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1 INTA EEA Balcarce, Argentina
2 National Institute of Agricultural Technology (INTA), Experimental Station of Balcarce, CC 276, (7620), Balcarce, Argentina
3 Instituto de Biología y Medicina Experimental (CONICET), V. Obligado 2490. Buenos Aires, Argentina
Corresponding author: G. A. Gagliostro; e-mail: ggagliostro{at}balcarce.inta.gov.ar.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: hydrogenated oil • grazing • milk composition • ruminal digestion
Abbreviation key: C2:C3 = acetate:propionate, control = no supplemental fat, fat = 0.73 kg/d of hydrogenated oil, FCM = 4% fat-corrected milk, IVDMD = in vitro DM digestibility, J-M = jugular-mammary differences, ME = metabolizable energy, PUN = plasma urea nitrogen, SFD = subcutaneous fat depth, WSC = water-soluble carbohydrates
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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In many countries that produce milk from pasture, the main source of supplemental energy used is nonfiber carbohydrates from cereal grains. When considerable amounts of these grains are fed to lactating dairy cows in combination with pastures of highly fermentable NDF, ruminal pH may decrease, resulting in a decrease in milk fat content (Schroeder et al, 2004).
Although in confinement systems milk production is often increased after fat supplementation (Schingoethe and Casper, 1991), the response in pasture-based diets is still poorly described (Schroeder et al., 2004). Besides, the response to nonfermentable energy supply may be different in grazing conditions because the specific nutrients that limit milk yield may not be the same (Schroeder et al., 2002). Supplemental fat can be used to increase the energy density of diets and substitute, in part, for cereal grains, although experimental data on effects of substitution of fermentable energy with rumen inert energy are scarce. Although the effect of feeding fat to dairy cows on milk production has been examined under many nutritional circumstances (Palmquist, 1984; Gagliostro and Chilliard, 1992; Wu and Huber, 1994), in this investigation the diets were based on pasture and were isocaloric, with starch substituted for fat. When hydrogenated oil was added to a basal concentrate of grazing dairy cows in early lactation, the production of FCM and milk fat secretion were increased and pasture fiber digestion was not affected (Schroeder et al., 2002).
Although the hormone profile may play an important role in coordinating the partition of dietary fatty acids between milk fat secretion, deposition in adipose tissue, and body lipid mobilization (Palmquist, 1984), the effects of fat addition on plasma hormones are poorly understood in grazing dairy cows.
The objective of this experiment was to determine whether the partial replacement of energy-yielding substrates to ruminal microbes (corn grain) for nonfermentable energy (hydrogenated oil) may improve milk yield in early lactation cows. Because the control and fat supplied the same amount of energy to grazing cows, we examined the opportunity for fat to affect lactation performance when energy intake is held constant. Effects of fat on milk composition, body condition parameters, plasma metabolites, and hormone concentrations were also studied. The effect of the hydrogenated fat on ruminal digestion was also evaluated in grazing dairy cows.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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).
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The fat was obtained by hydrogenation of vegetable oils. The hydrogenated fat was deodorized and solidified in flakes that were free of moisture (95.9% DM). The medium size of these particles was 2 to 3 mm, and the thickness about 0.3 mm. The flakes were composed of triglycerides of high melting point (58 to 60°C) that may remain solid at ruminal temperature (39 to 40°C). The fatty acid composition of the partially hydrogenated oil was C14:0 (2.4%), C14:1 plus iso C15:0 (0.7%), C15:0 (0.9%), C15:1 (0.4%), C16:0 (30.3%), C16:1 (0.2%), C17:0 (1.2%), C17:1 (0.1%), C18:0 (34.9%), C18:1 (21.8%), C18:2 (3.3%), C18:3 (0.9%), and others (2.9%) as previously described (Schroeder et al., 2002).
