Effects of induced clinical mastitis during preovulation on endocrine and follicular function.

The objective of the study was to determine if experimentally induced clinical mastitis before ovulation resulted in alterations of endocrine function, follicular growth, or ovulation. On d 8 (estrus = d 0), cows were challenged (TRT; n = 19) with Streptococcus uberis or were not challenged (control; n = 14). Forty-eight hours after induction of luteal regression on d 12, blood samples were collected to determine estradiol-17beta, LH pulse frequency, and occurrence of the LH surge. Ovaries were scanned to monitor follicular growth and ovulation. Cows with clinical mastitis (n = 12) had elevated rectal temperatures, somatic cell counts, and mammary scores. Estrus and ovulation occurred in 4 of 12 clinically infected cows and in all control cows. Cows that were challenged but did not develop clinical mastitis (n = 5) displayed estrus and ovulated. Due to differences in expression of estrus, cows were further subdivided for analyses into 4 groups: control, TRT-EST (infected cows that displayed estrus; n = 4), TRT-NOEST (infected cows that did not display estrus; n = 8), and NOMAS (cows that were inoculated but did not develop mastitis; n = 4). Ovulation rate was 100% for CON, NOMAS, and TRT-EST compared with 0% for TRT-NOEST cows. Size of the ovulatory follicle ("presumed" ovulatory follicle in TRT-NOEST cows) was similar for all groups. Frequency of LH pulses was decreased in TRT-NOEST compared with CON, TRT-EST, and NO-MAS. Estradiol-17beta increased over time in CON, NO-MAS, and TRT-EST cows, but did not increase in TRT-NOEST cows. Cows with clinical mastitis may exhibit estrus and ovulate normally or have disruptions in normal physiology including decreased LH pulsatility, absence of an LH surge and estrous behavior, suppressed estradiol-17beta, and failure to ovulate.

Endocrine profiles of dairy cows following experimentally induced clinical mastitis during early lactation.

Concentrations of LH, cortisol, estradiol-17beta (E(2)), prolactin and 13,14-dihydro-15-keto-prostaglandin F(2alpha) (PGFM) were determined in cows with experimentally induced clinical mastitis during early lactation. Cows free of intramammary infection (IMI) and in the luteal phase of the estrous cycle were balanced by lactation number and days in milk and assigned to either control (n=5) or treatment (n=5) groups. Treated cows were infected experimentally (day 0), in two mammary quarters, with Streptococcus uberis and developed clinical mastitis within 60 h after inoculation as evidenced by increased mastitis scores, elevated rectal temperatures, mammary swelling and isolation of S. uberis pathogen. Four days following bacterial challenge, blood samples were collected every 20 min for 8 h for determination of PGFM and LH following administration of oxytocin and GnRH, respectively. Blood samples were also collected on days 0, 4 and 7 of the experiment to determine concentrations of E(2), prolactin and cortisol. Four days after bacterial challenge, concentrations of cortisol were higher (P=0.04) in experimentally infected cows than controls. Experimentally challenged cows had increased (P=0.02) concentrations of cortisol on days 4 and 7 compared with day 0. Control cows had no significant increase in blood cortisol during the experimental period. Baseline concentrations of PGFM did not differ between groups; however, peak concentrations of PGFM following oxytocin challenge were elevated (P=0.006) in cows with clinical mastitis compared with control animals. Prolactin, E(2) and LH did not differ between cows with clinical mastitis or controls. Experimentally induced mastitis during early lactation elevated concentrations of cortisol during the luteal phase of the estrous cycle. Furthermore, mastitic cows demonstrated an increased PGFM response following oxytocin administration. Altered reproductive efficiency in cows with clinical mastitis caused by Gram-positive pathogens may be the result of increased uterine sensitivity to prostaglandin F(2alpha) (PGF(2alpha)).

Antibiotic Residues in Milk Following Antimicrobial Therapy During Lactation.

Milk was collected at 24 h intervals through 120 h after treatment from lactating dairy cows treated with antibiotics. Antibiotic residues were determined qualitatively by microbiological assays utilizing Bacillus stearothermophilus . Intrauterine infusion of antibiotics resulted in the lowest percentage of milk samples positive for residues. A high percentage of samples was positive for residues 24 and 48 h after intramuscular injection of antibiotics; however, most samples were negative by 72 h after treatment. Intramammary therapy resulted in a high proportion of samples positive for residues at 24 and 48 h after treatment, and some samples were positive 96 to 120 h after treatment. Samples from treated quarters were often positive when corresponding composite milk samples were negative. Treatment with more than one antibiotic by multiple routes resulted in the highest percentage of samples positive for residues for the longest time. Any variation of antibiotic dosage, duration of treatment, or use of multiple antibiotics should alert dairy producers of the probability of antibiotics being excreted in milk beyond recommended withdrawal times.

