Influence of prepubertal dietary regimen on mammary growth of Holstein heifers

One hundred-sixteen Holstein heifers (mean BW, 175 kg) were randomly assigned to diets of alfalfa silage or corn silage and were fed to gain approximately 725 or 950 g/d in order to study the influence of prepubertal diet and rate of gain on mammary growth and milk production. Blood was collected before puberty for hormone determination, and 8 heifers per group were killed at puberty for evaluation of tissue variables. Serum growth hormone was reduced, and IGF-I was increased, in the group of heifers reared at a high rate of gain on the corn silage diet. Accompanying the decline in growth hormone, total mammary parenchymal DNA and RNA was reduced in heifers reared at a high rate of gain on the corn silage diet. Mammary parenchyma in heifers of the latter group contained a greater volume of adipocytes and a lower volume of epithelial cells than did mammary parenchyma in heifers of other groups. Data are consistent with previous investigations that showed a deleterious effect of prepubertal rapid weight gain on mammogenesis when accompanied by excess body fat deposition. However, this effect did not cause a decline in subsequent milk production.

Milk fat yield and composition during abomasal infusion of cis or trans octadecenoates in Holstein cows

The role of trans-C18:1 fatty acids in milk fat depression was examined. Six rumen-cannulated Holstein cows were assigned to two Latin squares with 21-d periods. The common basal diet contained 40% forage and 60% concentrate. Treatments were the uninfused control, 750 g/d of a mixture of cis fat (65% high oleic sunflower oil and 35% cocoa butter), and 750 g/d of a mixture of trans fat (93% shortening and 7% corn oil) infused into the abomasum via a tube that passed through the rumen cannula. Milk yield was similar among treatments. Milk fat percentage and yield were lower, and milk citrate concentration was higher, for the trans than the cis treatment. Changes in the fatty acid composition of milk were similar for the cis and trans treatments compared with the control except for trans-C18:1. The concentration of trans-C18:1 was greater for the cis and trans treatments than for the control and was greater for the trans than for the cis treatment. These data clearly demonstrated that infusion of trans-C18:1 fatty acids into the abomasum depressed milk fat percentage and yield. We speculate that reduced synthesis of fatty acids and reduced activity of acyl transferase in mammary tissue contributed to depressed milk fat percentage for the trans treatment.

Direct analysis of body composition of dairy cows at three physiological stages

Holstein cows were killed at three physiological stages, prepartum (-7 d, n = 10), early lactation (63 d, n = 7), and late lactation (269 d, n = 8), for determination of chemical composition and prediction of energy changes during lactation. Cows were weighed, slaughtered, and separated into five or six fractions, including carcass, gastrointestinal tract, mammary gland, uterus, and fetus (if present); the remainder was noncarcass. Live BW and weight of all empty body components except fat were unaffected by physiological stage. Empty body fat was reduced 42.3 kg for the early lactation cows compared with that of prepartum cows. Fat-free matter was similar across physiological stages; however, the water content of fat-free matter was greater for the prepartum and early lactation cows than for late lactation cows. In early lactation cows, the percentages of total protein were less in carcass and greater in gastrointestinal tissue than in prepartum and late lactation cows, but fat distribution was not affected. The energy values of 9.2 and 5.57 Mcal/kg for fat and protein in tissue, respectively, were determined by regression and used to apportion energy associated with fat, .925, and protein, .07, during lactation using data adjusted for ash. A maximum loss of 442 Mcal of tissue energy by approximately 77 DIM was determined by regression of empty body energy on DIM).

Milk fat depression, the glucogenic theory, and trans-C18:1 fatty acids

Metabolic and endocrinological characteristics were compared for cows that differed in the extent of milk fat depression. Forty-one multiparous Holstein cows were fed control (40% concentrate and 60% forage) and high concentrate (80% concentrate and 20% forage) diets in a doubale-reversal design. Cows showing one or more percentage units of depression in milk fat were arbitrarily classified as responders (n = 26); those remaining were classified as nonresponders (n = 15). Compared with nonresponders, responders had greater increases in DMI, estimated NEL intake and balance, BW, milk yield, protein and lactose yields in milk, weight percentage of trans-C18:1 fatty acids in milk, and concentrations of triiodothyronine and thyroxine in serum when switched from the control diet to the 80% concentrate diet. Lack of an increase in concentrations of glucose and insulin in serum of cows with the greatest decline in percentage of milk fat casts doubt on the ability of the glucogenic theory to explain milk fat depression completely.

