Statistical difference between linear regressions

Question

I have a statistical question on which I am stuck:

Imagine you have 5 corn fields. You know the number of corn plant there is in each fields. You know want to compare the true number of corn plant, with some estimations of this number (for example taking the weight harvested). Lets call z1 the true number of plant and z2 the estimated number of plant.

My question is how do I test that my estimated number of plants are not statistically different from the true number of plants? I started by doing a linear regression between z1 and z2 for all the fields. My correlations have a pvalue < 0.05, but how do I test that this regression if not different from a regression I would have if z2 = z1 in all fields?

I hope I was clear.