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,3
* Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, Quebec, Canada J1M 1Z3
Department of Plant and Animal Sciences, Nova Scotia Agricultural College, Truro, Nova Scotia, Canada B2N 5E3
Département des Sciences Animales, Université Laval, Quebec, Quebec, Canada G1K 7P4
2 Corresponding author: benchaarc{at}agr.gc.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: essential oil • monensin • metabolism • dairy cow
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Monensin (MO), a monocarboxylic acid ionophore is widely used in ruminant diets, and its positive effects on feed efficiency, nitrogen, and energy utilization are well known (Plaizier et al., 2000; Tedeschi et al., 2003). Recently, the use of MO premix (Rumensin Premix; Elanco Division, Eli Lilly Canada Inc., Guelph, Ontario, Canada) has been approved in Canada in dairy cow rations at a level of 16 mg/kg of DM. Several reports have been published on the effects of MO supplementation on the digestion, metabolism, and milk production of dairy cows, but only a few studies have investigated the effectiveness of MO fed at the dose of 16 mg/kg of DM on feed intake, nutrient utilization, milk production, and milk composition (Ramanzin et al., 1997; Phipps et al., 2000; Ruiz et al., 2001). Essential oils have been shown to inhibit the activity of ruminal bacteria that are sensitive to MO (McIntosh et al., 2003; Newbold et al., 2004). Therefore, this study was undertaken to determine the effects of 2 antimicrobial agents, EO and MO, on digestion, ruminal fermentation characteristics, milk production, and milk composition in lactating dairy cows.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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. Cows were fed for ad libitum intake, housed in individual tie stalls, and had free access to water during the experiment. Adaptation to experimental treatments was from d 1 to 14, in sacco ruminal degradability measurement from d 14 to 18, ruminal sampling on d 17, and milk yield and sampling and total fecal and urine collection from d 21 to 28. Cows were cared for in accordance with the guidelines of the Canadian Council on Animal Care (CCAC, 1993).
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Apparent Total-Tract Digestibility and N Balance
On d 21 of each experimental period, cows were fitted with harnesses and tubes allowing the collection of feces and urine separately. For 7 consecutive days, feces were weighed and mixed daily, and a representative sample (2%) was taken, stored at –20°C, and subsequently thawed, dried at 55°C for 48 h, and ground through a 1-mm screen (Wiley mill) for chemical analysis. Total urine was collected daily into stainless-steel containers and acidified with H2SO4 (50% vol/vol) to maintain pH <2.0. A representative sample (2%) was taken and kept frozen at –20°C until analysis.
Milk Production and Milk Composition
Cows were milked twice daily in their stalls (0500 and 1700 h), and milk yield was recorded at each milking. During the last week of each 28-d period, milk samples were taken from each cow at each milking, pooled on a yield basis, and stored at +4°C with a preservative (bronopol-B2) until analyzed for fat, protein, urea N, lactose, and total solids. Composite milk samples without preservative were frozen at –20°C until analyzed for the milk FA profile.
Ruminal Fermentation Characteristics
Ruminal fluid was collected from the anterior dorsal, anterior ventral, medium ventral, posterior dorsal, and posterior ventral locations within the rumen at 0, 1, 2, 4, 6, and 8 h after the 0800 h feeding. Samples (250 mL/site) were withdrawn using a syringe screwed to a stainless-steel tube ending with a probe covered by fine metal mesh (RT Rumen Fluid Collection Tube, Bar Diamond Inc.). Ruminal fluid pH was measured immediately after sampling (Accumet pH meter; Fisher Scientific, Montreal, Quebec, Canada), and samples were acidified to pH 2 with 50% H2SO4 and frozen at –20°C for later determination of VFA and NH3-N concentrations.
Protozoa Enumeration
Protozoa counts were carried out on ruminal fluid samples collected 2 h after the a.m. feeding. Ruminal fluid (1 L) and solid digesta (500 g) samples were collected from different locations of the rumen of each cow. Samples were mixed thoroughly, and subsamples of 500 mL of ruminal fluid and 250 g of solid digesta were blended anaerobically under oxygen-free CO2 and strained through 2 layers of cheesecloth. A 3-mL portion of the ruminal fluid strained was preserved using 3 mL of methyl green formalin–saline solution for protozoa enumeration (Ogimoto and Imai, 1981). Protozoa samples were stored at room temperature in the dark until counted.
