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Relationships of milk culture status at calving with somatic cell counts and milk production of dairy heifers during early lactation on a Californian dairy

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Relationships of milk culture status at calving with somatic cell counts and milk production of dairy heifers during early lactation on a Californian dairy
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  ELSEVIER Preventive Veterinary Medicine 28 (1996) 187-198 PREVENTIVE VETERINARY MEDI INE Relationships of milk culture status at calving with somatic cell counts and milk production of dairy heifers during early lactation on a Californian dairy John H. Kirk ap*, Jim C. Wright b, Steven L. Berry ‘, James P. Reynolds d, John P. Maas e, Abbas Ahmadi ’ a Department of Large Animal Surgery and Medicine, College of Veterinary Medicine, Auburn University, AL 36849, USA ’ Depurtment of Pathobiology, College of Veterinary Medicine, Auburn University. AL 36849, USA ’ Animal Science Extension and Veterinary Medicine Extension, University of California, Davis, CA 95616, USA ’ 14177 Market Street, Suite E, Walnut Grove, CA 95690, USA ’ Veterinary Medicine Extension. University of California, Davis, CA 95616, USA Accepted 28 January 1996 Abstract Four-quarter, composite milk samples were collected from 339 heifers calving for the first time in a large Californian dairy which consistently had low herd somatic cell counts and low prevalence of major mastitis pathogens. The milk samples were collected on average at 6.4 days post partum (range 1-17). Thirty-nine percent of the heifers were subclinically infected with coagulase-negative Staphylococcus spp. Other results were: no growth, 29%; coliform only, 16%; Streptococcus spp. only, 11%; Streptococcus spp. and coagulase-negative Staphylococcus spp. only, 4%; others, 1%. The somatic cell counts and milk production were not significantly different (P > 0.05) between culture groups over the first five monthly Dairy Herd Improvement test periods. Somatic cell counts decreased significantly after the first test period within the Strepto- coccus spp. group (P < 0.002) and Staphylococcus spp. group (P < 0.01). Milk production was higher during most of the second and third test periods (P < 0.03) compared with the first period. The frequency of heifers lost to analysis was not different between the culture groups. Results from this dairy indicate that subclinical infections with minor pathogens (primarily coagulase- * Corresponding author. Tel.: 334:844-6407; fax: 334:844-6715. 0167-5877/96/ 15.00 Published by Elsevier Science B.V. PII SO167-5877 96)01037-9  188 J.H. Kirk et d./Preuentiue Veterinary Mrdicinr 28 19961 187-198 negative S~qhylocorcus spp.) had no significant effect on average somatic cell counts or milk production during early to middle lactation. Kryw~rdst Mastitis; Heifers: Bacteriology; Somatic cell counts; Production; Early lactation 1 Introduction Dairy heifers calving for the first time may be infected subclinically with a variety of mastitis pathogens. Somatic cell counts (SCC) and milk-production responses to these pathogens may vary. Most of the studies on heifer mastitis have focused on the major pathogens Srreprococcus agalacfiae and Staphylococcus aureus since these bacteria cause chronic infections resulting in reduced milk production (Matthews et al., 1992). Less is known about subclinical infections with the minor pathogens environmental treptococcus spp. and coagulase-negative taphylococcus spp. There are few reports of the relationships of culture status at calving, SCC and milk production for heifers in dairy herds with consistently low herd SCC and very low prevalence of the major pathogens. The purpose of our study was to determine the relationship among culture status at calving, SCC and milk production during early to middle lactation for primiparous heifers. 