All cows grazed together on mixed pastures that contained (DM basis) alfalfa (Medicago sativa, 49%), orchardgrass (Dactylis glomerata L., 15%), bromegrass (Bromus catharticus L., 15%) and perennial ryegrass (Lolium perenne L., 21%). Cows had first access to the pasture in a vegetative stage of maturity of the grazed forage, and we tried to maintain it using a daily strip-grazing system. The area of the strip was regulated using a temporary electric fence to achieve an herbage allowance of about 30 kg DM/cow per day to allow a high pasture intake (Minson, 1990). This figure was calculated assuming an optimum herbage allowance of 45 g OM/kg BW (Minson, 1990), the OM content of the pasture (89.1% of DM) and the initial BW of cows that averaged 599 kg.
The cows were moved to a new strip every day, and after grazing each strip was clipped-out of nongrazed forage to about 6 cm to allow a clean and uniform pasture regrowth.
Samples Collection and Analysis
After an adaptation period of 21 d (–14 to 15 DIM), milk production was measured at each milking daily. Milk samples were collected every 15 d at a.m. and p.m. milkings, composited according to the corresponding volume measured at each milking time and analyzed for fat, total protein, lactose, total solids, and SNF (AOAC 1997) by midinfrared spectrophotometry (Foss 605B Milko-Scan, Foss Electric, Hillerød, Denmark). Milk urea nitrogen (Wiener Lab., Rosario, Argentina; Clinical Chemistry, 18/8, 829–840, 1972) and cholesterol (Colestat, Wiener Lab., Rosario; Clinical Chemistry, 20/4, 470–475, 1974) were determined using commercial enzymatic kits as described in Schroeder et al. (2002). Casein was determined as proposed in AOAC (1997).
Total herbage mass was determined every week by cutting 16 quadrats (0.125 m2 per quadrat) of pasture samples to ground level. Each sample was dried at 60°C in a forced-air oven. The quality of the grazed herbage was estimated from samples obtained by hand-plucking at random transects every 10 d. Samples of concentrates were collected every 30 d. Pasture and concentrates samples were dried (60°C in a forced-air oven), ground through a 1-mm screen (Wiley mill, Philadelphia, PA), and analyzed for OM, NDF, and ADF (Goering and Van Soest, 1970), CP (AOAC, 1997), in vitro DM digestibility (IVDMD) (Tilley and Terry, 1963), water-soluble carbohydrate (WSC) (Morris, 1948), and ether extract (AOAC, 1997).
Pasture DMI was estimated on 9 cows per treatment from 45 to 60 DIM using Cr2O3 as an indigestible fecal marker as previously described (Schroeder et al., 2002). After each milking, cows were dosed twice daily with 3 gelatin capsules containing 2 g of Cr2O3 during a period of 15 d (12 g of Cr2O3/cow per day). Fecal grab samples were collected after milking on d 10 to 15. Total fecal DM production (kg/d) was estimated by dividing the total Cr dosed (g/d) by the Cr concentration in fecal DM (g/d) determined by absorption spectrophotometry. Fecal DM output due to concentrate was estimated as concentrate intake x (1 – concentrate IVDMD). This quantity was subtracted from the total fecal DM production and the remaining fecal DM material was attributed to pasture. Pasture DMI was calculated as the ratio between fecal DM yield due to pasture and pasture indigestibility (1 – IVDMD). Total energy intake was calculated from DMI of forage and concentrates and their NEL content estimated according to NRC (2001). Lipid metabolizable energy (ME) content (5.18 Mcal/kg DM) was calculated as proposed in Schroeder et al. (2002), assuming that FA digestion in the total tract was 56.5% for total C16 and 63.6% for total C18 (Pantoja et al., 1995).
Cows were weighed on 2 consecutive days after the a.m. milking at the start (3 and 4 DIM), in the middle (30 and 31 DIM), and at the end (74 and 75 DIM) of the period of fat supplementation. An additional record of BW was taken at the end of the residual period (104 and 105 DIM). On the same days, BCS and the subcutaneous fat depth (SFD) were also recorded. Body condition score was estimated by 2 independent observers using the 5-point scale (1 = thin to 5 = fat) and SFD was measured between the 12th and 13th ribs using a Pie Medical 480 scanner (Holland). The mean value of the 2 records was used to calculate changes in BW gain, BCS, and SFD.
