I want to do a linear regression model in R to test if a treatment (3 levels) has a significant effect on the blood counts 24 hours after treatment. Fixed effects: treatment, sex (2 levels) and age (8 levels). covariates: blood cell counts before treatment (BC_T0), the order the blood sampling was taken from an animal before sampling (Seq_code_T0) and the order that the blood sampling was taken from the animal 24 hours after sampling (Seq_code_T24). This experiment was performed across 3 years and males and females were done on different days. I generated a sequence code of 10 digits which consists of year (***), sex () males are 01 and females are 02, and slide order (), and a code to differentiate between time of sampling (1 for before treatment and 2 for 24 hours after treatment).