As of Sunday, July 1, 16 percent of Iowa’s corn crop had silked, two weeks ahead of normal (USDA July 1). A good share of the crop will silk in the next few days to two weeks. This is the most critical time for corn in terms of effects on yield potential (see Elmore ICM News), a time when water use maximizes (see Al-Kaisi et al., ICM News). Unfortunately, this timing coincides with dry soils across most of Iowa and a forecast for unseasonably high temperatures at least through the end of this week. Many wonder about the impact of this ‘perfect storm’ on corn yield potential. How will this rare combination of circumstances - corn at the most critical developmental stage, dry soils and hot dry weather persisting for several more days – impact yield potential? The corn simulation model, Hybrid-Maize, helps us address these questions.
- Has 2012 weather from planting through July 2 affected corn yield potential?
No, at least not at the three sites modeled in the historical as well as the 2012 weather data recorded to date at those sites. Obviously, some Iowa fields have experienced afternoon leaf rolling for a week or more already. At least some of these are related to rootworm feeding (see Erin Hodgson’s July 2 ICM News article). Other fields like this perhaps experienced soil compaction, excessive tillage or other issues that affected root growth and soil moisture (see Elmore’s ICM News on emergence issues). Yields are already affected in those fields. However, fields that have not had significant leaf rolling as of July 2 still had high yield potential.
- What effect will this week’s extremely hot and dry weather have on yield potential?
That depends on the location and the kind of year we have for the rest of the season.
Best-case: If the best year in the weather database occurred at each of the sites, estimated yields in Gilbert/ Ames could be reduced by about 11 percent compared to yield estimates at the same site on July 2 (Figure 1). Crawfordsville would experience a 4 percent reduction, and Sutherland would actually see a 3 percent yield increase over that of July 2. In the best year at Sutherland, significant rain events occurred before mid-July. I need to say here that these are best year – best year comparisons. That is the estimated yields from July 8 are as if the best possible year started on July 9 and the same is true for the July 2 estimates. That also means that the best possible year could and does differ between July 2 and July 8; at Gilbert/Ames for example, the best year following July 2 was 2009, and for July 8 was 2004 (see Table 2 for the specific years at each site).
Worst-case: The encouraging part of the analysis is that forecast conditions between July 2-8 are actually better than the worst year weather for all three sites. This corresponds with measurements of stress-days discussed by Elwynn Taylor in the ICM News and as shown on the July 5 daily feature on Mesonet.
Figure 1. Yield advantage for July 2 conditions over those of July 8 within yield groups and locations.