Blood samples were collected from the jugular vein immediately after the a.m. milking at 15, 30, 45, 60, 75, and 105 DIM. On the days of sampling, cows received the concentrates immediately after bleeding. On d 30, blood was simultaneously drawn from the external mammary abdominal vein to determine jugular-mammary (J-M) differences in metabolites and estimate apparent mammary uptake. Blood was collected in tubes containing EDTA (0.342 mol/L, pH 7.2, Wiener Laboratory, Rosario, Argentina), immediately placed on ice and plasma was obtained (2000 x g at 4°C for 10 min) and stored frozen (–24°C) until analysis. Commercial enzymatic kits were used for plasma urea nitrogen (PUN) (Wiener Laboratory Clinical Chemistry, 18/8, 829–840, 1972), NEFA (Wako Pure, Chemical Industries USA, Inc., Dallas, TX), glucose (Wiener Laboratory Clinical Chemistry, 21/12, 1754–1760, 1975), triglyceride (Wiener Laboratory, Clinical Chemistry, 28/10, 2077-2080-1982) and total cholesterol (Wiener Laboratory; Clinical Chemistry, 20/4, 470–475, 1974) as described in Schroeder et al. (2002).
Plasma IGF-I was measured by radioimmunoassay with previous acid-ethanol extraction as previously described (Schroeder et al., 2002). Insulin-like growth factor-I antibody (UB2-495, Hormone Distribution Program of the NIDDK, National Institute of Diabetes and Digestive and Kidney Diseases, Rockville, MD) was used. The intraassay coefficient of variance was 7% and assay sensitivity was 60 pg/tube. Somatotropin (ST) and insulin were measured in the same samples. Concentration of somatotropin was determined by radioimmunoassay using an anti-ovine antibody as described in (Schoeder et al., 2002). The intraassay coefficient of variance was 7.2% and minimum detectable concentration was 0.8 ng. Insulin was measured by radioimmunoassay using anti-bovine insulin antibody (Sigma, St. Louis, MO) and standard human insulin provided by Laboratories Beta (Buenos Aires, Argentina). Minimum detectable concentration was 0.05 ng.
Rumen Environment and in Situ Pasture NDF Degradability
The effects of fat supplementation on ruminal environment and parameters of pasture DM and NDF degradation were evaluated using 4 lactating Holstein cows in midlactation fitted with ruminal cannulas in a 2 x 2 crossover design with 15-d periods. These cows received the same treatments and were in a single herd under strip grazing conditions together with nonfistulated cows.
During the 15th day of each experimental period, samples of ruminal content were taken from the ventral rumen at 0 (0600 a.m.), 4, 8, 12, 16, and 20 h after the first sample. Ruminal fluid (200 mL) was obtained by straining through 4 layers of cheesecloth, and pH was measured immediately (Orion portable pH meter 250A, Orion Research Inc., Boston, MA). A sample (100 mL) was acidified with 1 mL of 1 N H2SO4 and frozen (–24 °C). Samples were later thawed and centrifuged at 10,000 x g for 10 min (0°C), and the supernatants were analyzed for concentrations of ammonia nitrogen (NH3-N) (Autoanalyser Tecator, model Kjeltec 1030) and VFA.
The VFA were determined in a gas chromatograph (Shimadzu model GC-14) using a gas N2 as a carrier at a flow rate of 50 mL/min and a glass column (2 m long x 2 mm of internal diameter) packed with 80/120 Carbopack B-DA/4% Carbowax 20 M (Supelco, Inc., Bellefonte, PA). Temperatures for the oven, injector port, and detector were 155, 185, and 190°C, respectively.
At the start of each experimental period pasture samples were obtained by hand-plucking and were cut to a final length of 1 cm. The wet material was placed (5 g of DM/bag) in Dacron bags (15.5 x 7.5 cm, 52-µm average pore size, Ankom, Fairport, NY). The bags were incubated in the ventral sac of the rumen by duplicate for 0, 4, 8, 12, 16, 20, 26, 32, 40, and 48 h. After incubation, the bags were rinsed in a pipette washer for 1 h and then hand-washed with cold tap water. Bags were squeezed until the water was clear and then oven dried at 60°C until constant weight. The residues from each bag were weighed, ground through a 1-mm screen, pooled within cow for each time of incubation and analyzed for DM and NDF content.
