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ANIMAL PRODUCTION |
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* Dipartimento di Patologia, Diagnostica e Clinica Veterinaria,
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Dipartimento di Scienze Biopatologiche ed Igiene delle Produzioni Animali e Alimentari, Via S. Costanzo 4, 06126 Perugia, Università degli Studi di Perugia, Italy;
and
Dipartimento di Ingegneria Industriale, Via Pentima Bassa 21, 05100 Terni, Università degli Studi di Perugia, Italy
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: cattle • electromagnetic field • ruminal transponder • ruminal bacteria • rumination
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Rossing, 1999). Recently, the traceability concept, linking animal identification to information, has become a very sensitive topic for the consumer and public. In cattle, application of transponders into the rumen for the electronic identification has been successfully tested in several studies since 1976 (Hanton, 1976; Ribó et al., 2001; Caja et al., 2003; Saa et al., 2005).
However, the biological impact of the device because of its long-term persistence within the forestomachs has rarely been studied. In fact, transponder-dependent side effects may arise from the mechanical action of the bolus on ruminal mucosa as well as from exposure to electromagnetic fields generated by the reading procedure. Although these electromagnetic fields are weak and short in duration, their potential for direct or indirect biological effects on both cells and ruminal bacteria cannot be ruled out. Electromagnetic fields of different intensity were recently found to stimulate cell proliferation and to modify gene expression of E. coli cultured in vitro (Potenza et al., 2004b).
Therefore, the present work investigated the long-term effects of passive ruminal transponders on 1) health and performance of cattle over a 2-yr period, 2) patterns of reticulorumenal motility, and 3) in vitro growth and metabolism of bacterial populations in the rumen exposed to an electromagnetic field induced by prolonged transponder activation.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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A total of 16 Italian Holstein-Friesian male calves, 64 Chianina and Italian Holstein-Friesian bulls, and 20 heifers and 82 Italian Holstein-Friesian cows from the experimental farms at the University of Perugia were used. At the beginning of the experiment, the age of the calves and bulls spanned from 3 d to 3 wk and from 6 to 14 mo, respectively; heifers and cows were from 18 to 23 mo and 2 to 4 yr of age, respectively. The calves were kept in individual crates up to 60 d of age and then transferred to pens with outdoor paddocks. Calves received a pelleted feed that averaged (as-fed basis) 14% CP, 17% NDF, and 13% ADF. This concentrate, along with mixed hay (9% CP, 61% NDF, and 40% ADF) and wheat straw (3% CP, 75% NDF, and 48% ADF), was provided in quantities sufficient for ad libitum DMI.
Dairy heifers and cows were raised in a conventional loose housing system and had free access to a total mixed ration, which was formulated with corn silage (6% CP, 37% NDF, and 18% ADF), alfalfa hay (15% CP, 45% NDF, and 33% ADF), and concentrate (18% CP, 16% NDF, and 7% ADF), in different proportions according to their physiological state. Bulls were kept in group pens (8 animals/pen; pen dimensions: 10 x 4.5 m) and their ad libitum diet (10.24% CP, 23.38% NDF, 7.95% ADF) contained (as-fed) 80% steam-rolled barley grain and 20% barley silage. A dietary supplement containing vitamins and minerals was available to all of the cattle. Feed was provided at 0830 and 1330 daily.
Within each category, the cattle were randomly allocated to 2 groups (control and treated), balanced for age, BW, BCS, lactation number, and genetic merit. Treated animals (8 calves, 32 bulls, 10 heifers, and 40 cows) were cared for exactly as the controls except that they were provided with a ruminal transponder by means of a metallic bolus gun at the beginning of the trial. The bolus transponder was delivered into the retisculorumen by inducing the swallowing reflex following its delivery over the caudal portion of the tongue. The operators were previously trained. After transponder administration, animals were observed for a 4-h period to detect any behavioral alteration or regurgitation of the transponder. The day of transponder application was considered d 0.