Net absorption and oxygen consumption by Holstein steers fed alfalfa or orchardgrass silage at two equalized intakes

The objective of this experiment was to compare net nutrient absorption and oxygen consumption by portal-drained viscera (PDV) of catheterized Holstein steers (333 kg) when fed alfalfa or orchardgrass silage at two equalized intakes. The design was a 4 X 4 Latin square with a 2 X 2 factorial arrangement of alfalfa or orchardgrass fed at 65 or 90 g dry matter/kg.75 live weight daily. Blood flow through PDV (dilution of p-aminohippurate), net nutrient absorption and oxygen consumption (venoarterial concentration differences times blood flow) were measured hourly for 12 h, followed by measurement of N and energy balance over 7 d. Compared with orchardgrass, steers when fed alfalfa absorbed more NH3-N (P less than .05), branched-chain volatile fatty acids (P less than .10) and n-valerate (P less than .05). Silage type did not affect (P greater than .10) blood flow to or O2 consumption by PDV or net absorption of glucose, L-lactate, acetate, propionate, urea-N, alpha-amino N or most amino acids. Oxygen consumption by PDV as a percentage of whole-animal O2 consumption was not different (P greater than .10) for steers when fed orchardgrass (27.2) or when fed alfalfa (23.6). Interrelationships between N and energy metabolism were responsible for the increased (P less than .05) metabolizable energy/kilogram silage dry matter and increased (P = .10) N retention by steers when fed alfalfa compared with orchardgrass. The PDV accounted for a substantial portion of whole-animal O2 consumption.

Prediction of body composition of dairy cows at three physiological stages from deuterium oxide and urea dilution

To develop equations for predicting body composition, mature Holstein cows (n = 21) were slaughtered at three physiological stages (-7, 63, and 269 d postpartum) after consecutive intravenous dosing with urea and D2O. Blood was sampled at 0 and 12 min after dosing with urea for determination of urea space and from 0 to 72 h after dosing with D2O. Empty body water and total body water were estimated by dilution kinetics for D2O using two- and one-compartment models, respectively. At slaughter, body components were ground, sampled, and freeze-dried for chemical analysis. Prediction of empty body water by urea space was not an improvement over the prediction by body weight alone. Prediction by D2O dilution explained 73 and 87% of the variation in empty and total body water, respectively. Estimated body protein, as determined from empty body water, predicted actual body protein with an error of 4.7 kg. Daily DMI explained 84% of the variation in the DM of the gastrointestinal tract contents (DM fill). Estimations of empty body fat (R2 = .85) and empty body energy (R2 = .89) from D2O dilution were capable of detecting significant differences in body fat (42.9 kg) and body energy (375 Mcal) across physiological stages and might be useful for prediction of body composition changes during the lactation cycle.

Components of growth in Holstein heifers fed either alfalfa or corn silage diets to produce two daily gains

Growth components were compared in an experiment with a 2 x 2 factorial design. Eight replicates of Holstein heifers were fed diets based on either alfalfa or corn silage for daily gain of either 725 or 950 g from 181 to 334 kg of body weight (BW). Mean daily gains from corn diets were greater than gains from alfalfa diets for BW, udder, empty body, fat, fat-free matter, protein, H2O, C, and energy but were less than gains from alfalfa for gut contents and ash. High daily gain produced higher mean values for all of these variables than did lower daily gain. The percentage of protein in fat-free matter was not affected by either main effect. The percentage of fat in empty body gain was increased as daily gain increased. Energy concentration of fat-free matter that was gained was not affected by either diet or daily gain. Energy concentration of empty body gain increased as daily gain increased. The percentage of gut contents in daily BW gain was higher for heifers fed the alfalfa diet than for those fed the corn diet. Energy concentration in daily BW gain was affected by diet and daily gain. Variations of fat and gut contents in daily BW gain must be considered when requirements or expected growth are described.

