There are many varieties of mustards around the world, most developed for seed or leaf consumption. They vary greatly in their growth characteristics and glucosinolate concentrations. Since 2000, we have conducted variety trials to determine those varieties that work well as green manures in the Columbia Basin. We are also looking at other plants, mainly Brassicas, and blends of different species, as potential green manures.
With mustard green manures, we are looking for:
- High biomass production
- High glucosinolate concentrations, probably genetic, but also associated with pre-bloom stages of crop. Therefore we are looking for varieties with delayed bloom.
- Lack of weedy characteristics, such as dormant seed
- Low seed cost
Factors to consider when choosing varieties:
- All glucosinolates are not equal. In general the glucosinolates in the shoots of oriental mustard (B. juncea) are better against soil-borne fungal pathogens than those in white mustard (S. alba). However the reverse is true for weeds.
- There is no consensus on whether the concentration of the total amount of glucosinolates is most important for pest control. Concentration increases until just before flowering and then decreases, while the total amount continues to increase with increasing biomass.
- The concentration of glucosinolates within varieties can vary with plant part (shoots vs. roots), season (spring vs. fall), plant health, and soil fertility.
- In most studies, only a small percentage of the glucosinolates in a green manure are converted to the chemicals active against soil-borne pests. Glucosinolates in the roots seem to be converted more readily than those in the shoots.
- Isothiocyanates (ITCs) are the most studied product of glucosinolate conversion, but other chemicals are formed. The effects of these other chemicals, and of the mixtures of chemicals formed has not been fully evaluated.
- The effects of the glucosinolates when incorporated into the soil on soil-borne pests is termed biofumigation. However this is not the only mechanism involved in green manures that affects these pests. Other mechanisms arise from the addition of organic matter to the soil, in which case biomass production is the most important factor. The relative importance of these different mechanisms has not been determined.
- As always with variety trials, they are the result of one set of field and climatic conditions. Other conditions may give different results. Use all of the information available, including data from multiple years, and the experience of other farmers in your area when choosing a variety.
On-Farm Research Results
Variety Trials (pdf) – biomass measurements
Glucosinolate Levels (pdf) – concentation and production estimates