In Sacco Degradability
Ruminal degradabilities of soybean meal, corn grain, and grass silage were determined using the nylon bag procedure, and these feeds were chosen to represent feed ingredients commonly used as sources of protein, starch (cereals), and fiber (forages), respectively. Feeds were freeze-dried and ground through a 2-mm screen in a Wiley mill (standard model 4; Arthur M. Thomas) and 5-g (DM basis) samples were weighed in duplicate into polyester bags (17 x 9 cm; 53 µm pore size) made of monofilament PeCAP polyester (B. & S. H. Thompson, Ville Mont-Royal, Quebec, Canada). Bags were placed in large mesh (20 x 30 cm) retaining sacs with 3 x 5-mm pores that allowed ruminal fluid to circulate freely. Bags were soaked in 37°C water for 5 min before being placed in duplicate in the ventral sac of the rumen for 0, 8, 16, 24, and 48 h (soybean meal); 0, 8, 16, 24, 48, and 72 h (corn grain); and 0, 8, 16, 24, 48, 72, and 96 h (grass silage). On removal from the rumen, bags were immediately immersed in ice water to stop microbial activity, then thoroughly rinsed with cold tap water and frozen at –20°C. Afterward, bags were thawed, washed in a domestic washing machine, and dried at 55°C for 48 h. Bags and contents were weighed, and residues were ground through a 1-mm screen in a Wiley mill and stored for subsequent analysis of DM (soybean meal, corn grain, and grass silage), total N (soybean meal), NDF and ADF (grass silage), and starch (corn grain). Zero-time disappearance was obtained by washing unincubated bags in a similar manner.
Kinetics of DM, CP, ADF, NDF, and starch degradabilities were calculated using a nonlinear model (Mc-Donald, 1981). The NLIN procedure of SAS (SAS Institute, 2000) was used to fit the following model:
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where p is the disappearance rate at time t, a is an intercept representing the fraction of the constituent that is rapidly degradable (%), b is the fraction of the constituent that is slowly degradable (%), c is the fractional degradation rate of disappearance of fraction b in the rumen (%/h), t is the time of incubation (h), and L is the lag time (h).
Effective ruminal degradabilities (ERD) of DM, CP, ADF, NDF, and starch were calculated using the equation:
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where kp is the ruminal passage rate, calculated at 5.0%/h for silage and 6.1%/h for soybean meal and corn grain, from the equations developed by the NRC (2001) for wet forages (silage) and concentrates.
Chemical Analyses
Analytical DM content of TMR, orts, and fecal samples was determined by oven drying at 105°C for 48 h (AOAC, 1990; Method 930.15). Ash content of the TMR, orts, and feces was determined by incineration at 550°C overnight, and the OM content was calculated as the difference between 100 and the percentage of ash (AOAC, 1990; Method 942.05). The total N content of TMR, orts, and feces was determined by thermal conductivity (LECO model FP-428 Nitrogen Determinator, LECO, St. Joseph, MI). Crude protein was calculated as N x 6.25. The concentration of NDF in TMR, orts, and feces was determined as described by Van Soest et al. (1991) without the use of sodium sulfite and with the inclusion of heat-stable 
-amylase. The ADF content in TMR, orts, and feces was determined according to AOAC (1990; Method 973.18). The NDF and ADF procedures were adapted for use in an ANKOM200 Fiber Analyzer (ANKOM Technology Corp., Fairport, NY). The starch concentration of TMR, orts, and feces was determined colorimetrically using a commercial kit (#10 207 748 035; Boehinger Mannheim, Burgessville, Ontario, Canada; Keppler and Decker, 1974). The ether extract content in TMR and orts samples was determined using a Soxlec system HT6 apparatus (Tecator, Fisher Scientific) according to AOAC Method 920.39 (AOAC, 1990). The concentration of N in acidified urine samples was determined by micro-Kjeldahl analysis (AOAC, 1990; Method 960.52). Concentrations of NH3-N and VFA in ruminal fluid were analyzed by colorimetry using the phenyl-hypochlorite reaction (Weatherburn, 1967) and by GLC (Varian 3700; Varian Specialities Ltd., Brockeville, Ontario, Canada), respectively. Protozoa were enumerated microscopically in a Levy–Hausser counting chamber (Hausser Scientific, Horsham, PA). Each sample was counted twice, and if the average of the duplicates differed by more than 10%, the counts were repeated. Protein, fat, lactose, total solids, and urea N concentrations in milk samples were analyzed by infrared spectrophotometer (System 4000 MilkoScan; Foss Electric, Hillerød, Denmark; AOAC, 1990). Milk FA composition was determined by GLC (HP 5890A Series II; Hewlett-Packard, Palo Alto, CA) according to the method described by Chouinard et al. (1997). Composition of FA in feed samples was analyzed according to the procedure of Sukhija and Palmquist (1988).