Kinetic parameters of ruminal DM degradation were estimated with the equation proposed by Ørskov and McDonald (1979): D = A + B (1-e–kdt), where D = disappearance at time (t), A = soluble fraction (%, wash value at 0 h), B = insoluble potentially digestible fraction (%), kd = fractional rate of degradation (%/h), and t = time of incubation. Total potentially degradable fraction of NDF was estimated as A + B. The effective degradation of DM was calculated with the following equation: effective degradation = A + B (kd/(kd+kp)), where kp = rate of passage (assumed to be 0.03, 0.05, and 0.07/h) (Ørskov and McDonald, 1979).
Kinetics parameters of NDF degradation were estimated with the equation proposed by Mertens and Loften (1980): R = D e–k(t-L) + U, where R = cell wall residue (at time after incubation = t), D = digestible fraction, k = digestion rate constant, L = lag time, and U = indigestible fraction. The effective degradation of NDF was calculated as: effective degradation = (D/100)*(k/k+kp))*-((e(–kp/100)*L)).
Statistical Analysis
Milk production and composition, changes in BW, BCS, SFD, and plasma metabolite and hormone concentration were evaluated by the GLM procedure of SAS (1996) using the following model:
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where:
| Yijk | = | the dependent variable
| µ | = | overall mean
| Ti | = | treatment effects
| A(i)j | = | random effects of animal within treatments
| Dk | = | effects of sampling date or time
| (TxD)ik | = | interaction effects of treatment and sampling date or time, and
|  ijk | = | the residual error associated with the ijk observation.
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As no interaction between treatment and week of lactation was detected (P > 0.05) for any parameter measured, milk yield and composition was also analyzed as a completed randomized design introducing data obtained over the early phase (7 to 17 DIM) of the previous lactation as the covariate. The model used was :
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where:
| Yij | = | the dependent variable (average values over the first 75 DIM)
| µ | = | overall mean
| Ti | = | treatment effects
| Cov | = | covariate (milk yield and composition over 7 to 17 DIM of the previous lactation), and
| ij | = | residual error.
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Data from DMI and J-M differences in metabolites were analyzed with the GLM procedure of the SAS (1996) program using the following model:
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where:
| Yij | = | the dependent variable
| µ | = | overall mean
| Ti | = | treatment effects, and
| ij | = | residual error.
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The rumen parameters were analyzed in a 2 x 2 crossover design using the following model:
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where:
| Yijk | = | the dependent variable
| µ | = | overall mean
| Ti | = | treatment effects
| A(i)j | = | random effects of animal within treatments
| Pk | = | the effects of the experimental period
| Hl | = | effects of hour of sampling
| (TxH)ik | = | interaction effects of treatment and hour, and
| ijkl | = | the residual error associated with the ijkl observation.
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Kinetics parameters of DM and NDF degradation were analyzed in a 2 x 2 crossover using the following model:
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where:
| Yijk | = | the dependent variable
| µ | = | overall mean
| Ti | = | treatment effects
| A(i)j | = | random effects of animal within treatments
| Pk | = | the effects of the experimental period, and
| ijk | = | the residual error associated with the ijk observation.
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Differences were considered significant with P < 0.10 unless otherwise stated. All results are reported as least square means.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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. Pasture CP, NDF, and ether extract composition for every 10 d is shown in Figure 1.
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). An interaction between treatment and week of lactation was not detected. Peak yield of FCM occurred at wk 4 of lactation (control = 23.7; fat = 25,6 kg/cow per day, Figure 2).
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). Milk composition of protein, casein, lactose, SNF, MUN, and total cholesterol were not affected by fat supplementation (Table 3).
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). Pasture and total DMI were not affected by fat feeding. Changes in energy intake from pasture or total energy were not observed (Table 4).
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). Total FCM produced over this period averaged 584 (control) and 596 kg (fat) without significant differences between treatments (P < 0.35).
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), BW gain or SFD (Table 6) during the period of fat supplementation. After the end of fat feeding at 75th day of lactation, the cows began to gain BCS, (Figure 3), BW, and SFD in both treatments (Table 6).
