An extremely important source of data for epidemiological studies in dairy cattle is milk recording. Outside of Europe, milk recording is sometimes called dairy herd improvement (DHI). In France, it is called contrôle laitier. International guidelines on how to perform milk recording are published by the International Committee for Animal Recording (ICAR).
The basic principles are the following:
- Recording is performed on all the lactating cows of a herd on a regular basis. The reference method consists in 4 week intervals with at least 11 recordings per herd per year.
- Two consecutive milkings are used per recording
- Milk yield is measured on all lactating cows
- A milk sample is taken from each cow for the determination of butterfat content, protein content and somatic cell count.
The data are used for different purposes. They allow farmers to know how much milk each cow produces as well as the content of the milk. Milk somatic cell count is a marker of inflammation and is used to detect mastitis. Milk recording is also used for the evaluation of the genetic value of bulls and cows.
For us epidemiologists, it is an invaluable source of information. It can be used to assess the effect of diseases on milk production (e.g. mastitis, bluetongue), to describe how variable herds are at a national level (e.g. mastitis).
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