4 acre pond north of Dallas. Other side of the pond is a ranch with about 50-70 head of cattle on it.

At first I couldn't figure out why pond had such bad algae/plants in it but I'm pretty sure it has to be the manure that is deposited on the other side of the pond, the rain washes it into the pond and sure enough the algae bloom explodes.

Other than trying to buy the land from the ranch (which won't happen), is it worth it to go out to the pond every day and shovel the manure away so it doesn't wash into the pond?

Trying to find a solution....

Would it be possible to build a diversion terrace to keep the runoff from the cow field from going into the pond?

If your pond stays full diverting some shouldn't be a problem. But you have to keep in mind any diverted water is water not going in your also. Just my opinion never dealt with that problem. You might get some farms to chime in that know better solutions.

Am I correct that the cattle side of the pond is not your property?

Yes thats correct

If the cattle are free to enter the pond, you could offer to build a fence and install an alternative method of watering the cattle. https://www.agry.purdue.edu/ext/forages/rotational/watering/watering-hole.html

One of the reasons given for Lake Fork production/success with large bass was due to the number of dairy farms on the lake.

Deal with the algae when necessary, that's an every year deal for me and we don't have a cow within 10 miles of us.

You're going to have to try to work with your neighbor if you hope to keep the cows away from the pond.

If the "downhill" (dam) side is on your neighbor's half, a gravity feed water line could be installed to water the cows downhill from the pond, with it fenced off to keep them out. (We have 1" black plastic waterlines buried from filters suspended inside our 2 ponds to freezeout hydrants located below the dams which we use to water our cattle in the Summer.)

Your help with installation (parts, labor, and/or $) may well be needed since your neighbor may not think there is a problem.