Abstract
Over the last two decades the dairy industry has encountered some major changes and as a result the farmer’s attention has shifted from a cow- towards a herd-level approach by means of close monitoring of herd level parameters. The overall aim of this thesis was to critically review how useful
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current somatic cell count (SCC) parameters are for monitoring udder health on herd level by producers and processors, and if necessary to suggest improvements and recommendations for the use of these parameters. For a large number of farms worldwide the bulk SCC (BMSCC) is the SCC parameter available to monitor trends in udder health. In this thesis it was assessed if bulk SCC (BMSCC) reflects the average SCC of all individual lactating cows, the correlation with the prevalence of cows with a subclinical mastitis, and if the sampling interval and variation of the BMSCC data affected the accuracy to describe measured detailed series of BMSCC data. Subsequently it was examined which cow factors, management practices, and seasonal fluctuations contributed to the average herd somatic cell count (HSCC) and if these factors found in studies in Europe and the United States also applied on farms in a different environment such as on Australian dairy farms. Finally, to support decision regarding udder health a model to predict of the average herd somatic cell count of the next month was developed. Briefly stated, the main conclusions of the studies described in this thesis are: - BMSCC is a less useful tool to monitor udder health when individual herd recording data are available; - The difference between BMSCC and the average SCC of all lactating cows is with up to 40%; - When BMSCC data are the only data available to monitor udder health at herd level, it should be anticipated that it is likely that it will underestimate the prevalence of subclinicalmastitis; - When BMSCC is used to detect a trend in average HSCC, the error in describing a series of BMSCC data increases significantly when sampling interval exceeds 4 d; - When BMSCC data is used as the outcome variable in management studies, a proportion of farms may be inaccurately classified as too low and as a consequence, association between BMSCC and certain management practices could be incorrect; - The contribution of management, herd characteristics and seasonal variables are different per level of the average HSCC and therefore when advice on alterations in management is given, the level of average HSCC should be taken into account; - Management style might be equally important for the impact of a management practice as the implementation itself; - Management practices applied in the EU and United States will have a similar effect on udder health under Australian conditions; - An accurate prediction of the average HSCC for the subsequent month is possible when information on management, herd characteristics and seasonal variation is taken into account.
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