Concentrations of trypsin inhibitor and immunoglobulins in colostrum of Jersey cows.

Colostrum samples from 49 Jersey cows were analyzed for concentrations of trypsin inhibitor, IgG, IgM, IgA, TS, fat, specific gravity, and N fractions. Colostrum (100 ml) was sampled from each cow as soon as possible after parturition. Mean concentrations of IgG, IgM, and IgA were 84.6, 3.4, and 4.5 g/L, respectively. Mean concentration of trypsin inhibitor was 56 mg of trypsin inhibited/dl of colostrum. Concentration of trypsin inhibitor was unaffected by lactation number and averaged 60, 53, and 54 mg of trypsin inhibited/dl of colostrum for cows in first, second, and third or later lactations, respectively. Colostral trypsin inhibitor and IgG were correlated (.54), although correlations between trypsin inhibitor and IgM and IgA were not significant. Trypsin inhibitor in colostrum was also positively correlated with fat, total N, protein N, noncasein N, and TS in colostrum. Variation in concentration of trypsin inhibitor from first-milking colostrum was closely related to colostral IgG concentration and may serve to protect IgG and other proteins from proteolytic degradation in the intestine of the neonatal calf.

Response of lactating jersey cows fed concentrate twice daily by computer or in a total mixed ration.

Two trials were conducted with 36 Jersey cows each to compare intake, production response, and relative economics of four concentrate management systems. Cows were assigned for 25 wk to concentrate fed: 1) for ad libitum consumption twice daily for 10 min, 2) twice daily according to production, 3) via computer feeder according to production, or 4) as part of a total mixed ration. Concentrate was fed at the rate of 1 kg/3.5 kg 4% FCM for treatments 2 and 3 and at the same rate, but to group average FCM, for treatment 4. Concentrate allotments were updated weekly. Cows were individually fed blended corn silage and whole cotton-seed (84:16, DM basis) plus 2.2 kg alfalfa hay/d (Trial 2 only). Concentrate DM intake was 2.3 kg lower and forage DM intake 2.3 kg higher when cows were fed to production (treatments 2, 3, and 4). Milk production was highest in both trials for treatment 2. Fat test was lowest for twice daily ad libitum feeding with no differences found between other methods. Body weight change was similar across treatments and trials. Milk income was similar, fed costs higher, and income over feed cost lower for treatment 1 than for other methods in both trials. Three methods of feeding concentrate according to production were equally effective in maximizing profit.

Influence of prepartum antibiotic therapy on intramammary infections in primigravid heifers during early lactation.

Jersey heifers were assigned alternately to three groups: 1) negative control (n = 41), 2) intramammary infusion of 200 mg of sodium cloxacillin (n = 38) at 7 d before expected parturition, and 3) intramammary infusion of 200 mg of cephapirin sodium (n = 36) at 7 d before expected parturition. The percentage of mammary glands infected prior to treatment was 62.2, 50.0, and 70.1 for groups 1, 2, and 3, respectively. The percentage of mammary glands infected during early lactation was 44.5, 8.6, and 2.1 for groups 1, 2, and 3, respectively. Most infections (87.1%) were due to Staphylococcus species other than Staphylococcus aureus. Thirty-six of 460 quarters were infected with major pathogens before treatment, 3 of 22 persisted following antibiotic treatment, and 9 of 14 persisted in the control group. Infusion of sodium cloxacillin resulted in antibiotic residues in 17.4% of samples obtained .5 d postpartum. All samples were negative at 3 and 10 d postpartum. Infusion of cephapirin sodium resulted in antibiotic residues in 84.7, 28.2, and 0% of samples obtained at .5, 3, and 10 d, respectively. Prepartum antibiotic therapy was effective in eliminating many IMI, especially those caused by coagulase-negative staphylococci, but there is the potential for antibiotic residues in milk.

Immunoglobulin concentration, specific gravity, and nitrogen fractions of colostrum from Jersey cattle.

