A 100-Year Review: Fat feeding of dairy cows.

Over 100 years, the Journal of Dairy Science has recorded incredible changes in the utilization of fat for dairy cattle. Fat has progressed from nothing more than a contaminant in some protein supplements to a valuable high-energy substitute for cereal grains, a valuable energy source in its own right, and a modifier of cellular metabolism that is under active investigation in the 21st century. Milestones in the use of fats for dairy cattle from 1917 to 2017 result from the combined efforts of noted scientists and industry personnel worldwide, with much of the research published in Journal of Dairy Science. We are humbled to have been asked to contribute to this historical collection of significant developments in fat research over the past 100 years. Our goal is not to detail all the work published as each development moved forward; rather, it is to point out when publication marked a significant change in thinking regarding use of fat supplements. This approach forced omission of critically important names and publications in many journals as ideas moved forward. However, we hope that a description of the major changes in fat feeding during the past 100 years will stimulate reflection on progress in fat research and encourage further perusal of details of significant events.

Kinetics of fatty acid biohydrogenation in vitro.

Biohydrogenation (BH) of fatty acids (FA) from fresh alfalfa and alfalfa hay with varying levels of supplemental sucrose and media pH was evaluated in vitro. A multicompartmental model was then developed to estimate pool size and flux of vaccenic acid (VA) during BH of FA in fresh alfalfa. To vary incubation pH, alfalfa samples were inoculated with rumen fluid in 2 media differing in molarity of the bicarbonate buffer. Samples were incubated for 0, 1, 2, 3, 4, 6, 9, and 12 h; pH was measured and tubes were put in ice and stored until analysis. The BH rates of linoleic acid (18:2) and linolenic acid (18:3) were estimated by PROC NLIN of SAS (single pool, first-order kinetic model) and SAAM II (multiple pools, first-order kinetic model). Both methods gave similar estimates for the BH rates of 18:2 and 18:3 as well as the temporal pool size of VA. The BH rates (%/h) in the strong (SB) and weak buffers (WB) were 27.4 (+/-0.7) and 23.5 (+/-0.9) for 18:2, and 43.8 (+/-0.2) and 30.3 (+/-0.6) for 18:3, respectively. The WB decreased the BH rates of 18:2 and 18:3 for both forage sources. However, BH rates of 18:3 were higher from fresh alfalfa than alfalfa hay. There was no effect of sucrose addition on the BH rates of 18:2 and 18:3. Moreover, there was no effect of buffer on the BH of VA estimated by the multiple pools model between the SB and WB (12.5 +/- 2.1 and 14.1 +/- 3.7%/h, respectively). The BH rates of the conjugated linoleic acid isomers were not different between the SB and WB treatments (36.7 +/- 19.8 and 25.9 +/- 27.2, respectively). Because we could estimate fluxes as well as mass of the VA pools, more information was generated from the data when a multiple pools model was used compared with a single pool, first-order kinetic model.

Tissue fatty acid profiles can be used to quantify endogenous rumenic acid synthesis in lambs.

Proportions of vaccenic (trans-11 18:1) and rumenic (cis-9, trans-11 18:2) acids in mesenteric adipose, subcutaneous adipose, and longissimus muscle tissue lipids from lambs fed varying proportions of forages and concentrates were used to develop a mathematical model to predict exogenous and endogenous contributions to rumenic acid (RA) in the several tissues. From the model, we were able to estimate the proportion of absorbed RA, the proportion of vaccenic acid (VA) desaturated, the original proportion of VA in the tissue (before desaturation), and finally the proportion of RA synthesized endogenously. Estimates of endogenous RA were in the range of published data estimated by independent procedures. An independent data set of VA and RA in milk fat was used to challenge the model. Predictions were concordant with observations, although estimates of endogenous RA synthesis were lower than previous reports. Changing the amount of exogenous RA through manipulation of the diet influenced desaturation of VA inversely, so that endogenous RA synthesis was decreased when exogenous supply was increased (r = -0.80). The model should be challenged with data from human and nonruminant, as well as ruminant studies.

