Which stats should I apply on my research data? Confused!

I need to know which stats should I apply to my research data. My research data contains four groups of animals. Each groups milk was tested for a particular chemical presence for four weeks. So I have three things which are 4 groups of animals, chemical presence in milk and weeks. Should I apply one way anova t test and duncan test? If I take groups as factor and chemical amount in milk as dependent value then where would days/weeks go? My fellow applied this test but I am so confused. I have seen a lot of tutorials but of no use.
It sounds like you have one between-subjects (or independent-measures) independent variable, and that is which group the animal is in.

And one within-subjects (or repeated-measures) independent variable, and that is time (measured in number of weeks).

Then your dependent variable is chemical presence in milk.

So I would do a 2-factor mixed ANOVA.