Dietary fish oil supplements depress milk fat yield and alter milk fatty acid composition in lactating cows fed grass silage-based diets.

The potential of dietary fish oil (FO) supplements to increase milk 20:5n-3 and 22:6n-3 concentrations and the associated effects on milk fatty acid (FA) composition, intake, and milk production were examined. Four multiparous lactating cows offered a grass silage-based diet (forage:concentrate ratio 58:42, on a dry matter basis) supplemented with 0, 75, 150, or 300g of FO/d (FO0, FO75, FO150, and FO300, respectively) were used in a 4×4 Latin square with 28-d experimental periods. Milk FA composition was analyzed by complementary silver-ion thin-layer chromatography, gas chromatography-mass spectrometry, and silver-ion HPLC. Supplements of FO decreased linearly dry matter intake, yields of energy-corrected milk, milk fat and protein, and milk fat content. Compared with FO0, milk fat content and yield were decreased by 30.1 and 40.6%, respectively, on the FO300 treatment. Supplements of FO linearly increased milk 20:5n-3 and 22:6n-3 concentrations from 0.07 to 0.18 and 0.03 to 0.10g/100g of FA, respectively. Enrichment of 20:5n-3 and 22:6n-3 was accompanied by decreases in 4- to 18-carbon saturated FA and increases in total conjugated linoleic acid (CLA), trans FA, and polyunsaturated FA concentrations. Fish oil elevated milk fat cis-9,trans-11 CLA content in a quadratic manner, reaching a maximum on FO150 (from 0.61 to 2.15g/100g of FA), whereas further amounts of FO increased trans-10 18:1 with no change in trans-11 18:1 concentration. Supplements of FO also resulted in a dose-dependent appearance of 37 unique 20- and 22-carbon intermediates in milk fat. Concentrations of 16-, 18-, 20-, and 22-carbon trans FA were all increased by FO, with enrichment of trans 18:1 and trans 18:2 being quantitatively the most important. Decreases in milk fat yield to FO were not related to changes in milk trans-10,cis-12 CLA concentration or estimated milk fat melting point. Partial least square regression analysis indicated that FO-induced milk fat depression was associated with changes in the concentrations of multiple FA in milk. Even though a direct cause and effect could not be established, a decrease in 18:0 supply in combination with increased mammary uptake of cis-11 18:1, trans-10 18:1, and trans 20- and 22-carbon FA may contribute. In conclusion, dietary FO supplements enrich 20:5n-3 and 22:6n-3 in milk, but also elevate mono- and polyenoic trans FA concentrations, and in high amounts alter the distribution of individual trans FA isomers.

Characterization of the disappearance and formation of biohydrogenation intermediates during incubations of linoleic acid with rumen fluid in vitro.

Dietary unsaturated fatty acids are extensively hydrogenated in the rumen, resulting in the formation of numerous intermediates that may exert physiological effects and alter the fat composition of ruminant-derived foods. A batch culture method was used to characterize the hydrogenation of linoleic acid (LeA) by strained rumen fluid in vitro. Incubations (n = 5) were performed in 100-mL flasks maintained at 39 °C containing 400mg of grass hay, 50 mL of buffered rumen fluid, and incremental amounts of LeA (0, 1.0, 2.5, 5.0, or 10.0mg) for 0, 1.5, 3.0, 4.5, 6.0, and 9.0 h. The fatty acid composition of flask contents was determined using complimentary silver-ion thin-layer chromatography, gas chromatography mass-spectrometry, and silver-ion high-performance liquid chromatography. Linoleic acid was extensively (98.1, 97.6, 98.0, and 89.8% for additions of 1.0, 2.5, 5.0, and 10.0mg of LeA, respectively) hydrogenated over time. Complete reduction of LeA to 18:0 was inhibited in direct relation to the amount of added substrate, the extent of which was greatest for the highest amount of LeA addition. Recoveries of 1.0, 2.5, 5.0, and 10.0mg of added LeA as 18:0 averaged 73.6, 65.0, 57.3, and 10.7%, respectively. Incubation of incremental amounts of LeA resulted in a time-dependent accumulation of geometric isomers of 9,11 and 10,12 conjugated linoleic acid, several nonconjugated 18:2 isomers, and a wide range of cis 18:1 and trans 18:1 intermediates. Several unusual intermediates including cis-6,cis-12 18:2; cis-7,cis-12 18:2; and cis-8,cis-12 18:2, were found to accumulate in direct relation to the amount of added LeA, providing the first indications that hydrogenation of LeA by ruminal bacteria may also involve mechanisms other than hydrogen abstraction or isomerization of the cis-12 double bond. Fitting of single-pool, first-order kinetic models to experimental data indicated that the rate of LeA disappearance decreased with increases in substrate availability. Reduction of 18:1 and 18:2 intermediates occurred at much lower rates compared with conjugated linoleic acid and nonconjugated 18:2 isomer formation. In conclusion, the extent of LeA biohydrogenation in vitro was shown to be time- and dose-dependent with evidence that LeA is hydrogenated by ruminal bacteria via several distinct metabolic pathways. The accumulation of several unusual 18:2 isomers indicates that biohydrogenation of LeA also proceeds via mechanisms other than isomerization of the cis-12 double bond.

