Bulk tank milk selenium and its association with milk production parameters in Canadian dairy herds

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1 Article Bulk tnk milk selenium nd its ssocition with milk production prmeters in Cndin diry herds Alejndro Cebllos-Márquez, Hermn W. Brkem, Henrik Stryhn, In R. Dohoo, Gregory P. Keefe, Jeffrey J. Wichtel Abstrct An observtionl study ws conducted in 63 Cndin diry frms to evlute the ssocition of bulk tnk milk selenium (Se) concentrtion (BTSe) with verge dily milk yield, milk ft nd milk protein, bulk tnk somtic cell count (BTSCC), nd the probbility of being Stphylococcus ureus-positive herd. Bulk tnk milk smples collected between Mrch 2007 nd Februry 2008 were evluted for BTSCC, S. ureus culture sttus, nd BTSe. Men BTSe ws mmol/l; no herds were clssified s deficient or mrginl bsed on BTSe. Bulk tnk milk Se ws unconditionlly ssocited with milk production; however, djusting by region indicted confounding effect of this vrible. There ws no reltionship between BTSe nd BTSCC. Higher vlues of BTSe were ssocited with lower risk of being S. ureus-positive herd, possibly s result of more robust udder immune response, or s result of providing Se t higher rte s one component of n udder helth progrm tht reduces risk of being S. ureus positive. Résumé Sélénium dns le lit de réservoir et son ssocition vec les prmètres de production du lit dns les troupeux litiers cndiens. Une étude pr observtion été rélisée dns 63 fermes litières cndiennes fin d évluer l ssocition entre les concentrtions de sélénium (Se) dns le lit de réservoir (SeLR) et l production quotidienne de lit, des mtières grsses du lit et des protéines du lit, l numértion des cellules somtiques de réservoir (NCSR) et l probbilité de l présence d un troupeu positif pour Stphylococcus ureus. Des échntillons prélevés dns le lit de réservoir entre mrs 2007 et février 2008 ont été évlués pour l NCSR, le sttut de culture de S. ureus et de SeLR. Le SeLR moyen étit de 0,51 6 0,15 mmol/l; ucun troupeu n été clssé comme ynt une crence ou des doses minimes de SeLR. Le Se dns le lit de réservoir étit inconditionnellement ssocié à l production de lit; cependnt, l justement pr région indiqué un effet prsite de cette vrible. Il n y vit ucune reltion entre le SeLR et l NCSR. Des vleurs supérieures de SeLR ont été ssociées à un risque inférieur d être un troupeu positif pour S. ureus, peut-être en rison d une réction immunitire robuste du pis ou en rison de l jout d un tux supérieur de Se comme composnte d un progrmme de snté des pis qui réduit le risque d un résultt positif pour S. ureus. (Trduit pr Isbelle Vllières) Cn Vet J 2012;53:51 56 Introduction Soils of the estern nd western Cndin costs contin reltively low selenium (Se) concentrtions. Consequently, low concentrtion of Se [, 0.05 mg/kg dry mtter (DM)] hs been reported in pstures nd crops grown on these soils (1,2). A low Se concentrtion in the forge, which constitutes t lest 50% of the totl dry mtter intke of the diet, hs been ssocited with mrginl serum Se concentrtions in ruminnts (1). Tken together, these results indicte tht Se deficiency my be widespred problem in Cnd, prticulrly in the costl provinces where livestock re fed entirely on loclly grown feeds. Centre for Veterinry Epidemiologicl Reserch, Deprtment of Helth Mngement, Atlntic Veterinry College, University of Prince Edwrd Islnd, Chrlottetown, Prince Edwrd Islnd C1A 4P3 (Cebllos-Márquez, Stryhn, Dohoo, Keefe, Wichtel); Deprtmento de Sistems de Producción, Universidd de Clds. Mnizles, Colombi (Cebllos-Márquez); Deprtment of Production Animl Helth, Fculty of Veterinry Medicine, University of Clgry, Clgry, Albert T2N 4N1 (Brkem); Deprtment of