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). Average plasma levels of glucose, NEFA, and PUN were not affected by fat feeding. Concentration of plasma triglyceride and total cholesterol was higher in fat supplemented cows compared with controls (Table 7). Average plasma levels of IGF-I tended to be lower (P < 0.11) in cows that received the supplemental fat, whereas plasma somatotropin and insulin concentrations did not change (Table 7). The highest plasma somatotropin concentrations were detected at 15 DIM in control cows (3.66 ng/mL) and at 30 DIM in fat-supplemented cows (3.07 ng/mL). Concentrations of somatotropin decreased gradually in both treatments until 75 d postpartum where the lower levels of the hormone were observed (control = 1.62 ng/mL and fat = 1.33 ng/mL). Plasma IGF-I gradually increased from 15 d postpartum (control = 128.8 ng/mL and fat = 94.9 ng/mL) until 60 d postpartum (control = 164 ng/mL and fat = 136 ng/mL). Plasma insulin increased from 15 d postpartum (control = 0.31 ng/mL and fat = 0.31 ng/mL) until 45 d postpartum in fat supplemented cows (0.87 ng/mL) and 60 d postpartum in control cows (0.80 ng/mL).
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). No residual effects of fat supplementation were detected for the somatotropin/insulin ratio or for any other parameter measured (Table 8).
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). A higher apparent mammary uptake for triglycerides was detected in cows supplemented with hydrogenated oil.
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). The acetate:propionate (C2:C3) ratio tended to be higher (P < 0.06) in the cows that received the hydrogenated oil. The highest pH values were detected at 0600 a.m. (control = 6.23 and fat = 6.40) and the lowest were observed at 1400 p.m. (control = 5.54 and fat = 5.56).
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). The partial hydrogenation of the oil demonstrated no effect on ruminal digestion.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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) was below the values (40 to 50%) considered as adequate for well-managed pastures (Minson, 1990; Muller and Fales, 1998) and close to the value of 34 to 36%, which would not affect voluntary DMI due to rumen fill (Mertens, 1994). The pasture NDF also showed a high rate and extent of rumen digestion (Table 11), suggesting a good quality of the forage apparently consumed by cows. Average pasture CP content (Table 2) was in the range of 15 to 25% proposed by Minson (1990) to obtain high values of forage DM digestibility.
The partial replacement of corn grain with hydrogenated oil increased the energy density of the concentrate (+18%) and reduced its starch content (-22%, Table 1). The energy provided by the concentrate was slightly higher in the control treatment (+4.5%, Table 4) and the main difference between treatments was the nature of this energy being fermentable (starch) in control and nonfermentable (fatty acids) in fat concentrate (Table 1).
In fat-supplemented cows, microbial protein synthesis may be reduced as the consequence of a lower intake of fermentable energy from the concentrate (Clark et al., 1992). However, providing hydrogenated fat might also allow the microbes to use preformed fatty acids, which might improve efficiency of growth (D. J. Schingoethe, personal communication). In this experiment, the inclusion of fishmeal as a source of RUP could prevent a decrease in milk protein content observed when supplemental fat is supplied to lactating dairy cows (Gagliostro and Chilliard, 1992; Wu and Huber, 1994). A soluble source of Ca was added to the concentrate to compensate for any potential loss in Ca digestibility due to fat supplementation (Palmquist, 1984).
Milk Production and Composition
Among all the previous grazing experiments conducted at INTA Balcarce on the effects of fat feeding on lactating dairy cows performance, this was the first one in which nonfermentable energy partially replaced starch in the concentrate and total energy intake was held constant. The results showed that some starch may be replaced with fat increasing milk yield in dairy cows grazing spring pastures (Table 3). This may contribute to prevent ruminal acidosis and reduction in milk fat content often observed with excessive amounts of fermentable energy in the diet.
