Features Agronomy Fertility and Nutrients
Low input cropping with black medic

Black medic is a self-seeding annual legume cover crop.     Photos courtesy of Bill May, AAFC.

At Agriculture and Agri-Food Canada’s (AAFC) Indian Head Research Farm, researchers are trying to determine whether or not black medic could have a fit in current cropping systems. Black medic Medicago lupulina, is a self-seeding annual legume crop that can be used as an underseeded cover crop in rotation to fix nitrogen and potentially increase the availability of nitrogen (N) during the growing season. Although considered to be a weed by some, researchers are interested in finding out its value in no-till cropping systems. “Our objective with this study was to evaluate the benefits and problems of black medic as a self-seeding legume cover crop in a no-till system,” explains Bill May, research scientist. “We started the project in 2002, seeding black medic to get the cover crop established, which then annually reseeds itself.”

In 2003, the first trials were started using a winter wheat–oat–flax rotation established on blocks with and without black medic. Three rates of N were compared, at 20 percent, 60 percent and 100 percent of recommended N rates for each crop, taking into account the residual N supply in the soil. At Indian Head, the target N rates including soil residual N, are 135 kg per hectare (120 lbs per acre) for wheat, 110 kg per hectare (98 lbs per acre) for flax and 100 kg per hectare (89 lbs per acre) for oat.

Black medic is similar to other forage crops in terms of seeding and establishment. Seeding rates of about 10 lbs/ac  similar to alfalfa, are used in the establishment year. Black medic emerges later and is not very competitive for the first 60 days of growth. That gives other crops a chance to get established, although flax is less competitive than oats and winter wheat. Seeding crops early and controlling annual weeds early is a good strategy because it allows for the herbicides to be applied before the black medic germinates, protecting the cover crop. Residual herbicides should not be used, as they will significantly reduce the black medic biomass. Herbicide products with chlopyralid will significantly damage the black medic.

Oat and wheat are competitive enough to prevent the medic from causing a yield loss if the medic is set back by a herbicide such as MCPA sodium salt. This also allows for an increase in black medic biomass and seed production. However, black medic does have to be controlled aggressively in flax and Buctril M is a good choice that will allow new medic seedlings to germinate under the flax later in the season. “Weed control for perennial weeds should be done at pre-harvest or in the fall, to minimize the effect on the black medic cover crop,” says May. “In most crops, managing black medic isn’t a huge problem,” says May. “However, once you have black medic in rotation, eradicating it can be difficult, so you will have to plan to manage for it. Managing black medic in peas and lentils is where knowledge is currently lacking.”

After six years and two complete cycles of the study, researchers are starting to see some interesting results. In 2007, the winter wheat crop showed a significant yield increase at the 20 percent N rate, doubling from 1023 kg/ha (15.2 bu/ac) with no medic to 2085 kg/ha (31 bu/ac) with medic. In 2008, the medic treatment increased flax yields by almost 50 percent at the 20 percent N rates. In 2009, the medic at 20 percent N treatment outyielded the non-medic by 29 percent in the oats and by 57 percent in the wheat.

“Overall, the research shows that at that low N rate, black medic is providing enough N to improve the yield stability of the crops,” explains May. “The 20 percent recommended N rate for crops grown on black medic does increase and stabilize crop yields. Therefore, low input and organic growers can benefit from using black medic as a self-seeding cover crop to increase crop yields and improve soil residual N.”

However, at the higher rates of recommended N at 60 percent and 100 percent, the yields benefits were not significant. In fact, at higher N rates the black medic does not perform as well and impacts the biomass growth of the crop. As N rates increased, the biomass of the medic consistently decreased. “The research also shows significant changes in the residual soil N levels,” says May. “As higher rates of N are applied, higher levels of residual N are observed. Of the three crops, it appears that oat is the most effective at scavenging the extra residual N.”
Black medic may also have a fit for livestock operations such as beef cattle or sheep. “Although we haven’t done any research in this area, it is something we are interested in and would like to consider for the future,” explains May. “For livestock growers, black medic may be used for grazing during the growing season, instead of growing another crop, or perhaps for fall grazing. An assessment of whether or not grazing has an impact on N fixation and soil residual N levels would be required.”

Researchers are continuing the project and from now through to 2012, they are interested in finding out if they can show a benefit at the 60 percent recommended N rate and be able to establish the medic to be productive enough to supplement enough N for subsequent crop need. “We also want to conduct more intensive studies on various soil factors to determine their impact on N fixation and soil residual N levels,” adds May. “In the oilseed year, canola will replace flax to find out how this N-sensitive crop responds to black medic and the three rates of N.” In the future, researchers hope to conduct another experiment to demonstrate whether or not black medic can be successfully managed in less competitive crops such as peas or lentils.