Challenges with fats and fatty acid methods.

The content and chemical nature of lipids in feedstuffs is heterogeneous. It has long been known that ether extraction by the Weende procedure inadequately characterizes the fat content of feedstuffs, yet it remains the official method. Diethyl ether (or hexanes that are often used) extracts significant amounts of nonnutritive, nonsaponifiable lipids from forages, and often incompletely extracts lipids of nutritional value, especially fatty acids present as salts of divalent cations. Preextraction hydrolysis of insoluble fatty acid salts with acid releases these fatty acids, and this step is included in the official procedure for certain feedstuffs in the United Kingdom; however, acid hydrolysis increases analysis time and decreases precision. Acid hydrolysis also causes confusion as to the proper definition of the fat content of feedstuffs. A preferred method of fat analysis determines the total fatty acid concentration in feed samples by converting fatty acid salts, as well as the acyl components in all lipid classes, such as triacylglycerols, phospholipids, and sphingolipids, to methyl esters using a simple, direct one-step esterification procedure. Fatty acid methyl esters are then quantified by GLC, which provides information on both fatty acid quantity and profile in a single analysis. Adjustments in conditions and reagents may be necessary to overcome difficulty in quantitatively preparing esters from certain types of fatty acids and their derivatives in commercial fat supplements. After correction for glycerol content, analysis of oils by this procedure provides information on the content of nonsaponifiable material, such as chlorophyll, waxes, and indigestible polymers formed from heat- or oxidatively damaged fats. The correct description of feedstuffs for energy value of fats is the content of total fatty acids.

Effect of supplemental tallow on performance of dairy cows fed diets with different corn silage:alfalfa silage ratios.

A study was conducted to investigate the response to supplemental tallow of lactating cows fed basal diets with different alfalfa silage:corn silage ratios. We postulated that supplemental tallow will have decreasing negative effects on rumen fermentation, dry matter intake (DMI), and milk fat percentage as the dietary ratio of alfalfa silage:corn silage is increased. Eighteen Holstein cows averaging 134 +/- 14 d in milk were used in a replicated 6 x 6 Latin square design with 21-d periods. Treatments were arranged as a 2 x 3 factorial with 0 or 2% tallow (DM basis) and three forage treatments: 1) 50% of diet DM as corn silage, 2) 37.5% corn silage and 12.5% alfalfa silage, and 3) 25% corn silage and 25% alfalfa silage. Cows were allowed ad libitum consumption of a total mixed ration. Diets were formulated to contain 18% crude protein and 32% neutral detergent fiber. No fat x forage treatment interactions were observed. Fat supplemented cows had lower DMI and produced more milk with less milk fat content relative to non-supplemented cows. Concentration of trans-octadecenoic acids was higher in milk fat of tallow-supplemented cows. Tallow supplementation had no effect on ruminal pH and acetate:propionate ratio, but tended to decrease total volatile fatty acid (VFA) concentration in the rumen. Increasing the proportion of alfalfa silage increased DMI, milk fat percentage, and milk fat yield regardless of the fat content of the diet. Total VFA concentration and acetate:propionate ratio in the rumen were increased in response to higher levels of alfalfa in the diets. These results suggest that replacing corn silage with alfalfa silage did not alleviate the negative response of dairy cows to tallow supplementation at 2% of diet DM.

Effects of propylene glycol drenching on energy balance, plasma glucose, plasma insulin, ovarian function and conception in dairy cows.

