You say you have 100 records in the data if the cow lives 100 days. That doesn't make sense to me because you also say that a cow might have GrowCost = 1200 for the first day but then you assign GFrowCost=0 for the other 99 days. That doesn't lead to a sound model.
It seems that none of your variables are changing during the life of a cow, so you should have one observation per cow and drop the weight variable.
If you want to compute the daily cost per cow, you can add LifeSpan the data and model the rate. See how to model rates in SAS.
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