Effect of abomasal infusions of a mixture of octadecenoic acids on milk fat synthesis in lactating cows.

Diets causing milk fat depression (MFD) are known to alter ruminal lipid metabolism leading to the formation of specific biohydrogenation intermediates that exert antilipogenic effects. Several isomers of conjugated linoleic acid (CLA), namely trans-10, cis-12 CLA, cis-10, trans-12 CLA, and trans-9, cis-11 CLA, inhibit mammary lipogenesis in the lactating cow, but ruminal outflow of these biohydrogenation intermediates does not account entirely for the reductions in milk fat synthesis during diet-induced MFD. Milk fat trans-10 18:1 concentrations are consistently increased on diets that cause MFD, suggesting a possible role in the regulation of milk fat secretion. Three rumen-fistulated cows in mid lactation were used in a 3 x 3 Latin square to evaluate the effects of a mixture of 18:1 fatty acid methyl esters (FAME) on milk fat synthesis. Experimental treatments consisted of abomasal infusions of ethanol (control), 6 g/d of trans-10, cis-12 CLA (positive control; CLA), or 247 g/d of a mixture of 18:1 FAME containing (% fatty acids) cis-9 (9.45), cis-12 (3.35), trans-10 (37.3), trans-11 (37.4), and trans-12 (2.66) as major isomers (T181 treatment). Administration of the T181 treatment supplied 92.1 g/d of trans-10 18:1. Infusions were conducted over a 5-d period with a 9-d interval between treatments. Treatments had no effect on dry matter intake, milk yield, or milk protein. Relative to the control, abomasal infusion of T181 and trans-10, cis-12 CLA treatments reduced milk fat secretion by 19.5 and 41.5%, respectively. Even though a direct cause and effect on mammary lipogenesis could not be established, comparisons with published data and considerations of the relative abundance of constituent FAME in treatment T181 implicated trans-10 18:1 as the isomer responsible. In conclusion, current data suggest that trans-10 18:1 potentially exerts antilipogenic effects and may contribute to the reduction in milk fat synthesis during diet-induced MFD in the lactating cow.

Effect of incremental levels of sunflower-seed oil in the diet on ruminal lipid metabolism in lactating cows.

