Early termination, inclusion of legume species and a little patience might be what’s needed to make green manures and cover crops work in the drier areas of the U.S. without dinging yields and profits.

As you’ve likely seen or read, there’s growing interest among no-tillers in the semi-arid U.S. in replacing fallow periods with legume green manures (LGMs) and cover-crop mixes (CCMs) to improve soil health.

But there are many questions surrounding the effect these tools have on soil moisture reserves, crop yields, protein, economic returns and, of course, soil quality.

Montana State University recently shared the results of three different studies —both short-term and long-term — that evaluated LGM and CCM residues in wheat production systems in both tilled and no-tilled systems. You can read the entire article by clicking here.

• Short-term trials: Gain yields over 3 years weren’t different between LGM (pea or lentil) and fallow treatments in no-till in Study 1.

Grain protein following a pea green manure crop (GM) was higher than after fallow at all N rates and in all tillage systems, yet grain protein following a lentil GM crop was not different than after fallow. The different responses between pea and lentil can be attributed to higher N fixation by pea than lentil (McCauley et al., 2012).

In Study 2 on cover-crop mixes, spring wheat grain yield was higher following fallow conditions than following any of the 10 cover crops seeded at Dutton. Grain yield was higher after the pea and N-fixing treatments than after the full eight-species mix when averaged across the three N treatments.

Protein was higher (more than 1 percentage point) after pea and N-fixing treatments than any of the other CCMs when no N was applied. Soil-quality parameters such as soil penetration resistance, potentially mineralizable N temperature, soil enzyme activity, etc., weren’t different between pea and the full mix after one cycle, but will be measured again after two cycles.

In the companion field study, grain yields following CCMs were up to about15 bushels an acre lower than following fallow, likely because of high water and N use by late terminated cover crops (often approaching seed set).

• Long-term trial: In the first three cycles of no-till, LGM wheat at Bozeman (annual precipitation of about 16 inches), wheat yield was the same between wheat grown after LGM or fallow.

However, after four LGM cycles, grain yield at the full N rate was 5 bushels an acre higher following LGM than following fallow, and was 16 bushels an acre higher following LGM than fallow at the ½ N rate (Figure 2).

Most notably, grain yield was the same following LGM with the ½ N rate (only 5 pounds of N per acre applied) as following fallow that received 129 pounds of N per acre. Moist conditions in 2010 were apparently ideal for substantial N release from decomposition of accumulated legume residue.

Perhaps more importantly, researchers say, the range in 4-year net returns were much narrower for the LGM-wheat system ($378-$441 an acre) than for the fallow-wheat system ($303– $477 per acre), suggesting LGMs can reduce uncertainty about economic returns without reducing the expected value of those returns.

Management Implications

Here are some of the key points researchers shared:

• In dry regions of the western U.S., cover crops need to be terminated early (e.g., by first pea bloom) to avoid large yield losses of the following crop.
• The cover crop should contain at least 50% legumes to enhance subsequent wheat grain protein.
• At least three cycles of cover crops are needed before they provide economic benefits.
• Although diverse cover-crop mixes may have some benefits to pollinators or reduced risk of crop failure, they don’t, in the short term, appear to increase yield, protein, or soil quality.

I think this research shows that growers don’t have to be beholden to established wheat-fallow paradigms and can make cover crops and green manures part of a dynamic no-till system that is more balanced and profitable.