Cover Crops from No Till On The Plains

Our knowledge of Sunn Hemp is by no means complete, but here is what we have learned so far.

Sunn hemp (Crotalaria juncea) can be planted up to 2 to 2.5" deep. Seeding rates of 3 to 5 lbs/a appear adequate for Kansas. Well-adapted to low-pH soils (but less tolerant of high pH, such as over 7.5) and loves warm (hot) weather. Will tolerate drought and low fertility.

Inoculate with cowpea-type inoculant. Use plenty of viable inoculant, as this Rhizobium is not native and the Rhizobium from soybeans does not colonize sunn hemp roots. No inoculant = no N fixation.

In the rotation: It looks like a natural fit between two wheat crops (in a long rotation), as it can reach a height of 40" in 35 to 40 days of growth, with adequate moisture & warm weather. This is enough growth to produce a substantial amount of nitrogen. We have observed no detrimental effects on the following wheat crop, other than it can use too much moisture if left too long and if it is a dry fall. According to Rolf Derpsch, a very common No-till crop sequence in Paraguay is to seed sunn hemp ahead of wheat, which is a good agronomic practice for them.

Sunn hemp also looks like a good possibility after wheat, for fields going to corn the following year. In two trials conducted in 2001/2002, we observed no detrimental effects on the corn following sunn hemp. In fact, the corn following sunn hemp produced slightly better stands & higher yields than adjacent strips where no cover crop was grown. Note, however, that we do not know if this will be true for milo following sunn hemp (according to Derpsch, sunn hemp is not allelopathic to milo, so we don't expect problems).

Sunn hemp is easily terminated by 2,4-D and glyphosate. It does not appear affected by dicamba. Freezing temperatures in the fall will also kill it. Producers should plan on terminating it after 45 days of growth or sooner (we're guessing as to the optimum), since sunn hemp left to grow on into the late fall will become very fibrous and difficult to plant into, especially as the stalks 'cure' over the winter. The main problem with sunn hemp left to grow for too long is that its fibrous nature causes it to wrap around any and all rotating parts on the planter or drill the following spring. Earlier terminating prevents this -- producers in Kansas have seeded hundreds of acres into sunn hemp that was properly terminated after 45 days (30 to 60") without any problems. If producers wish to have a tropical legume cover crop growing for more than 45 days, we suggest cowpeas or mung beans instead of sunn hemp.

We have no good estimates of N fixation or credits for our climate. Some extremely high numbers (200 lbs/a) have sometimes been bandied about, but those data pertain to very tropical (warm and wet) climates where the sunn hemp may reach a height of 10 feet or more in 85 days of growth. For the plantability reasons previously mentioned, we cannot let it grow that long. Moisture, heat, inoculation, and other growing conditions will also affect the amount of N accumulated in the sunn hemp plants per acre. We do not expect our N credit following sunn hemp to be very large for KS, especially not for the following wheat crop (not enough time for decomposition).

Do not feed sunn hemp to livestock, as it contains poisonous alkaloids. Certain varieties of sunn hemp have been selected to not contain these substances (such as USDA's Tropic Sun variety, and some others grown in Paraguay), but the varieties commonly available in the U.S. are generally considered to be toxic to livestock.