Tree/Swale spacing

I'm getting ready to set up my contour tree/swale lines on my 8+ acre pasture. The land is essentially 800ft long and 400ft wide, with the long (800ft) side heading downhill at 3-12% grade to a pond. The soil is silty loam, and there are currently no trees, just overgrown pasture and weeds. There are two primary valleys on the down slope, which I plan to eventually dam up for some small ponds along the ridge.

My plan was to space the tree lines by somewhere between 50 and 100 feet as I worked my way down the hill. The alleys will be used for sheep rotationally grazing as my primary animal, with some other animals here and there. If I do spacing every 100 feet, then the area for grazing one paddock will be 400x100, but if I do 50 feet then it will be 400x50. The latter seems maybe too long and narrow, but I'm really not sure.

Anyways, I was leaning towards 100 foot spacing between tree rows, but now I'm reading some more traditional material on silvopastures and it seems like 50ft is a pretty standard recommendation for maximum photosynthesis. This has me thinking I should do 100 foot spacing for now, and I can add rows between the rows to create 50 foot spacing in a year or 3.

So my question: Does anyone have an opinion on spacing? Or things I should be considering? I already know that my swales won't hold all the rainfall because I have a pretty massive watershed from neighbor properties that flood my land and pond anytime we get an inch or so of rain, and we get 45 inches per year on average. So I'll have a method to divert the flood water down to my lowest pond, which also floods to my neighbors pond down stream after mine fills. So I don't know whether I need to consider rainfall in my planning, since I don't see too much need in capturing more than what I'll get even with 100 foot spacing. But I could be wrong...

For the record, I plan on a 4 foot wide swale, with about a 1 or 2 foot depth, although that's still up in the air.

Are you sure you even need to build the swales? It sounds like maybe some keyline plowing and on contour planting might be more appropriate. I guess if you're making ponds then swales might be called for, but I would size the ponds to take advantage of all of the water you have coming onto your property. Build the swales to direct the water into the ponds, build big enough ponds to catch and hold it all then utilize the swales again to redirect water out of the ponds for irrigation of the pasture. I guess I'm tracking you now... sorry!

As far as spacing goes you might want to consider the possibility of utilizing haying equipment so space the plantings accordingly. I've seen 80 foot spacing mentioned on a couple different occasions by I believe Mark Sheapard and Jack Spirko in their silvo-pasture designs specifically for this reason. Just something to think about, at least to keep the option open.

That's a god point, I'm not even totally sure I need swales. It's possible that I could use swales at the top where there are no good dam options for ponds and use keyline plowing below. The problem is that I don't have a tractor nor do I plan to get one, and I haven't found anyone nearby that understands keyline plowing. Swales, I figured, I could do once. I know a guy that is an artist with an excavator, so I was planning to mark contour lines and describe the swale structure to him, then let him get to work. If you have a better idea, please let me know.

I don't plan on cutting hay and if I do I will likely use a scythe in just one or two areas since I don't have a tractor and plan on year-round grazing of my sheep. With that said, 80 feet might be a good option anyways. Do you happen to know where Mark Shepard or Jack Spirko talks about that? I've read Mark's book, and I've specifically searched around Google for his recommendation on spacing but I had never come up with anything other than making sure there's enough space to get through whatever harvest machinery you might use (in my case, that harvest machinery is just me and whatever hand tools I bring). I just highly doubt that my 8 acres of polyculture silvopasture will ever benefit from large machinery harvesting, although I've been wrong before.

Here's a video where Spirko is talking about swale spacing and the sort of hybrid keyline/swale system:

I'm sure there's more out there on this, but it's a start.