Dan 1962, of course the goals of the owner are paramount. Assuming he simply wants to clean up the pond (i.e. reduce algae & odors, increase clarity) these cattle need to be kept out of the pond. As mentioned above, the simple churning of the sediments by the cattle will create tutbidity. Furthermore, because they would be stirring up bottom sediment, this material would likely be high in nutrients which in turn could create algae problems. Finally, cattle relieving themselves in the pond is another source of nutrients. I heard or read somewhere that 1 pound of phosphorous will create 500 pounds of algae (wet weight). I'm not sure if this is true or not but it certainly puts things into perspective if it is.
It's important to realize that having a nice looking pond is just as much a function of your watershed management as it is your actual pond management. I've seen small changes in land use make significant changes in water quality. Some of these changes include creating vegetated buffers around the pond, riparian exclusion for cattle (at least 15' from shore), and aerated buffer strips around the outside the of pond for better water infiltration on land to name a few.
