Previous experiments from our group have demonstrated that abomasal infusion of unsaturated free fatty acids (FFA) markedly decreases dry matter intake (DMI) in dairy cows. In contrast, experiments from other groups have noted smaller decreases in DMI when unsaturated triglycerides (TG) were infused postruminally. Our hypothesis was that unsaturated FFA would be more potent inhibitors of DMI than an equivalent amount of unsaturated TG. Four Holstein cows in late lactation were used in a single reversal design. Cows were fed a total mixed ration containing (DM basis) 23% alfalfa silage, 23% corn silage, 40.3% ground shelled corn, and 10.5% soybean meal. Two cows received soy FFA (UFA; 0, 200, 400, 600 g/d) and 2 received soy oil (TG) in the same amounts; cows then were switched to the other lipid source. Cows were abomasally infused with each amount for 5-d periods. The daily amount of lipid was pulse-dosed in 4 equal portions at 0600, 1000, 1700, and 2200 h; no emulsifiers were used and there was no sign of digestive disturbance. Both lipid sources linearly decreased DMI, with a significant interaction between lipid source and amount. Slope-ratio analysis indicated that UFA were about 2 times more potent in decreasing DMI than were TG. Decreased DMI led to decreased milk production. Milk fat content was increased linearly by lipid infusion. Milk fat yield decreased markedly for UFA infusion but was relatively unaffected by infusion of TG. Contents of short- and medium-chain fatty acids in milk fat decreased as the amount of either infusate increased. Contents of C(18:2) and C(18:3) in milk fat were increased linearly by abomasal infusion of either fat source; cis-9 C(18:1) was unaffected. Transfer of infused C(18:2) to milk fat was 35.6, 42.5, and 27.8% for 200, 400, and 600 g/d of UFA, and 34.3, 39.6, and 34.0% for respective amounts of TG. Glucagon-like peptide-1 (7-36) amide (GLP-1) concentration in plasma significantly increased as DMI decreased with increasing infusion amount of UFA or TG. Plasma concentration of cholecystokinin-octapeptide (CCK-8) was unaffected by lipid infusion. These results indicate that unsaturated FFA reaching the duodenum are more potent inhibitors of DMI than are unsaturated TG; the effect may be at least partially mediated by GLP-1.

Fat is often fed to dairy cows to increase the energy concentration of their diet; however, feeding fat often reduces dry matter intake (DMI), which limits its impact on metabolizable energy (ME) intake. To investigate the effects of postruminal fat infusion on intake, feeding behavior, and milk production of dairy cows at two stages of lactation (55 and 111 d postpartum), six Holstein x British Friesian cows were infused into the abomasum, with a mixture of rapeseed and sunflower oils supplying predominantly unsaturated long-chain fatty acids (LCFA). Dry matter intake was significantly depressed by oil infusion, but estimated ME intake was unchanged, and thus there was no effect of oil infusion on milk yield. There was no effect of stage of lactation on the DM or ME intake response to oil infusion. Milk fat concentration was increased by oil infusion in mid-lactation but not in early lactation, suggesting that the infused LCFA were utilized differently in early compared with midlactation. Cows spent an average of 654 min idling, 462 min ruminating, and 248 min eating during the last 22.8 h of each infusion. There were no significant effects of oil infusion or stage of lactation on the total time spent engaged in these activities. An assessment of the circadian pattern of feeding behavior suggested that the depression in DMI in response to oil infusion occurred after the 1630 and 2230 h feeding times. This may reflect differences in mechanisms regulating feed intake behavior and appetite during the day. Comparison of the results of the present study with the results of other trials involving postruminal fat infusion suggests that polyunsaturated nonesterified fatty acids have the most potent effect on DMI intake.