Statistical Analysis
Data were analyzed using the MIXED procedure of SAS (SAS Institute, 2000) according to the model: Yijk = µ + ai + ßj + 
k + eijk, where Yijk is the response variable, µ is the overall mean, ai is the random effect of cow i, ßj is the effect of period j, k is the effect of treatment k, and eijk is the random residual error. The residual effect was initially included in the model but was removed because it was not significant. For the statistical analysis of ruminal fermentation characteristics (pH, VFA, NH3-N), sampling time and sampling time x treatment were added to the model and analyzed as repeated measures using PROC MIXED, and the compound symmetry was used as the covariance structure. Factorial contrasts were used to test the main effects of EO supplementation (+EO vs. –EO), MO supplementation (+MO vs. –MO), and their interaction. Results are reported as least squares means ± SEM. Significance was declared at P 
0.05 and a trend at P < 0.10 unless otherwise stated.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). Benchaar et al. (2003b) observed no change in DMI when lactating dairy cows were fed a mixture of EO alone (750 mg/d; Crina Ruminants), whereas feeding greater amounts (2 and 4 g/head per d) of a blend of EO compounds (Vertan; IDENA, Sautron, France) increased DMI of growing beef cattle fed silage-based diets (Benchaar et al., 2006). Although the effects of including MO as a premix or controlled-release capsule in dairy cattle rations have been extensively investigated, results have been variable in terms of DMI. The addition of MO either did not influence (Ramanzin et al., 1997; Broderick, 2004) or decreased (Sauer et al., 1998) the DMI of lactating dairy cows. Tedeschi et al. (2003) speculated that the variability between studies is probably due to differences in the stage of lactation and that a reduction in DMI is a consequence of animals eating to their energy requirement. When cows are in positive energy balance (late lactation or dry cows), supplementation of the diet with MO may increase the energy available per unit of feed consumed (Mcal/d), thus resulting in lower DMI. On the other hand, when cows are in negative energy balance (early lactation) the additional energy available because of MO supplementation is used to improve performance, reduce body reserve losses, or both (Tedeschi et al., 2003). Cows used in the present experiment would have been in positive energy balance because they averaged 98 DIM at the initiation of the experiment, but they did not show any decrease in DMI. Therefore, although feeding MO alone could have a negative impact on the DMI of cows in a positive energy balance, as hypothesized by Tedeschi et al. (2003), feeding EO at the same time could contribute to alleviating the negative effect of MO. These results would suggest that the addition of both EO and MO simultaneously in ruminant diets stimulates DMI compared with feeding either one separately.
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Apparent Total-Tract Digestibility and N Balance
There was no interaction between EO and MO for digestibility of nutrients (Table 3). Apparent digestibilities of DM, OM, and NDF averaged, respectively, 66.6, 68.3, and 47.9%, and they were not influenced by EO and MO supplementation. Apparent CP digestibility averaged 64.1% and did not differ between cows fed EO and those not receiving EO. This would agree with the results of Castillejos et al. (2005), who observed no change in DM, OM, NDF, and CP digestibility when a Crina Ruminants EO mixture was added at the dose of 3.8 mg/L of ruminal fluid in continuous-culture fermenters. The digestibility of NDF was not affected in lactating dairy cows supplemented with a mixture of EO compounds (Benchaar et al., 2003b). In the current study, digestibility of ADF was significantly increased by 3 percentage points when diets were supplemented with EO (48.9 vs. 46.0%). However, ruminal in sacco degradation of ADF was not affected by EO addition (see subsequent discussion; Table 7), suggesting that EO supplementation altered total-tract digestibility by enhancing ADF digestion at postruminal sites. The digestibility of ADF was not changed by EO addition in lactating dairy cow diets (Benchaar et al., 2003b) or in continuous-culture fermenters (Castillejos et al., 2005).
