A highly credible scientific review of worldwide research assessing single species (monoculture) cover crops versus multispecies ones has largely found little gain from changing to multispecies.
Productivity, soil biology, competitiveness against weeds, scavenging of soil nutrients like nitrogen and following crop yield performance were among the factors assessed.
Along similar lines, NSW DPI, with MLA funding, is conducting research across the state assessing dual-purpose production of single species compared to multispecies combinations.
Crop and animal productivity are being assessed, as well as aspects like soil carbon.
NSW DPI senior research agronomist Dr Mark Norton leads the program in collaboration with other researchers, farmers, LLS and agribusiness.
The NSW study includes comparing multispecies that include canola, other brassicas, legumes (pasture and crop species), cereals and ryegrass.
So far, the research indicates winter feed may not be much improved via multispecies compared to the best single species for a given situation.
A big part of the study will examine if livestock achieve improved weight gains from multispecies compared to single ones.
Anecdotally some farmers and agronomists feel greater gains occur from multispecies crops, but research is yet to verify this conclusion.
The review, published in June 2020 Agronomy Journal, is titled 'Do diverse cover crop mixtures perform better than monocultures? A systematic review', authored by A M Florence and A M McGuire.
The 27 studies compared cover crop mixtures (at least three species) to all their constituent species, although it is important to note that the trials measured agronomic factors, not animal production.
The studies contained 119 cover crop plantings.
From these, 243 full comparisons of the best-performing mixture and best-performing monoculture were assessed for selected metrics.
Experiments were conducted in 12 US states, Poland, Hungary, Japan, Brazil, Switzerland, Turkey, Italy, Canada and Germany.
In 88 per cent of these comparisons, monoculture and mixture performed comparably.
In 10pc, monoculture performed best, and in 2pc of comparisons, mixtures performed better.
Overall, there are few published studies documenting the superiority of cover crop mixtures over monocultures for the selected metrics.
Where the elevated performance of cover crop mixtures occurred, it was felt elevated seeding rate was the significant factor.
Biomass, weed, nitrogen, water, biology, yield, and stability effects were evaluated in the studies.
The treatments comprised mixtures of three to 18 species compared to single species.
Mixtures were "functionally rich" as well as species-rich.
The Florence and McGuire review noted that it was common for grasses and non-leguminous broadleaved crops (e.g. brassicas) to be the most biomass productive.
Leguminous species tended to produce less biomass than other cover crops.
It was common to see mixtures of legumes with non-legumes dominated by non-leguminous species.
In 55 of 56 weed suppression comparisons, there was no difference between the most weed suppressive mixture and monoculture.
The remaining comparison noted benefits from mixture related to elevated sowing rate compared to that of its constituent monocultures seeded at normal rates.
In above-ground nitrogen accumulation, there was mainly no significant difference between the best monoculture and the best mixture.
Likewise, there was manly little difference between the best mixture and monoculture for soil nitrogen reduction.
In most cases, there was no significant difference between the most soil water conserving cover crop mixture and monoculture.
Mostly there was no significant difference between the best mixture and best monoculture in terms of soil biology promotion.
In crop yield comparisons, there was mainly no significant difference between the best cover crop mixture and the best monoculture.
The absence of clear advantages in these agronomic factors does not mean that there are not likely to be benefits from species mixtures.
The one "stand-out" likely benefit is animal production.
In the 15 to 20 years since annual single species fodder crops have assumed such a major role in feedbase production, there have been widespread animal nutrition problems, sometimes resulting in serious stock losses.
The key hypothesis behind this new project is that mixtures will provide a more balanced grazing diet which will lead to greater production.
Next week: Soils researcher urges caution on seeking soil carbon credits.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.
