Does anybody have any experience with planting Sunn Hemp? We experimented with some this summer. Deer grazed on it heavily. now wondering how it fairs thru the winter?

First hard frost will kill it.

Yea, first hard frost and it toast.

Great biomass for soil though, as soon as it dies, mow it down with a bush hog, throw turnips and mustard on top of it and disk it in during sping planting.

We planted it on several plots this summer. I was not impressed with it but maybe we did something wrong.

It provides a great deal of N to the soil while growing. I've noticed that deer only seem to browse it when young and tender shoots are avilable. Mowing at an appropriate plant height may promote more tender shoots and increased browsing. After the plant is mature I think you're just making N for the soil at something like 100 lbs of N/acre/60days. That's the beauty of the sunn hemp...I think it is better suited as a companion planting for a more prefered deer browse that requires lots of Nitrogen.

Every 60 days, when it reaches 5-6', mow it down to 18" and let it regrow. I like to include pea's and buckwheat, with mine. I think i'll throw in some EW and milo.

I don't think this is accurate. If I recall correctly from my discussion with Mr. Petcher, sunn hemp is not a nitrogen fixer like a legume. It is not fixing nitrogen in the soil. It is pulling nitrogen and storing it in the leaves of the plant. The nitrogen is then released to the subsequent crop upon its decomposition.

We planted sunn hemp and had mixed results. The smaller plots were hammered to the point that nothing was visible but stems where sunn hemp used to grow. The larger plots about 1 acre and bigger were able to outgrow the browse pressure. We've never been able to grow soybeans or corn as the deer just hammer it too hard and kill it. The sunn hemp was great in that regard. We plan to plant much more next year.

This is taken from a USDA article from Auburn University on Sunn Hemp. Here is the link to the full article. http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053283.pdf

Nitrogen Producer
In Hawaii, Rotar and Joy (1983) reported ‘Tropic Sun’ added
134 to 147 pounds of nitrogen per acre after 60 days of growth. In
Alabama, Mansoer et al. (1997) reported an average of 126 pounds of
nitrogen in 9 to 12 weeks. With this large amount of nitrogen
produced, whether in spring, summer or fall, a crop such as small grain
should follow sunn hemp to utilize the symbiotically produced
nitrogen. However, in the study conducted by Mansoer et al. (1997),
38 percent of the N in biomass remained available for corn planted in
mid-April, 16 weeks after the sunn hemp was mowed.

It is a legume:
Link to Sunn Hemp.pdf
Sunn Hemp Uses and Benefits
Sunn hemp has been used extensively as a soil improvement or green
manure crop in the tropics because of its ability to produce large amounts of
biomass in as little as 60 to 90 days. Because of this, it has the potential to
build organic matter levels and sequester carbon. Also, as a legume it can fix
large amounts of nitrogen. Used as a cover crop, sunn hemp can improve soil
properties, reduce soil erosion, conserve soil water, and recycle plant nutrients.
‘Tropic Sun’ is also resistant to root-knot nematodes. Other potential uses for
sunn hemp are forage, paper fiber, and as alternative fuel crop (Rotar and Joy,
1983).

I tried putting it in 3 acres this summer. The only place it was able to survive was in the exclusion cages. All other plants were browsed heavily after it emerged and never made it.

So it depends on how much other food sources you have and how dense your deer population is on whether it will survive or not.

You are correct that it is in the legume family. It also purports to do some atmospheric nitrogen fixation, although I haven't found any reports as to how much.

I will stand by my statement that the bulk of the nitrogen is stored in the leaves. Here's a quote from the linked study where they are measuring the nitrogen found in the biomass. That's what biomass is, living, biological matter.

"Sunn hemp plants need not be mowed during the middle of summer. However, mowing to remove the very top of the plant can be beneficial. For example, in an experiment at Homestead, Florida, in which 5-foot-tall plants were mowed at 1 foot, 2 feet, and 3 feet above the ground and then allowed to grow 70 more days, the total yields of dry biomass were 8.3, 11.0, and 13.8 tons per acre, respectively, compared to 11.4 tons per acre in the uncut control. The corresponding numbers of pounds of nitrogen in the biomass were 259, 255, 277, respectively, compared to 238 pounds of nitrogen in the uncut control."

If you plant it as a companion crop, I'm not sure you'll get the results you're looking for. If the bulk of the nitrogen is in the biomass, you won't see any benefit until you cut it down and it begins to decay.

http://edis.ifas.ufl.edu/m/#publication?id=TR003

per your cited report 180lbs/acre nitrogen fixation and 700lbs/acre nitrogen from biomass under optimal conditions:
Via atmospheric nitrogen fixation, sunn hemp may accumulate as much as 180 pounds of nitrogen per acre. This crop also adds 2.5 to 11 tons of organic matter when incorporated into the soil. When environmental conditions are optimal, as much as 16 tons of aboveground fresh biomass and 700 pounds of nitrogen can be accumulated in subtropical regions, such as south Florida (Fig. 1). To grow sunn hemp in subtropical regions, seeds should be sown at a soil depth of 1 inch or less in June through July. At greater seeding depths, emergence is poor. Seeds should be sown in soil temperatures greater than 68°F for successful germination. Seeds usually germinate readily within 3 days, and seedlings rapidly develop a dense ground cover. To establish as a cover crop, sunn hemp should be sown at a rate of 10 to 40 pounds of seed per acre. Lower seeding rates can promote lateral branching. Seeds can be inoculated with cowpea inoculant to improve nitrogen fixation.
180 lbs of free N is awesome. I don't think I have ever seen a soil test come back that called for much more N than that per acre.