Problem:

I have collected some data to compare the biodiversity of a field in which the plagioclimax is maintained by machine mowing with a field in which the plagioclimax is maintained by sheep grazing.
What I found was that there was a significant increase in floral biodiversity in the field which was managed by agricultural grazing when compared to the floral biodiversity of the machine-mown field.

I am trying to explain my findings, and I believe that it is due to the comparitively short amount of time that machine-mowing has been a grassland management technique, so there has been little time for natural selection to take place and for species to develop adaptations to machine-mowing (such as being low-lying, or extremely fast-growing). Whereas the field which is managed through sheep-grazing, contains multiple competing species which have adapted over time to grazing sheep, such as Cirsium arvense.

Moreover, the field which is managed by grazing has nutrients returned to it through defecation of the grazing animals, allowing for more favorable conditions for growth.

Question: Is my explanation valid?

Question: Are there any other reasons why the floral biodiversity of a field managed by sheep grazing would be higher than one managed by machine-mowing?

Please give your suggestion and explain something new if anyone knows about it.