Based on the potential benefits of cis-9, trans-11-conjugated linoleic acid (CLA) for human health there is interest in developing sustainable nutritional strategies for enhancing the concentration of this fatty acid in ruminant-derived foods. Most evidence to date suggests that endogenous synthesis is the major source of cis-9, trans-11 in milk fat and ruminal outflow is limited and largely independent of dietary 18 : 2n-6 supply. Four lactating cows fitted with a rumen cannula were used in a 4 x 4 Latin square with 14 d experimental periods to examine the effects of sunflower-seed oil (SFO) as a source of 18 : 2n-6 on ruminal lipid metabolism. Cows were offered grass silage-based diets supplemented with 0, 250, 500 or 750 g SFO/d. Supplements of SFO had no effect on DM intake, milk fat or protein secretion, but increased linearly (P < 0.01) milk yield and milk lactose output and shifted (P < 0.001) rumen fermentation towards propionate at the expense of acetate. SFO supplements increased linearly (P < 0.05) the flow of 18 : 0, 18 : 1, 18 : 2n-6 and total CLA at the omasum and enhanced ruminal cis-9-18 : 1, 18 : 2n-6 and 18 : 3n-3 metabolism. Flows of all-trans- (Delta4-16) and cis- (Delta9-16) 18 : 1 isomers were elevated, while increases in ruminal CLA outflow were confined to trans-8, trans-10 and geometric 9,11 and 10,12 isomers. It is concluded that supplementing grass silage-based diets with plant oils rich in 18 : 2n-6 enhances ruminal outflow of trans-11-18 : 1 and cis-9, trans-11-CLA in lactating cows.

Trans-9, cis-11 conjugated linoleic acid reduces milk fat synthesis in lactating dairy cows.

Under certain dietary situations, rumen biohydrogenation results in the production of unique fatty acids that inhibit milk fat synthesis. The first of these to be identified was trans-10, cis-12 conjugated linoleic acid (CLA), but others are postulated to contribute to diet-induced milk fat depression (MFD). Our objective was to examine the potential role of trans-9, cis-11 CLA in the regulation of milk fat. In a preliminary study, we used gas-liquid and high-performance liquid chromatography techniques to examine milk fat samples from a diet-induced MFD study and found that an increase in trans-9, cis-11 CLA corresponded to the decrease in milk fat yield. We investigated this further using a CLA enrichment of 9, 11 isomers to examine the biological effect of trans-9, cis-11 CLA on milk fat synthesis. Four rumen-fistulated Holstein cows were randomly assigned in a 4 x 4 Latin square experiment involving 5-d treatment periods and abomasal infusion of 1) ethanol (control), 2) a 9, 11 CLA mix (containing 32% trans-9, cis-11, 29% cis-9, trans-11, and 17% trans-9, trans-11), 3) a trans-9, trans-11 CLA supplement, and 4) a trans-10, cis-12 CLA supplement (positive control). The trans-9, trans-11 CLA and trans-10, cis-12 CLA supplements were of high purity (>90%), and all supplements were infused at a rate to provide 5 g/d of the CLA isomer of interest. Milk yield and dry matter intake did not differ among treatments. Compared with the control treatment, milk fat yield was reduced by 15% for the 9, 11 CLA mixture and by 27% for the trans-10, cis-12 CLA treatment. We also found that trans-9, trans-11 CLA had no effect on milk fat yield, and previous research has shown that milk fat yield is unaltered when cows are infused with cis-9, trans-11 CLA. When all treatments were considered, results suggested that trans-9, cis-11 was the CLA isomer in the 9, 11 CLA mix responsible for the reduction in milk fat synthesis, although the magnitude was less than that observed for trans-10, cis-12 CLA. Interestingly, trans-9, trans-11 CLA altered the milk fat desaturase index, further demonstrating that alterations in desaturase can occur independently of effects on milk fat synthesis. Overall, our investigations identified that an increase in milk fat content of trans-9, cis-11 CLA was associated with diet-induced MFD and provided evidence of a role for this isomer in MFD based on the 15% reduction in milk fat yield with abomasal infusion of a CLA enrichment that supplied 5 g/d of trans-9, cis-11 CLA.

Short communication: indigestible markers reduce the mammary Delta9-desaturase index and alter the milk fatty acid composition in cows.

