Drought Carry-Over May Not Impact 2013 Yields

March 26, 2013 10:54 PM
Drought Carry-Over May Not Impact 2013 Yields

The ground may be dry now, but rain and heat in June and July will ultimately decide crop yields.

Will last year’s extreme drought drag down this year’s yields? Many farmers believe that drier-than-normal soil conditions, especially in Western states still suffering from drought conditions, will have a carry-over impact on corn and soybean yields this year. But if agronomical economists at the U. S. Department of Agriculture (USDA) are correct, last year’s drought will have a minimal impact on the success of this year’s crops.

Paul Wescott, an USDA agricultural economist, presented the counter-intuitive findings in a panel discussion at the recent 2012 Agricultural Outlook Conference in Arlington, Va. Wescott, along with USDA’s Michael Jewison, developed an agronomic model that accurately projected crop yields in eight, key corn-producing states and seven soybean-producing states from 1988 to 2012.

The pair found that yields are largely determined by growing-season weather conditions in a given year. The biggest factors in the success of corn crops, they found, are spring planting progress by mid-May, rain in June, and rain and heat in July. To gauge the impact of drought conditions during planting season, the economists evaluated overlaid data from the Palmer Modified Drought Index, a measure of long-term drought, and cumulative monthly precipitation, a measure of soil moisture recharge.

"In no case did we come up with a statistically significant effect," Wescott says. Though it’s important to monitor states with continuing drought, he adds: "It (previous drought) doesn’t appear to have an impact."

What does have a bearing – a rather large one – is whether corn crops receive enough rainfall in June. A 2-inch shortfall, like much of the Corn Belt experienced last year, is enough to reduce yields by 20 bu. per acre. "Then we got a hot and dry July – the hottest July on record for the United States -- which took away another 22.7 bu. per acre," Wescott says.

The 2012 season got off to a pretty good start. Thanks to mild weather last spring, corn plantings by the end of April were ahead of schedule. So, the other big yield variable, mid-May planting progress, was in a positive mode. Early planting, of course, allows the critical stages of crop development to occur earlier, before the most severe summer heat.

The model shows that heat and moisture in the key month of July have the biggest impact on corn yields. "However, extreme weather deviations from normal in June can have larger impacts, as seen in 2012 and in 1988," the economists report.

The model for soybeans is somewhat different from corn. The soybean model doesn’t include a variable for planting progress and uses average July and August weather variables, rather than just July weather. Plus, the researchers evaluated data in only seven states. The model explains 80% of the variation in soybean yields.

"Overall, the model’s weather variables have lower statistical significance in explaining soybean yields than the corn yield model, likely reflecting the longer reproductive period for soybeans," the economists wrote in their report.

USDA meteorologist Brad Rippey, who also spoke on the panel provided a second opinion.

"There’s very little relationship between drought this time of year and output for corn," Rippey says, noting that in other years, like 2000 in Iowa, when there was an early spring drought and it didn’t affect the eventual crop outcome. He notes: "The real difference is what happens in late spring and early summer."


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