By Rico Balzano, UVM Extension Agronomy Outreach Professional
There is growing consensus that cover crops have many environmental and agronomic benefits including reducing soil erosion, adding valuable organic matter, and improving overall soil health. But how do cover crops fit into a weed control program? And how may they effect other soil-borne pests and diseases?
In 2015, I received a SARE farmer grant to explore the use of mustard cover crops to help control plant parasitic nematodes, weeds, and soil-borne diseases. Varieties of two species of mustard (Sinapis alba and Brassica juncea) have been identified as producing chemical compounds known as glucosinolates that have been shown to reduce fungus and nematodes populations when mowed and incorporated into the soil. This process is known as biofumigation.
Six varieties of mustard were trialed to test glucosinolate production and overall biomass yield. The yields were measured by weighing samples in the field, and glucosinolates were measured by a lab at the University of Idaho. The varieties were: Kodiak (Brassica juncea), Pacific Gold (Brassica juncea), Ida Gold (Sinapis alba), Caliente 119 (S.alba and B. juncea blend), Caliente 199 (S.alba and B. juncea blend), and Nemat (Eruca sativa– also a Brassica, bred as a nematode trap crop). They were planted in the spring of 2015 and allowed to grow for 60 days before incorporation and measurements were taken. It was found that ‘Caliente 199’ had the highest biomass yield and highest levels of the glucosinolate ‘sinigrin’, a volatile compound that has been shown to have anti-fungal and anti-nematode properties. Interestingly, ‘Ida Gold’ contained another gluscosinolate, ‘sinalbin’. This non-volatile compound has shown the ability to inhibit weed seed germination. Although measurements were not taken, it was observed there was less overall weed pressure in the ‘Ida Gold’ plots. This is similar to observations in trials of ‘tillage radish’, another Brassica species. It was not determined whether weed suppression was a result of biofumigation or a dense cover crop outcompeting weeds. Planting rate (density) in other cover crops such as winter rye and oats has been shown to effectively suppress weeds. Further study is needed to determine how planting rates of mustards and other Brassica species effect glucosinolate production, disease suppression, and weed control.
As with any biological control, results can be variable. In trials in Idaho, higher soil moisture improved fungus and nematode suppression, while increasing weed pressure. It is necessary to macerate and incorporate the mustard plants for the glucosinolates to be effective. This can be accomplished by mowing and disking in the plants. For fall planted mustards and Brassicas, freezing and thawing may effectively macerate and release the glucosinolate ‘sinalbin’, potentially explaining weed suppression the following spring. Further study is needed to determine how these bio-chemicals and cover crops perform under different management.