Reproduction, Obstetrics nd Herd Helth, Fculty of Veterinry Medicine, Ghent University, Merelbeke, Belgium (Brkem). Address ll correspondence to Alejndro Cebllos-Márquez; e-mil: lejndro.cebllos@uclds.edu.co Dr. Cebllos-Márquez s current ddress is Qulity Milk Production Services, Cornell University, 430 Millenium Drive, Genesco, New York 14454, USA. Use of this rticle is limited to single copy for personl study. Anyone interested in obtining reprints should contct the CVMA office (hbroughton@cvm-cmv.org) for dditionl copies or permission to use this mteril elsewhere. CVJ / VOL 53 / JANUARY

2 ARTICLE Milk yield is reported to be sensitive indictor of Se sttus in diry cttle, thus low milk yield my be n importnt economic consequence of subclinicl Se deficiency in cttle (3). Milk yield incresed by 4.6% in cows in New Zelnd tht were supplemented with Se (4). In Prince Edwrd Islnd (PEI), milk yield ws, on verge, 7.6% higher in Se-dequte diry herds thn in herds with mrginl Se sttus (5). Further, 2 North Americn clinicl trils hve indicted n increse between 2% nd 5% in milk yield in diry cows supplemented with orgnic Se t the onset of lcttion (6). A dietry Se concentrtion, 0.1 mg Se/kg DM hs been ssocited with n incresed risk to Se-responsive disorders, such s mstitis in cttle (7), nd tht risk is higher when dietry Se decreses to 0.05 mg/kg DM (8). Two North Americn herdlevel studies identified negtive ssocition between Se sttus nd bulk tnk somtic cell count (BTSCC) in diry herds with good control of clinicl mstitis (CM) cused by mjor pthogens (9,10). Stphylococcus ureus is frequently isolted in herds with high BTSCC nd there is herd-level prevlence of S. ureus of 83% in Cndin diry herds, with higher geometric men BTSCC in S. ureus-positive herds thn in negtive herds (11). Although 59% of PEI diry herds were mrginl for Se sttus in , there ws no ssocition of bulk tnk milk Se concentrtion (BTSe) with ny mesure of udder helth t herd level (5). Selenium mngement prctices hve chnged over the pst decde, nd more Se sources re vilble, but little is known bout the ssocition between herd-level Se sttus mesured by BTSe, nd udder helth in Cndin diry herds. Becuse no studies of the ssocition of BTSe with milk yield nd udder helth hve been conducted in Cnd, nd becuse of previous evidence indicting tht Se intke by diry cows my be lower thn the current recommended level (8), we hypothesized tht Se sttus my be ssocited with milk production prmeters nd mesures of udder helth. The objectives of this study were: 1) to determine the Se sttus in selected Cndin diry herds by mesuring BTSe; nd 2) to evlute the ssocition between BTSe concentrtion nd milk production prmeters, BTSCC, nd the probbility of being S. ureus-positive herd. Mterils nd methods Tble 1. Brekdown by region nd seson of unvilble smples for the evlution of bulk tnk milk selenium concentrtion in Cndin diry herds Region Seson Avilble Unvilble Western Cnd Winter 7 8 Spring 6 9 Summer 7 8 Fll 0 15 Ontrio Winter 1 2 Spring 0 3 Summer 3 0 Fll 0 3 Québec Winter 28 0 Spring 22 6 Summer 0 28 Fll 0 28 Atlntic provinces Winter 11 6 Spring 17 0 Summer 15 2 Fll 6 11 A totl of 63 diry frms from Albert (n = 15), Ontrio (n = 3), nd Québec (n = 28), which were considered s one region, nd Atlntic Cnd (n = 17) prticipting in the Cndin Bovine Mstitis Reserch Network (CBMRN) cohort study were selected s strtified convenience smple to represent, s closely s possible, the Cndin commercil diry frm popultion. Selection criteri, production dt, nd housing mngement hve been described elsewhere (12). The selection process ws bsed on the reltive contribution of ech region to Cndin milk production, nd on strt of low, intermedite nd high 