University Extension Looks At How Planting Dates Relate To Yield

University Extension Looks At How Planting Dates Relate To Yield

Iowa's wet spring, late planting and stress degree days as of mid-May 2013 are being compared to 1947 and 1995.

As of May 12, only 15% of Iowa's 2013 corn crop was planted, according to USDA's weekly survey. This is a slower pace than what Iowan's experienced even in the flood year of 1993, when 20% was planted at this time in May. The 8% planted before May 5 was subjected to dramatic soil temperature changes as well as up to a foot of snow on top of that newly-planted corn in parts of Iowa. Many news articles, blogs, Twitter posts, and other media document spring 2013 conditions and concerns about late planting.

COMPARING 2013: Some people are comparing the wet spring, late planting of 2013 to the wet spring, late planting years 1947 and 1995. "Although analog years for weather and corn planting dates are interesting to compare and help us understand potential hazards as well as opportunities, they may or may not be accurate predictors of what will happen in 2013," says ISU Extension agronomist Roger Elmore. "Weather patterns the rest of the growing season will dictate that."

To the surprise of many, soil moisture profiles across the state are filled or closely filled as of now. The U.S. Drought Monitor weekly report shows that drought has receded to the west and northwest every progressive week. Based on what has already happened, what does the 2013 cropping season look like from this mid-May perspective? Iowa State University Extension corn agronomist Roger Elmore and ISU Extension climatologist Elwynn Taylor provide the following observations.

An analog is a person or thing that is seen as comparable to another

When faced with different weather patterns like those we've experienced so far this year, we often turn to analog years. These are years considered as having similar trends in temperature and/or precipitation as those we are currently experiencing.~~~PAGE_BREAK_HERE~~~

Whether we agree or not, people use analog years to compare data like planting progress as well. It is often tempting to speculate that if we've had similar trends in weather or planting dates historically, we can use that as an analog year to forecast conditions -- and thus corn yields -- the rest of the current growing season. Even if the analog year is not totally reliable for the yield outlook, it does at least provide an idea of the risks and potentials that could be experienced during the growing season.

Spring weather analog year for 2013 is 1947; this year looks comparable to 1947

Figure 1 displays Iowa state average Growing Degree Day (GDD) accumulations, Precipitation, and Stress Degree Days (SDD) for April 1 through May 8 for 1947, 1995 and 2013. The year 1947 is suggested as a spring-weather analog year for 2013 by a number of weather experts. This is probably based on certain similarities of April weather in both years. April through June 1947 was cold and wet. Corn stands were reduced by 'water damage' across the state. The cold-wet weather transitioned to drought in July (Figure 2). By July 20, 1947, heat stress set in and GDD exceeded normal (Figure 2).

Planting dates in 1947 ranged from May 5 to May 21 (Elmore unpublished data). About half of the 1947 corn crop was planted by May 15. This may seem late to us now, but this range of planting dates and the 50% planted date were normal for that era. However, early rains probably resulted in shallow rooting. This coupled with stress later in the 1947 growing season probably reduced corn yield (Figure 2). Mid-season heat coupled with limited rain reduced Iowa average corn yields to 31 bushels per acre, 39% below the 1937-2012 trendline yield (Figure 3).~~~PAGE_BREAK_HERE~~~

Another way to think about analog years is planting progress. As we mentioned previously, as of Sunday, May 12, only 15% of Iowa's 2013 corn crop was planted. That was an increase from 8% the week before. Little corn planting occurred May 6 to 10 because of mid-week rains that unfortunately resaturated fields just when they were nearly dry enough for planting -- following the early May snow; this delayed planting once more.

Since 1980, planting progress in early May of three years - 1984, 1993 and 1995 - was similar to what we've experienced this year. Because flooding resulted in delayed planting in 1993, let's focus on the other two as possible analog years for planting date.

What happened in 1984: 6% of Iowa's corn was planted by May 6; however, one week later, by May 13, Iowa farmers planted another 29% (Figure 4). Planting in 1984 began by April 29 and ended by June 10. Half the crop was planted by May 15, 10 days behind the 1978-1987 average of May 5 (Elmore, 2013 in press). By May 12 of this year, 2013, we only have 15% of our corn planted in Iowa for the reasons we mentioned above. Corn yields in 1984 were 3% above trendline (see Pierson & Elmore) (Figure 3). The delayed planting in early May was correlated with increased yields. Although 1984 SDD or stress degree days were slightly greater than normal, there were fewer SDD in that year compared to either 1947 or 1995.

What happened in 1995: Only 10% of Iowa's corn was planted by May 7, 1995 (Figure 4). Another 20% was added the next week and 62% of the crop was in by May 21. Although planting progress was similar to that of 1984, yields in 1995 were 9% below trendline (see Pierson & Elmore) (Figure 3). Planting delays in 1995 were correlated with reduced yields; please note, however, this doesn't mean delays that year actually reduced the yields.

A word of caution: ISU's Roger Elmore and Elwynn Taylor leave us with this conclusion: "Although analog years for weather and corn planting dates are interesting and help us understand potential hazards as well as opportunities, they may or may not be accurate predictors of what will happen in 2013. Weather patterns the rest of the growing season will dictate that."

TAGS: USDA Extension
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