Accurate determination of the flow of nutrients at the omasum requires the use of a triple marker system. Typically, a system based on ruminal administration of the lithium salt of CoEDTA, ytterbium acetate (Yb-Ac), and chromium-mordanted straw (Cr-S) has been used. However, there is evidence to suggest that product:substrate ratios for stearoyl-coenzyme A desaturase (Delta(9)-desaturase) are lower in milk fat from cows administered a combination of CoEDTA, Yb-Ac, and Cr-S, indicating reduced Delta(9)-desaturase activity. To evaluate this hypothesis, samples of milk were collected 1 d before, and on d 2, 6, and 9 of administering the CoEDTA, Yb-Ac, and Cr-S triple marker system into the rumen of 4 cows. A 4 x 4 Latin square with 28-d experimental periods was used to assess the effects of 0, 75, 150, and 300 g/d of fish oil in the diet on ruminal and mammary lipid metabolism. Irrespective of the amount of fish oil in the diet, concentrations of all milk fatty acids containing a cis-9 double bond were reduced after markers were given. Milk fatty acid pairs dependent on Delta(9)-desaturase were decreased over time, with responses reaching a nadir within 6 d of marker administration. Overall, administering markers into the rumen was associated with a mean decrease in milk cis-9 10:1/ 10:0, cis-9 12:1/12:0, cis-9 14:1/14:0, cis-9 16:1/16:0, cis-9 17:1/17:0, cis-9 18:1/18:0, and cis-9,trans-11 conjugated linoleic acid/trans-11 18:1 concentration ratios of 44.6, 52.7, 58.7, 36.8, 37.2, 44.3, and 43.0%, respectively. In conclusion, one or more of the markers administered altered milk fatty acid composition and may act as an inhibitor of Delta(9)-desaturase in the bovine mammary gland.

Modulation of conjugated linoleic acid-induced loss of chicken egg hatchability by dietary soybean oil.

The main objective of the present study was to determine the minimum level of dietary plant oil supplementation that results in full recovery from loss of hatchability induced by conjugated linoleic acid (CLA). Another objective was to define the changes in egg yolk fatty acid composition associated with the loss and recovery of hatchability. Shaver hens were assigned to groups of 8 and were fed a diet containing either no CLA plus 0.5% soybean oil (control) or 0.5% CLA (1:1 mixture of cis-9, trans-11 and trans-10, cis-12 CLA) plus 0, 2, 4, 6, or 8% soybean oil for 15 d. Supplementation with CLA (CLA plus 0% soybean oil) resulted in complete loss of hatchability of fertile eggs. Hatchability was progressively improved by increasing doses of soybean oil, and full recovery of hatchability compared with the control levels was achieved at 6% soybean oil. There was no further improvement in hatchability when 8% soybean oil was added to the CLA-supplemented diet. Loss of hatchability was associated with a 2- to 3-fold decrease in desaturase ratios (cis-9 C16:1/C16:0 and cis-9 C18:1/C18:0) in the egg yolk total lipids, indicating marked inhibition of delta-9 desaturase in the chicken liver. In addition, the concentration of arachidonic acid was observed to decrease. Recovery of hatchability was associated with an increased proportion of linoleic acid and linolenic acid in the egg yolk. However, there was no change in desaturase ratios, suggesting that delta-9 desaturase inhibition persisted. Increased incorporation of dietary linoleic and linolenic acids might have compensated for the reduced levels of palmitoleic and oleic acid, thus allowing for the improvement in hatchability.

Examination of the persistency of milk fatty acid composition responses to fish oil and sunflower oil in the diet of dairy cows.