12-month rolling verge BTSCC in 2006, with cut-offs, , between nd , nd cells/ml, respectively. The frms were grouped ccording to the diry production system s follows: intensively mnged herds with cows housed in tie-stll or free-stll brns over the yer, nd intensively mnged herds with cows on psture during the grzing seson (between spring nd summer). All frms hd. 80% Holstein-Friesin cows, the cows were milked twice dily, nd the frms subscribed to Diry Herd Improvement (DHI) recording (12). Smpling nd dt collection Smpling procedures were discussed between the CBMRN personnel nd diry producers before the beginning of the study (12). Technicins from the CBMRN project septiclly collected bulk tnk milk smples from the selected frms on monthly bsis. Immeditely fter collection, smples were frozen nd sent to the Mritime Qulity Milk lbortory of the University of Prince Edwrd Islnd (Chrlottetown, PEI, Cnd) for nlysis. Smples were cultured for S. ureus, nd then preserved with Bronopol nd refrigerted or re-frozen in preprtion for SCC nlysis. An liquot (5 ml) of the smples collected in April, July, October of 2007 nd Jnury 2008, which typiclly represented spring, summer, fll, nd winter periods, respectively, ws used to evlute BTSe concentrtion. However, full set of the sesonl smples from ech herd ws not lwys vilble for BTSe nlysis (Tble 1). Specific frm dt, such s herd size (number of lctting cows), verge dys in milk (DIM), nd dily milk yield corresponding to the study period (from Mrch 2007 to Februry 2008) were obtined from the regionl DHI orgniztions. Lbortory nlysis For ech bulk tnk smple, n liquot of 0.05 ml ws plted on blood esculin gr to detect S. ureus (13). Pltes were exmined fter 24 nd 48 h of incubtion, then colonies were counted when # 10/plte (equivlent to # 200 cfu/ml). Stphylococcus ureus ws identified by Grm stin, positive ctlse test, nd double-zone hemolysis on blood gr. Preserved smples were nlyzed for SCC using Foss 4000 cell counter (Foss Electric, Hillerød, Denmrk). Milk Se concentrtion in fresh smples ws evluted by grphite furnce tomic bsorption spectroscopy (14). Redings were crried out using spectrometer (PerkinElmer AAnlyst 52 CVJ / VOL 53 / JANUARY 2012

3 Tble 2. Mesures of herd productivity bsed on DHI informtion for 63 Cndin diry frms between Mrch of 2007 nd Februry of 2008 Vrible Men s Medin Rnge Herd size Lctting cows Dry cows Culled cows Averge dys in milk Dily milk yield (kg/cow) Milk ft (%) Milk protein (%) BTSCC (1000 cells/ml) Clving to first service (dys) Clving to conception (dys) Bulk tnk somtic cell count. s stndrd devition. 800; PerkinElmer, Wlthm, Msschusetts, USA), reporting milk Se concentrtion s micromoles per liter (mmol/l). Herd Se sttus ws interpreted using previously suggested cut-off points (5). Briefly, the reference rnge for BTSe ws estblished by fitting regression model of the men herd serum Se concentrtion on the BTSe concentrtion in 15 diry herds from PEI with widely differing BTSe concentrtions. The cutoff points for BTSe were selected by clculting the point t which the reference vlues for serum Se from the peer-reviewed literture intersected the regression line of the model. Thus BTSe concentrtion, 0.12 μmol/l ws considered to represent deficiency, 0.20 μmol/l divided the mrginl rnge eqully into high- nd low-mrginl rnges, nd 0.28 μmol/l or higher ws tken to represent n dequte sttus. Sttisticl nlysis A totl of 192 smples were collected to evlute BTSe; however, due to the uneven yerly distribution of smples within herds, single nnul BTSe vlue ws estimted for ech herd from the vilble dt for the period between Mrch 2007 nd Februry Unconditionl ssocitions between independent nd dependent vribles were