Bolus Transponders and Reading Procedure with Transceivers
A passive half-duplex, glass-encapsulated transponder was enclosed in each ceramic bolus (51.4 g, 67 x 17 mm, Innoceramics, Teramo, Italy). The ceramic composition was 91.4% Al2O3, 2.5% CaO, 2.2% MgO, and 3.9% SiO. The specific density was 3.65 g/cm3. Technical specifications of the transponders conformed to the International Organization for Standardization, standards 11784 and 11785 (ISO, 1996), and worked at a frequency of 134.2 kHz.
Readability of the transponder was tested throughout the experimental period by the same trained personnel, using a portable reader (Mod P3000, Innoceramics) with a stick antenna (field strength 108 db µV/m at 3 m; reading distance 
25 cm). Reading checks of dairy cows were done in the milking parlor during the afternoon milking session. Reading checks of calves, heifers, and bulls were done in corresponding pens. For each treated animal, readings of the transponder were performed immediately after administration of the bolus and were repeated at 1 h, 24 h, and 1 wk. Continued readings were taken every 2 wk during the first 3 mo and then at approximately monthly intervals for the following 21 mo. For each animal category, the average duration of treatment is reported in Table 1. During the experiment, a total of 2,085 readings were carried out.
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). Production records were adjusted to twice daily milking for 305 d, and age-corrected to a mature equivalent basis (Bagnato et al., 1994). Milk fat and protein analyses were carried out by infrared spectroscopy (DairyLab2 A7S Foss Electric, Hilleröd, Denmark). Reproductive indexes (days open, number of inseminations per pregnancy, conception rate) of heifers and cows were recorded. Chianina bulls were weighed at monthly intervals using a digital scale (precision, 500 g) until they reached the slaughter age (approximately 22 mo).
Rumination Behavior
One objective of this experiment was to evaluate possible modifications of rumination behavior induced by the presence of the transponder in the forestomachs. All 4 animal categories (calves, bulls, heifers, and cows) were used in this experiment. Observations were carried out at the experimental farms according to the focal animal sampling technique (Lehner, 1996). The number of regurgitations in 10 min and chewing movements for each bolus were recorded during each session in randomly selected animals. The observation sessions (1 in July and 1 in December) began no less than 30 d after application of the ruminal transponder and were conducted for 3 consecutive weeks by 2 trained operators. Observations (250 for calves, 491 for bulls, 112 for heifers, and 520 for cows) were stratified by taking into consideration the time of day (0800 to 0900, 1130 to 1230, and 1530 to 1630), season (July and December), operator (1 or 2), animal category (calves, bulls, heifers, and cows), and group (treated or control). Within each session, each animal was observed at least once.
Postmortem Examination
Postmortem localization of ruminal transponders was evaluated in 43 treated animals (8 calves, 29 bulls, and 6 cows) at the abattoir. The forestomachs, opened and cleared of ruminal contents, were carefully examined to find any gross lesion of the mucosa that could be related to the presence of the transponder. Samples from 27 treated animals (8 calves, 14 bulls, 5 cows) were taken from the reticulorumen wall in close proximity to the site of transponder retrieval or where a tissue lesion was evident. Samples of reticulorumen wall were also obtained from the same regions of 23 control animals (8 calves, 14 bulls, 1 cow). Tissue samples were fixed by immersion in 10% neutral buffered formalin for 48 h and then processed for embedding in paraffin following routine tissue preparation procedures. Serial 5-µm-thick sections were prepared and stained with hematoxylin and eosin (Thompson and Hunt, 1966).
In Vitro Studies of the Electromagnetic Field
This in vitro experiment was designed to assess the effects of the electromagnetic field generated by the transponder on rumen microbial numbers and their metabolism. Samples of ruminal contents were collected approximately 30 min before the morning feeding from the same ruminally fistulated dairy cow, which was fed 9 kg daily of mixed hay (12% CP, 62% NDF, 38% ADF) plus 1 kg of concentrate (22% CP, 20% NDF, 5% ADF) containing a vitamin-mineral supplement. The liquid fraction, obtained by filtering the ruminal contents through 4 layers of cheesecloth, was immediately taken to the laboratory, where all subsequent techniques were performed under anaerobic conditions.