Based on the potential benefits of cis-9, trans-11 conjugated linoleic acid (CLA) for human health, there is a need to develop effective strategies for enhancing milk fat CLA concentrations. Levels of cis-9, trans-11 CLA in milk can be increased by supplements of fish oil (FO) and sunflower oil (SO), but there is considerable variation in the response. Part of this variance may reflect time-dependent ruminal adaptations to high levels of lipid in the diet, which lead to alterations in the formation of specific biohydrogenation intermediates. To test this hypothesis, 16 late lactation Holstein-British Friesian cows were used in a repeated measures randomized block design to examine milk fatty acid composition responses to FO and SO in the diet over a 28-d period. Cows were allocated at random to corn silage-based rations (8 per treatment) containing 0 (control) or 45 g of oil supplement/kg of dry matter consisting (1:2; wt/wt) of FO and SO (FSO), and milk composition was determined on alternate days from d 1. Compared with the control, the FSO diet decreased mean dry matter intake (21.1 vs. 17.9 kg/d), milk fat (47.7 vs. 32.6 g/kg), and protein content (36.1 vs. 33.3 g/kg), but had no effect on milk yield (27.1 vs. 26.4 kg/d). Reductions in milk fat content relative to the FSO diet were associated with increases in milk trans-10 18:1, trans-10, cis-12 CLA, and trans-9, cis-11 CLA concentrations (r(2) = 0.74, 0.57, and 0.80, respectively). Compared with the control, the FSO diet reduced milk 4:0 to 18:0 and cis 18:1 content and increased trans 18:1, trans 18:2, cis-9, trans-11 CLA, 20:5 n-3, and 22:6 n-3 concentrations. The FSO diet caused a rapid elevation in milk cis-9, trans-11 CLA content, reaching a maximum of 5.37 g/100 g of fatty acids on d 5, but these increases were transient, declining to 2.35 g/100 g of fatty acids by d 15. They remained relatively constant thereafter. Even though concentrations of trans-11 18:1 followed the same pattern of temporal changes as cis-9, trans-11 CLA, the total trans 18:1 content of FSO milk was unchanged because of the concomitant increases in the concentration of other isomers (Delta(4-10) and Delta(12-15)), predominantely trans-10 18:1. In conclusion, supplementing diets with FSO enhances milk fat cis-9, trans-11 CLA content, but the high level of enrichment declines because of changes in ruminal biohydrogenation that result in trans-10 replacing trans-11 as the major 18:1 biohydrogenation intermediate formed in the rumen.

Effect of safflower oil, flaxseed oil, monensin, and vitamin E on concentration of conjugated linoleic acid in bovine milk fat.

Conjugated linoleic acid (CLA) refers to a mixture of conjugated octadecadienoic acids of predominantly ruminant origin. The main isomer in bovine milk fat is the cis-9, trans-11 CLA. Interest in CLA increased after the discovery of its health-promoting properties, including potent anticarcinogenic activity. Two experiments were conducted to evaluate dietary strategies aimed at increasing the concentration of CLA in bovine milk fat. Both experiments were organized as a randomized complete block design with a repeated measures treatment structure. In Experiment 1, 28 Holstein cows received either a control diet or one of 3 treatments for a period of 2 wk. The control diet consisted of 60% forage (barley silage, alfalfa silage, and alfalfa hay) and 40% concentrate on a dry matter (DM) basis, fed as a total mixed ration (TMR). The concentrate was partially replaced in the treatment groups with 24 ppm of monensin (MON), 6% of DM safflower oil (SAFF), or 6% of DM safflower oil plus 24 ppm of monensin (SAFF/M). Average cis-9, trans-11 CLA levels in milk fat after 2 wk of feeding were 0.45, 0.52, 3.36, and 5.15% of total fatty acids for control, MON, SAFF, and SAFF/M, respectively. In Experiment 2, 62 Holstein cows received either a control diet or one of 5 treatment diets for a period of 9 wk. The control diet consisted of 60% forage (barley silage, alfalfa silage, and alfalfa hay) and 40% concentrate on a DM basis, fed as a TMR. The concentrate was partially replaced in the treatment groups with 6% of DM safflower oil (SAFF), 6% of DM safflower oil plus 150 IU of vitamin E/kg of DM (SAFF/E), 6% of DM safflower oil plus 24 ppm of monensin (SAFF/M), 6% of DM safflower oil plus 24 ppm of monensin plus 150 IU of vitamin E/kg of DM (SAFF/ME), or 6% of DM flaxseed oil plus 150 IU of vitamin E/kg of DM (FLAX/E). Average cis-9, trans-11 CLA levels during the treatment period were 0.68, 4.12, 3.48, 4.55, 4.75, and 2.80% of total fatty acids for control, SAFF, SAFF/E, SAFF/M, SAFF/ME, and FLAX/E, respectively. The combination of safflower oil with monensin was particularly effective at increasing milk fat CLA. The addition of vitamin E to the diet partially prevented the depression in milk fat associated with oilseed feeding, but had no significant effect on the concentration of CLA in milk.