We postulated that daily drenching of propylene glycol to cows in early lactation would increase plasma glucose and insulin concentrations and improve fertility in postpartum cows. Thirty-six Holstein cows were assigned to treatment or control groups. Each treatment cow was given 500 ml of propylene glycol by drenching daily from 7 to 42 days of lactation. Blood samples for glucose, insulin, nonesterified fatty acids (NEFA), and plasma urea N were collected at 0, 30, and 90 min postdrenching once weekly during 1-6 weeks. Blood samples were collected for progesterone analysis and cows were palpated three times per week until 11 weeks to assess ovarian status. Propylene glycol did not affect dry matter intake (DMI), milk yield or energy balance in treatment cows. After drenching, propylene glycol increased (P<0.01) plasma glucose and insulin and decreased (P<0.01) NEFA; plasma urea N of the treatment group tended (P=0.07) to be higher than that of the control group through 90 min. Days to first service, days open, and services per conception were not different between groups. Conception rates to first insemination were 33% in the control group and 57% in treated cows, but these were not significantly different. First ovulation of treatment cows occurred earlier than that of control cows (32.3 versus 44.5 days, P=0.06) and the length of the first luteal phase was longer in treated cows (13.1 versus 7.3 days, P<0.05). These data are consistent with the hypothesis that insulin is important for normal ovarian function. During negative energy balance, treatment with propylene glycol, which induced small increases in plasma concentrations of insulin, prevented the short luteal phase characteristic of the first estrous cycle in control cows.

Relationships among dietary roasted soybeans, milk components, and spontaneous oxidized flavor of milk.

Relationships among dietary roasted whole soybeans (RSB), milk fatty acid profile, and the development of spontaneous oxidized flavor of milk were investigated by using 20 commercial dairy herds. Diets contained 0 to 15.3% of dry matter as RSB. Concentrations of dietary RSB were correlated positively with concentrations of C18:2 and C18:3 in milk fat. Concentrations of alpha-tocopherol, beta-carotene, and ascorbic acid in milk decreased from 0 to 3 d of storage (4 degrees C), and oxidized flavor in milk increased linearly between 0 and 8 d of storage. Milk fatty acid profile did not change during storage. The development of oxidized flavor at 8 d postsampling was correlated (r) with increased concentrations in milk fat of C18:2 (0.49), C18:3 (0.55), total polyunsaturated milk fatty acids (0.50), and dietary concentrations of RSB (0.38). Multiple regression was used to quantify relationships between variables and oxidized flavor (samples stored 8 d). All significant models included milk concentrations of Cu and dehydroascorbic acid. Concentrations of C18:2, C18:3, or total polyunsaturated fatty acids in milk fat, or dietary RSB concentrations, and interactions of those variables with Cu were included in individual models. Milk with high concentrations of polyunsaturated fatty acids and Cu were most susceptible to oxidation. Feeding RSB increased polyunsaturated fatty acid concentrations in milk fat, which increased the likelihood of oxidized flavor, especially when milk had high concentrations of Cu.

Effects of amount and source of fat on the rates of lipolysis and biohydrogenation of fatty acids in ruminal contents.

Because the percentage loss of unsaturated fatty acids across the rumen has varied considerably in previous in vivo studies, we conducted five experiments to identify potential factors that might affect the in vitro rates of lipid lipolysis and biohydrogenation in ruminal contents. The factors examined included the amount of fat added to the substrate, the source of added fat, the diet fed to the donor fistulated cow, and the time of collection of inoculum from the donor cow. Lipolysis and biohydrogenation were expressed as the rates of disappearance of neutral lipid and unsaturated fatty acids, respectively, from the culture contents over time using a first-order model. The rate of lipolysis of soybean oil declined from 44%/h to less than 30%/h as the percentage of soybean oil in the culture substrate increased from 2 to 10%. The overall rate of biohydrogenation of C18:2 was 14.3%/h, but declined 1.2%/h for each percentage unit increase in C18:2 added to the substrate. Compared with C18:2, the rates of biohydrogenation of C18:1 were generally lower (averaged 3.6 %/h) for all fat sources. The rate of biohydrogenation of C18:2 in soybean oil was not affected by the amount of grain or fat fed to the donor cow, or the time after feeding that ruminal inoculum was collected. Based on these findings, high linoleic acid concentrations in the diet would possibly reduce biohydrogenation and increase the postruminal flow of this unsaturated fatty acid. Also, lipolysis may vary considerably due to amount and source of lipid added to the diet, but this has little influence on the initial disappearance rates of linoleic or oleic acids from ruminal contents.

Cholecystokinin mediates depression of feed intake in dairy cattle fed high fat diets.