The increase in milk yield over the first 75 DIM in fat-supplemented cows (+1.3 kg/d, Table 3) was close to the mean effect observed (about 1.0 kg/d) when grazing experiments were reviewed and fat supplementation ranged from 0.2 to 1.0 kg/cow per day (Salado, 2000; Schroeder et al., 2004). It was suggested that positive effects of fat feeding would be mainly obtained in high yielding dairy cows owing to their higher requirements of energy and long-chain fatty acids (Gagliostro and Chilliard, 1992). In the present experiment (replacing starch by fat) and in our previous work (adding fat to the concentrate, Schroeder et al., 2002), a positive response was detected using cows of moderate milk production potential (26 to 27 kg/d at peak of lactation). An increase in milk yield in early lactation (+11.3%) was also detected when 0.4 kg/d of calcium salts of saturated fatty acids were fed to grazing cows of moderate genetic merit (22 to 24 kg of milk/d) for milk production (Gagliostro, 1998). The difference between fat and control cows for FCM (+2 kg/d, Table 3) can be mainly explained by an increase in milk fat production (+12.8%) rather than in milk yield (+5.5%, Table 3). The effect of fat supplementation on milk and FCM production in grazing dairy cows also depends on the degree of saturation of the fat. Whereas supplementation with unsaturated FA sources did not significantly increase milk or FCM production, both parameters were increased by saturated FA supplements (Schroeder et al., 2004).
As cows had similar total NEL intake (Table 4), the increase in milk and in FCM yields (Table 3) after fat feeding were not apparently explained by higher energy absorption (Table 4). The supplementation with hydrogenated oil did not induce a different pattern of variation in BW gain, SFD (Table 6) or plasma NEFA concentrations (Table 7). These results suggest that body tissue mobilization did not seem to contribute in a different way to milk energy between fat supplemented and control cows. As was observed here (Table 4), our previous study also showed a higher efficiency (expressed as FCM production per kilograms of total DMI or per Mcal of NEL consumed) in grazing dairy cows receiving the hydrogenated oil (Schroeder et al., 2002). It has been proposed that maximum efficiency of milk production occurs when about 12 to 16% of total ME requirements are supplied as dietary fat (Brumby et al, 1978). In the present experiment, ME provided by supplemental fat was estimated to be about 8.2%, and in the previous experiment it represented about 11% (Schroeder et al. 2002). Although the 2 estimations were below the range proposed as the most efficient for milk production, positive milk responses were detected.
It has been suggested that supplemental fat in early lactation may improve the persistency of milk and milk fat production through a carryover effect that may occur after the end of fat feeding (Palmquist, 1984; Schingoethe and Casper, 1991). In the present study, fat supply was stopped at 75 DIM and no residual effects of fat supplementation were detected (Table 5, Figure 2) as previously observed by Schroeder et al. (2002). The lack of a positive residual effect in the present experiment should be taken with care because the pasture quality decreased towards the end of the experiment.
The observed increase in milk fat content after fat feeding (+0.22 g/100 g, Table 3) was close to the mean response of 0.17 g/100 g (P < 0.01) observed when grazing experiments were reviewed by Salado (2000) but lower than that observed in nongrazing experiments (+0.40 g/100 g, P < 0.01) using saturated fat (Gagliostro and Chilliard, 1992). The response in total milk fat output (+110 g/d, P < 0.01, Table 3) was higher than the mean response obtained in grazing experiments (+85 g/d, P < 0.01) (Salado 2000). Concentration and yield of milk fat depends on the balance between the increase in exogenous fatty acid transfer to the mammary gland and the decrease in de novo synthesis. A reduction of fatty acid synthesis within the mammary gland may be expected when supplemental fat is added to the diet of lactating dairy cows (Chilliard, 1993). The reduction may be mediated at the ruminal level through a lower rumen acetate and butyrate production or through the inhibitory effect of long-chain FA on mammary lipogenic enzymes (Palmquist, 1984; Chilliard, 1993). In our experiment, production rates of acetate and butyrate were not measured. As changes in the acetate and butyrate proportions were not observed after fat feeding (Table 10) and pasture DMI was not decreased (Table 4), the production of acetate and butyrate was probably not affected. The saturated source of fat used in the experiment could also contribute to maintain the secretion of short- and medium-chain fatty acids, because the inhibition of de novo mammary synthesis is more sensitive to unsaturated fatty acid supply (Palmquist, 1984). The higher apparent mammary uptake of triglycerides in the fat treatment (Table 9) was also consistent with the observed increase in milk fat output (Table 3). According to the results obtained here, when the hydrogenated oil (1 kg/d) was supplied to early lactation cows fed spring pastures, milk fat content (+0.34 g/100 g), and milk fat output (+160 g/d) were increased (Schroeder et al. 2002). The results were explained by a higher yield of C16:0, C18:0, and C18:1 FA with no changes in the secretion of short and medium-chain FA (Schroeder et al. 2002).
