Cows only spawn on grass, even if they're coming from a mob spawner. You need to surround the spawner with a nice 9x9 field to get maximum output. If you still want water you'll have to elevate the streams on signs so that it will still push cows but the grass doesn't decay to dirt.

Science!
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I scienced up some cow spawners to demonstrate this, and to test whether water streams were a viable transport method. You can see the initial results here:

![enter image description here][1]

As you can see, both the spawners on grass spawn cows, but the one on sand to the left has spawned no cows, despite being created first. Note that the top-right spawner is floating 1 block in the air and it's spawning cows just fine.

Apart from the need for grass and light, the normal spawner-block rules apply:

* Only air blocks 1 above, on the level with, and 1 below the spawner are valid spawn locations. (Note that the extra need to be on a grass block means that unlike other spawners, cows will not spawn in mid-air, so only 1 vertical layer of the normal 3 actually spawns cows.)
* Mobs spawn in an 8x8 area centred on the NW corner of the spawner block. (I've done 9x9 paddocks in the demonstration because I couldn't be bothered with the weird "centre" of spawner blocks.)
* The spawner will not generate any new mobs so long as there are any of the same type in a 17×9×17 area centred on the same corner of the spawner block.

Making a collection point
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For moving the cows out of the spawner's detection area to enable continuous spawning, I experimented with a few configurations of water streams. To my dismay I found that cow spawning logic dictates that only grass blocks that can see the sky are legal spawn locations, even from a spawner block. Therefore, *any* non-air block above your grass, including transparent ones like signs and water flow, prevents the spawner from spawning cows on those blocks. For instance, you can't just cover all the grass with water streams, as no cows will spawn. Elevating the water streams with signs doesn't work either – the signs themselves prevent spawning, and even if they didn't, the elevated water streams block the direct view of the sky from the grass blocks and prevent spawning anyway.

The best I could come up with was making a moat of water streams that would transport any cows that wandered out of the 9x9 area to a collection point 16 blocks away horizontally from the spawner block. (Effectively, I replaced the fences with the moat.) This was very inefficient because the cows don't like falling into moats, so they hang out in the spawning area for quite a while. (I've heard tell that placing signs below a cliff edge will make mobs think it's safe to walk there, but I didn't see a noticeable increase in cow-falling.) However, it does generate more than it would by itself (which is only 6), so it's still an improvement. It's just not as fast as we are used to grinders working.

  [1]: https://i.sstatic.net/CJ9uQ.jpg