Two experiments were conducted to investigate the effects of 1) different concentrations of dietary fat and 2) i.v. administration of a cholecystokinin receptor antagonist (MK-329) on feed intake and plasma concentrations of hormones and metabolites in dairy cattle. In Experiment 1, 4 lactating Holstein cows were used in a 4 x 4 Latin square design. Treatments were diets with 1) no fat added, 2) 30 g fat/kg feed (calcium salts of long-chain fatty acids as fat supplement), 3) 60 g fat/kg, and 4) 90 g fat/kg added. Cows were fed once daily a diet of concentrate, corn silage, alfalfa haylage, and alfalfa pellets. Dry matter intake decreased linearly with increasing concentrations of dietary fat (P < 0.0001). Overall plasma concentrations of nonesterified fatty acids (P < 0.0001), triacylglycerol (P < 0.0006), and cholecystokinin (P < 0.02), increased linearly with each level of dietary fat, but there was a linear decrease in plasma insulin (P < 0.0008). In Experiment 2, 4 nonpregnant and nonlactating Holstein heifers were used in a cross-over design in a 2 x 2 factorial arrangement of treatments. Treatments were diet (fatty acids, 27 g/kg vs 103 g/kg diet dry matter) and i.v. injections (MK-329 vs vehicle). Heifers were fed once daily a total mixed ration of corn silage, cracked corn and soybean meal with or without fat supplement. Diets were switched by period and either MK-329 (70 microg/kg body weight) or its vehicle was injected i.v. at 2 hr postfeeding. Daily dry matter intake was decreased by feeding the high fat diet (P < 0.02) but was not affected by injections. Injection of MK-329, however, increased dry matter intake by 92% in heifers fed the high fat diet during the first 2 hr postinjection compared to vehicle injection. Plasma pancreatic polypeptide concentration was increased by the high fat diet at 2 hr postfeeding (P < 0.02) but was lowered by MK-329 at 1 hr postinjection (P < 0.001). Plasma insulin was lowered by the high fat diet (P < 0.01) but was not affected by injections. The elevated plasma cholecystokinin concentration may have mediated depressed feed intake of dairy cattle fed the high fat diets.

Short communication: influence of dietary tallow and fish oil on milk fat composition.

Four midlactation Holstein cows in a 4 x 4 Latin square design were fed one of four diets that contained 3% added fat (dry matter basis) as 100:0, 67:33, 50:50, or 33:67 tallow:fish oil. Feed intake and yield and composition of milk were not affected by treatment. The proportion of fatty acids 18:0 and cis-18:1 decreased and trans-18:1, conjugated linoleic acid, 18:3, 20:4, and 20:5 increased in milk fat with increasing fish oil. The efficiency of transferring dietary 20:5 and 22:6 to milk fat was < 7%.

Effects of fat source and copper on unsaturation of blood and milk triacylglycerol fatty acids in Holstein and Jersey cows.

Fatty acid composition of plasma triacylglycerides and milk fat was analyzed from Holstein and Jersey cows with control or depleted copper status and fed roasted whole soybeans or tallow. Conjugated linoleic acid in plasma was higher in Jersey cows. Dietary fat source influenced the proportions of all fatty acids in plasma and in milk, except for conjugated linoleic acid in milk. Feeding soybeans increased plasma C14:1, C18:0, C18:2, and conjugated linoleic acid, and decreased C14:0, C16:0, C16:1, and cis- and trans-C18:1 compared with feeding tallow. Low copper diets decreased C18:0 and increased cis- and trans-C18:1, and conjugated linoleic acid in plasma. A fat source x copper status interaction occurred for cis-C18:1 in plasma. Proportions of C4:0 to C14:0 were higher, and cis16:1, cis- and trans-C18:1, and conjugated linoleic acid were lower in milk fat of Jersey compared with Holstein cows. Generally, the effects of copper depletion were less apparent in milk than in plasma. Copper depletion increased C4:0, trans-C18:1, and conjugated linoleic acid, and decreased C16:1 in milk. Feeding whole soybeans increased C4:0 to C14:0, C18:0, C18:2, and C18:3, and decreased C14:1, C16:0, C16:1, and cis- and trans-C18:1 in milk. Breed x fat interactions occurred for C4:0, C14:1, C16:1, and conjugated linoleic acid in milk. Copper status x fat source interaction occurred for trans-C18:1. The breed x copper status interaction was apparent in milk fat for C16:1 and C18:0 and conjugated linoleic acid in milk. Both C18:0 and trans-C18:1 were desaturated by mammary tissue; however, whereas desaturation of C18:0 was linear, desaturation of trans-C18:1 reached a plateau that could have been caused by presence of the trans-10 isomer, which is not desaturated and was not separated from trans-11 C18:1 in our analysis. Comparison of the plasma triacylglycerol fatty acid profile with the milk fat profile was useful to interpret separate events of biohydrogenation in the rumen and desaturation by the mammary gland.

