As the 2025-26 grain cycle unfolds, global weather developments are casting a long shadow over yields, commodity prices, and ag-related decision-making. According to Kevin Marcus of Marcus Weather Consulting, climate variability is becoming the most consistent disruptor in the world's top-producing regions: the United States, Brazil and Argentina, and China.

In the U.S., much of the Corn Belt is reliving a version of 2024. Warmer and drier-than-average conditions through September and October have raised concerns, particularly in the eastern Corn Belt, where extended nighttime heat in July and August likely impaired grain fill. "We had some of the warmest nights since 2010 and 2011," says Marcus. High respiration rates during pollination can result in shallow kernels and reduced yield potential-a hidden loss that's already showing up in early test weights and farmer reports.

On the western side of the Corn Belt, the issue is less about temperature and more about disease pressure. Southern rust, a persistent pathogen that thrives in warm, humid conditions, has spread aggressively. Fields left untreated are seeing losses of 40 to 50 bushels per acre, with Iowa identified as a major hotspot. Marcus estimates that even a 10% under-treatment rate could drag down statewide yields by five bushels and knock a full bushel off the national average-a significant hit at scale.

Complicating the picture further is the emerging influence of La Niña, along with anomalous warm waters in the eastern Indian Ocean. These patterns are injecting fresh energy into the polar jet stream, a signal Marcus interprets as an indicator of sluggish harvest progress in the final weeks. More alarmingly, he forecasts heavy, wet snow across the Northern Plains starting in November-a dramatic shift from last year that could stall equipment in the field and place strain on grain storage logistics.

In South America, both Brazil and Argentina are off to a rocky start. Rainfall that would typically come every other day is arriving just once a week. At the same time, early-season temperatures above 100°F are putting young crops under severe stress. While Marcus isn't sounding the alarm just yet, he emphasizes the elevated yield risk. If planting delays continue or early stressors worsen, it could open the door for higher U.S. export demand, though any benefit would come with heightened market volatility and input cost uncertainty for American growers.

Meanwhile, China's grain outlook is dominated by one factor: unrelenting rain. The northern plains have endured seven weeks of near-constant rainfall, rendering cornfields unharvestable and grain quality increasingly questionable. Marcus draws a comparison to 2022, when similar weather resulted in estimated losses of 30 million metric tons. The implications go beyond China's borders: reduced domestic output could trigger increased imports, potentially swinging global trade flows and influencing U.S. grain prices just as marketing decisions are being finalized.

For U.S. farmers, agronomists, and co-op leaders, these developments represent more than meteorological curiosities-they're direct inputs into risk management, input purchasing, crop insurance calculations, and supply-chain planning. Precision agriculture tools, long relied upon to optimize margins, now face an added burden: accounting for unpredictable climate inputs while still ensuring sustainable productivity. And with farm bill negotiations on the horizon, the ag sector's appetite for stronger disaster relief mechanisms and climate-adaptive programs may grow even more urgent.

Looking ahead, the final phase of the U.S. harvest and the progression of the Southern Hemisphere's planting season will offer more data-but little comfort. As Marcus notes, "We're not seeing any catastrophic red flags yet, but we're absolutely in a higher-risk environment." For professionals across the ag value chain, this year's weather trends are a vivid reminder that climate risk is now a permanent variable in grain production economics.