Longitudinal data

Hi,

I'm analyzing a longitudinal data set with 387 cows were 
observed in 63 days divided into 6 groups, and every 30 days 
was found to produce milk. Does not aim to model the time 
using regression. Only compare the groups differ in terms of 
milk production. There are many missing observations. 
Because the data are correlated I used the SAS program:

    proc mixed data=univar method=reml;
    class RACA GRUPO APELIDO Dias;
    model Prod = GRUPO / solution DDFM=BW;
    repeated Dias / type=arh(1) subject=APELIDO r rcorr;
    lsmeans GRUPO / pdiff adjust=tukey;
    run ;

But, I want use R. What would be the equivalent in R?

Thank you.

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Silvano Cesar da Costa
Departamento de Estat?stica
Universidade Estadual de Londrina
Fone: 3371-4346