estblished. All independent vribles unconditionlly ssocited (P # 0.10) with the dependent vribles were included in multivrible model which ws mnully reduced by bckwrds selection of the significnt vribles (P # 0.05). The estimtion ws bsed on multivrible liner mixed model for BTSe, including ll independent vribles: region, seson [winter (December 21st to Mrch 20th), spring, summer, nd fll], housing system (tie-stll, free-stll, nd bedding pck brns), nd whether or not cows hd ccess to psture in the grzing seson. Clustering within herds ws ccounted for by including herd rndom effects (15). The dependent vribles used in the sttisticl models were: yerly verge of dily milk yield nd BTSCC (log scle), s well s isoltion of S. ureus in bulk tnk milk smples during the study period (Mrch 2007 to Februry 2008). Milk production prmeters nd the probbility of being S. ureuspositive herd were nlyzed by fixed-effects liner nd logistic regression models, with region nd the single estimted BTSe vlue s predictors. A S. ureus-positive herd ws defined by the isoltion of S. ureus ($ 1 cfu/0.05 ml) in 2 consecutive bulk Tble 3. Finl multivrible regression model of the ssocition between bulk tnk milk selenium concentrtion nd region, seson, housing system, nd whether or not cows hd ccess to psture in 63 diry frms from 3 Cndin regions Effect Coefficient 95% CI P-vlue Intercept , 0.68, 0.01 Region 0.10 Atlntic Cnd 20 Albert , 0.06 Ontrio/Québec , Seson, 0.01 Fll 20 Winter , 0.23 Spring , 0.28 Summer , 0.16 Housing system, 0.01 Bedding pck 20 Tie-stll , Free-stll , Grzing 0.04 No 20 Yes , 0.00 Tble 4. Averge bulk tnk milk selenium concentrtion (BTSe in mmol/l) nd yerly men BTSe estimted for 63 diry frms from 3 Cndin regions BTSe BTSe observed estimted Region Number of frms Men s Men s Albert Ontrio/Québec Atlntic Cnd s stndrd devition. tnk milk smples collected no more thn 2 mo prt during the study period (16). All model ssumptions were evluted by exmining the stndrdized residuls. A nturl logrithmic trnsformtion of SCC vlues (1000 cells/ml) ws used to pproximte the norml distribution. Anlyses were crried out in SAS version 9.2 (SAS Institute, Cry, North Crolin, USA) using the MIXED, GLM, nd LOGISTIC procedures. Results Most frms used tie-stll brn system (57%), while free-stll nd bedding-pck brn systems were used in 35% nd 8% of the frms, respectively. Cows were housed throughout the yer in 71% of the frms. Most of the frms (56%) where cows hd ccess to psture during the grzing seson were locted in Atlntic Cnd. Some herd-level mesures of productivity re presented in Tble 2. No BTSe vlues comptible with Se deficiency were found; however, 8 smples (4%) were considered mrginl (BTSe, 0.28 mmol/l). These mrginl vlues were found in 1 frm from Québec, nd in 7 frms from Atlntic Cnd; ll of them used tie-stll brn with cows hving ccess to psture during the grzing seson. Five of the 8 BTSe mrginl vlues were found in the summer, 2 in the spring, nd 1 in the winter. ARTICLE CVJ / VOL 53 / JANUARY

4 Tble 5. Unconditionl ssocitions nd multivrible model of estimted bulk tnk milk selenium concentrtion (BTSe in mmol/l) nd region with yerly verge of milk production prmeters in 63 diry frms from 3 Cndin regions Dily milk yield (kg/d) Milk ft (%) Milk protein (%) Coefficient 95% CI P-vlue Coefficient 95% CI P-vlue Coefficient 95% CI P-vlue ARTICLE Unconditionl ssocitions BTSe , , , Region 0.05, Albert Ontrio/Québec , , , 0.11 Atlntic Cnd , , , 0.04 Multivrible model Intercept , 35.1, , 3.72, , 3.31, 0.01 BTSe , , , Region 0.05, Albert Ontrio/Québec , , , 0.12 Atlntic Cnd , , , 0.04 Univrible results were shown only to illustrte the confounding effects of region. Bulk tnk milk Se concentrtion tended