Ruminal fluid was added to prewarmed (39°C) McDougall’s artificial saliva (McDougall, 1948) at a 1:4 ratio and buffered to between pH 6.8 and 7.0 with 20% NaOH or 20% H3PO4 if needed. Anaerobic conditions were maintained by bubbling with CO2. Five hundred milliliters of buffered ruminal fluid were dispensed into six 500-mL Erlenmeyer flasks (base diameter, 10 cm), each containing 2 g of alfalfa hay (15% CP, 44% NDF, 32% ADF) and 0.2 g of corn (10% CP, 12% NDF, 2% ADF), both ground to pass through a 1-mm screen. Immediately after addition of the inoculum mixture, the flasks were gassed with CO2 and closed with rubber stoppers that were equipped with Bunsen valves.
The flasks were randomly allocated to control and treated groups and equally distributed in 2 corresponding 205-L incubators (Termodigit, PBI International, Milan, Italy) at 39°C for 48 h. The 2 incubators were close to each other in the same laboratory; nevertheless, the flasks were isolated from radiation interferences by the stainless steel case of the incubator. A transponder was dropped into each treated flask. Inside one of the incubators, the stick antenna (27-cm long) of the portable reader was placed centrally, at the same distance (5 cm) from the 3 surrounding treated flasks, located in the frontal, right-lateral, and left-lateral position. Every 2 h, except between 2400 and 0800, all the flasks were gently mixed by swirling, and their position was changed in a clockwise order.
The intensities of the electric and magnetic fields generated by the transceiver were measured using a 9-cm-long monopole and a 6.5-cm loop antenna connected to a spectrum analyzer (model E4407B - S/N MY41441068, Agilent Technologies Italia S.p.A., Cersnusco sul Naviglio, Italy) through a low-noise preamplifier (model BA 011000-35 - S/N 04-1978, RFPA, Artigues près Bordeaux, France). To improve accuracy, all measurements were performed inside an anechoic chamber by reproducing the same experimental conditions used in the incubators for the in vitro experiments. The effect of reflection from the metallic case of the incubator was considered negligible compared with the direct radiations generated by the radiator caused by the frequency of the electromagnetic field.
The power density components were calculated using the electric and magnetic field data. The electric fields were 0.785, 0.558, and 0.300 V/m. The magnetic fields were 0.300, 0.380, and 0.088 A/m. The power density measured was 0.187, 0.173, and 0.017 W/m2 in the right lateral, left lateral, and frontal position, respectively. Differences between the 3 positions were in accordance with the theoretical radiation pattern of a dipole antenna (Paul, 1992).
After incubation for 24 or 48 h, fluid samples were collected from treated and control flasks to measure pH and to evaluate NH3, lactate, and VFA production. The pH was evaluated by immersing a probe (Microprocessor pH Meter "pH 213", Hanna Instruments s.r.l., Ronchi di Villafranca, Italy) into each flask. The NH3-N content was evaluated using a colorimetric assay (Beecher and Whitten, 1970), and the lactate concentration using a commercial kit (Randox Laboratories Ltd., Ardmore, UK) with a UV-Visible spectrophotometer (DMS 90, Varian Techtron Pty., Ltd., Mulgrave, Australia). Volatile fatty acid production was evaluated by gas chromatography (4810 Gas Chromatograph, Perkin-Elmer, Norwalk, CT) according to Huntington et al. (1998). At the end of the incubation period, anaerobic culture media for enumeration of total, amylolytic, and cellulolytic bacteria were inoculated according to the most-probable-number procedure (Dehority et al., 1989). The experiment was replicated 6 times. Exposures to the electromagnetic field were always carried out with alternating incubators so that slight differences between incubators were averaged.
