Pinkeye, as it is usually named and defined in textbooks, is caused by a bacterial infection of the surface of the eyeball and the inner surfaces of the lids. Its technical name is infectious bovine keratoconjunctivitis (infectious=caused by an infecting organism; kerato=referring to the cornea of the eye; conjunctiv = referring to the pink tissues of the eyelids and the other soft red tissues of the eyeball; and "itis" = inflammation). The cause for this disease is usually given as the bacteria, Moraxella bovis. In recent years a couple of additional bacteria have been incriminated as causing pinkeye. Unfortunately, the bacteria that cause this disease are commonly carried by a few animals that show no signs of disease and serve to introduce it to a new herd when those animals are added to it or that keep the bacteria on the farm to expose a new group of susceptible animals to it. Most animals that recover from the disease clear the infection from their eye tissues; sometimes all of them do. The body's immune response is responsible for this and serves to help keep the animal from getting it again. This is why sometimes it appears in a cow herd with both cows and calves affected by it, but it eventually disappears by the end of the summer. If one or more carriers remain, it sometimes appears again the next year but usually only in the animals that weren't affected the previous year such as new calves or heifers housed at another location the previous year.
The presence of seed heads on tall stems does not, by itself, cause pinkeye. The presence of seed heads can be a factor causing some eye irritation as the cows and calves graze. Of course, so are dusts, pollen, strong sunlight, and face flies - all considered predisposing factors. But the causative bacteria have to be present, and at least a few non-immune animals have to be present, before the disease appears. And not all the predisposing factors have to be present to have the disease appear. Pinkeye in a non-grazing dairy herd can be a nightmare to deal with to which I can attest from personal experience; several times.
The disease often disappears from a herd after a couple of grazing seasons without any special preventive efforts like vaccination or pasture clipping. I suspect that this is caused by the eventual exposure and development of a good immune response by almost all the cows with the carrier cows eventually clearing the infection from their eyes. Unfortunately, there are several strains of Moraxella bovis out there, and it is a pretty safe bet that the available vaccines do not provide good protection for all of them. Therefore, it is not uncommon to see the disease reappear in a herd previously infected if new animals carrying a different strain are introduced or if the herd grazes close to another herd that has the disease. Face flies can carry the bacteria on their body for up to three days, and can transmit it between animals and between herds. The disease can also be spread cow-to-cow by close facial contact such as around feeders. The bacteria are in the tears. I have never seen any evidence to support that seed heads or other inanimate objects are involved in the spread of the disease from animal-to-animal, but the bacteria certainly are mechanically transmitted by flies, so I suppose it could occur in just the right circumstances.