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Conservation ag may not reap hoped for global yield gains

No-tillage is lauded as a key sustainable farming practice around the world, and indeed it is. By avoiding plowing and disturbing the soil, no-till improves soil structure, promotes growth of beneficial microbes, and curbs erosion, all of which are vital to the long-term productivity of agricultural lands.

No-till farmfield

But in a sweeping analysis of 610 peer-reviewed studies comparing no-till and conventional tillage systems, a team led by the University of California, Davis found that no-till reduced crop yields by a global average of 5.7%, although it still shows promise for yield gains in dryland agricultural areas.

This isn’t to say that no-till is a bad practice, says Cameron Pittelkow, who led the study as a UC Davis postdoc and is now on the faculty at University of Illinois. Rather, the study suggests that no-tillage by itself may not be as critical to food security and the “sustainable intensification” of agriculture as people have believed.

“When we say sustainable intensification in this context, what we mean is further increasing food production on existing land while also the reducing environmental impacts of farming,” Pittelkow says. If no-till can only reliably meet the environmental goal, he continues, “the common assumption that it’s going to play a large role in the sustainable intensification of agriculture doesn’t necessarily hold true, according to our research findings.”

The analysis was published recently in the journal Nature.

Lingering questions

Some form of no-tillage is practiced on an estimated 9% of global arable land today, along with varying degrees of the other two principles of conservation agriculture: crop rotation and retention of crop residues. And there’s no sign adoption rates will slow. Conservation agriculture is being heavily promoted in the developing world as a means to ensure long-term farm productivity, profits, and food security, especially in the face of climate change.

Yet, while no-till is one of the world’s most widely applied and heavily researched management practices, Pittelkow and his postdoc adviser, UC Davis agronomist Chris van Kessel, were surprised by the lingering debate around its impact on yield. This came home for van Kessel during the review of another paper. As in the Nature study, he and his co-authors had found a 5 to 6% yield decline under no-tillage. But the dataset was much smaller, and before the paper was finally accepted, one reviewer raised serious concerns about the team’s ability to make that conclusion.

That’s why in the current work, van Kessel, Pittelkow, and their collaborators identified every sound, peer-reviewed scientific paper they could that included side-by-side comparisons of conventional tillage and no-tillage systems. Eventually, they assembled more than 5,000 side-by-side observations from 610 studies of 48 crops in 63 countries. The tool they used in their assessment was meta-analysis, a method that allows researchers to quantitatively examine an entire body of research at once to arrive at an overall mean response.

It ended up being the largest meta-analysis van Kessel has ever been involved in, and possibly the biggest one yet in agriculture. But assembling the huge dataset was necessary, he explains.

“The only way you can tackle a problem [like this] is that you have to go across all environments, regions, and crops. You cannot have one single study dominating the outcome,” he says. “So you want to go as big as you can.”

And no matter how big they went—2,000, 3,000, 5,000 comparisons—the result remained the same. Overall, no-tillage dropped yields by 5 to 6% compared with conventional tillage.

Counteracting the yield penalty

There were nuances, though. For example, use of all three conservation ag principles—no-till, residue retention, and crop rotation—still caused a yield decline compared with conventional tillage. However, yield losses were not as severe when the three principles were practiced together versus when no-till was implemented alone.

What this means, Pittelkow says, is that “you want to be careful about promoting conservation agriculture if the other two principles are not going to be applied.” Unfortunately, he adds, smallholder farmers often struggle to rotate crops and, especially, to leave crop residues on fields, since they rely on residues for many other things, such as cattle fodder and fuel.

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Moreover, no-till performed slightly better than conventional tillage in dry areas without irrigation, the team found, most likely through a positive effect on soil moisture levels. This suggests that conservation agriculture may indeed become a key strategy for adapting to the more frequent droughts predicted under climate change.

But even in humid climates, there are ways for farmers to minimize the yield penalty associated with adoption of no-till. For one, yield declines are generally greatest in the first two to three years, van Kessel says, suggesting one important strategy is just to hang in. Another tactic is to add extra nitrogen during the first years of no-till. Pittelkow and colleagues are now writing up another meta-analysis that indicates more nitrogen is needed during this initial period to offset the yield decline.

There are also many excellent arguments for implementing no-till that have nothing to do with yield. The practice saves on fuel and labor costs, for example, and it may curb the emission of greenhouse gases from agricultural soils. But if an agency or scientist recommends no-till to a farmer, the recommendation must include mention of a possible yield loss, van Kessel stresses—not because that’s his opinion or what a few studies have concluded, but because of the weight of the evidence.

“[The advice] has to be evidence-based. There is too much anecdotal evidence,” he says. “And ours is an evidence-based conclusion from analyzing the peer-reviewed literature from all over the world.”

Pittelkow thinks there’s a lesson here for the public, as well. Buzz terms like sustainable intensification are relatively simple to grasp and talk about; what he and his co-authors have shown, however, is how hard they are to actually achieve on farms.

“Sustainable intensification essentially means that for any management practice or technology with the potential to increase crop yields, we also have to demonstrate that it has lower environmental costs,” he says. “But the reality is it’s not always easy to accomplish both simultaneously, so we need to focus more research in this area.”