Changes in the metabolism of nutrients by the portal-drained viscera (PDV) and liver may contribute to the reduction in dry matter intake (DMI) and other production responses generally observed in lactating dairy cows fed supplemental long-chain fatty acids (LCFA). In the present study, effects of a 7-d abomasal infusion of vegetable oil on arterial concentration and splanchnic (PDV and liver) metabolism of nutrients were measured in six cows at 55 (early lactation [ELAC]) and 111 (midlactation [MLAC]) d postpartum. Cows were fed for ad libitum DMI at 8-h intervals, and blood samples for measurement of splanchnic metabolism were obtained over 8 h beginning 2 h before feeding at 0830 h. Blood flow for the PDV and liver was increased by oil infusion and was greater in ELAC, despite similar-feed DMI during blood sampling. Increased blood flow in ELAC was associated with greater liver oxygen removal and glucose release that accompanied greater milk yield. In contrast, oil infusion had no effect on splanchnic oxygen use. Greater blood flow during oil infusion may have been due to specific effects of intestinal LCFA supply on PDV blood flow. Net PDV release and liver removal of branched-chain volatile fatty acids (VFA) and ammonia were increased by oil infusion. Net PDV release of longer-chain (4 and 5 C) VFA and NEFA was greater in ELAC, but net PDV flux of other nutrients was not affected by lactation stage, possibly due to the similarity of feed DMI. Oil infusion increased arterial concentration and net PDV release and liver removal of NEFA, and it decreased net liver release and arterial concentration of glucose. Effects of oil infusion on liver glucose release were associated with decreased daily DMI. In ELAC, arterial concentration and net liver removal of NEFA were also increased, but liver release of glucose was greater than in MLAC. Oil infusion and ELAC both increased net liver removal of L-lactate. The resulting decrease in net total splanchnic release of L-lactate in ELAC reflects decreased tissue energy balance of the cows. Generally, stage of lactation and relative milk yield had greater effects on metabolism of the liver than the PDV, in which metabolism was largely dictated by DMI. In the present study, there was little evidence to suggest an effect of stage of lactation on the metabolic response ofthe PDV and liver to postruminal LCFA supply.

Pancreatic and gut peptide hormones are potential mediators of the reduction in dry matter intake (DMI) often observed in lactating dairy cows fed supplemental fat. We investigated the effects of 7-d abomasal infusions of a rapeseed and sunflower oil mixture providing mostly unsaturated long-chain fatty acids (LCFA) on arterial concentration and splanchnic (portal-drained viscera [PDV] and liver) metabolism of insulin, pancreatic (PAN) and gut (GUT) glucagon, glucagon-like peptide-1 (7-36) amide (GLP-1), and cholecystokinin (CCK) in six cows at 55 (ELAC) and 111 (MLAC) d postpartum. Plasma flow for the PDV and liver were greater in ELAC and increased by oil infusion. Arterial concentrations of insulin and PAN were greater in MLAC, whereas arterial concentrations of GLP-1 and CCK were greater in ELAC. Abomasal oil infusion increased arterial concentration of GUT and GLP-1 but decreased arterial insulin concentration. These differences in peripheral hormone concentration were due largely to changes in their net PDV release and (or) liver removal. In addition, net liver removal of PAN was increased by oil infusion. There was no effect of oil infusion on splanchnic metabolism or arterial concentration of CCK. Lower concentrations of CCK in MLAC were attributable to net liver removal, emphasizing the importance of liver metabolism in determining peripheral concentrations of gut and pancreatic peptide hormones. Results of this study suggest a role for products of proglucagon processing (PAN, GUT, and GLP-1) as mediators of the reduction in DMI caused by postruminal supply of LCFA.

Eight Angus steers (290 +/- 8 kg), surgically prepared with pancreatic pouch-duodenal reentrant cannulas and abomasal infusion catheters were used in a replicated 4 x 4 Latin square experiment to investigate the effects of abomasal infusion of starch hydrolyzate (SH) and/or casein on pancreatic exocrine secretion and plasma concentration of hormones. Steers were fed a basal diet of alfalfa (1.2 x NEm) in 12 equal portions daily. Abomasal infusion treatments (6-L total volume infused per day) were water (control), SH [2.7 g/(kg BW x d)], casein [0.6 g/(kg BW x d)], and SH + casein. Periods were 3 d for adaptation and 8 d of full infusion. Pancreatic juice and jugular blood samples were collected over 30-min intervals for 6 h on d 11. Weight and pH of pancreatic samples were measured, and a 10% subsample was composited and frozen until analysis of total protein and pancreatic enzyme activities. The remaining sample was returned to the duodenum. Plasma was harvested and frozen until analyzed. Pancreatic juice (67 mL/h) and protein (1.8 g/h) secretion rates were not affected by nutrient infusion. There were SH x casein interactions for all pancreatic enzyme secretions (U/h; alpha-amylase, P < 0.03; trypsin, P < 0.08; and chymotrypsin, P < 0.03) and plasma insulin concentration (P < 0.10). Secretion of pancreatic enzymes was increased by SH (trypsin) and casein (alpha-amylase, trypsin, and chymotrypsin) but not when SH + casein were infused together. Glucose (P < 0.10) and cholecystokinin octapeptide concentrations (CCK-8; P < 0.05) were increased by SH, but glucagon was decreased (P < 0.10). Casein decreased (P < 0.10) plasma CCK-8 concentrations. These data indicate that positive effects of postruminal casein on enzyme secretion were inhibited by SH, emphasizing the complexity of the regulatory mechanisms involved in dietary adaptation of pancreatic exocrine secretion. Changes in hormone concentration may not relate directly to changes in enzyme secretion.