||The objectives of the study were to compare the efficacy of six intramammary dry-cow antibiotic preparations for curing existing IMI and for preventing the introduction of new IMI during the dry period. The effect of factors such as parity, milk yield, udder depth, teat canal integrity, number of infected quarters per cow, quarter-site infections and somatic cell count at drying off on cure-rates and new IMI as well as the length of the dry period and rainfall during the dry period were examined. The possible influence of treatment on the SCC during the subsequent lactation was also examined. Prior to the study a herd survey was performed to determine the basic udder health status of the trial herd. Cows due for drying off were clinically evaluated for enrollment and randomly allocated to receive one of the six dry-cow intramammary products under investigation. The six products that were studied were Nafpenzal DC (procaine benzylpenicillin, nafcillin and dihydrostreptomycin), Rilexine 500 DC (cephalexin and neomycin sulphate), Orbenin Extra DC (cloxacillin), Cepravin Dry Cow (cephalonium), Bovaclox DC (cloxacillin and ampicillin) and Dispolac Dry Cow (procaine benzylpenicillin and dihydrostreptomycin). Quarter milk samples were collected at drying off, (prior to treatment), and 1 to 6 days after calving for determination of the SSC and presence of micro-organisms in the milk. Cows were closely observed during the dry period and udders were clinically examined on a weekly basis and post-calving till udder oedema disappeared. Data from 162 cows and 648 quarters are summarized. The following were determined · Prevalence of pathogens at drying off and post-calving · Overall cure-rate and new intramammary infection rate. · Comparative efficacy of dry-cow treatment on cure-rate and prevention of new IMI. · Effect of parity, milk yield at drying off, udder depth, teat canal integrity, SCC, quarter-site infection, number of infected quarters at drying off, length of the dry period and rainfall during the dry period. · Effect of treatment on three SCC post calving during the subsequent lactation. Clinical mastitis developed in two quarters of two cows (1,03%) during the dry period which was less than described in literature, and they were removed from the trial. The prevalence of pathogens at drying off was 29,78%, of which 7,87% and 21,91% were due to major and minor pathogens respectively. The prevalence of pathogens post-calving was 22,22%, a net reduction of 7,56%, of which 4,47% and 17,75% were due to major and minor pathogens respectively. The overall cure-rate was 83,94% for quarters and varied between 72,3% and 93,9% for the various products. The overall difference in the percentage of cases cured, compared to the different micro-organisms, was found to be only marginally significant (p<0,057). High cure- rates were observed for STA, SAG and non-agalactiae pathogenic streptococci compared to studies previously done, while lower cure-rates than described were observed with CNS. Antimicrobial products used in this trial differed substantially in their efficacy to cure Gram-positive IMI. Cure rates for Cepravin Dry Cow was 93,9%, Orbenin Extra DC 91,5%, Rilexine 500 DC 85,7%, Nafpenzal DC 79,2%, Bovaclox DC 75,0% and Dispolac Dry Cow 72,4%. Interestingly, but not of any significance, was an observation that the two products with the highest overall cure-rates both contained only one antimicrobial agent compared to the other 4 products which were combinations of two or more antimicrobials. There was no significant association between parity, milk yield, teat canal integrity, number of infected quarters per cow, quarter-site infections at drying off, length of the dry period and rainfall during the dry period and the cure-rate. However, there was a significant association between udder depth (p<0,0056) and SCC (p<0,005) at drying off and cure-rate. The cure-rate was significantly less in cows with udder depth scores of 1 and 2 or SCC of more than 750 000 cells per ml milk at drying off. The overall rate of new IMI during the dry period was 17,44%. The new intramammary infection rate for quarters which received dry-cow treatment varied between 13,4% and 24,1% for the different products. The majority (70,8%) of new IMI were caused by CNS during the dry period. Almost all (96,6%) major pathogens isolated post-calving were new IMI, while 74,1% of minor pathogens were new IMI. Antimicrobial products differed in their efficacy in preventing new IMI during the dry period. The percentages of new IMI observed in cows treated with the six products were: 13,2% for Cepravin Dry Cow, 16,3% for Rilexine 500 DC, 16,7% for Dispolac Dry Cow, 17,3% for Bovaclox DC, 21,4% for Nafpenzal DC and 25,9% for Orbenin Extra DC. The probability of quarters developing new IMI during the dry period was significantly increased when cows were dried off with milk yields higher than 18kg (p<0,0037) or had low udder depth (scores of 1 or 2) (p<0,0003). Higher parity cows (p<0,005) and those that had a teat canal score of 4 (p<0,039) and above at drying off were also at an increased risk for new IMI. Marginally significantly (p<0,06) more IMI were contracted on the left side of the udder than the right side of the udder and cows at drying off and cows with dry periods longer than 80 days had marginally (p<0,06) more new IMI during the dry period. A positive correlation was found between low SCC (<250 000 cell per ml milk) and number of infected quarters per cow and parity (less than 3 lactations) at drying off. There was no significant association between the SCC at drying off and new IMI during the dry period. Significantly fewer new IMI (p<0,05) were observed when no rain fell during the dry period, compared to cows that experienced rain during their dry period. A comparison between the six antibiotic intramammary dry-cow products in relation to their efficacy in curing existing IMI and preventing new IMI showed qualitative differences between them, ranked as follows: Dispolac Dry Cow was the most effective and Orbenin Extra dry the least effective in preventing new IMI with major pathogens and Cepravin Dry Cow was to most and Nafpenzal DC the least effective in preventing new IMI with minor pathogens. Cepravin Dry cow was the most effective in the overall prevention of new IMIduring the dry period. Due to the random selection of cows the percentage of IMI differed for each product at the start of the trial. To compensate for this initial variation, percentage point improvement from drying off until calving was calculated for each antimicrobial product, taking both the cure-rates and new IMI into account. The efficacy of antimicrobial products during the dry period differed substantially when percentage point improvements were utilised: from an increase in IMI post-calving of 11,73% with Nafpenzal DC to a decrease of more than 50% in IMI at calving with both Cepravin Dry Cow and Rilexine 500 DC. Cows dried off with Rilexine 500 DC had a significantly lower SCC for the first two months post-calving than those dried off with the other five products in this trial. It is concluded that a substantial difference in efficacy exists between antimicrobial intramammary dry-cow products in their ability to cure and prevent new IMI during the dry period. Dry cow therapy should form part of a holistic approach towards the dry period, which also includes cow factors, dry-cow management, micro-organisms and the environment of the dry cow. Cow factors (milk yields, udder depth, parity, TCS and quarter-site infections) mainly have an influence on new IMI but a few cow factors (udder depth and SCC) are associated with the cure-rate of IMI. Management and rainfall during the dry period mainly affected new IMI rather than the cure-rate of existing IMI.