Milk fat composition of Holstein and Jersey cows with control or depleted copper status and fed whole soybeans or tallow.

We studied effects of breed, dietary fat source, and dietary copper intake as factors known to influence unsaturation of milk fat and its potential for development of spontaneous oxidized flavor in milk. Twelve Holstein and 12 Jersey cows were allotted to three blocks with four cows of each breed. Cows within breed were allotted randomly within blocks and fed control or copper-depleting diets for 2 mo to achieve stable or depleted liver copper stores. Cows then were fed tallow or roasted whole soybeans in a two-period switchback (5 wk per period); during the last week of each period additional vitamin E (2000 IU/d) was added. Copper depletion for 2 mo decreased concentrations of copper in liver. Feed intake and milk yield were influenced only by breed. The proportions of C4:0 to C14:0 and C18:0 in milk fat were higher, whereas C16:1 and cis-C18:1 were lower in Jersey cows. Feeding soybeans increased C4:0 to C14:0, C18:0, C18:2, and C18:3 in milk, and decreased C14:1, C16:0, C16:1, trans-C18:1, and cis-C18:1. Depleted copper status increased conjugated linoleic acid in milk. Several breed x fat source interactions for individual milk fatty acids occurred. Feeding soybeans decreased plasma concentrations of copper and zinc, and increased concentrations of alpha-tocopherol in plasma and milk. The concentration of zinc was higher in milk of Jersey cows. Depleted copper status tended to increase copper concentration in plasma and decreased copper in milk. Fat source did not influence plasma copper concentration when status was adequate, but plasma copper concentration was higher when tallow was fed to cows with depleted copper status. Supplementing vitamin E increased concentration of alpha-tocopherol in plasma and milk and decreased concentration of zinc in milk. Factors influencing the potential for oxidized flavor development in milk can be manipulated by changing the diet of the cow.

Trans-vaccenic acid is desaturated to conjugated linoleic acid in mice.

Mice were fed pure trans11 octadecenoic acid (trans-vaccenic acid; TVA) to determine whether it is desaturated to cis9, trans11 octadecadienoic acid, a predominant isomer of conjugated linoleic acid (CLA). In a preliminary trial, 12% of the TVA consumed during a 2-wk feeding period was recovered in the carcass as CLA. As a proportion of TVA in the tissues available for bioconversion, 48.8% was desaturated. We tested whether desaturation could be modified by supplementing no modifier, 0.5% clofibric acid to stimulate desaturation, or increasing the polyunsaturated fatty acids (PUFA) (10% corn oil vs. 4% corn oil) to inhibit desaturation in diets with or without 1% TVA. These diets were fed to six groups of mice in a 3x2 factorial arrangement of treatments. Feeding 1% TVA with 10% corn oil decreased feed intake (2.70 vs. 3.73 g/d, SEM 0.23; P<0.05). Bioconversion of dietary TVA was 12.0, 7.5 and 5.1% for mice fed no modifier of desaturation, clofibrate and increased PUFA, respectively. Conversion based on TVA available for desaturation was 52.6, 55.5 and 37.0%, respectively. Thus, clofibrate did not increase bioconversion, but increasing PUFA decreased conversion by 30%. To test whether TVA decreases food intake directly or after conversion to CLA, four groups of mice were fed diets containing 1% stearic, TVA, elaidic or conjugated linoleic acid. Dietary CLA decreased food intake and body fat, but did not change body protein. CLA was found in the carcass only when TVA or CLA was fed. CLA was found in both triacylglycerol and phospholipids when CLA was fed, but only in triacylglycerol when TVA was fed, suggesting that bioconversion occurred in the adipose tissue. In three trials, conversion of dietary TVA to CLA was 11.4+/-1.25%; conversion of stored TVA was 50.8+/-1.91%. Similar bioconversion of TVA in humans would increase current estimates of CLA available for the general population by 6- to 10-fold.