Milk production of Holstein heifers fed either alfalfa or corn silage diets at two rates of daily gain

Seventy-five prepubertal heifers were fed diets based on either alfalfa silage or corn silage plus soybean meal for daily gains of either 725 or 950 g/d in a 2 x 2 factorial. Heifers were fed from 175 to 325 kg of body weight (BW). The alfalfa diet contained more digestible protein and less digestible energy than did the diet containing corn silage plus soybean meal. Actual gains were preexperimental BW gain, 633 g/d; lowest experimental BW gain, 785 g/d; highest experimental BW gain, 994 g/d; lowest postexperimental BW gain, 494 g/d; and highest postexperimental BW gain, 546 g/d. Compensatory postexperimental BW gains of heifers fed a common diet allowed the heifers to calve at 732 d of age. The postcalving BW was 508 kg, and precalving height at withers was 134 cm. A total mixed diet containing 17.1% CP and digestible energy at 3.12 Mcal/kg of dry matter was fed during lactation. Feed intake, milk and milk component production, and milk composition were not affected by either experimental diet or growth rate. As covariates, milk production was related to age at calving and was more strongly related to BW after calving, but no differences were observed among growth diets. Differences in protein and energy concentrations in experimental growth diets did not affect lactation performance. About 75% of total BW gains during the treatment period occurred before puberty, but rate of gain did not affect milk production. This lack of an effect of prepubertal growth rates on the milk production of primiparous heifers is consistent with six other similar studies that were conducted recently.

Duodenal nutrient flow and digestibility in Holstein steers fed formaldehyde- and formic acid-treated alfalfa or orchardgrass silage at two intakes

Formaldehyde- and formic acid-treated alfalfa or orchardgrass silage were fed at 65 and 90 g DM/kg BW.75.d) to growing Holstein steers (209 +/- SE = 35 kg) fitted with permanent ruminal and duodenal cannulas in a 4 x 4 latin square. Alfalfa had higher (P less than .01) concentrations of cell solubles, total N and rumen-soluble N than did orchardgrass. Digestible energy (Mcal/d), total N and soluble N intake (g/d) were higher (P less than .05) for steers fed alfalfa than for those fed orchardgrass. Total duodenal OM, DM, NDF, N and non-NH3-N flows were greater (P less than .001) for steers fed alfalfa than for those fed orchardgrass and were greater (P less than .001) at high vs low intake. Duodenal bacterial N flow (g/d) was greater (P less than .001) for steers fed alfalfa than for those fed orchardgrass, and bacterial N synthesis (g/kg DM truly digested in the rumen) was 58 and 32, respectively (P less than .001). Ruminal concentrations of NH3-N (P less than .001) and VFA (P less than .05) were greater for steers fed alfalfa than for those fed orchardgrass. Total tract DM, energy and N digestibilities were higher (P less than .05) for steers fed alfalfa vs orchardgrass, whereas total tract NDF digestibility was lower (P less than .01). Tissue N retention tended to be greater (P less than .1) for steers fed alfalfa than for those fed orchardgrass. Regression analysis indicated that duodenal non-NH3-N flow was related to intake of metabolizable energy and soluble N (R2 = .939). Improved performance and higher efficiency of use of ME for tissue gain by steers fed alfalfa rather than orchardgrass is related to lower ruminal acetate:propionate, higher microbial efficiency and greater duodenal DM and N flows.

Utilization of nitrogen and energy by Holstein steers fed formaldehyde- and formic acid-treated alfalfa or orchardgrass silage at two intakes

Our objective was to measure the utilization of energy (E) and N by Holstein steers when fed alfalfa and orchardgrass silages offered at 65 and 90 g DM/kg live weight (LW).75 daily. Twelve steers adapted to the Beltsville respiration chambers were assigned to three Latin squares with 42-d periods. Steers in one square had permanent catheters in the portal and two mesenteric veins. Energy and N balance were measured during a 7-d collection of feces and urine that included a 3-d measurement of respiratory exchange. Energy and N variables were not different between catheterized and uncatheterized steers. Apparent digestibilities of DM, OM, CP, neutral detergent solubles and permanganate lignin were higher (P less than .01) and digestibilities of NDF and ADF, hemicellulose and cellulose were lower (P less than .01) for steers consuming alfalfa compared to orchardgrass silage. When fed alfalfa, steers' daily gross energy intake, DE, urine energy, ME, heat production and tissue energy retention were greater (P less than .01) and fecal energy losses were less (P less than .01) than when they were fed orchardgrass. Partial efficiency of ME use for tissue energy (TE) was greater (P less than .01) for steers when fed alfalfa (46.1%) than when fed orchardgrass (35.6%). Apparent ME (kcal/LW.75) required for maintenance of TE was similar for steers when fed alfalfa (133.9) and orchardgrass (131.2) silages. Nitrogen retention (g/d) was 48% greater (P less than .01) for steers when fed alfalfa (30.6) than when fed orchardgrass (20.7). This study demonstrates that steers used ME from alfalfa more efficiently for TE deposition than ME from orchardgrass.