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Apparent digestibility of starch was slightly but significantly (P < 0.05) higher in cows fed EO compared with those fed diets not supplemented with EO (97.0 vs. 96.4%). No effect of MO supplementation was observed on the apparent digestibility of starch. This would agree with the results of Ali-Haïmoud et al. (1995), who observed no change in total-tract digestibility of starch in lactating cows supplemented with MO.
Apparent digestibility of CP was higher for cows fed diets supplemented with MO compared with those not receiving MO (65.0 vs. 63.6%). Earlier studies showed inconsistent effects of MO on N digestibility in dairy cows. For example, Plaizier et al. (2000) reported an increase in N digestibility when early lactating dairy cows were fed MO in the postcalving period, whereas Ali-Haïmoud et al. (1995) observed no effect of MO on N digestibility in lactating dairy cows. Discrepancies between studies could partly be due to differences in diet composition. According to Plaizier et al. (2000), MO administration increases N digestibility in cows fed high-concentrate diets, whereas it has no effect in those fed high-forage diets. Diets used in the present experiment contained approximately 52% concentrate (Table 1
), which would corroborate that hypothesis.
The interaction between EO and MO tended (P = 0.06) to be significant for N intake (Table 3) as a result of differences in DMI (Table 2). Outputs of N in feces, urine, and milk were similar among treatments. As a result, retention of N, expressed in grams per day, averaged 27.1 g/d and was not affected by additive treatments. Data on the effect of EO on the retention of N by dairy cows are scarce. The retention of N was not affected in lactating dairy cows (Benchaar et al., 2003b) or in beef cattle (Benchaar et al., 2006) fed different dose levels of a mixture of EO compounds.
Monensin premix supplementation has increased N retention in postcalving cows, but not in precalving cows (Plaizier et al., 2000). The increased N retention has usually been explained by improved N digestibility (Tedeschi et al., 2003). Although CP digestibility was increased by MO supplementation in the present experiment (Table 3), N retention was similar between treatments. Assuming that the body tissue contains about 20% protein (NRC, 2001), the retention of 27.3 g of N/d (i.e., 170.3 g of protein/d) should have resulted in a BW gain of 0.85 kg/d. Considering the inherent error associated with N balance studies (Spanghero and Kowalski, 1997), true N retention was likely overestimated, as illustrated by the modest BW change recorded (Table 2).
Ruminal Fermentation Characteristics
There was no interaction (P < 0.05) between sampling time and treatment on any measurement of ruminal fermentation characteristics; therefore, only dietary effects are reported (Table 4). No interaction between EO and MO supplementation was observed for ruminal fermentation characteristics. Ruminal pH was increased (6.50 vs. 6.39) by the addition of EO. Similarly, Benchaar et al. (2003a) reported that ruminal pH tended (P = 0.07) to increase in lactating dairy cows fed EO. These findings can be interesting for controlling ruminal pH when cows are fed high-grain diets and are at risk for developing acidosis.