Statistical Analysis
The data were statistically evaluated by ANOVA using the GLM procedure of SAS (release 8.02, SAS Inst. Inc., Cary, NC). Performance and reproductive traits were analyzed within each animal category. Adjustment was made for the year of birth, calving month, and age at first parturition, in the case of dairy cattle, or for the BW at the beginning of the experiment in the case of bulls. As for the model used for rumination behavior, independent factors other than experimental treatment and operator (i.e., time of the day, season, and animal category) were not included because they were found to be not significant (P >0.05). Factors considered in the model for the in vitro fermentation experiment were treatment (with or without electromagnetic stimulation), time of incubation (24 and 48 h), and in vitro run. Interactions among factors were included if significant (P <0.05). Comparisons among the individual treatments were made by the Tukey test where significance (P 0.05) had been indicated by the ANOVA.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Readability (readable transponders/transponders administered x 100) at d 0 (100% in all animal categories) differed from that observed at the end of the experiment (Table 1). The portable reader failed to read the transponder in 51 occasions (1 in calves, 5 in bulls, 20 in heifers, and 25 in cows) corresponding to 2.4% of the total readings. Twenty-six of the 51 missing readings were associated with 1 calf and 1 bull whose transponders (unreadable at d 360, 352 kg of BW, and d 270, 645 kg of BW, respectively) were not recovered at the abattoir, and with 1 heifer not slaughtered (unreadable at d 120, 517 kg of BW). The remaining reading failures occurred sporadically within the cows.
Animal Performance
Over the 2-yr study period, the presence of the transponder did not affect (P >0.05) annual milk yield [10,268 ± 81.0 vs. 10,318 ± 141.4 kg of mature equivalent milk (n = 1,567)], milk fat yield [311 ± 1.9 vs. 313 ± 2.1 kg (n = 1,566)] and milk protein yield [285 ± 1.8 vs. 280.2 ± 1.9 kg (n = 1,573)] for treated and control cows, respectively. Similarly, treatment did not influence the reproductive traits (149 ± 1.2 vs. 170 ± 11.9 d open and 2.2 ± 0.15 vs. 2.6 ± 0.22 inseminations per pregnancy, for treated and control cows, respectively) except conception rate, which was greater in treated cows (73 ± 2.6 vs. 66 ± 2.7 %; P <0.05).
Body weight gain of bulls up to 22 mo of age was not influenced by treatments (final BW: 785 ± 73.4 vs. 771 ± 49.8 kg; and ADG: 1.4 ± 0.15 vs. 1.3 ± 0.13 kg; for treated and control group, respectively). Animals’ mortality was unaffected by treatment. Four treated cows died from postpartum complications during the experimental period. Two control calves died at the age of 4 wk from acute gastroenteritis and were immediately replaced by 2 newborn calves.
Rumination Behavior
Treated bulls, heifers, and cows showed a lower number (P <0.01) of chewing movements per bolus. Treated animals tended to have a greater frequency of regurgitations (P = 0.07; P = 0.26; P = 0.08 for bulls, heifers, and cows, respectively) when compared with control animals (Table 2). These effects were not observed in calves.
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. Two boluses were never recovered (1 calf and 1 bull).
Postmortem gross visual examination of reticulorumen revealed clear marks of the reticulum mucosa in 8 animals (6 bulls and 2 calves carrying transponders, or 25 and 67% of cases, respectively, according to the retrieval in the reticulum) characterized by dystrophy and flattening of folds and papillae over a limited area (Figure 1). In 1 case, a dystrophic lesion of the papillae, closely reproducing the negative image of the transponder, was found on the ruminal mucosa (Figure 2). Histological examination revealed no specific lesions in the mucosa of the dystrophic areas (Figure 3).
