Study questions
- Consider that it has been decided to import some exotic high yielding cattle to mate with the indigenous breeds. Use the results of this survey to suggest within which areas this might most usefully be done. In which areas might the Ankole breed be best conserved as an indigenous breed?
- The data have been analysed by a nested analysis of variance with fixed effects for herd group and herd within herd group. Repeat the analysis using REML and choose herd within group to be random. Compare the mean values obtained for herd group with those that occur when using the nested analysis of variance approach. Explain what any differences mean. Consider the way the study was designed and decide which model you think is the more appropriate.
- The calculation of YLD210 attempted to account, to a limited degree, for variations in lactation length. An alternative method would have been to include LACLEN as a covariate in an analysis of TOTYLD. Run this model for TOTYLD and produce a new set of top cows. Compare this list with the list developed in this case study. Explain why the list might be different. Discuss whether you think this a good approach.
- Some herds had few cows recorded throughout lactation with one having as few as four cows. The more individuals in a group the more precise is the sample mean and hence the more precise a residual estimate. Look at a summary of the herd sizes, and decide on a minimum number of cows per herd that you would recommend for the purposes of calculating herd means which can then be used to identify elite cows. Remove the herds with fewer cows than that defined in your criterion and see what difference it makes to the final selection.
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