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The addition of EO had no effect on ruminal fluid concentration of NH3-N. This would agree with the results of Castillejos et al. (2005) and Busquet et al. (2005), who reported that EO had no influence on NH3-N concentration in continuous-culture fermenters. On the other hand, McIntosh et al. (2003) and Newbold et al. (2004) observed a reduction in the rate of NH3-N production when a CN acid hydrolysate (i.e., free AA) was incubated for 24 to 48 h in strained ruminal fluid collected from cows and sheep fed 1 g and 100 mg/d of a commercial mixture of EO compounds (Crina Ruminants), respectively. McIntosh et al. (2003) observed no further decrease in NH3-N production when MO was added in ruminal fluid incubations, suggesting that EO reduced ammonia production in ruminal fluid by inhibiting the activity of the same group of bacteria that is sensitive to MO. This group of bacteria, called hyper-ammonia-producing ("HAP") bacteria, was originally identified by Russell et al. (1988) and characterized as having high deaminative activity and as being responsible for a significant proportion of ammonia produced in the rumen. Benchaar et al. (2003a) observed no effect of EO on the NH3-N concentration in the rumen of lactating cows fed silage-based diets. This discrepancy between studies could be due to the procedure used (in vivo vs. in vitro) and therefore to the length of exposure of ruminal bacteria to EO (e.g., 24 to 48 h in vitro vs. 2 to 4 wk in vivo). Moreover, ruminal bacteria could adapt to EO, because Cardozo et al. (2004) and Busquet et al. (2005) have recently observed that the effects of different EO compounds on rumen microbial fermentation disappeared after 6 d of incubation in a continuous-culture system. As a consequence, Cardozo et al. (2004) and Busquet et al. (2005) warned that data from short-term in vitro fermentation studies may lead to erroneous conclusions and must be interpreted with caution. The variable effects of EO on the activities of ruminal bacteria could also be explained by the different doses used. Results from recent in vitro studies (Cardozo et al., 2004; Busquet et al., 2005; Busquet et al., 2006) revealed that EO are effective at high doses (300 to 3,000 mg/L of culture fluid) but at lower doses (0.22 to 2.2 mg/L of culture fluid), EO have little effect on rumen microbial fermentation. Assuming a rumen volume of 100 L for an adult lactating dairy cow, 300 to 3,000 mg of EO per liter of rumen fluid would correspond to an intake of 30 to 300 g of EO/d, which is 30 to 300 times higher than the doses generally used in dairy cow rations (i.e., 1 g/cow per d). Therefore, longer term in vivo experiments are required to clearly establish the effects of EO supplementation at more normal feeding doses.
In the present study, NH3-N concentration in ruminal fluid was reduced (12.7 vs. 14.3 mg/100 mL) for cows fed MO compared with those not supplemented with MO, which would agree with the results of Ali-Haïmoud et al. (1995) and Plaizier et al. (2000). In other studies, MO supplementation had no effect on the ruminal NH3-N concentration of dairy cows (Ramanzin et al., 1997; Broderick, 2004).
Supplementation with EO had no effects on the ruminal total VFA concentrations and on molar proportions of individual VFA, as previously reported by Benchaar et al. (2003a) and Newbold et al. (2004). Castillejos et al. (2005) observed an increase in total VFA concentrations and no change in molar proportions of individual VFA when a mixture of EO compounds (Crina Ruminants) was added to continuous-culture fermenters.
The addition of MO had no effect on the proportions of individual VFA and on the acetate-to-propionate ratio. Ali-Haïmoud et al. (1995) observed no effect of MO supplementation on the acetate-to-propionate ratio, whereas Ruiz et al. (2001) reported that MO decreased the acetate-to-propionate ratio in dairy cows. These discrepancies between studies could be explained by differences in dietary inclusion levels of MO and to interactions between diet composition and MO. Broderick (2004) observed that the acetate-to-propionate ratio decreased when lactating cows were fed MO at 10 mg/kg of DM, but this change was much smaller than has been observed when using a higher dose of MO (Sauer et al., 1998). The level of MO fed in the present study (i.e., 16 mg/kg of DM) was lower than the dose of 24 mg/kg of DM administered in the study of Sauer et al. (1998). This would contribute to explaining the moderate changes in the acetate-to-propionate ratio observed in our study. Surber and Bowman (1998) reported that MO decreased the acetate-to-propionate ratio to a greater extent when beef steers were fed a barley-based diet than when they were fed a corn-based diet. Similarly, Jenkins et al. (2003) observed in vitro (i.e., continuous-culture fermenters) that MO had a greater effect on reducing the acetate-to-propionate ratio when the diet contained barley instead of corn grain. In the current study, the experimental diets contained 30% corn grain (Table 1), thus explaining the absence of significant effects of MO on the acetate-to-propionate ratio. The efficacy of MO in modifying the VFA pattern has also been shown to vary with diet composition, particularly with reference to the dietary proportion of concentrate. Ramanzin et al. (1997) reported that MO decreased the acetate-to-propionate ratio to a greater extent when lactating cows were fed a low-forage diet (50:50) than when they were fed a high-forage diet (70:30). However, no interaction between the forage-to-concentrate ratio of the diet and MO was found when Van Maanen et al. (1978) measured propionate production by the isotope-dilution technique in beef steers fed either a low- or a high-forage diet (20 vs. 70%), suggesting that molar VFA proportions do not accurately reflect changes in VFA production in the rumen (Van Maanen et al., 1978), as was demonstrated subsequently by Rogers and Davis (1982).