Demonstration of a role for insulin in the regulation of leptin in lactating dairy cows.

In lactating dairy cows, the onset of negative net energy balance (EB) at parturition causes a reduction in plasma leptin and is also associated with increased concentration of growth hormone (GH) and decreased concentration of insulin. These observations raise the possibility that opposite changes in plasma insulin and GH are partly responsible for reduced plasma leptin. To test this hypothesis, we first examined the effects of undernutrition without the confounding influence of parturition by using late lactating dairy cows fed 120% of their nutrient requirements or restricted to 33% of maintenance requirements. Plasma leptin was reduced within 24 h of feed restriction, and was associated with increased plasma GH and decreased plasma insulin. Complete food deprivation for 48 h caused similar changes in the plasma concentration of leptin. To determine if an elevation in GH is responsible for the fall in plasma leptin, dairy cows were treated with excipient or bovine somatotropin in early lactation or in late lactation. Growth hormone treatment had no significant effect on plasma leptin irrespective of stage of lactation. Finally, the effects of insulin were studied by performing euglycemic hyperinsulinemic clamps in mid-lactating dairy cows. After 96 h of hyperinsulinemia, plasma leptin was increased significantly. These data indicate that insulin regulates plasma leptin in lactating dairy cows. They also suggest that, in undernourished lactating dairy cows, reduced plasma insulin could account for a portion of the decline in plasma leptin but that elevated plasma GH is unlikely to have a major effect.

Technical note: production of butter with enhanced conjugated linoleic acid for use in biomedical studies with animal models.

Cancer models utilize massive doses of carcinogen so that investigations of anticarcinogenic effects require equally large doses. Conjugated linoleic acids (CLA), predominately consumed in dairy products, are thought to be anticarcinogenic. Our objective was to naturally produce a CLA-enhanced butter for use in biomedical studies with animal models. To do this, we fed cows a low forage diet supplemented with sun-flower oil. This resulted in increases in content of CLA of milk fat, but the markedly elevated concentrations were transient and declined over a 3-wk period. By collecting milk fat over the first few days on the diet (d 7 to 10) and selecting cows with the greatest CLA concentrations, we were able to produce a butter in which CLA content was enhanced sevenfold over control butter (41 vs. 5 mg/g of fatty acids) and the cis-9, trans-11 isomer predominated (91%). Thus, butter produced by this method can be used to investigate the preventive role of CLA in natural foods with biomedical models of different types of cancer. Furthermore, the butter allows examination of the other beneficial health effects of CLA reported with animal models.

Conjugated linoleic acid is synthesized endogenously in lactating dairy cows by Delta(9)-desaturase.

Conjugated linoleic acid (CLA) is a naturally occurring anticarcinogen found in milk fat and body fat of ruminants. Although CLA is an intermediate in ruminal biohydrogenation of linoleic acid, we hypothesized that its primary source was from endogenous synthesis. This would involve Delta(9)-desaturase and synthesis from trans-11 18:1, another intermediate in ruminal biohydrogenation. Our first experiment supplied lactating cows (n = 3) with trans-11 18:1 by abomasal infusion and examined the potential for endogenous synthesis by measuring changes in milk fat CLA. By d 3, infusion of trans-11 18:1 resulted in a 31% increase in concentration of cis-9, trans-11 CLA in milk fat, demonstrating that an active pathway for endogenous synthesis of CLA exists. Our second experiment examined the quantitative importance of endogenous synthesis of CLA in lactating cows (n = 3) by abomasally infusing a putative stimulator (retinol palmitate) or an inhibitor (sterculic oil) of Delta(9)-desaturase. Infusion of retinol palmitate had no influence on milk fatty acid desaturation, and yield of CLA in milk fat was not altered. However, sterculic oil infusion decreased the concentration of CLA in milk fat by 45%. Consistent with Delta(9)-desaturase inhibition, the sterculic oil treatment also altered the milk fat concentration of other Delta(9)-desaturase products as indicated by the two- to threefold increase in the ratios of 14:0 to 14:1(,) 16:0 to 16:1 and 18:0 to cis-18:1. Using changes in the ratio of 14:0 to 14:1 as an indication of the extent of Delta(9)-desaturase inhibition with the sterculic oil treatment, an estimated 64% of the CLA in milk fat was of endogenous origin. Overall, results demonstrate that endogenous synthesis of CLA from trans-11 18:1 represented the primary source of CLA in milk fat of lactating cows.