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.

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.

Tracing gluconeogenesis with deuterated water: measurement of low deuterium enrichments on carbons 6 and 2 of glucose.

The contribution of gluconeogenesis to glucose production in vivo can be measured by enriching body water with 0.5% 2H2O and measuring the glucose labeling ratio C6/C2 (Landau et al., J. Clin. Invest. 95, 172-178, 1995). We present further refinements of the measurements of the 2H enrichments on C6 and C2 of glucose. The transfer of 2H from C6 of glucose to hexamethylenetetramine (HMT) and extraction in preparation for gas chromatography-mass spectrometry can be done in a single test tube, without distillation of the intermediate formaldehyde. In addition, extraction of small amounts of HMT is greatly improved by making a HMT-iodine adduct. For C2, glucose is reduced to sorbitol, and 2H on C2 is transferred enzymatically to [U-13C3]pyruvate, forming [U-13C3,2-2H]lactate. The latter is assayed by negative chemical ionization gas chromatography-mass spectrometry of the pentafluorobenzyl derivative. The natural enrichment of the [U-13C3]lactyl ion is only 0.4%, allowing measurements of 2H enrichment down to 0.1%. These techniques were used in dogs infused with 2H2O and in isolated rat livers perfused with buffer containing 1 to 5% 2H2O. Our data reveal a difference in the rate of labeling of C6 and C2 of glucose in vivo. Lastly, in cows infused with [6,6-2H2]glucose, we show that the turnover of glucose can be economically measured by assaying low tracer enrichment (down to 0.1%) via hexamethylenetetramine.

Sodium mercaptoacetate is not a useful probe to study the role of fat in regulation of feed intake in dairy cattle.

Inhibition of fatty acid oxidation by mercaptoacetate stimulates food intake of rats fed dietary fat. To study regulation of feed intake of ruminants fed fat, dry matter intake and plasma concentrations of insulin and metabolites were determined in eight nonpregnant Holstein heifers in a cross-over design with two 14-d feeding periods by using a 2 x 2 factorial arrangement of treatments. Treatments were combinations of diet (27 or 103 g fatty acids/kg food dry matter) and injection (mercaptoacetate or saline). Half the heifers were fed each diet in Period 1, and diets were reversed in Period 2. On d 10 of each period, two animals per treatment were injected intravenously with either mercaptoacetate (300 mumol/kg body weight 0.75) or saline at 2 h postfeeding. Injections were reversed on d 12. Dry matter intake was suppressed by the high fat diet. Intravenous injection of mercaptoacetate decreased dry matter intake to 25% that of the control during 4 h postinjection. Both the high fat diet and mercaptoacetate injection increased plasma non-esterified fatty acid concentration, whereas plasma beta-hydroxybutyrate concentration was lowered by the high fat diet and by mercaptoacetate injection. Plasma triglyceride concentration was increased by the high fat diet, but was decreased by mercaptoacetate injection. Mercaptoacetate elevated plasma glucose concentrations at 2 and 3 h postinjection, possibly because plasma insulin concentration was lower. Effects of mercaptoacetate on plasma insulin and metabolite concentrations may have been confounded by the effects of decreased feed intake. Therefore, direct effects of mercaptoacetate injection were not separated from effects of feed intake on plasma insulin and metabolite concentration. Because mercaptoacetate injection decreased dry matter intake it was not a useful probe to study mechanisms of feed intake regulation in dairy cattle fed fat.