Protozoa counts averaged 4.88 x 105/mL and were not affected by the addition of EO and MO to the diet (Table 4). Other studies (Benchaar et al., 2003b; McIntosh et al., 2003; Newbold et al., 2004) reported no effects of EO on the number and composition by genera of the ciliate protozoal populations.
In Sacco Ruminal Degradation Kinetics
There was no interaction between EO and MO for ruminal degradation kinetics of soybean meal (Table 5). The rapidly (a) and the slowly (b) degradable fractions of DM averaged 36.9 and 62.8%, respectively, and they were not affected by additive treatments. The rate of DM degradation (c) tended (P = 0.08) to be higher (7.1 vs. 6.4%/h) and the ERD of DM was increased (67.5 vs. 65.9%) for cows fed MO compared with those not receiving MO. Feeding MO decreased the rapidly degradable fraction (a) of CP of soybean meal (18.0 vs. 16.8%), whereas feeding EO increased it (17.9 vs. 16.8%). The slowly degradable fraction (b) of CP tended (P = 0.07) to increase with MO supplementation. The degradation rate of CP was higher with than without MO (6.0 vs. 5.5 %/h) and tended (P = 0.07) to increase slightly for cows fed EO as compared with those fed no EO (5.9 vs. 5.6 %/h). The ERD of CP was higher for diets containing MO than for diets not supplemented with MO (55.6 vs. 53.0%). Although some changes were noted in the kinetics of in sacco degradation of soybean meal in the rumen of cows supplemented with MO, these changes were too small to have any significant nutritional impact on ruminal N metabolism. Similarly, the kinetics of degradation of soybean meal protein were not changed in growing heifers (Molero et al., 2004) and sheep (Newbold et al., 2004) supplemented with 700 and 110 mg of the EO mixture (Crina Ruminants), respectively.
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). The addition of EO had no effect on degradation kinetics and ERD of DM. The addition of MO did not affect the rapidly degradable fraction of DM (33.7%), but it tended (P = 0.09) to decrease the slowly degradable fraction of DM of corn (64.4 vs. 65.7%) and increased the degradation rate of DM (6.0 vs. 5.4 %/h) compared with when no MO was fed. The ERD of DM of corn tended (P = 0.07) to increase for cows fed diets supplemented with MO (66.2 vs. 64.0%). The ruminal degradation kinetics and ERD of corn starch were not affected by the addition of EO or MO to the diet. Similarly, Surber and Bowman (1998) reported no effect of MO on the degradation rate or the ERD of corn grain.
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). The rapidly degradable fraction (a), the slowly degradable fraction (b), the rate of disappearance (c) of fraction b, the lag time (L), and the ERD of DM were not different among treatments. Similarly, supplementation with EO and MO had no effect on the kinetics of ruminal degradation of ADF and NDF. Although ADF total-tract digestibility was increased by feeding EO (Table 3), results from ruminal in sacco incubations indicated that degradation of ADF and NDF from grass silage was not affected by EO supplementation. These results suggest that cellulolytic bacterial populations in the rumen were not sensitive to the amount or the mixture of EO used. In fact, Benchaar et al. (2003a) observed no effect of EO supplementation on counts of ruminal cellulolytic bacteria in dairy cows.
The lack of any significant effect of MO addition on apparent digestibilities of NDF and ADF (Table 3) would agree with the similar in sacco ruminal degradation of fiber from grass silage observed for cows supplemented or not supplemented with MO.
Milk Production and Milk Composition
Production of milk averaged 33.6 kg/d and was not affected by additives (Table 8). However, 4% FCM yield tended (P = 0.06) to be affected differently by EO and MO, as shown by a trend for an interaction between EO and MO. The yield of 4% FCM decreased when cows were fed EO alone, whereas it increased when cows were fed the diet containing both additives. This would suggest that MO supplementation contributes to lessening the negative effect of EO on 4% FCM yield. Very little work has been published to date on the effects of EO supplementation on the milk performance of dairy cows. Benchaar et al. (2003b) observed no change in milk production when cows were supplemented with 750 mg/d of EO mixture (Crina Ruminants).