Replacement of starch with fat (Table 1) decreases the amount of energy that is available for growth of ruminal microorganisms and may reduce microbial protein synthesis (Clark et al., 1992). As pasture DMI was not decreased by fat (Table 4) and the WSC content in the grazed forage was relatively high (13.4%, Table 2), microbial protein synthesis in the rumen was probably not decreased. Concentration of MUN (11.6 mg/dL, Table 3) was in the range of 10 to 16 mg/dL proposed by Jonker et al. (1998), which may reflect a high N efficiency and a low N excretion. This fact and the inclusion of fishmeal as a source of RUP probably contributed to preventing a possible decrease in amino acid availability to the mammary gland. Other results obtained in grazing conditions (King et al, 1990; Gagliostro 1998; Schroeder et al., 2002) also showed no decrease in milk protein content when saturated fat was added to the diet. In grazing experiments (n = 14), the overall effect of fat feeding on milk protein content (0.53 ± 0.17 kg fat/cow) was near zero in 86% of the studies reviewed by Salado (2000), and reductions in milk protein content were only detected in 7% of the experiments. Decreases in the casein N in milk is often observed when fats are fed and a simultaneous increase in NPN was also reported (DePeters and Cant, 1992). Both in this study (Table 3) and in our previous one (Schroeder et al., 2002), casein N content and MUN were not affected by fat feeding.
Changes in BCS and BW
As it was observed here (Table 6), dietary fat did not seem to decrease the loss of BW or lipid mobilization in early lactation cows (Palmquist, 1984; Chilliard, 1993, Komaragiri et al., 1998). Similar results were observed in grazing dairy cows in early lactation (Gagliostro, 1998; Schroeder et al., 2002).
Total energy intake was not increased (Table 4), and a higher response in milk yield was observed in fat fed cows (Table 3). As the basal NEFA concentrations remained unchanged (Table 7), it seems that energy balance was probably not decreased in fat supplemented cows throughout the experiment. During the period of intake measurements (45 to 60 DIM), calculated energy balance was similar between treatments (Table 4). Agreeing with the results of this experiment, when the hydrogenated oil was added to the concentrate of grazing cows, a higher response in milk yield was observed, but changes in BCS, BW, total energy intake, or basal NEFA concentration were not detected (Schroeder et al., 2002). Komaragiri et al. (1998) suggested that the hormonal profile may play a more important role than the type of diet (fat) in regulating body lipid mobilization in the early-lactation dairy cow. In our study (Table 7) and in the previous experiment by Schroeder et al. (2002), supplemented fat did not change the plasma hormone profile of cows or the somatotropin/insulin ratio. (Table 4). The improvement in the efficiency of energy utilization for lactation when fat is added to the diet has been attributed to a lower energy loss as methane, a greater efficiency arising from the direct use of long-chain FA for milk fat secretion and a higher efficiency of ATP generation from long-chain FA rather than acetate (Chilliard, 1993).
Plasma Concentration of Metabolites and Hormones
Fat supplementation had no consistent effects on circulating glucose and insulin concentrations (Chilliard, 1993). However, as fat replaced starch in the concentrate, a decrease in the entry of propionic acid to maintain hepatic glucose synthesis and pancreatic insulin secretion could be expected. The absence of negative effects of fat on pasture DMI and the high WSC content in the grazed forage (13.4%, Table 2) probably contributed to maintain ruminal propionate, plasma glucose, and insulin levels. Fatty acids absorbed from the hydrogenated oil may have also contributed to maintaining glycemia by reducing total (CO2) or partial (to NADPH2) glucose oxidation (Chilliard, 1993).
