US corn yields are growing, but so is sensitivity to drought

Carol Von Canon

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.