Colostrum samples from 88 Jersey cows were analyzed for concentrations of IgG, IgM, IgA, total solids, specific gravity, and N fractions. Colostrum (50 ml) was sampled from each cow as soon as possible after parturition, and specific gravity was determined immediately using a hydrometer. Samples then were frozen prior to analysis of Ig, fat, and N fractions. Mean concentrations of IgG, IgM, and IgA were 65.8, 2.4, and 1.7 g/L, respectively. Concentration of IgG was lower, and IgA was higher, in colostrum from second lactation cows than from first lactation cows or from cows in third or later lactations; IgM increased linearly as lactation number increased. Total N, protein N, noncasein N, and fat contents also were lower in second lactation cows. Regression of total Ig (grams per liter) on specific gravity was -1172 + 1180 x specific gravity (r2 = .38). Relationship of total Ig to specific gravity differed from colostrum of Holstein cattle and may have been related to differences in fat and noncasein N concentrations. Use of specific gravity hydrometer to estimate Ig concentration using equations derived from Holstein cattle appears to underestimate Ig concentration in colostrum from Jersey cattle.

Evaluation of uterine body and bilateral uterine horn insemination techniques.

Uterine body and bilateral horn insemination techniques were compared in 364 Jersey cows and 138 heifers by two inseminators. For horn inseminations, approximately one-half of the semen was placed in left uterine horn just prior to the point of resistance. The remainder of semen was then placed in the right uterine horn just prior to the point of resistance. Animals were inseminated in the uterine body on even days of the month throughout the study. On odd days of the month, animals were inseminated in both uterine horns. A total of 62.9% of 286 animals inseminated in the uterine body were pregnant versus 54.2% of 216 animals inseminated in both uterine horns. Differences were greater for cows than for heifers and at first and second services than at third or greater services. These results favor current recommended techniques to deposit semen into the uterine body correctly.

Antibiotic residues and prevalence of mastitis pathogen isolation in heifers during early lactation following prepartum antibiotic therapy.

The present study was conducted to determine if antibiotic treatment of heifer mammary glands earlier in the prepartum period reduced the occurrence of residues in milk without compromising efficacy in treatment of intramammary infections. Heifers were assigned randomly to two groups: 1. untreated negative control (n = 42); and 2. intramammary infusion of 200 mg cephapirin sodium (n = 40) 14 days prior to expected calving. Mammary secretions were collected before treatment and during early lactation, and were analyzed for residues by the Bacillus stearothermophilus disc assay. Only four of 127 samples (3.1%) obtained from cephapirin-treated mammary glands at 3 days after calving were positive for residues and most (three of four) were from a heifer that calved within 3 days of treatment. Mammary secretions were also collected before treatment, and at 3 and 30 days after calving for microbiological evaluation. For untreated control heifers, mastitis pathogens were isolated from 67.3% of samples obtained from mammary glands 14 days prior to expected calving, 55.6% obtained 3 days after calving and 36.4% of samples obtained 30 days postpartum. A similar percentage of samples (63.8%) was positive for mastitis pathogens at 14 days before expected calving prior to antibiotic treatment. However, only 15.1% of samples obtained at 3 days after calving and 7.9% of samples obtained 30 days postpartum from mammary glands of antibiotic-treated heifers contained mastitis pathogens. Coagulase-negative staphylococci were isolated most frequently. Intramammary infusion of antibiotics earlier in the prepartum period markedly reduced the occurrence of residues in milk during early lactation without affecting efficacy.

Analysis and frequency of bovine lymphocyte antigen DRB3.2 alleles in Jersey cows.

Jersey cows (n = 172) were genotyped for the bovine lymphocyte antigen (BoLA)-DRB3.2 allele by polymerase chain reaction and restriction fragment length polymorphism analysis. Bovine DNA was isolated from aliquots of whole blood. A two-step polymerase chain reaction followed by digestion with restriction endonucleases RsaI, BstyI, and HaeIII was conducted on the DNA from Jersey cattle. Twenty-four BoLA-DRB3.2 alleles were identified with frequencies ranging from 0.3 to 22.9%. Thirteen allele types were similar to those reported previously; and 11 were new allele types that have not been reported previously. Allele types reported previously include: BoLA-DRB3.2*2, *8, *10, *15, *17, *20, *21, *22, *23, *25, *28, *36, and *37. Their frequencies were 0.3, 11.3, 22.9, 13.6, 5.5, 3.7, 10.7, 3.5, 0.9, 0.3, 4.7, 9.3, and 0.9%, respectively. Of the new allele types detected, *ibe occurred at the highest frequency (6.1%) in Jersey cows from this herd. The six most frequently isolated alleles (BoLA-DRB3.2*8, *10, *15, *21, *36, and *ibe) accounted for 73.9% of the alleles in the population of this herd. Results of this study demonstrate that the BoLA-DRB3.2 locus is highly polymorphic in Jersey cattle.