High fat diets increase plasma cholecystokinin and pancreatic polypeptide, and decrease plasma insulin and feed intake in lactating cows.

High fat diets often decrease feed intake in dairy cows; however, mechanisms underlying fat-induced depression of feed intake are yet to be established. The postulate that high fat diets decrease feed intake by increasing concentrations of lipid metabolites or satiety hormones in blood was tested by using eight multiparous Holstein cows in a simultaneously replicated 4 x 4 Latin-square design. Treatments were control diet with 1) no fat added, 2) 30 g/kg calcium salts of long-chain fatty acids, 3) 60 g/kg calcium salts of long-chain fatty acids, and 4) 90 g/kg calcium salts of long-chain fatty acids. Cows were fed once daily a diet of concentrate, corn silage, alfalfa haylage and alfalfa hay (50:25:14:11 on a dry matter basis). Dry matter and energy intakes were decreased by inclusion of calcium salts of long-chain fatty acids >30 g/kg of total diet dry matter (P = 0.0001). Plasma nonesterified fatty acids and triglyceride concentrations were increased linearly by feeding increasing amounts of fat (P < 0.003 and P = 0.0001, respectively), whereas plasma beta-hydroxybutyrate and glucose concentrations were not influenced by supplemental fat. Fat supplementation increased postfeeding plasma cholecystokinin concentrations and linearly increased plasma pancreatic polypeptide concentrations. Highest concentrations of plasma cholecystokinin (P < 0.001) and pancreatic polypeptide (P < 0.05) were observed in cows fed the 90 g/kg fat supplement. Plasma insulin was lowered linearly by feeding fat (P = 0.0001). Increased concentrations of cholecystokinin and pancreatic polypeptide were associated with decreased intakes of feed and energy, whereas insulin may not be involved in the control of feed intake in cows fed fat.

Responses of dairy cows supplemented with somatotropin during weeks 5 through 43 of lactation.

Beginning at wk 5 of lactation, 136 cows (34 per treatment) were supplemented daily for 38 wk with 0, 10.3, 20.6, or 41.2 mg of recombinantly derived bST monomer. Cows were obtained from University of Kentucky, University of Minnesota, University of Pennsylvania, and The Ohio State University. Nine cows (4 at 0 mg/d, 1 at 10.3 mg/d, 1 at 20.6 mg/d, and 3 at 41.2 mg/d) did not complete the experiment because of health problems. Data from these cows were included in the reproduction and health databases but not in the production database. Cows supplemented with bST produced more milk, consumed more feed, had lower rates of BW gain, and had improved efficiencies of milk production (conversion of feed and NEL to milk). Additional increases in productivity were modest at 20.6 and 41.2 mg/d versus productivity at 10.3 mg/d of bST. Concentrations of fat, protein, and TS in milk were unaffected. At 10.3 mg/d, bST did not adversely affect reproduction or health.

Differential effects of high fat diets on fatty acid composition in milk of Jersey and Holstein cows.

Effects of increasing dietary intake of calcium salts of palm fatty acid distillate (0, .25, .50, and .75 kg/d) on milk yield and milk fat composition were investigated for Jersey and Holstein cows. Increased dietary fat decreased DMI but did not influence milk yield or fat and protein contents. Jersey milk fat contained a higher proportion of short- and medium-chain fatty acids and lower proportions of palmitic and oleic fatty acids than did Holstein milk fat. With few exceptions, increased dietary fat altered the proportions of milk fatty acids in a parallel manner in both breeds. Except for butyrate, for which an effect was inconsistent, and palmitate, which was increased, additional dietary fat inhibited de novo synthesis of the milk fatty acids. The inhibition increased as the chain length of the fatty acids increased. Additional dietary fat increased the ratio of C18:1:C18:0 in Holstein cows, but the ratio was unchanged by dietary fat in Jersey cows. The regulation of fluidity of milk fat may differ between the two breeds.