In the US, crop yields just keep increasing. It’s the result of farming techniques, new technology, improved cultivars (including genetically modified ones), and the aggressive application of fertilizers, herbicides, and pesticides. But despite all the advances, farmers are still dependent on the weather, as they have been since the dawn of agriculture. Some areas rely heavily on irrigation, but it’s generally rainwater that feeds the crops in most places. The reliance on rain is one of the factors projected to work against agricultural progress as Earth’s climate continues to warm.

Improved drought tolerance has been one aim of crop breeding, but US corn (“maize” to much of the world) is actually becoming more sensitive to drought—likely because of one of the farming techniques being used to raise yields.

Stanford’s David Lobell and a group of collaborators set out to examine recent harvests for evidence of changing drought sensitivity. They took advantage of a detailed US Department of Agriculture database that started tracking yields by field (rather than state or national totals) in 1995, and focused on Iowa, Illinois, and Indiana. They found that the amount of moisture available to plants in July was the best predictor of each year’s harvest. They broke down yields at each location according to that July moisture and averaged them together to get yield trends at various levels of wetness or drought.

Even for the driest conditions, corn yields increased over that time period. However, they increased significantly less than yields in wet conditions. That is, the difference between yields in wet and dry areas is now greater than it was in 1995. Repeating the same analysis for rain-reliant fields in South Dakota, Nebraska, and Kansas—where drought tolerant strains are likely a higher priority for farmers—showed a similar, but even larger, disparity.

Soybean yields displayed no such pattern; yields have increased just as fast in dry years.

So why is corn taking a bigger hit during droughts? The researchers suspected it was due to the practice of planting corn more densely, which has been made possible by newer cultivars and genetic modifications that reduce root damage from insects. In Illinois, for example, density has increased from around 24,000 corn plants per acre in 1995 to 30,000 by 2012. That hasn’t happened for soybeans.

To test their hypothesis, the researchers used a computer model that simulates crop growth. The simulated yields matched the observed trend pretty well. The higher plant density exacerbated the loss of soil moisture in the model, resulting in greater sensitivity to dry conditions.

Although regional climate model projections are pretty uncertain, parts of the Midwest are projected to experience drier July conditions in the future. Using the crop model, the researchers simulated the impacts of that climate change. If corn’s sensitivity to drought were to remain constant, simulated corn yields in 2060 would be about 15 percent less. (That doesn’t mean yields won’t keep increasing, but that this particular factor will have a negative impact.)

If that drought sensitivity were to continue increasing at the rate it has since 1995, however, the simulated reduction would reach about 30 percent. That could mean, the researchers write, that “climate change effects may be more severe than predicted by models that assume current crop genetics and management.”

“Our results,” they note, “agree with the general notion that, as farmers become more adept at removing all nonwater constraints to crop production, the sensitivity to drought generally increases.” Farmers (and the rest of the agriculture industry) can do a lot, but they still can’t make it rain.

Science, 2014. DOI: 10.1126/science.1251423 (About DOIs).