Supplemental niacin and heat-treated whole soybeans for Jersey cows during early lactation.

Fifty-six multiparous Jersey cows were used in a replicated trial to investigate the effect of supplemental niacin and heat-treated whole soybeans on milk production and composition. Dietary treatments included 0 or 6 g/d of supplemental niacin beginning 21 d prior to expected parturition and raw or heat-treated whole soybeans fed at 10% of the dietary DM beginning immediately after parturition. Data were collected for 18 wk following a 2-wk postpartum period during which cows were adjusted to experimental diets and allowed to recover from parturition. Based on index values for protein dispersibility, heat-treated soybeans were marginally heated in yr 1 (12.1) and underheated in yr 2 (17.9). Intake of DM from corn silage plus concentrate, alfalfa hay, or soybeans averaged 15.5, 1.5, and 1.7 kg/d, respectively, and was not different because of niacin supplementation or form of soybean. Percentage of milk fat was highest when raw soybeans plus niacin were fed and lowest for heat-treated soybeans plus niacin. No difference was observed in milk production, milk percentage, or milk protein, lactose, and SNF. No advantage was observed from less than optimally heated whole soybeans or supplemental niacin in diets for Jersey cows during early lactation.

Addition of soybean trypsin inhibitor to bovine colostrum: effects on serum immunoglobulin concentrations in Jersey calves.

Secretion of trypsin by the neonate may reduce absorption of Ig from colostrum and increase failure of transfer of passive immunity, particularly for calves fed after 12 h of age. Jersey calves (n = 48) were used in a 2 x 2 factorial arrangement of soybean trypsin inhibitor (0 or 1 g) added to 1 L of maternal colostrum at the first two feedings and time of initial feeding (.65 or 12 h of age). A second colostrum feeding was offered 12 h after the initial feeding. Maternal colostrum was analyzed for Ig by radial immunodiffusion and trypsin inhibitor by radial protease diffusion. Jugular blood was sampled at 0, 12, 24, and 48 h after initial feeding, and serum was analyzed for IgG and IgM. Addition of trypsin inhibitor increased serum Ig and total serum protein concentrations, regardless of age at first feeding. Concentrations of serum IgG and IgM were increased 16 and 30%, respectively, when trypsin inhibitor was fed. Age of first feeding did not affect concentrations of IgG or IgM in serum. Supplementation of colostrum with trypsin inhibitor improved transfer of passive immunity to neonatal Jersey calves.

Sodium bicarbonate and yeast culture effects on ruminal fermentation, growth, and intake in dairy calves.

Sodium bicarbonate and yeast culture effects on ruminal fermentation, intake, and growth were evaluated in young calves. In trail 1, nine ruminally cannulated Holstein calves averaging 12 wk of age were fed control starter (17% CP) or starters containing 3% sodium bicarbonate or .2% yeast (Saccharomyces cerevisiae) culture in a 3 x 3 Latin square. Calves were fed for ad libitum consumption for 10 d and then at 85% of ad libitum intake to d 14. Ruminal fluid taken at 0 h postfeeding tended to have higher pH and a greater proportion of acetate when calves were fed sodium bicarbonate, but other ruminal and blood parameters did not differ among treatments. By 4 h after feeding, ruminal VFA had increased to 120.7 mM, molar proportions of individual acids were altered, and blood ketones and VFA increased in treated calves. In trial 2, 42 Jersey calves were fed experimental starters for ad libitum consumption during a 12-wk study. Calves began the study at 3 to 5 d of age. There were no significant effects of yeast culture or sodium bicarbonate on DMI or intake of starter, rates of gain, or feed efficiency. Plasma urea N was reduced when sodium bicarbonate was fed. Both sodium bicarbonate and yeast culture affected blood and ruminal metabolites when calves were limit-fed but did not influence intake or daily gain when calves were fed for ad libitum consumption.

Survival of coliform bacteria in static compost piles of dairy waste solids intended for freestall bedding.

Dairy waste solids separated from a slurry by a centrifugal separator were composted in 12 static piles. Seven of the compost piles were naturally aerated, and five were aerated by a fan that forced air through the piles of solids. The natural aeration process aged the manure solids in an unconfined pile. The fan in the forced aeration process forced air into a perforated plenum beneath the compost piles. Dairy waste solids in each compost pile were heated into the thermophilic temperature range and generally composted well. At most sampling points, coliform bacteria declined to low or undetectable numbers early in the composting period. However, as the composting process proceeded, bacterial numbers increased to approximately those present in raw dairy waste solids. Findings of this study suggest that composting offers little benefit toward net reduction in coliform bacterial numbers in dairy waste solids.