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). After 48 h of incubation, electromagnetic exposed flasks had greater bacterial concentrations than control flasks (Table 3). A slightly greater (P <0.001) NH3-N content was detected in the electromagnetic exposed flasks after 48 h of incubation, whereas no difference was detected in lactate concentration between treatments. Total VFA production as well as most individual VFA did not vary by effect of the treatments, except for propionate and butyrate, which slightly increased (P <0.05) and tended to increase (P = 0.071), respectively, when expressed as a percentage of total VFA concentration (Table 3).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In Vivo Study
The overall readability recorded in the current study was below values reported by other authors. Physical characteristics of the transponder can affect its retention and localization in the reticulorumen. Caja et al. (1999) found that transponders with a weight of 65 g (specific weight >3.3 g/cm3) were characterized by 99.7% readability values. Hasker and Bassingthwaighte (1996) reported 100% retention rate in feedlot steers when similar transponders (65 g, 3 g/cm3) were used. Ghirardi et al. (2003) reported a minimum weight of 65 g and a specific gravity higher than 3.0 g/cm3 to reach the 98% retention rate established by ICAR (2003). In an 8-yr study conducted on 161 beef cows, Ghirardi et al. (2004) found 98.8% readability when using a ceramic transponder with a weight of 75 g (3.36 g/cm3 specific gravity).
Transponder localization in the rumen did not appear to affect its readability. Nevertheless, when the antenna placed beneath the reticulum projection area failed to read the transponder, further attempts were made by moving the stick antenna along the abdomen of the animal. In 1 case (1 heifer), the transponder was unreadable since d 120 after administration, but it was impossible to determine whether it was lost or simply not functioning because the animal was not slaughtered.
Administration of transponders to young (3 to 21 d of age) calves was easily carried out by using the same instrument and technique employed for older animals. Nevertheless, our data suggest that some caution is needed when evaluating the feasibility of the system in these young animals; most transponders administered to calves were found in the rumen, or in 1 case, never recovered at slaughter (Table 1). However, because of the relatively low number of animals used in the current study, readability values and slaughterhouse recovery rates are not directly comparable with those obtained in large-scale experiments.
Overall performance and reproductive traits were not affected by the presence of ruminal transponders, suggesting that the modification of ruminal motility patterns were not able to exert negative effects on digestive and reproductive physiology. In our in vivo study, effects of the electromagnetic field were probably negligible because each animal was subjected to a maximum of 30 readings over the experimental period. At present, no explanation is available for the increased conception rate found in treated cows and heifers. Similarly, an increase of conception rate, although not significant, was reported in bolused sheep by Caja et al. (1999). However, before excluding any side effects caused by transponder-induced changes in ruminal motility of cattle, performance and reproductive traits should be evaluated over a longer period of time with more sophisticated procedures to analyze clinical and chemical aspects of ruminal functions.
Effects of electromagnetic field exposure on animal health have been investigated in several studies, leading to contradictory results. Burchard et al. (2003) reported a moderate decrease in milk yield (4.97%) and milk fat content (16.39%), together with increased DMI (4.75%), in nonpregnant dairy cows exposed to an electromagnetic field (10 kV, 30 µT, 60 Hz) for 3 consecutive estrous cycles, whereas Raleigh (1988) did not observe any increase in feed consumption in beef cattle exposed to electromagnetic field under a direct current 500 kV transmission line for a 3-yr period. Studies with rats (Takebe et al., 1999) and lambs (Thompson et al., 1995) chronically exposed to electromagnetic fields (0.5 mT for rats and 60 Hz for lambs) failed to demonstrate an influence on BW or ADG. These data may suggest that exposure effects can vary with time because of an adaptive response by the animal. The electromagnetic field generated by the transponder in our experiment is lower or greater than those applied in the cited works.
To our knowledge, we have shown for the first time that the presence of a ruminal transponder in the reticulum stimulated rumination in dairy cows and bulls. The increased number of regurgitations per unit time was probably due to mechanical stimulation by the transponder of vagal epithelial receptors, localized near to the basement membrane of luminal reticulorumen epithelium. In fact, these receptors are characterized by a low threshold to mechanical inputs and are sensitive to even light tactile stimulus. In addition, when an inert mass of appropriate dimensions is inserted into the rumen or the reticulum, rumination is stimulated (Nocek and Kesler, 1980; Campion and Leek, 1996).
