Questions to be addressed

We shall approach the questions as follows:

• For the purposes of identifying elite cows, we shall use cows in their second or subsequent lactation. The reason for this is that the first parity cows (i.e. cows in their first lactation) have lower milk offtakes than older cows. From the second parity onwards variation in milk production varies less with age; we assume in this analysis that the age of a cow has little effect on milk production beyond its first lactation.
• We shall investigate two alternative ways of estimating total lactation offtake.

Method 1. Total milk offtake using all recorded milk offtakes.

Method 2. Total milk offtake up to a fixed point of lactation in order to reduce some of the influence of variations in length of lactation on total milk offtake.

• We shall use least squares analysis of variance to account for mean differences in milk offtakes among herds.