Its looking for a relationship between growth and milk yield in cows. So I have weight, height and girth measurements of the cows when they were calves at birth, 3 months, 6 months, 9 months and pre-service.
My outcome variable is 305 day milk yield during first lactation.
I figured its not much use to relate the actual size of a cow with its milk yield but better to look for an assocaition between growth rate and milk yield.
So I have growth rate (daily rate of change in height / weight / girth) for the periods birth to 3 months, 3 to 6 months, 6 to 9 months and 9 months to preservice.
Its not really suitable to calculate an overall daily growth rate for the whole period as their growth rate is quite different for the different periods.
So I was wondering if there is an analysis in which I could treat these as repeated measures of an explanatory variable and relate them to the outcome - milk yield.?
I have tried a linear mixed model but you cant use this unless the outcome measure and explanatory measures are repeatedly measured at the same time - as far as I know?
I have also analysed it treating each growth period as an independent explanatory variable but I think I am losing some of the power of the study by doing this?