A computer model to predict empty body weight in cattle from diet and animal characteristics

A computer model was developed to predict empty BW in cattle as a function of diet (forage NDF, physical form of forage [hay vs silage and pasture], proportion of dietary concentrates) and animal (full BW) characteristics. The model was empty BW = full BW * (1 - GFILL), where GFILL is gut fill expressed as a fraction of full BW. An equation obtained from published data (GFILL = .05354 + .329 * NDF) was used to provide a base prediction of GFILL from the fraction of NDF in the forage. Predicted GFILL was then corrected for full BW, physical form of forage, and fraction of concentrates using multiplicative factors obtained from published data. The model was evaluated with data from 11 published experiments. Several breeds of cattle, a wide range of forage types, and diets with 0 to 93% concentrates were represented in these data. Observed values for empty BW were compared to model-predicted values and to values predicted by systems published by the Agricultural Research Council (ARC) and National Research Council (NRC). Sums of squared deviations of predicted values from observed (n = 64) were 3,074, 37,327, and 25,920 for the model, ARC, and NRC systems, respectively. After fitting predicted empty BW values to observed values, proportion of concentrates and forage NDF accounted for a significant (P less than .01) amount of the residual variation with the ARC and NRC systems, but not for the model. This finding suggests that the model will predict empty BW more accurately than the ARC and NRC systems with diets similar to those used in the evaluation.

Prediction of body composition in Holstein steers using urea space

Our objective was to evaluate urea space estimate of body composition in Holstein steers by examining prediction equations based on chemical composition and by comparison of these equations to those developed for beef steers. Urea space was determined in 38 Holstein steers ranging in full-fed live weight from 143 to 404 kg by the change in plasma urea N concentration before and 12 min after the midpoint of an intravenous infusion of 130 mg urea/kg live weight (20% solution in .9% saline). The relationship between empty body water (mean, 159.9 kg) and urea space in Holstein steers (Sy.x = 7.9 kg, R2 = .95) showed slope (.93) and intercept (20.6) similar to those of beef steers (.92 and 21.8). Predictions of empty body water, fat, and protein were slightly improved by including urea space in multiple regression equations with full-fed live weight over equations based on full-fed live weight alone. Predicted mean body protein in Holstein steers based on previous equations developed with beef steers was not different from chemically determined empty body protein. Beef steer equations underestimated mean empty body water (155.4 vs. 159.9 kg) and overestimated mean empty body fat (29.8 vs. 25.8 kg). These data suggest that urea space can be used to improve predictions of body composition on a mass basis in Holstein steers over that using of live weight measurements alone and that prediction equations may be different for different types of cattle.

Energy components of growth in Holstein steers fed formaldehyde- and formic acid-treated alfalfa or orchardgrass silages at equalized intakes of dry matter

Energy retention was compared in Holstein steers fed either alfalfa or orchardgrass silages for 164 d at either 65 or 90 g DM/kg.75 BW daily in a 2 x 2 factorial. Energy retention was estimated by slaughter-balance using an initial kill of eight steers at 216 kg and a final kill of eight steers per treatment at 326 kg. The ADG was not affected (P greater than .05) by silage, but steers fed alfalfa gained less (P less than .001) gut fill (they lost gut fill) and gained more (P less than .001) of the following than steers fed orchardgrass: empty body, 23%; fat, 50%; fat-free matter, 18%; protein, 16%; water, 17%; ash, 43%; gross energy, 31%; and carbon, 38%. With retained energy at 1.15 Mcal/d, retained energy was equally distributed between fat and protein. Increments of daily retained energy greater than 1.15 Mcal were deposited as 76% to fat and 24% to protein; this distribution was not affected by silage. The energy requirement for maintenance, with BW adjusted to equal gut fill, was not different (P greater than .05) at 130 kcal ME/kg.75 BW for steers fed alfalfa vs 125 for steers fed orchardgrass. Although not significant (P greater than .05), retained energy/ME intake above maintenance was 13% greater for steers fed alfalfa (.261) than for steers fed orchardgrass (.230), which supports the difference observed by calorimetry. The difference in dietary protein (25.6 vs 20.5%) did not contribute to the difference in energy retention because the differences in fat and protein retention could be explained totally by differences in daily energy deposition. The higher NDF of orchardgrass, or other fiber components, seems to be the most probable cause of its somewhat lower partial energetic efficiency relative to alfalfa.