This hypothesis is further supported, although indirectly, by the findings that calves fitted with transponders did not show any change in rumination patterns. Indeed, most animals in this latter group were found at slaughter to have the transponder positioned in the rumen and not in the reticulum. Therefore, the different localization of the bolus might be responsible for the different rumination pattern. In addition, an adaptation of the mucosal receptors because of early stimulation in calves cannot be ruled out.
Garín et al. (2003), in a study performed on lambs bolused within the first week of life, reported 20% retrieval of transponders in the reticulum of animals slaughtered at weaning. When lambs were slaughtered at the end of fattening period (24 kg of BW) the transponder retrieval in the reticulum was 91.7%. The authors hypothesized that the transponders, located in the ruminal cranial sac after application, were later transferred to the reticulum by ruminal motility.
No information is available in the literature on the effect of transponders on rumination patterns and rate of passage of digesta. Caja et al. (1999) have only hypothesized that the presence of the transponder might modify the rumination process; nevertheless, they did not detect any significant disturbance to intake or digestibility by adult sheep. Moreover, the presence of the transponder did not affect intake in lambs (Garín et al., 2005).
In 9 cases (a total of 41 transponders retrieved in the reticulum or in the rumen), the presence of the transponder for a period of 461 ± 27 d caused mild dysplasia of the mucosa. This effect is probably the consequence of repeated mechanical friction, compression exerted by the transponder, or both on the mucosa itself.
To our knowledge, these kinds of alterations in association with transponders have never been described before. Garín et al. (2005) did not find alterations of the reticulorumen mucosa in lambs with a rumen bolus inserted before or after weaning and slaughtered at 24 kg of BW. Previously, Garín et al. (2003) observed a mild reduction in the keratinization of the reticuloruminal wall when a "mini bolus" (9.3 x 37.4 mm, 5.2 g) was administered to lambs the first week after birth, but no effects on papillae and crest sizes. When the mini bolus was retrieved in the rumen (<10% of lambs), the papillae size was greater than in control animals. The authors attribute these findings to friction and stimulation effect of ruminal motility introduced by the bolus.
In our study, when the transponder was found in the rumen, the lack of any alteration of the mucosa, except in 1 case, was probably a result of the size of the transponder in relation to ruminal volume. On the contrary, the relatively small size of the reticulum, together with the different consistency of its contents and the morphology of the wall, might explain the compressive action of the transponder on the mucosa. In addition, the time elapsed between transponder administration and postmortem observation could further explain the discrepancy with findings reported in lambs.
Castro et al. (2004) detected a lower number of reticular crests per centimeter in the reticuloruminal wall of kids fitted with 50-g transponders and slaughtered at 24 kg of BW, but no modifications of the crest size. Although the experimental conditions were different from those reported in the current work, their and our results confirm that the ratio between reticulum size and transponder dimensions is a relevant parameter to be considered when evaluating the safety and reliability of the system.
In Vitro Study
Values obtained in the in vitro study are not easily comparable with data by other authors, which were mostly obtained in continuous culture fermenters, using different substrates or substrate:inoculum ratios. Nevertheless, results of our study lead to the conclusion that electromagnetic fields of 134.2 kHz, when constantly applied for 48 h, can stimulate bacterial growth and activity, as indirectly confirmed by the decrease of pH and by the increase in bacterial numbers and NH3-N concentration in the medium.
Microbial efficiency decreases when pH is lowered (Strobel and Russell, 1986). In the current study, the decrease in pH was limited by the buffered medium, allowing microbial growth and substrate fermentation. The higher fermentation rate in electromagnetic exposed flasks possibly resulted in increased production of NH3-N, considering the large amount of soluble protein contained in the lucerne hay and the mismatch between the rate at which energy and N were released by the forage. Although bacterial growth was greater in electromagnetic field-exposed flasks, VFA production was not affected as one would expect. The increase in propionate (P = 0.03) percentage was consistent with the greater number of amylolytic bacteria found in treated cultures; this might also have contributed to the decrease in pH of the medium. The relatively high percentage of propionate and butyrate found in control and electromagnetic field-exposed flasks remains unclear.