Dietary fatty acid sources affect conjugated linoleic acid concentrations in milk from lactating dairy cows.

Conjugated linoleic acid (CLA), a naturally occurring anticarcinogen found in dairy products, is an intermediary product of ruminal biohydrogenation of polyunsaturated fatty acids. Our objective was to determine the effect of different dietary oils, which vary in fatty acid composition, on CLA concentrations in milk from lactating dairy cows. Twelve Holstein cows were randomly assigned to a 3 x 3 Latin square design. Dietary treatments were the addition (53 g/kg dietary dry matter) of peanut oil (high oleic acid), sunflower oil (high linoleic acid) and linseed oil (high linolenic acid). Each treatment period was 2 wk, and milk samples were collected on the last 4 d of each period. Milk yield (34.2 +/- 1.3 kg/d) and milk fat (2.25 +/- 0.06%) were not different among treatments. Milk protein during the sunflower oil treatment (mean, 3.44% protein) was significantly higher (P < 0.01) than during the other treatments. Milk fat concentration of CLA during the sunflower oil treatment was significantly different from other treatments (P < 0.001) and approximately 500% greater than typically observed when cows consume traditional diets. CLA concentrations (mg/g of milk fat) were 13.3, 24.4 and 16.7 during peanut oil, sunflower oil and linseed oil treatment, respectively. CLA concentration in milk fat can be enhanced by the addition of polyunsaturated fatty acids to the diet, especially oils high in linoleic acid.

Trans-octadecenoic acids and milk fat depression in lactating dairy cows.

We examined the role of trans-octadecenoic acids in milk fat depression when low fiber diets were fed. The study consisted of four experimental periods with a 2 x 2 factorial arrangement of treatments to test the effects of dietary fat (saturated vs. unsaturated) and rumen fermentation (high fiber diets vs. low fiber diets) on milk fat depression. Dietary fiber concentration and type of fat had significant effects on milk fat. Effects were most pronounced when unsaturated fat was added to the low fiber diet. When the low fiber diet plus unsaturated fat was fed, milk fat percentage and yield were decreased by 30 and 35%, respectively, compared with the percentage and yield when the high fiber diet plus saturated fat was fed. Alterations in rumen fermentation caused by differences in dietary fiber concentrations had little effect on the amount of trans-octadecenoic acids in milk fat, and the total amount did not correlate with changes in milk fat percentage. Further examination of the isomeric profile of trans-octadecenoic acid revealed substantial differences among the dietary treatments. Although the addition of unsaturated fat resulted in marked increases in the milk fat content of trans-11-octadecenoic acid, regardless of dietary fiber concentration, the low fiber diet plus unsaturated fat increased the content of trans-10-octadecenoic acid. This combination was also associated with a significant decrease in milk fat content and yield. When the low fiber diets were fed, circulating insulin concentrations were elevated, regardless of the type of fat supplement. However, marked milk fat depression occurred only when the low fiber diet was supplemented with unsaturated fat.

The role of insulin in the regulation of milk protein synthesis in dairy cows.

