Effects of method of barley grain processing and source of supplemental dietary fat on duodenal nutrient flows, milk fatty acid profiles, and microbial protein synthesis in dairy cows.

The objective of this study was to determine the interactive effects of barley grain processing and source of supplemental fat on ruminal nutrient digestion, duodenal nutrient flow, microbial protein synthesis, and milk fatty acid (FA) profiles in dairy cows. Four Holstein cows (656 ± 28 kg of BW; 80 ± 12 d in milk) fitted with permanent ruminal and duodenal cannulas were used in a 4×4 Latin square design with a 2×2 factorial arrangement of experimental treatments, in which barley was either dry-rolled (DRB) or pelleted (PB) and supplemented with full-fat canola seed (canola) or full-fat flaxseed (flaxseed). Cows fed DRB consumed 1.8 to 3.5 kg/d more dry matter compared with those fed PB. Mean ruminal pH was lower and the duration (min/d) and area (pH × min) of total ruminal acidosis (RA; ruminal pH <5.8), mild RA (5.8 >ruminal pH >5.5), and severe RA (5.5 >ruminal pH >5.2), and the duration of acute RA (ruminal pH <5.2) were greater in cows fed PB compared with those fed DRB. Milk yield was unaffected by diet. Milk fat content and yield were greater in cows fed DRB than in cows fed PB. Cows fed PB had greater duodenal flows of C18:1 trans, C18:1 cis, C18:2 trans-9,trans-12, C18:2 trans-9,cis-12, C18:2 trans-11,cis-15, and C18:2 trans-10,cis-12 compared with those fed DRB. Duodenal flows of C18:1 cis, C18:1 trans, total C18:1, C18:2n6, C18:2 cis-9,trans-11, C18:2 trans-10,cis-12, C18:2 trans-11,trans-13, total conjugated linoleic acid (CLA), and C18:3n3 were greater in cows fed flaxseed compared with those fed canola. Milk concentrations of C18:3n3 and total CLA tended to be greater in cows fed PB compared with those fed DRB. When compared with cows fed canola, milk concentrations of C18:2 trans-11,cis-15, C18:2 cis-9,trans-11, C18:2 trans-11,trans-13, total CLA, and C18:3n3 were greater in cows fed flaxseed. Ruminal digestion of neutral detergent fiber and acid detergent fiber was greater in cows fed DRB compared with those fed PB. Duodenal flow of ammonia-N tended to be greater in cows fed PB compared with those fed DRB. Duodenal flow of nonammonia nonmicrobial N was greater in cows fed flaxseed compared with those fed canola; however, barley grain processing did not affect duodenal nonammonia nonmicrobial N flow. Duodenal flow of microbial nonammonia N and microbial efficiency were not affected by diet. In summary, barley grain processing and source of fat altered ruminal FA biohydrogenation, and this was reflected in changes in duodenal FA flow and milk FA profiles; however, ruminal microbial protein synthesis was unaltered.