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1 Department of Veterinary Preventive Medicine, The Ohio State University, Columbus 43210
2 Department of Animal Sciences, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691
Corresponding author: P. J. Rajala-Schultz; e-mail: rajala-schultz.1{at}osu.edu.
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ABSTRACT |
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Key Words: milk yield • dry-off • intramammary infection
From the udder health perspective, the goal of dry cow management is to have cows starting a new lactation with a healthy, uninfected mammary gland (Eberhart, 1986; Dingwell et al., 2003; Leslie and Dingwell, 2003). The rate of new IMI is higher during the dry period than during the lactation; 2 highly susceptible times for cows getting an IMI during the dry period are soon after cessation of milking and during lactogenesis (Cousins et al., 1980; Smith et al., 1985). Even though the epidemiology of mastitis during the dry period has been intensively studied, milk yield at dry-off as a risk factor for IMI has not received much attention, especially with the current high production levels. The objective of this study was to assess the association between milk yield at dry-off and the probability of an IMI at calving.
Duplicate quarter milk samples were obtained within 3 d after calving from 116 lactations of 96 multiparous cows in one study herd over a 16-mo study period to estimate the prevalence of IMI at calving (IDF, 1987). Milk samples were examined microbiologically according to the guidelines of National Mastitis Council (NMC, 1999). A quarter was diagnosed as infected when the same pathogen was isolated from both of the duplicate milk samples from that quarter (Hogan et al., 1995). More specifically, a quarter was classified as infected with environmental pathogens if Escherichia coli, Klebsiella spp., Citrobacter spp., Enterobacter spp., Serratia spp., or species of streptococci other than Streptococcus agalactiae were isolated from that quarter. A quarter was classified as infected with CNS if only CNS were isolated. A cow was classified as infected if it had at least one quarter infected at calving based on the above criteria.
Generalized estimation equations (GEE) in SAS version 8.2 (SAS Institute, Inc., Cary, NC) with a logit link and binomial error distribution were used to model the probability of a cow or a quarter being infected at calving, separately for CNS and environmental organisms. Mixed infections with CNS and environmental organisms were included in the analyses for both types of infections, however, additional analyses were also run by omitting mixed infections. Comparison was with the noninfected cows (quarters). Compound-symmetry correlation structure was used to account for the clustering of quarters within cows and lactations within cows (Littell et al., 1996). The milk yield used in the analyses was the actual daily milk weight of a cow on the day before dry-off. Milk yield was centered at 12.5 kg (median in the data) and scaled to reflect a 5-kg change. Somatic cell count at the end of lactation, parity of a cow, calving season, DIM at dry-off, and dry period length were considered potential confounding variables (Dohoo et al., 2003).
A total of 29 of the 116 lactations (25%) started with a cow being infected with environmental pathogens (gram-negative organisms or streptococci), 14 of which were mixed infections with CNS. Thirty-nine (39) quarters were infected either with streptococci or gram-negative bacteria. The gram-negative isolates included E. coli, Klebsiella spp., Citrobacter spp., and Serratia spp. Thirty-eight (38/116, 32.8%) cows and 76 quarters were infected with CNS only. One cow had one quarter infected with Staph. aureus, however, this was omitted from the analyses. No cows in this herd were infected with Strep. agalactiae.
Milk yield at dry-off was significantly associated with environmental infections at calving, both on a quarter and a cow level. The results from the quarter- and cow-level analyses were very similar and because milk yield at dry-off was the main risk factor of interest in this study and is typically measured on a cow level, only results from the cow-level analysis are presented. With mixed infections included in the analysis, for every 5-kg increase in milk production at dry-off above 12.5 kg, the odds of a cow having an IMI caused by environmental organisms at calving increased by 77%, odds ratio = 1.77 (Table 1
). None of the potential confounders were significantly associated with environmental IMI at calving (P >0.05). However, including SCC in the model meaningfully changed (>20%) the parameter estimates for milk yield at dry-off, suggesting a confounding effect (Dohoo et al., 2003) and thus, SCC was kept in the model. When mixed infections with CNS were omitted from the analysis for environmental infections, the negative effect of high milk yield at dry-off became even more pronounced (odds ratio = 2.13, P <0.001, results not shown). Association between milk yield at dry-off and CNS infections at calving was not significant, regardless of whether mixed infections were included in the analyses or not and whether analyses were done on a quarter or cow level. Parity of a cow was the only factor significantly associated with CNS infections at calving (results not shown).
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ACKNOWLEDGEMENTS |
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Received for publication July 5, 2004. Accepted for publication November 6, 2004.
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