The effect of artificial insemination once versus twice per day.

The objective of this study was to determine the effect of inseminating Jersey cows and heifers once per day or according to the a.m.-p.m. rule. A total of 337 artificial inseminations (AI) were completed by three technicians at the University of Tennessee Dairy Experiment Station at Lewisburg for 6 mo. Cows and heifers were inseminated at estrus using the a.m.-p.m. rule on even days of the month. On odd days of the month, AI were once daily between 0800 and 1200 h. Estrus detection was conducted two to three times daily. Pregnancy was confirmed by rectal palpation 60 to 80 d after AI. Herd DHIA averages were a 12.2-mo calving interval, 76 d to first AI, 83% observed estruses, and a 50% conception rate during the trial. Pregnancy data were analyzed with a model including treatment, AI, lactation number, parity, technician, and group. This study grouped cows and heifers according to when they were in estrus and inseminated (a.m.-a.m., a.m.-p.m., or p.m.-a.m.); means were 43.7, 57.9, and 59.0%, respectively. The a.m.-p.m. AI versus once per day AI yielded a pregnancy rate of 55.6% versus 51.3%. These results show no difference among Jersey cows or heifers that were inseminated artificially once daily in the a.m. However, those cows and heifers inseminated in the a.m. of first estrus detection had a lower pregnancy rate.

Effects of gonadotropin-releasing hormone and human chorionic gonadotropin on pregnancy rate in dairy cattle.

This study was to determine if gonadotropin-releasing hormone, human chorionic gonadotropin, or their interaction would affect pregnancy rate or milk progesterone profiles in dairy cattle. Cows and heifers eligible for first, second, and third services were assigned to four treatments: 1) saline i.m. within 5 min after AI (d 0) and saline 15 d after AI (n = 222 AI); 2) gonadotropin-releasing hormone (100 micrograms) i.m. within 5 min after AI and saline on d 15 (n = 223); 3) saline after AI and human chorionic gonadotropin (3500 IU) i.m. on d 15 (n = 196); 4) gonadotropin-releasing hormone at AI and human chorionic gonadotropin on d 15 (n = 195). Progesterone was quantified in milk samples collected twice weekly for 5 wk from all cows. Neither gonadotropin-releasing hormone, human chorionic gonadotropin, gonadotropin-releasing hormone x human chorionic gonadotropin interaction, age, nor service number affected pregnancy rate, which averaged 46.2%. Pregnancy and human chorionic gonadotropin affected milk progesterone profiles. Pregnancy maintained progesterone concentrations and human chorionic gonadotropin on d 15 increased them. This study does not support the general use of gonadotropin-releasing hormone at AI or human chorionic gonadotropin 15 d after AI as methods for enhancing pregnancy rates of dairy cattle.

Efficacy of chlorhexidine as a postmilking teat disinfectant for the prevention of bovine mastitis during lactation.

A natural exposure trial was conducted for 12 mo in a herd of 150 lactating Jersey cows to determine efficacy of a .35% chlorhexidine teat dip containing a glycerine emollient for the prevention of bovine intramammary infections. Right teats of cows were dipped in the experimental teat dip after milking machine removal and left teats were not dipped. The herd was free of Streptococcus agalactiae and had a low prevalence of Staphylococcus aureus. Most new major pathogen intramammary infections resulted from Streptococcus species, primarily Streptococcus uberis and Streptococcus dysgalactiae. New infections by Streptococcus species were significantly lower in teats dipped in chlorhexidine than in undipped teats. Overall efficacy of the chlorhexidine teat dip against major mastitis pathogens was 50%. The experimental teat dip also reduced coagulase-negative Staphylococcus species infections 49.0% and Corynebacterium bovis infections 65.2%. Overall efficacy against minor mastitis pathogens was 54.0%. No irritation or chapping of teats dipped in the experimental teat dip was observed.

Prepartum antibiotic treatment of heifers: milk production, milk quality and economic benefit.

