Despite the slow start to planting, the US corn crop has entered July in one of its most mature states ever for this time of year. The rapid development by itself does not necessarily dictate yield outcomes, but it highlights the importance of weather over the next couple of months.
As of July 1, some 17 percent of US corn was in the silking stage, according to data from the US Department of Agriculture's statistics service. This is the second-largest amount of corn silking on July 1 since at least 1981, and is 9 points ahead of the five-year average. The fastest July 1 silking was 25 percent in 2012.
Silking is important because it marks the beginning of the reproductive period. The corn silks allow the immature corn ear to be pollinated and grow kernels. Favorable weather during this time is necessary to maximize yield potential, but too hot or too dry conditions can be big setbacks.
The 17 percent of US corn silking by July 1 was heavily anchored by No. 2 producer Illinois, in which 40 percent of corn was silking on Sunday, some 29 points ahead of normal.
The progress comparison with 2012 emphasizes how extraordinary this year's fast development has been. As of May 5, US farmers had planted only 39 percent of the corn compared with 71 percent on the same date in 2012. The five-year average for the date is 44 percent.
But the crop is progressing as if it had been planted early, and this is owing to to extremely warm temperatures over the past two months. May 2018 was the warmest May for the contiguous United States in records dating back to 1895. June was also among the warmest.
Midwestern states have piled up growing degree days (GDDs), which relate temperature to plant development, as a result. In Iowa and Illinois, which produced a third of US corn last year, May and June GDDs were 130 percent and 131 percent of normal, respectively.
By comparison, May and June GDDs in 2012 were 119 percent of normal in Iowa and 117 percent in Illinois. The warmer the temperatures, the more GDDs will be accumulated, and the crops generally advance more quickly.
Since 1981, there have been seven other years besides 2018 in which more than 10 percent of corn was silking by July 1. Three of those years featured poor to terrible yields, three had good yields, and the remaining yield was close to average.
All but one of those seven years had warmer than normal temperatures during July and August. The exception, 2004, was near-record cool. Corn yield soared 10 percent above the long-term trend that year in what was one of the best performances of all time.
The other two years with fast silking and good yields were 1987 and 2016. July and August of 2016 featured record rainfall across the Midwest, and the same period in 1987 was near-record wet. This offers a good lesson for 2018.
Forecasts for July and August of 2018 suggest anomalous warmth is likely. Even though much of the Corn Belt currently has ample moisture, and in some cases too much, the wet trend would likely need to continue for the next two months to reach new record high yields.
Warmer-than-normal temperatures during July and August have not historically led to the best corn yield outcomes, especially when the overnight temperatures are high. This causes corn plants to effectively waste resources that would have been used to fatten the kernels for a bigger yield.
Since 1981, only 1987 and 2016 got away with warm nighttime temperatures while producing convincingly good yields, and the abundant rainfall was likely the key.
While corn yield scenarios are still wide open in terms of early silking, late silking may be more of a problem. Since 1982, there have been 14 years in which silking progress as of July 31 was below 80 percent, the long-term average. Only three of those 14 produced above-average yields: 1982, 1992 and 2009.
The US Climate Prediction Center sees good chances for July to be warmer than normal across the Corn Belt, especially during the next two weeks. The next two weeks may also be drier than normal in the western half of the Corn Belt, but wetter in the east.
Longer term, the CPC predicts equal chances for above or below normal precipitation across the primary growing regions over the next three months. This means market-watchers may want to keep a particularly close eye on the forecast in the coming weeks as the rainfall outlooks may shift along the way.