At the beginning of April, the Mexican army took US military officials, UN personnel, and officials from the US embassy on a tour of a half-dozen sites in the Golden Triangle — the heart of Mexico's heroin country — to witness the destruction of opium poppies.

The trip marked the first time in at least 10 years that the Mexican army had allowed US and UN officials to observe poppy eradication, and, according to Reuters, more trips were being arranged.

The visit could put Mexico on the same footing as anti-drug operations in places like Colombia and Afghanistan.

It also coincides with US President Donald Trump's efforts to address a withering opioid epidemic that has claimed tens of thousands of lives in the US.

But if the US and Mexico are about to make the opium-poppy eradication a joint effort, they may find their progress stymied by conditions on the ground and the nature of their foe.

A US official told Reuters that the area of poppies under cultivation in 2016 was estimated at 32,000 hectares in 2016. In 2015, US data showed there were 28,000 hectares estimated to be under cultivation — triple the area under cultivation recorded in 2012.

Last year, Mexico for the first time released its own data on cultivation, reporting an average of 24,800 hectares under cultivation between July 2014 and June 2015. The Mexican defense secretariat also said it destroyed 22,235 hectares of opium poppies in 2016 and 26,249 hectares in 2015. (Through the end of April 2017, the Mexican government says it eradicated 15,179 hectares of poppies.)

What effect this eradication has on the overall crop level is not clear. Data on production and eradication is varied and, in some places, incomplete. Estimates of cultivation in Mexico are also based on satellite imagery, not like census-based programs that more confidently measure cultivation and eradication.

"We don't know with any level of precision how heroin production has evolved in recent years," Mexican security analyst Alejandro Hope wrote in February 2016.

Moreover, according to Mike Vigil, former chief of international operations for the US Drug Enforcement Administration, it can be hard to accurately gauge how many plants exist and are destroyed, especially when relying on aerial observation and fumigation.

"People automatically do a formula that there's so many plants per meter and therefore they destroyed so many plants that would have produced a certain tonnage of opium, for example, and that is not correct either, because a lot of times they spray fields where the plant density is just very loose," Vigil, who, as a DEA official, worked in Mexico in the 1990s, told Business Insider.

"We're not talking about plants that are densely concentrated per square meter. You could have one plant per square meter. So they destroy 10 hectares and probably destroyed very few plants," Vigil said.

Manuel eradication has been common in Mexico's war on drugs. Military units have been stationed in areas of high cultivation like Sinaloa (which is also a hub of organized-crime activity) and Guerrero states, and they are frequently pictured crawling poppy-covered hillsides rooting out the pink-tipped bulbs that would eventually yield opium.

But aerial fumigation has also been used in places with a high incidence of cultivation, like the states Chihuahua, Durango, Sinaloa, and Guerrero, at times of elevated cultivation activity.

Fumigation squadrons are deployed in MD-530F helicopters and Cessna 182SL planes. Bell 407 and Bell 206 helicopters are also used when aerial-fumigation teams need to access remote or hard-to-reach spots.

In Guerrero, where poppy cultivation — and criminal activity related to it — has grown precipitously in recent years, manual eradication has been replaced in some cases by fumigation. One official told Excelsior the chemical used was only harmful to poppy plants, not to other crops or underground water systems. (The Mexican defense secretariat has in the past classified the exact chemicals used in aerial fumigation, calling it a matter of national security.)

"It would be very difficult for them to verify [eradication totals] because they're not going to be out there on every spray mission, and even if they were, they really don't know how many fields they're going to spray. They could take them to the same field the following day and spray that same field that they observed 24 hours previously," leading observers to think a different field had been sprayed, Vigil added.

This summer, the Mexican army is reportedly set to roll out hardware and software developed by the UN Office on Drugs and Crime that would permit troops to send information on destroyed fields in real time.

And, Brownfield said, the US could offer more vehicles or fund helicopter flights to reach the remote parts of the country — typically mountain highlands — where poppies are grown.