2 Materials and methods Primiparous Holstein heifers calving in an open-shed, sand bedded free-stall dairy operation were used in the study. Heifers were from a California dairy of approximately 1300 milking cows with milk production ranging from 55 to 60 lb per cow per day with twice-daily milking. The bulk-tank SCC ranged seasonally from 180 000 to 300000 cells ml-‘. Individual-cow composite SCC and test-day milk production were obtained from the North Central Valley Dairy Herd Improvement Association. SCCs were electroni- cally counted using the Fosometric method (Foss Electric, Hillerod, Denmark). Milk weights are from a morning and evening milking and were taken by DHIA-approved meters. Equal amounts of milk were aseptically taken from each quarter to form a composite milk sample for each heifer. Composite milk samples were collected from individual heifers by dairy personnel that had been trained as part of a herd-health program provided by their veterinarian. After the udder and teats had been washed and dried prior to milking, teat ends were disinfected using alcohol-soaked cotton balls. Milk was aspirated from the teats into sterile, snap-top vials. Vials were kept under refrigeration until transported to the laboratory about 15 min away, where they were kept under refrigeration until examined. Milk samples were plated on blood agar, mycoplasma agar and MacConkey agar. With the exception of mycoplasma, presumptive identification is made 18-24 h later. Gram stain, KOH, hydrogen peroxide and taphylococcus aureus beta toxin are used according to the National Mastitis Council Field Handbook recommendations. Identifica-  J.H. Kirk et al./Preventive Veterinary Medicine 28 1996) 187-198 189 tion of the bacteria was by the herd veterinarian (Reynolds, 1995). Over 200 other milk samples were submitted to the Dairy Food Safety Laboratory at the School of Veterinary Medicine, University of California-Davis as part of a larger study which confirmed the absence of major mastitis pathogens. Isolates were classified as: no growth, coliform, other Sfrep~~coccus spp., other (coagulase-negative) Staphylococcus spp., and others. Milk samples were available from 346 heifers which had culture results and at least one DHIA test following calving. Heifers were included in the study if they calved for the first time during the calendar year 1993, had a milk culture result from a composite milk sample taken at the first calving, had a DHIA test with milk production and SCC data on the first DHIA testing date following calving, and the first DHIA test date was less than 40 days following calving. Cows calving previously, heifers without a culture result, heifers without DHIA information on the initial test after calving or with the first DHIA test 40 days or more following calving were not used in the study. Of the study heifers, three had culture results indicating other sporadically occurring bacteria and were eliminated from the study. For the statistical analysis, cultures with results of no growth or coliform only were combined to form a single group termed Group 1. Group 2 was formed from the samples with Streptococcus spp. only, Group 3 from samples with coagulase-negative Sruphylococcus spp. only, and Group 4 from cultures with both Streptococcus spp. and coagulase-negative Staphylococcus spp. The relationships of culture status (independent variable) to SCC and milk production (dependent variables) over test dates was determined using the general linear models procedure for repeated-measures analysis of variance (Statistical Analysis Systems Institute Inc., 1989). Chi-square test for independence was used to compare the proportion of heifers lost to followup in each culture group (Centers for Disease Control and Prevention, 1994). A similar test was done on the SCC grouped by days from calving to first DHIA test date. Alpha was 0.05. 