These bacteria can spread rapidly through a herd, so it is better not to introduce them in the first place.

Replacement cows may be purchased to increase the size of the herd, to assemble a new herd, or to maintain cow numbers after culling. You should carefully consider the disease history of cows before buying as replacements or assembling a herd from mixed-age cows or from multiple sources.

Guideline 21 and Technote 21 provide practical and technical information on:

• buying heifers rather than cows
• Not buying cows unless bulk milk SCC and mastitis records are available
• Checking udders and collecting milk samples before buying the cows
• Milking bought-in cows last
• Not milking other people's cows with your herd
• Assembling new herds