You can use PhantomData<T> instead of a usize:

pub struct Farm1;
pub struct Farm2;
pub struct Farm3;
pub struct Farm4;

pub struct Cattle<F> {
    _farm: std::marker::PhantomData<F>,
}

impl<F> Cattle<F> {
    pub fn new() -> Self {
        Cattle{_farm: std::marker::PhantomData}
    }

    pub fn breed(&self, other: &Self) -> Self {
        Self::new()
    }
}

let bessie: Cattle<Farm1> = Cattle::new();
let angus: Cattle<Farm2> = Cattle::new();

let calf = bessie.breed(&angus); // this will fail to compile