood flow probe, were assigned to a 4×4 Latin square to measure the effects of duodenal infusions of Leu (0, 40, 80 and 120 g/d) on lactational responses and mammary metabolism of nutrients. Cows were fed a diet of 67·0% corn silage, 5·3% grass hay, 14·2% peas, 7·8% maize starch, 1·0% each of molasses, urea, and sodium bicarbonate and 2·4% minerals and vitamins. Requirements of the remaining 9 essential amino acids were met by infusing into the duodenum 323 g/d of a mixture of amino acids including, Ile, Val, Met, Lys, Trp, Phe, His, Thr, Arg, Tyr and Glu. Milk protein content and yield were highest with 40 g/d of Leu and then progressively declined. Milk fat content and yield were significantly decreased over the entire range of Leu infusions. Arterial concentrations of Leu increased linearly and corresponding mammary arterio-venous differences increased to a plateau with 80 g/d of Leu. As Leu infusions increased, extraction rates of Leu decreased linearly, whereas those of Ile, Val, Lys, Arg, Thr and Tyr were significantly increased. Leu was taken up by the mammary gland according to milk output with 0 and 40 g/d of Leu infused and then largely in excess. Based on responses of protein yield and mammary uptake to milk output ratio, Leu concentration in total amino acids absorbed in the small intestine needs to be close to 8·9% for optimal milk protein synthesis.