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How to grow Sunn Hemp
By Wayne Smith
Agronomic Acumen
Sunn Hemp is Crotalaria juncea, one of the fastest growing legumes known. Saia oats (black oats) and
Sunn hemp are the two main cover crops grown around the world, and for good reason.
Sunn Hemp is called this because of its bright yellow flowers. It is an erect, branching, annual summer
legume. It is a rapid, vigorous grower with a strong taproot and branching root system. It reaches a
height of over 1.2m in 60 days when grown under favourable conditions. It can attain a height of over
1.8m in approximately 90 days. Plants are usually unbranched from the ground to 2 ft (60 cm) and
many branches develop above this height. It does not self-seed easily and is therefore a low weed risk.
Sunn hemp is drought hardy, fast growing, huge producer of nitrogen, not fussy with soil types,
diseases, water, inoculation etc…, and kills/reduces CCN, RLN, rhizoctonia and take-all.
My advice to you is to grow it first for seed production rather than for its performance as a cover crop.
Two Ag Dept researchers will do the trials of Sunn hemp as a cover crop. I have spent hours reading
all about this crop and have summarised the findings below.
Seed rate: For seed production, 5-10kg/ha. Higher would be needed when wanting to test it as a cover
crop. Sow with no-till and press-wheels.
Row spacing: I suggest 1m rows. Trials showed best yields occur at low seed rates and wide row
spacings, making the plants branch more and having more moisture to fill seed.
Inoculation: Most people say it does not need it. Americans suggest using the EL (cowpea)
inoculum. Because Sunn Hemp is in the fabaceae family, it might be possible to use the faba bean
or pea inoculum if you were to use any, but testing without inoculum would prove how robust it is.
Australian researchers are testing its nodulation requirements in the summer of 2001/2002.
When to sow: There are no hard and fast measurements of what temperature it needs to germinate. At
9am at 3-5cm depth, the temperature should be 15C and rising, but some people think ~14C would
be OK. We will have to see. Please take note of the soil temperature when sowing (you need a
thermometer in the ground). It certainly hates cold, but should emerge in 3-4 days if warm
enough.
Sowing depth: Again, varying reports on this, but aim for ~3cm. Use press-wheels.
Soil types & pH: This is a robust species. It will grow happily from 5.0-8.5pH, and on any soil type.
Down as low as pH 4.5 should be OK, though slightly higher would be better.
Fertilisers: Apply ~50-60kg/ha of an MAP based fertiliser, preferably something like No-till, No-till
Manganese, MAPSCZ, Maxi-TE, Multi-MAP etc…, or plain TSP. If potash is marginal, add 80kg
potash before sowing (top-dressed or drilled away from the seed).
Herbicides: This has been the hardest to find info on because no one needs to use a herbicide.
Common replies have been that it smothers all the weeds – except at low seed rates. I have found
some information and have made some educated guesses to make the following suggestions.
Chemicals that should be very safe. Spinnaker, Dual, Treflan, Stomp.
The following should be safe – Raptor.
May not be safe – Diuron, Simazine, Atrazine, Bladex.
Won’t be safe – knockdowns, 2,4-D’s, Lontrel, Dicamba, SU’s
One note about 2,4-D – a farmer in Paraguay tried killing Sunn Hemp with 700ml 2,4-D Amine,
but it did not affect it much, and then it fully recovered, so they grazed it. Interesting!
Finding out about herbicides will be very difficult. Please record all your experiences and I will
compile it all for all of us to learn from. I will see if we can get the Ag Dept to do a few spray
strips for us.☺
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Pests: It is quite tolerate to many pests because it out grows them, but before sowing, use something
like 200ml Chlorpyrifos + 75ml Cypermethrin just to make sure. If Bryobia mites are a problem,
use 150ml Lemat instead of Chlorpyrifos. Aphids will be attracted to it, and so may diamond-back
moths. Check closely for aphids and spray 200ml Chlorpyrifos + 150ml Cypermethrin if you see
them – note, when we grow them as a cover crop, we would not be doing any of this.
Harvesting: This is all I could find about header settings – “Initially set the concave clearance at 1/8
to 3/16 inches and the cylinder speed at 1150 to 1200 RPM. Adjust as needed according to crop
conditions.” I hope this means something.
When the pods rattle, the seed is ready to harvest. It can be desiccated with Gramoxone/Reglone if
needed, but it should dry down on its own. Planted by mid-October, I expect it to be harvested by
late February/early March. Time will tell.
Harvest is where I suspect we will have the biggest problem with Sunn Hemp. It does not produce
much seed in cool night areas. It needs hot days and hot nights to produce significant seed, but
until we try it, we wont know for sure. Likely yields may range from 200kg-800kg/ha, but well
grown crops in hot areas can yield up to 2.5t/ha.
As a cover crop: Maximum nitrogen production occurs by or just before flowering (~40-50 days from
sowing). No one lets it go much past this point before killing it as the stems become too fibrous.
Average nitrogen concentration is ~5% when young, so 50kgN for every tonne produced. 5t/ha
biomass (a small crop) would produce about 250N, or >500kg Urea equivalent. Huge!
Effects on Livestock: Seeds and forage of Sunn hemp are non-toxic in laboratory tests and feeding
trials. Other species of Crotalaria contain poisonous alkaloids and, under certain conditions, can
be toxic to animals, but not with Crotalaria juncea. Stock may find this variety unpalatable, but
should not suffer health problems. As always though, be cautious and do not put hungry stock into
it.
In summary then, please record all your observations and send them to me. I will compile it at the end
of the season and send a copy to all growing it. Most of you should try and produce seed and then on
sell it to others. As a guide, we can get >10tonne lots in from India for ~$2-3/kg. I would aim to sell
what you produce for $2-3/kg, depending on how well it grew, how much seed is produced, and how
big the demand is.
If Sunn Hemp grows like it does elsewhere in the world, demand by no-till farmers will be very high.
As far as I know, the 1000kg I brought in is the only bit growing in Australia. The seed has been
spread throughout WA wheatbelt, several areas in South Australia, and one in Victoria. It should give
us a good picture of how and where Sunn Hemp performs.
All the every best and I look forward to seeing most of you in your Sunn Hemp, including the eastern
state ones.
Wayne Smith
Ph 9842 1267
Fx 9842 1964
Mobile 0428 188 479
Email wsmith@agronomy.com.au
Web www.agronomy.com.au
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