to differ mong the 3 Cndin regions (Tble 3). Bulk tnk milk Se significntly differed between sesons, housing systems, nd grzing. Men BTSe ws significntly lower in the summer, in frms using tie-stll brns, nd in frms where lctting cows hd ccess to psture (Tble 3). The estimted yerly BTSe vlues were lower thn the observed vlues (Tble 4). Most of the bulk tnk smples from diry frms in Albert nd Ontrio/Québec were collected in the winter nd spring (Tble 1), corresponding to the time of the yer with higher BTSe vlues; therefore, the estimted yerly verge vlues were lower thn the observed verges. This difference ws less mrked in Atlntic Cnd where herds hd BTSe dt collected in the summer nd fll. There ws n unconditionl (univrible) positive ssocition between men dily milk yield nd BTSe (Tble 5). Dily milk yield incresed 12.3 kg for ech unit (mmol/l) increse in BTSe (P = 0.01). However, there ws no ssocition between BTSe nd other milk production prmeters, such s milk ft nd milk protein (Tble 5). The ssocition between BTSe nd milk production prmeters ws djusted by geogrphicl region, leding to substntil chnges in the coefficients (Tble 5), nd thus suggesting confounding effects of region on the reltionship between BTSe nd production. Unconditionlly, bulk milk Se concentrtion hd no effect on BTSCC (P = 0.29), while regionl differences were found (Tble 6). Frms in Ontrio/Québec hd higher men BTSCC thn did frms in Albert (P = 0.05) nd the Atlntic region (P = 0.03). No differences were observed between Albert nd Atlntic regions (P = 0.91). The djustment of the ssocition between BTSe nd BTSCC by the effect of geogrphicl region showed substntil chnge in the coefficient, suggesting confounding effects of region on tht reltionship (Tble 6). Stphylococcus ureus ws found t lest once in 47 (75%) herds during the study period. Bulk tnk milk Se concentrtion ws unconditionlly ssocited with the odds of being S. ureus-positive herd (P = 0.002). When BTSe increses, the odds of being S. ureus-positive herd re reduced (Tble 7). After controlling for region, the protective effect of BTSe ws slightly reduced, but remined significnt. Higher vlues of BTSe were ssocited with lower risk of being S. ureuspositive herd (Tble 7). Discussion Bulk tnk milk Se concentrtion responds rpidly to chnges in Se intke, nd hs been stisfctorily used s screening test for evluting lctting cows Se dequcy (5). In the United Sttes men milk Se concentrtions in cttle rnged from 0.09 to 1.11 μmol/l (17). In Cnd, men cow-level milk Se concentrtion ws 0.35 mmol/l (18), rnging between 0.23 nd 0.34 μmol/l in herd-level studies (5). In the present study, 8 of 192 smples hd BTSe, 0.28 μmol/l; 7 of these smples were from frms in Atlntic Cnd. Selenium-mrginl vlues my be relted to low Se concentrtions found in forges nd crops grown in the Atlntic provinces, where pproximtely 90% of forge smples contin, 0.05 ppm of Se (2), which is insufficient to meet the Se requirement in cttle. No BTSe vlues comptible with Se deficiency were found. These results must be interpreted with cution becuse Se supplementtion prctices hve chnged mrkedly since erlier studies estblished the reltionships between concentrtions of Se in blood nd milk. Erlier dt were derived from herds where the principl or only source of supplementry Se ws inorgnic (for exmple, sodium selenite). Orgnic sources of Se hve been widely used in North Americn diry herds in recent yers. At the recommended rte of Se supplementtion (0.3 mg/kg DM), replcing selenite with orgnic Se will lmost double the BTSe (19), while incresing plsm or serum Se concentrtions only 10% to 20%. Milk Se concentrtions observed in our study re generlly higher thn those previously reported in North Americ (5,17,20), supporting the rgument tht more orgnic Se is now being fed. The observtion of mrginl Se sttus in Atlntic Cnd herds my reflect supplementtion prctices rther thn the Se content of the bse feeds. Thus, in interpreting these results, we should cknowledge tht the reltionships between Se intke, milk Se concentrtion, nd Se sttus (bsed on estblished reference rnges for Se in blood) would differ between herds depending on the source of supplementry Se 54 CVJ / VOL 53 / JANUARY 2012

