Corn on sandy soil showing stress

By Dan Martens, University of Minnesota Extension

Weather and crop conditions are quite variable around Minnesota. Corn on non-irrigated sand and areas of field with coarse soil started to show serious dry weather stress during the hot Fourth of July week.

Corn on soil with good water holding capacity has been holding its own better.

Minnesota Extension state corn specialist Jeff Coulter offers the following discussion about the weather related to corn development and yield.

Tassels started in early-planted corn fields around Minnesota during the first week of July, coinciding with a week of unusually hot and continually dry weather. While some isolated areas in Minnesota received a little rain during the first two weeks of July, most did not.

A large percentage of the corn started pollinating the second week in July. Although temperatures for Minnesota during mid-July are forecast to be near optimal for corn (mid- to upper-80s), there is little chance of rain during this time. How will these weather conditions affect the corn crop?

The critical period for avoiding stress in corn is during the time span two weeks before and two weeks after tassel emergence, with the most important time being the eight days after tassels emerge, also known as the pollination period. Drought and heat stress around tassel emergence can affect the success of pollination and the number of kernels per ear.

Heat stress generally has less of an impact on corn at pollination than water stress, and does not occur until temperatures exceed 86 degrees with dry soils, or 92 degrees with adequate soil moisture and high humidity. With high temperatures, corn plants require more energy to maintain themselves. Temperature and/or water stress before pollination can reduce the number of potential kernels per row, while combined temperature and water stress shortly after tassels emerge can cause exposed silks to desiccate and not accept pollen.

Fortunately, much of the corn that was pollinating during the first week of July with high temperatures was not under severe moisture stress, as the roots in these early-planted fields were accessing water deep in the soil. Thus, the direct impact of last week’s high temperatures on pollinating corn in Minnesota was likely low. Instead, the amplified loss of soil moisture as a result of high temperatures likely had, and will continue to have a greater impact on the crop.

Water stress before pollination affects the number of potential kernels per row. It can also cause silk emergence and elongation to slow while hastening or not affecting pollen shed, resulting in mismatch between pollen shed and silk emergence. This mismatch can result in poor kernel set and ears with missing kernels. The success of kernel set can be evaluated throughout and soon after the pollination period by carefully unwrapping husks and gently shaking ears, as silks detach from the ear within a couple days after successful pollination.

However, water stress following successful pollination is more common and will likely be the main result of dry conditions this year. This water stress results in the loss of kernels at the tips of ears, but kernel loss can occur in other patterns on ears if water stress is intense enough or combined with other stresses.

Even so, the corn in Minnesota is in far better shape so far than that in east-central Corn Belt states such as Illinois, Indiana, and Ohio.

OTHER NOTES: Corn generally reaches maturity where a black layer forms on the tip of the kernel about 55 to 60 days after pollination. It normally takes about 45 days to go from silking to half milk line where the whole plant moisture could be around 65% and considered suitable for corn silage. We’ll see how those guidelines play out with this year’s conditions.

It might be good to make hay on low-land meadows as soon we’re able to get on them to harvest better quality hay where this feed is useful.

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