Prepartum intramammary antibiotic infusion of heifer mammary glands at 7 or 14 d before expected parturition is an effective procedure for eliminating many infections in heifers during late gestation and for reducing the prevalence of mastitis in heifers during early lactation and throughout lactation. Mastitis pathogens were isolated from 76% of samples obtained from untreated control quarters 7 d before expected calving, from 47% of samples obtained 3 d after calving, and from 29% of samples obtained 10 d postpartum. Mastitis pathogens were isolated from about 30% of control quarters through 240 d of lactation. A similar percentage of samples (70%) was positive for mastitis pathogens at C-7 before antibiotic treatment. However, only 8% of samples obtained at 3 d after calving and 4% of samples obtained at 10 d postpartum from quarters of antibiotic-treated heifers contained mastitis pathogens. Throughout the remainder of lactation, mastitis pathogens were isolated from an average of about 11% of quarters. The percentage of samples with mastitis pathogens was higher in untreated controls than in antibiotic-treated quarters at all sampling intervals during lactation. A similar response was observed in heifers that were treated with antibiotics at 14 d before expected parturition. Prepartum antibiotic-treated heifers produced significantly more milk than control heifers and had significantly lower somatic cell count scores than untreated control heifers. These observations are likely associated with or due to the lower prevalence of mastitis pathogen isolation in prepartum antibiotic-treated heifers throughout lactation. Prepartum antibiotic-treated heifers produced 531 kg more milk than heifers in the untreated control group. Multiplying this increase by a milk price of 0.407 dollars/kg yielded a 216.24 dollars per-heifer increase in gross revenue. The cost of treatment, including the cost of testing for antibiotic residues, was estimated at 15.60 dollars for a net revenue of 200.64 dollars per heifer. Prepartum antibiotic treatment to reduce the rate of mastitis in heifers during lactation was highly effective and economically beneficial.

Influence of clinical mastitis during early lactation on reproductive performance of Jersey cows.

The purpose of this study was to determine the influence of clinical mastitis on reproductive performance of high producing Jersey cows. Cows (n = 102) with clinical mastitis during the first 150 d of lactation were evaluated. Groups were balanced according to lactation number and days of lactation and sub-divided as follows: group 1, clinical mastitis before first artificial insemination (AI) (n = 48); group 2, clinical mastitis between first AI and pregnancy (n = 14); group 3, clinical mastitis after confirmed pregnancy (n = 40); and group 4, control cows (n = 103) with no clinical mastitis. No differences in reproductive performance were detected because of milk production or mastitis caused by Gram-positive or Gram-negative pathogens. The number of days to first AI was significantly greater for cows with clinical mastitis before first AI (93.6 d) than for all other groups (71.0 d). Artificial inseminations per conception were significantly greater for cows with clinical mastitis after first AI (2.9) than for cows with clinical mastitis before first AI (1.6), cows with no clinical mastitis, or cows with clinical mastitis after confirmed pregnancy (1.7). The number of days to conception for cows with clinical mastitis after first AI (136.6 d) was significantly greater than that for control cows and that for cows that developed clinical mastitis after confirmed pregnancy (92.1 d). Clinical mastitis during early lactation markedly influenced reproductive performance of Jersey cows.

Evaluation of Chlorhexidine as a Premilking Teat Disinfectant for the Prevention of Intramammary Infections During Lactation.

A study was conducted for 15 months to evaluate efficacy of a 0.35% chlorhexidine teat dip as a premilking teat disinfectant based on reduction of naturally occurring new intramammary infections. Predipping was compared with a negative control using a split-udder experimental design. All teats were dipped after milking with the same 0.35% chlorhexidine teat dip. Most new major pathogen intramammary infections were caused by Streptococcus species, primarily Streptococcus uberis and Streptococcus equinus and gram-negative bacteria, primarily Escherichia coli . Percentage of quarters newly infected by major mastitis pathogens was 30.6% lower in mammary glands with teats predipped and postdipped in chlorhexidine than in mammary glands with teats postdipped only, and differences between treatment groups approached significance. New infections by coagulase-negative Staphylococcus species were significantly lower in mammary glands with teats predipped and postdipped than in mammary glands with teats postdipped only. When all mastitis pathogens were combined, percentage of quarters newly infected by major and minor mastitis pathogens was significantly lower in the predipped and postdipped group than in the postdipped only group. No statistical differences in incidence of clinical mastitis between treatment groups were observed. No chapping or irritation of teats was observed and no adverse effects were detected using chlorhexidine as a premilking and postmilking teat disinfectant. Results of this study suggest that premilking teat disinfection with chlorhexidine in association with good udder preparation and postmilking teat disinfection can further reduce the occurrence of new intramammary infections during lactation.