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Milk protein and lactose concentrations averaged, respectively, 3.5 and 4.6%, and were unaffected by EO and MO addition. Very little work has been published on the effects of EO on milk composition in dairy cows. Benchaar et al. (2003b) reported no effect of EO (750 mg/d; Crina Ruminants) supplementation on milk composition.
Monensin supplementation had no influence on the milk protein content but decreased the milk fat concentration. Similarly, Sauer et al. (1998) reported a reduction in milk fat content following MO supplementation with no change in milk protein concentration, whereas Ramanzin et al. (1997) observed that feeding MO had no effect on milk concentrations of protein and fat. Conversely, concentrations of protein and fat in milk were decreased when MO was fed to dairy cows (Phipps et al., 2000; Ruiz et al., 2001; Broderick, 2004). In many of the studies in which MO reduced milk fat and protein concentrations, a parallel increase in milk production was observed, thus suggesting that a dilution effect was partly responsible for changes in milk composition (Phipps et al., 2000), although this was not the case in the current study.
The milk urea N concentration was increased by MO supplementation, but this effect was not apparent when MO was fed in combination with EO, as shown by the interaction (P = 0.05) between EO and MO. A higher milk urea N concentration has also been reported when cows were supplemented with MO (Duffield et al., 1998). According to Duffield et al. (1998), MO supplementation increases the amount of protein reaching the small intestine and the use of AA for gluconeogenesis (Plaizier et al., 2000), thus increasing deamination and the concentration of blood urea N. On the other hand, the addition of MO to the diet did not change the urea N concentration in milk (Ruiz et al., 2001). In general, yields of milk components were not changed by addition of additives to the diet, although there was an interaction (P < 0.05) between EO and MO for fat yield, which would agree with the results observed for the yield of 4% FCM.
Essential oils from plant extracts have been reported to have an antibacterial activity against gram-negative and gram-positive bacteria (Helander et al., 1998). Several of the gram-positive bacteria are involved in ruminal biohydrogenation of FA (Bauman et al., 1999), thus suggesting that feeding EO could lower biohydrogenation of FA because of a decrease in the number of bacteria involved in that process. Therefore, we were interested in evaluating whether the milk FA profile could be altered by feeding EO to dairy cows. However, the addition of EO had only minor effects on the milk FA profile (Table 9). The concentration of 14:0 was lower when cows were fed EO alone, but this effect was not apparent when EO was fed in combination with MO, as shown by the trend (P = 0.08) for the interaction between EO and MO. Feeding MO alone increased the concentration of 17:0, but this effect was not apparent when MO was administered in the presence of EO, which resulted in an interaction (P < 0.05) between EO and MO. The milk concentration of CLA tended (P = 0.10) to increase (+17%) in cows fed EO compared with those not supplemented with EO. More investigation is needed to assess the potential of using plant extract compounds to alter biohydrogenation of FA in the rumen.
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). The proportion of 16:1 tended (P = 0.08) to increase (+6%) in milk fat of cows fed MO compared with those fed no MO. The concentration of CLA in milk fat was similar for cows supplemented or not with MO, which would agree with the results of Mutsvangwa et al. (2003). There have been some suggestions that MO can modify milk FA profile by inhibiting biohydrogenation of linoleic acid (18:2) and increasing the content of monounsaturated 18:1 FA (Fellner et al., 1997; Sauer et al., 1998). Fellner et al. (1997) observed higher concentrations of linoleic acid (18:2), trans 18:1, and CLA in continuous cultures of ruminal bacteria following infusion of ionophores (MO, migericin, or tetronasin). Feeding MO had similar effects on enhancing 18:2 and trans FA in the milk of lactating cows (Sauer et al., 1998). Similar increases in trans FA concentration following MO supplementation were observed in the present experiment although there was no difference in 18:2 concentration. Jenkins et al. (2003) suggested that MO increased the concentration of unsaturated FA (e.g., 18:2) in the rumen when the diet contained grains with higher rates of ruminal digestion such as barley as compared with corn. In the present study, corn (grain and silage) was the main source of starch (Table 1), thus explaining the lack of effect of MO on the concentrations of 18:2 and 18:3 in milk fat. |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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3 Present address: Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada T6G 2P5.
Received for publication January 25, 2006. Accepted for publication June 13, 2006.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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