3. Results Coagulase-negative Staphylococcus spp. was the most frequent isolate (38.4%; Table 1). When the no-growth and coliform-only categories were combined they gave the most frequent isolation result (41%). Table 1 Culture results from composite milk samples collected at calving from heifers; California, 1993 Culture results Samples Number Percentage of total o growth 98 29 Colifonn only 55 16 Streptococcus spp. only 39 11 Coag-neg Staphylococcus spp. only 131 39 Streptococcus spp. and coag-neg Staphylococcus spp. only 13 4 Others 3 1 Total 339 100  I90 J.H. Kirk et al./Preuentiue Vetrrinury Medicine 28 1996) 187-198 Table 2 Frequency distribution of interval (days) from calving to milk sample collection for culture for heifers by group (n = 336); California, 1993 Interval Group Total Percentage of total Id 2b 3c 4d l-5 62 I2 54 8 136 40.5 6-10 70 23 73 5 171 50.9 II-17 21 4 4 0 29 8.4 Total I53 39 131 13 336 100.0 a No growth and coliform group. b Slreptococcus spp. only. ’ Stuphylococcus spp. only. ’ Streprococcus and Staphylococcus spp. Table 3 Mean number of days from calving to collection of first DHIA test date information and between DHIA tests for study heifers; California, 1993 Interval Group I a Group 2 b Group 3 ’ Group 4 d XkSD No. XfSD No. XiSD No. X+SD No. le 21 k8 153 21 i9 39 22+8 131 21 f8 I3 2 32*9 150 31 _t5 37 32k8 I28 33* IO I3 3 33+ IO 149 32+8 37 34*11 125 38i 19 13 4 32+7 I47 32+8 37 31+5 122 30+ I II 5 33+s I45 341- IO 37 35kll 122 34+9 II a No growth and colifotm group. b Slreprococcus spp. only. ’ Stuphylococcus spp. only d Streptococcus and Stcrphylococcus spp. ’ Calving to first DHIA test interval; all other intervals are between DHIA tests. Table 4 Distribution of heifers lost (No. lost) by test date from analyses and the percentage of heifers remaining (percentage in herd) by DHIA test date (n = 336); California, I993 DHIA test date Group I a (n = 153) Group 2 ’ (n = 42) Group3’ (n= 137) Group4d(n=14) No. lost % in herd No. lost % in herd No. lost % in herd No. lost % in herd 1 0 100 0 loo 0 100 0 100 2 3 98 2 95 3 98 0 IOil 3 I 97 0 95 3 95 0 100 4 2 95 0 95 2 93 2 86 5 2 95 0 95 0 93 0 86 a No growth and coliform group. b Srreptococcus spp. only. ’ Staphy1ococcu.s spp. only. d Streptococcus and Stahylococcus spp  J.H. Kirk et al./Preventive Veterinary Medicine 28 1996) 187-198 191 Table 5 Seasonal frequency distribution of calvings by culture groups for study heifers, California, 1993 Season Group 1 a Group 2 b Group 3 ’ Group 4 * January-March 42 18 50 5 April-June 42 8 44 5 July-September 33 5 25 2 October-December 36 8 12 1 a No growth and coliform group. b Streptococcus spp. only. ’ Staphylococcus spp. only. * Streptococcus and Staphylococcus spp. The average number of days from calving to the collection of the composite milk samples was 6.4 (range l- 17). The interval in days from calving to collection of the composite milk samples and the grouped culture results are independent (x2 = 9.95; d.f. = 6; P = 0.13; Table 2). The mean interval from calving until the first DHIA test dates was approximately 21 days and the interval between DHIA test dates was approximately 33 days (Table 3). The distribution of heifers which were lost to analysis was not significantly different between the culture groups (x2 = 1.49; d.f. = 12; P > 0.90; Table 4). The seasonal frequency distribution by culture group indicated significant differences (x2 = 17.63; d.f. = 9; P = 0.04; Table 5). These differences appear to be fewer than expected no-growth or coliform isolates during January-March and more no-growth or coliform isolates during October-December. There are disproportionately fewer than expected Sruphylococcus spp.-only isolates during October-December. There was no significant difference between the heifer culture groups by DHIA test date in SCCs and test-day milk production (Tables 6 and 7). Median and 75th percentile Table 6 General linear model of SCC by culture group for first 5 DHIA test days of heifers calving for the first time; California, 1993 Model Source d.f. Sum of squares F P F) SCCI a scc2 scc3 scc4 sees Culture group 3 3545791.2 2.18 0.09 Error 332 180045 104.7 Culture group 3 61438.7 0.27 0.85 Error 332 25313266.8 Culture group 3 228639.5 0.15 0.93 Error 332 168006873.2 Culture group 3 61716.3 0.25 0.86 Error 332 26856719.4 Culture group 3 257079.4 0.76 0.52 Error 332 37612785.6 a SCCI-SCCS, SCCs for DHIA test days l-5 after calving.
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