To our knowledge there is no additional information concerning effects of electromagnetic fields on ruminal anaerobic microorganisms. Extremely low-frequency and radio frequency electromagnetic fields greater than those generated by the transponder device used in the present work can modify metabolic pathways in aerobic bacteria. However, the literature is often conflicting, and the underlying biological mechanisms are not clear.
Potenza et al. (2004b) reported increased cell proliferation and changes in gene expression, caused by a 300 mT static magnetic field (magnetic field that does not vary with time) in Escherichia coli cultured in vitro. Point DNA mutations were also reported in a series of in vitro experiments where E. coli cultures and extracted DNA were exposed to a 200 to 250 mT static magnetic field (Potenza et al., 2004a).
Fojt et al. (2004) studied the biological effects of different exposure times to a magnetic field (50 Hz, 10 mT) on E. coli, Leclercia adecarboxylata, and Staphylococcus aureus. The decrease in bacterial viability depended on the species (E. coli was the most affected strain) and increased with the time of exposure. Extremely low-frequency electromagnetic fields (2–50 Hz, 1–10 mT) were used in different trials with E. coli, Proteus vulgaris, Photobacterium phosphoreum, and P. fisheri (Mittenzwey et al., 1996). Protein synthesis and the protein expression pattern were not affected by treatment, but growth rate and luminescence were significantly influenced only when the treatment was combined with heat stress.
The biological mechanisms assumed to be responsible for this large array of effects are not clear. Low-frequency electric fields do not penetrate cells very effectively because of the dielectric constant of the cell membrane, but low-frequency magnetic fields do penetrate. The electric fields that magnetic fields induce in cells are very small because of the dimensions of the cell, so an indirect route via the membrane has been assumed. However, it is also possible for the magnetic fields to interact directly (Blank and Goodman, 1997).
Because calcium ions play a major role in cell biology and membrane function, several experiments have focused on this subject. However, results are contradictory; Bauréus Koch et al. (2003) affirm that cell membrane is a site of interaction for weak low frequency magnetic fields, and more specifically, the opening of calcium channels is influenced by extremely low-frequency magnetic fields. Fojt et al. (2004) suggested that electromagnetic fields might affect permeability of ionic channels of the membrane. This can affect ion transport into the cell and result in a large array of biological changes in the organisms. Electromagnetic field-induced effects would be more evident in in vitro systems because they lack protective cellular responses present in living organisms (Potenza et al., 2004a).
A positive effect on glucose utilization and growth rate of E. coli was demonstrated by Nascimento et al. (2003), but under experimental conditions (5 G electromagnetic field generated by an alternate 60 Hz voltage source) very different from those used in the current study. In particular, the effect would be justified by an excitement of the periplasm-binding protein-dependent transport system and by a more precocious, prolonged log phase.
Considerable variation in cellular response to extremely low-frequency field exposure has also been reported in eukaryotic cells. Frequency, field strength, and AC-DC magnetic field combinations, as well as biological factors such as the cell status, have been shown to affect the magnitude of the response. However, factors implicated in this process and biological mechanisms can be different from those described for prokaryotic cells.
In conclusion, the present work demonstrated for the first time that ruminal boluses used for electronic identification of cattle may interact with reticuloruminal motility. The electromagnetic field generated by the transponder, continuously activated for 24 or 48 h in an in vitro ruminal system, modified bacterial growth rate and metabolism. The biological relevance of these findings should be considered with caution given the very long exposure time that was employed. In fact, under on-farm operating conditions, ruminal transponders will likely be activated for only few minutes a day, at performance recording or feed dispenser.
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Footnotes |
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2 The authors would like to thank C. Cavalletti for assistance with data collection and care of the animals, E. Del Rossi and E. Cassetta for technical help in laboratory analyses, and Patrick Raymer for revision of the English text.
3 Corresponding author: cristiano.boiti{at}unipg.it
Received for publication March 8, 2006. Accepted for publication June 8, 2006.
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