Assessing the real-world impact of new agronomic practices depends on good economic studies, says David Spielman.
Sustainable intensification hinges primarily on practices and technologies that help farmers systematically improve the management of their crops, soil, trees, livestock and water — what I will call 'new agronomy'. It is essentially a focused approach to helping farmers simultaneously improve yields and manage natural resources sustainably.
New agronomy covers a range of practices, including strategic crop rotations to minimise pest and disease threats; interspersed planting of nitrogen-fixing trees and shrubs with crops to improve soil health; and elimination of tillage to retain moisture under crop residues.
Improved cultivars and chemical fertilisers may play an important role in these systems, but they do not take centre stage, as they did in past decades. Rather, on-farm learning and adaptation are the key.
Some of these practices are actually not new. But others are new, in so far as they have attracted much attention in recent years; for example, agroecology, conservation agriculture (CA), and systems of rice intensification (SRI).
CA and SRI, in particular, have generated controversy over their technical performance and environmental footprint. Debates rage around precisely how these systems contribute to yields, soil carbon and nitrogen content, groundwater use, greenhouse gas emissions or reduced use of chemical inputs.
Debates also rage around the wider replicability of these systems. Can CA, which is now prevalent on large farms found in North and South America, address the soil and water constraints facing smallholders in eastern and southern Africa? Can SRI, with its humble beginnings among smallholder rice farmers in Madagascar, make its mark on rice yields in north-eastern and southern India?
Many already claim that these new agronomic ideas are a big success. But others cry foul. As a result, what seem like innocuous agronomic recommendations — changes in planting dates, sowing methods and irrigation practices — have become subjects of international controversy. 
How can these debates be settled?
Ultimately, future research into, and widespread adoption of, new agronomic practices for sustainable intensification will require more than just good agronomy. It will take effective evaluations and plausible forecasts of social and economic impacts that demonstrate where new practices might work, where they might falter and what the trade-offs might be for individuals, society and the environment.
But in many cases, evidence of impact tends to rely on agronomic studies conducted under near-ideal conditions — for example on a research station or in a progressive farmer's field where agronomists can control the experiment. These do not always capture 'on the ground' realities in developing countries.
Enter the economist, who brings an insistence on rigorous evidence under real-world conditions. Trudging through fields without having had schooling in plants, soils or water, economists might utter incomprehensible terms like internalised externalities, self-selection bias and endogeneity that most agronomists don't understand — and insist that what looks like a good yield may not make much economic sense when prices and costs are considered.
Don't get me wrong: there are many economists who work well with agronomists, providing some of the most compelling evidence in support of new agronomy. Economists' valuation of minimum-tillage wheat cultivation in north-west India, for example, shows how good agronomy can increase yields and reduce groundwater use among smallholders. 
And a similar valuation of minimum-tillage wheat and barley cultivation in Morocco illustrates the economic trade-off between using crop stubble to improve soil health or to feed livestock, particularly in drought years. 
But the economist is still fairly marginalised in the world of new agronomy — or maybe not enough economists have caught on to its potential importance.
The challenge is often that each discipline's methods seem confusing to the other. For example, economists are interested in farmers' behaviour given a particular agronomic practice, whereas agronomists hold people constant in their study of plant, soil and water behaviour.
Using economic inquiry
Given a chance, economists can do three things well: evaluate the economic impact of a new agronomic practice, forecast its welfare impacts at scale and shoot down bad ideas.
When it comes to evaluation, they can explore how diversity within a given population — in terms of wealth or education, for example — affects costs and benefits, or poses trade-offs for those willing to adopt the practice.
Economists can also assess promotional efforts such as demonstration trials, targeted input subsidies, or farmer field schools. Or they can investigate why farmers do not adopt new practices — exploring farmers' feelings about risk or their aspirations for the future, for example.
And economists can take findings from the field and peek into the future, simulating alternative scenarios about food prices, poverty or environmental outcomes even with the 'noise' that real-world conditions might add. [4, 5]
These methods are needed to rigorously demonstrate the benefits of new agronomic practices, which very few economists have had a chance to explore in great depth. If they are to do so in a way that adds useful evidence to international debates, agronomists and economists must collaborate more closely.