We examined the role of insulin in milk protein synthesis using the hyperinsulinemic-euglycemic clamp approach in combination with abomasal infusion of casein. The two experimental periods consisted of abomasal infusion of water or 0.5 kg/d of casein. An insulin clamp was conducted over the last 4 d of each period. During the insulin clamp, circulating insulin was elevated fourfold, and euglycemia was maintained by the infusion of exogenous glucose. Casein infusion increased milk yield so that milk protein yield was 10% greater than baseline values. Use of the insulin clamp combined with casein infusion increased milk protein yield by 230 g/d (28% greater than baseline values). Milk protein composition was not altered, but content was increased from 3.13% during the baseline period to 3.44% by d 4 of the clamp; calcium concentration in milk increased about 10% to 1.2 g/kg. During the clamp, circulating concentrations of essential amino acids were dramatically reduced. The most pronounced effects were noted for branched-chain amino acids (64% reduction from baseline values). The insulin clamp resulted in alterations in circulating insulin-like growth factor (IGF)-I concentrations (increase) as well as IGF-II and IGF-binding protein-2 concentrations (decreases). Overall, results indicated that the ability of the mammary gland to synthesize milk protein does not function at maximum capacity, and there is a previously unrecognized potential to enhance milk protein percentage and yield.

Role of insulin in the regulation of milk fat synthesis in dairy cows.

Five lactating Holstein cows were fitted with rumen fistulas and subjected to a hyperinsulinemic-euglycemic clamp and abomasal casein infusion to examine the effects on milk fat synthesis and the composition of milk fatty acids. The experiment consisted of two periods of abomasal infusions (water or 0.5 kg/d of casein); each period was divided into three 4-d intervals. The initial interval allowed for acclimation, and baseline measurements were established during the second interval. During the third 4-d interval, a hyperinsulinemic-euglycemic clamp was maintained, and insulin was infused continuously at the rate of 1 microgram/kg of body weight per h. Circulating concentrations of insulin were increased more than fourfold, and euglycemia was maintained by infusion of glucose at variable rates. Insulin had no effect on milk fat yield but casein infusion increased milk yield and tended to increase fat yield. A trend toward higher milk yield during the clamp, combined with a slight numerical decrease in milk fat yield, resulted in decreased fat percentage. Calculated net energy balance was positive throughout the study, although feed intake decreased during the insulin clamp, particularly for the water infusion period. Minor changes occurred in the composition of milk fatty acids during the clamp when the balance between de novo and preformed fatty acids shifted slightly toward de novo. Overall, results demonstrated that a relatively constant rate of milk fat synthesis was maintained during chronic hyperinsulinemia. Effects on milk fat yield and composition of fatty acids offered no support for the role of insulin on milk fat depression.

Role of insulin in the regulation of mammary synthesis of fat and protein.

Five lactating Holstein cows were subjected to a hyperinsulinemiceuglycemic clamp to examine the effects of insulin on milk yield and composition. Of special interest was the evaluation of the glucogenic-insulin theory of milk fat depression. Cows were fed every other hour to minimize postprandial effects, and blood samples were obtained via an indwelling jugular catheter every 4 h for 2 d to establish baseline glucose concentrations. For the 4-d clamp, insulin was infused continuously (1 microgram/kg of BW per h) into the contralateral jugular vein, and circulating insulin was increased approximately fivefold. Blood was sampled frequently, and blood glucose was maintained within 10% of baseline concentrations by infusion of exogenous glucose at variable rates (mean = .15 g/kg of BW per h). Dietary intake declined on the 4th d of the insulin clamp (23.0 vs. 16.3 kg/d). Milk yield, however, did not change (32.4 vs. 33.6 kg/d) in support of the lack of sensitivity of the mammary gland to insulin. Milk fat percentage (3.85 vs. 3.66) and yield (1.26 vs. 1.22 kg/d) did not change during the insulin clamp. Milk protein yield increased (.98 vs. 1.05 kg/d), and milk protein percentage tended to increase (3.04 vs. 3.14), during the insulin clamp. This modest increase in milk protein yield may have been constrained by a lack of available amino acids, as indicated by a decrease in circulating concentrations of essential amino acids, urea nitrogen, and plasma proteins. Overall, results offer no support for the glucogenic-insulin theory of milk fat depression but do indicate that the insulin infusion, either directly or indirectly, enhanced secretion of milk protein.