Nolinear mixed-effects models (nlme)

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Hello,

I am trying to fit a nonlinear model of the form of:

A*x^b*exp(-c*x)

This represents a lactation curve. I have a bunch of cows, so I want
COW to be a random effect.
You need to decide which of the model parameters (i.e. A, B and C)
should have a random effect grouped by COW and to specify this in your
call to nlme.
I have been trying the following code with very littel success:

 > fm1 <- nlme(yield ~ A*(DIM^B)*(exp(-C*DIM)),
+             data = group,
+             fixed = A + B + C ~ 1,
+             start = c(A = 20, B = 0.3, C = 0.03))

Does anyone know how to add the random effect of the cow? I have used
the command groupedData to have Cow as subject (i.e., yield~DIM |
cow). Is this a valid and sufficient approach? I have the feeling it
is not sufficient.

Also, does anyone know whether the formulation of the fixed effects
is correct?.

Thank you very much,

Alex

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