Health insurance and the elderly

The effectiveness of proposed changes to the Medicare program depends on consumers' responses to different market incentives, which vary according to the coverage the elderly possess to supplement their Medicare coverage. This Data Watch explores the extent of supplemental insurance among the elderly, based on a new data set from the Medicare Current Beneficiary Survey. Only 11 percent of Medicare beneficiaries have only Medicare as their source of coverage; the rest of the elderly population is covered by either private coverage (employer-sponsored retiree coverage or individually purchased coverage) or Medicaid. An increase in Medicare cost sharing would likely affect one-third of elderly beneficiaries, which calls into question the effectiveness of this approach to Medicare program reform.

Utilization of energy and nitrogen by yearling Holstein cattle fed direct-cut alfalfa or orchardgrass ensiled with formic acid plus formaldehyde

First-growth orchardgrass and alfalfa were harvested at two stages of maturity, treated with formic acid plus formaldehyde, and ensiled as direct-cut silage during 1978 and 1979. The 1978 silages were fed to eight yearling Holstein heifers (average BW 273 kg), and the 1979 silages were fed to eight yearling Holstein steers (average BW 264 kg) in replicated 4 x 4 Latin square experiments to measure total energy and N balance using the Beltsville open-circuit respiration calorimeters. Silage was offered daily at 70 g of DM/kg.75 BW, a rate that was essentially ad libitum for late-maturity orchardgrass, but restricted for the other three silages within each experiment. Cattle fed alfalfa used ME for growth with greater efficiency (55%) than did cattle fed orchardgrass (40%). Cattle fed orchardgrass achieved the same tissue N retention at a lower total N intake than did cattle fed alfalfa. Differences in tissue N retention were accounted for by differences in N intake insoluble in autoclaved ruminal fluid, but soluble in acid detergent, a fraction termed available N. At equal intake of ME and available N, cattle fed alfalfa gained more tissue energy than those fed orchardgrass and gained tissue protein similarly to cattle fed orchardgrass. Fractions composing digestible OM were different between forage types but similar within forage type between maturities at harvest. More efficient use of ME for growth by animals fed alfalfa compared with orchardgrass may be related to differences in digestible OM composition, load of digestive tract content, and composition of absorbed nutrients.

Effects of prepubertal growth rate and diet on lipid metabolism in lactating Holstein cows

The objectives were to determine the effects of rate of BW gain and type of silage fed before puberty on the partitioning of excess dietary energy between synthesis of milk and BW gain in second or third lactation. Accordingly, 41 Holstein heifers weighing 175 kg were fed diets containing either alfalfa silage or corn silage to gain either 725 or 950 g/d until BW was 325 kg and two estrous cycles were observed. Puberty occurred near 281 kg of BW. During second (n = 36) or third (n = 5) lactation, the cows were fed a control diet (60% forage and 40% concentrate) and a high energy diet (20% forage and 80% concentrate) in a double-reversal experimental design with three 6-wk periods. The rate of BW gain before puberty did not affect the magnitude of changes in DMI, milk yield, milk composition, or concentrations of thyroid hormones, insulin, bST, glucose, or lipids in serum when cows were switched from a control to a high energy diet during second or third lactation. However, compared with cows fed a corn silage diet, cows fed alfalfa silage between 175 and 325 kg of BW had more depressed yields of fat, total solids, and FCM when fed the high energy diet than when fed the control diet during second or third lactation. Increased deposition of fat in adipose and mammary tissues of cows with mean BW gain in excess of 950 g/d or fed a corn silage diet between 175 and 325 kg of BW did not result in more pronounced depression of milk fat percentage when cows were switched from a control to a high energy diet during second or third lactation. Overall, neither rate of BW gain nor type of silage fed between 175 and 325 kg of BW had a major influence on partitioning of excess dietary energy between synthesis of milk and BW gain during second or third lactation.