5 Tble 6. Unconditionl ssocition nd multivrible model of estimted bulk tnk milk selenium concentrtion (BTSe in mmol/l) nd region with log bulk tnk milk somtic cell count (BTSCC) in 63 diry frms from 3 Cndin regions BTSCC (1000 cells/ml) Coefficient 95% CI P-vlue Unconditionl ssocitions BTSe , Region 0.05 Albert 20 Ontrio/Québec , 0.44 Atlntic Cnd , 0.23 Multivrible model Intercept , 5.72, 0.01 BTSe , Region 0.08 Albert 20 Ontrio/Québec , 0.47 Atlntic Cnd , 0.23 Tble 7. Unconditionl ssocitions nd finl multivrible model of estimted bulk tnk milk selenium concentrtion (BTSe in mmol/l) nd region with the probbility of being Stphylococcus ureus-positive herd Odds rtio 95% CI P-vlue Unconditionl ssocitions BTSe , 0.288, 0.01 Region, 0.01 Albert 1 Ontrio/Québec , Atlntic Cnd , 11.5 Multivrible model BTSe , Region 0.02 Albert 1 Ontrio/Québec , 72.0 Atlntic Cnd , 28.4 Odds rtio clculted for n increse of 0.20 mmol/l in BTSe. ARTICLE Univrible results were shown only to illustrte the confounding effects of region. (orgnic or inorgnic) being fed (19). It ws beyond the scope of this study to collect informtion on Se supplementtion prctices in the study herds. Sesonl vritions in milk Se concentrtions hve been found, with greter risk of finding Se deficiency in the fll nd winter compred with other sesons (5). Nevertheless, our results concur with previous studies in the United Sttes (20), which found mrginl Se vlues in the summer. A sesonl vrition in Se concentrtion in pstures, nd differences in the mount of concentrte fed to diry cows my prtilly explin the sesonl chnges found in BTSe. No ssocition between BTSe nd yerly verge of dily milk yield ws found in our study fter djusting for the confounding effect of region. Milk yield cn be sensitive indictor of Se sttus, nd reduced milk yield my be the most importnt economic consequence of mrginl Se sttus (3). However, our results concur with former observtionl studies (21,22), nd Se supplementtion trils (23,24) which found no ssocition between Se sttus nd milk yield prmeters. Contrry to our results, in New Zelnd milk yield nd milk ft were higher in Se-dequte herds (blood Se mmol/l) compred with Se-deficient herds; dditionlly, milk yield in response to Se supplementtion ws higher when Se levels were low before supplementtion (4,25). Men BTSe in the herds of our study ws higher compred with milk Se concentrtion in other studies where positive response to Se supplementtion ws found, which my explin the lck of ssocition between BTSe nd milk yield. There ws no ssocition between BTSe nd BTSCC in our study, which concurred with results from Norwy (21), New Zelnd (26), nd Cnd (5). Nevertheless, other studies hve reported negtive ssocition between Se sttus nd BTSCC in herds with geometric men BTSCC cells/ml (9) or cells/ml (10). The diry popultion in our study hd Se sttus much higher thn tht in studies where negtive ssocition between Se nd udder helth ws first reported. A high BTSCC might induce the frmer to intentionlly supplement Se t higher rte resulting in higher BTSe. This ltter sitution hs resulted in observtions of wek reltionships between impired udder helth nd Se sttus (22). Moreover, geometric men BTSCC in our