Hormones may play an important role in coordinating the partition of dietary fatty acids between milk fat secretion, deposition in adipose tissue, and body lipid mobilization (Palmquist, 1984). Enhanced adipose tissue lipolysis is also associated with a higher plasma somatotropin/insulin ratio (Vernon, 1988). The lack of fat feeding effects on basal NEFA was in accordance with the absence of variations in the body parameters and with the similar somatotropin/insulin ratio. Plasma urea levels did not change in fat-supplemented cows in agreement with the similar MUN observed in both treatments, and the lack of fat effects on rumen NH3-N, and may be due to the similar pasture (and hence RDP) intake.
The higher apparent mammary uptake of triglycerides agrees with the higher milk fat content and yield as jugular concentration and apparent uptake are closely linked (Gagliostro et al., 1991). On the other side, in spite of the increase in plasma cholesterol levels in fat-supplemented cows, the apparent mammary uptake and milk cholesterol content were not affected as previously observed by Schroeder et al. (2002), suggesting a reduced mammary uptake of this metabolite (Christie, 1981).
In cattle, the main hormone stimulating milk production is somatotropin by direct and indirect mechanisms, including the involvement of the IGF system and interactions with metabolic hormones such as insulin and T4 (Vernon, 1989; Cohick, 1998). Circulating concentrations of these hormones are all regulated by food intake and/or nutritional status. No clear effects of dietary fat have been found on circulating somatotropin in lactating dairy cows (Wu and Huber, 1994). In this study and in the previous experiment (Schroeder et al., 2002), somatotropin concentrations were not affected after hydrogenated oil feeding to grazing dairy cows in early lactation, suggesting that the increase in milk production was not mediated by this hormone.
The somatotropin-IGF-1 axis is functional in lactating cows only when nutrient availability is not restricted (Cohick, 1998). In early-lactation cows, McGuire et al. (1998) reported that plasma IGF-I concentration was positively correlated with energy intake and plasma somatotropin levels. The lack of a significant effect of fat on IGF-I concentrations in the present experiment is consistent with the similar somatotropin levels observed, in accordance with the adequate nutritional level provided throughout the experiment (discussed above), and with the similar energy intake of cows.
Ruminal Environment and Forage NDF Digestion
When supplemental fat has negative effects on ruminal digestion, a reduced VFA production and a lower C2:C3 ratio may be expected (Jenkins, 1993). In this experiment, concentration of total or individual VFA were not affected (Table 10), suggesting that the elevation of the melting point above the ruminal temperature was effective in preventing any negative action of the hydrogenated oil on ruminal digestion as proposed by Chalupa (1986). Although the C2:C3 ratio tended to be higher in oil-supplemented cows (Table 10), the result was probably better explained by the replacement of corn grain with fat rather than a direct effect of the supplemented oil because parameters of pasture NDF fermentation were not affected (Table 11).
It was pointed out that replacement of starch with fat may increase ruminal NH3-N concentration owing to the higher RDP/nonstructural carbohydrate ratio (DePeters and Cant, 1992), but this result was not observed here (Table 10). The hydrogenated oil did not seem to affect ruminal forage CP degradation as reported in other grazing experiments (King et al., 1990).
Parameters of pasture NDF digestion were not affected by the hydrogenated oil (Table 11) and showed a high rate and extent of ruminal fiber digestion. The result is consistent with the good quality of the forage offered to cows (Table 2) and agree with previous studies concluding that negative effects were minimal in diets with a high proportion of forage (Palmquist, 1984). The high rate of passage generally observed when dairy cows are grazing high-quality pastures and the adequate levels of calcium of these pastures could alleviate some possible negative effects of fat supplements (Schroeder et al, 2004). More research including unsaturated fat sources and different types of pastures is needed before definitive conclusions can be obtained.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication February 27, 2003. Accepted for publication August 7, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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s anthrone reagent. Science 107:254–255.This article has been cited by other articles:
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  A. T. M. van Knegsel, H. van den Brand, E. A. M. Graat, J. Dijkstra, R. Jorritsma, E. Decuypere, S. Tamminga, and B. Kemp Dietary Energy Source in Dairy Cows in Early Lactation: Metabolites and Metabolic Hormones J Dairy Sci, March 1, 2007; 90(3): 1477 - 1485. [Abstract] [Full Text] [PDF]
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