Composition of formaldehyde- and formic acid-treated alfalfa and orchardgrass silages harvested at two maturities and their effects on intake and growth by Holstein heifers

Alfalfa and orchardgrass herbages were each harvested at two maturities (May 22 to 25 vs June 5 to 7) in primary growth and directly ensiled with 5 kg of a mixture of 30% formic acid and 25% formaldehyde per metric ton of fresh herbage in upright conventional silos. Alfalfa silage contained less NDF (71%) and more N (150%) and hot-water-insoluble N (117%) than orchardgrass silage did. Major differences between silages were that orchardgrass contained 20.4 percentage units more NDF and 1.10 percentage units less N than alfalfa. The NDF increased 104% and total N decreased to 86% with advancing maturity. Twenty-eight yearling Holstein heifers (223 kg BW) were given ad libitum access to the four silages with trace mineral salt, and growth rate was measured in a 119-d period. Daily DE intake was 297 kcal/kg.75 BW for heifers fed alfalfa silage compared with 202 kcal/kg.75 BW for heifers fed orchardgrass silage. Daily gain was 992 g for heifers fed alfalfa compared with 661 g for heifers fed orchardgrass. Gross efficiency, or gain per unit of DE, was similar for heifers fed orchardgrass and alfalfa. Rate of gain was primarily a function of ad libitum intake of DE. Heifers fed alfalfa had greater plasma essential amino acid concentrations (122%) than those fed orchardgrass did. Late alfalfa produced greater ADG than orchardgrass harvested 2 wk earlier.

Voluntary intake, growth rate, and tissue retention by Holstein steers fed formaldehyde- and formic acid-treated alfalfa and orchardgrass silages

Alfalfa and orchardgrass herbages of similar digestibility were harvested at early and late maturity from primary growth and conserved as direct-cut silage using formic acid and formaldehyde simultaneously. Major compositional differences between the silages were lower NDF (principally hemicellulose) and a greater N content in alfalfa than in orchardgrass. An initial group of eight steers was slaughtered with a mean BW of 222 kg, and each of the four silages was fed to comparable groups of eight Holstein steers. Ad libitum DMI per unit of metabolic BW for alfalfa silages was 128% of that for orchardgrass silages. The ADG of steers fed alfalfa silages was 132% of that of steers fed orchardgrass silages. Despite greater ad libitum intake, total gut fill, as a percentage of BW, on alfalfa silages was 77% of that on orchardgrass silages. Daily empty BW gain of steers fed alfalfa silages was 158% of that of steers fed orchardgrass silages. Daily energy retention of steers fed alfalfa silages was 180% of that of steers fed orchardgrass silages. Steers fed alfalfa silages retained 140% more protein than steers fed orchardgrass silages did, but steers fed alfalfa silages retained only 71% as much protein energy relative to their total energy retention compared with steers fed orchardgrass silages. Differences in composition of daily energy retained were almost totally a result of differences in the total daily energy retention. Late alfalfa silage produced a greater daily gain than orchardgrass silage cut 2 wk earlier because greater intake compensated for lower digestibility.

Glucose and norepinephrine challenges during abomasal infusion of cis or trans octadecenoates in Holstein cows

This experiment determined the effects of infusion of mixtures of fat containing predominantly cis-C18:1 or trans-C18:1 fatty acids into the abomasum on responses of cows to glucose and norepinephrine challenges administered i.v. Six lactating Holstein cows, each with a rumen cannula, were arranged in two Latin squares with 21-d periods. The common basal diet contained 40% forage and 60% concentrate. Treatments were the uninfused control, 750 g/d of a cis fat mixture (65% high oleic sunflower oil and 35% cocoa butter), and 750 g/d of a trans fat mixture (93% shortening and 7% corn oil) infused into the abomasum via a tube that passed through the rumen cannula. Glucose challenges (0.4 mg/kg of BW, administered i.v.) were conducted on d 18, and norepinephrine challenges (0.7 microgram/kg of BW, administered i.v.) were conducted on d 19 of each experimental period. Despite a lower percentage of fat in milk for trans than for cis treatment, disappearance rates of glucose, secretion of insulin after glucose challenge, and appearance rates of NEFA and triglycerides after norepinephrine challenge were similar between treatments. Thus, these data support the hypothesis that trans-C18:1 fatty acids affect the synthesis of milk fat in the mammary gland of lactating cows.