diry popultion ws lso much lower ( cells/ml) thn tht in diry herds 25 yers go, when negtive ssocition between Se sttus nd udder helth ws first reported. Higher BTSe levels were negtively ssocited with the odds of being S. ureus-positive herd in our study. For exmple, incresing BTSe by 0.2 mmol/l reduced the odds by fctor of Erlier studies found tht n dequte Se sttus ws ssocited with decresed prevlence of mstitis pthogens, nd lower BTSCC (9,10). Further, protective effect of supplementing minerls, especilly Se, on udder helth nd leukocyte function hs been found (27). Even though biomrkers of Se sttus (selenoproteins with specific functions, for exmple) were not evluted in our study, reduction in the odds of being S. ureus-positive herd cn be explined by the biologicl function of Se on the immune system. The effect of Se ginst mmmry pthogens is medited through severl mechnisms: more rpid nd mssive influx of polymorphonucler cells (PMN) to the udder (28), bcteri (such s S. ureus) being killed more efficiently by PMN (29), high nti-bcteril ctivity in whey sufficient to inhibit S. ureus growth (30), nd high expression of selenoproteins with ntioxidnt properties in the mmmry glnd (31). These mechnisms, cting together in Se-dequte cows, enhnce the immune response of the udder ginst mmmry pthogens, reducing their herd prevlence. Current men BTSe ws higher thn tht previously reported for Atlntic Cndin diry herds (5). The effectiveness of Se supplementtion prctices my hve incresed over time (22). In ddition, herds with higher BTSCC (nd higher odds of being S. ureus-positive) re often supplemented with Se, or provided with Se t higher rte, s mngement intervention to control BTSCC. The results of this study lrgely support the contention tht, wheres supplementing cows consuming Se-deficient diets my CVJ / VOL 53 / JANUARY

6 ARTICLE elicit fvorble production response or reduction in disese prevlence, super-supplementtion of Se-dequte diets my not result in dditionl benefits to niml performnce (6). Although no herds were considered to be Se-deficient bsed on ccepted reference rnges for Se sttus, reduction in the odds of being S. ureus-positive herd ws observed in this study in herds with the highest BTSe concentrtions. This ltter finding suggests tht feeding cows to increse the milk Se concentrtion (for exmple using orgnic Se) in excess of tht currently considered dequte for support of growth nd milk production, my optimize udder defense ginst S. ureus infection. Further study is required to confirm this hypothesis. Acknowledgments The Cndin Bovine Mstitis Reserch Network (CBMRN) nd the Atlntic Veterinry College Reserch Fund funded this reserch. Authors wish lso to thnk to Dr. Kristen Reyher, Dr. Rphël Vnderstichel, Ntsh Robinson, Orysi Dwydik, nd Doris Poole from the University of Prince Edwrd Islnd; nd Dr. Simon Dufour from the University of Montrel for their vluble help. We would like to lso thnk ll technicins who collected the smples. CVJ References 1. Fenimore RL, Adms DS, Puls R. Selenium levels of beef cttle in southestern British Columbi reltive to supplementtion nd type of psture. Cn Vet J 1983;24: Winter KA, Gupt UC. Selenium content of forges grown in Nov Scoti, New Brunswick, nd Newfoundlnd. Cn J Anim Sci 1979; 59: Wichtel JJ. A review of selenium deficiency in grzing ruminnts. Prt 2: Towrds more rtionl pproch to dignosis nd prevention. N Z Vet J 1998;46: Frser AJ, Ryn TJ, Sproule R, Clrk RG, Anderson D, Pederson EO. The effect of selenium supplementtion on milk production in diry cttle. Proc N Z Soc Anim Prod 1987;47: Wichtel JJ, Keefe GP, Vn Leeuwen JA, Spngler E, McNiven MA, Ogilvie TH. 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