Field Notes Offered Corn and Soybean Update

By Phyllis Bongard and Jeff Coulter et.al

A wet spring followed by dry conditions has resulted in a challenging growing season. Drs. Jeff Coulter and Seth Naeve, Extension agronomists, gave crop updates during the August 16 Field Notes session. They were joined by moderator Anthony Hanson, Extension crops educator.

Corn update

In general, Coulter feels the crop is looking better. However, conditions continue to vary widely around the state. Some areas have received timely rains, while others continue to be excessively dry. 

Conditions were warm and dry during pollination, resulting in some kernel abortion near the ear tips. While corn can continue to lose kernels through the end of the milk stage under stressful conditions, most of the crop is now at the late milk to early dough stage and kernel numbers are set.

As the crop transitions into grain filling, sufficient moisture will result in bigger kernels, while drier conditions will stress the crop and produce smaller kernels. If conditions remain favorable – sufficient moisture, not too high temperatures, and a long grain filling period – there is good potential to fill the remaining kernels.

Estimating yield

Given the variability of the crop, how is yield estimated? Once the crop reaches dough stage, one can start taking yield estimates. There are two major components to any yield estimate: 1) number of kernels, and 2) the weight of those kernels.

To estimate yield, follow these steps:

1. Count the number of rows on representative ears. Typically, that would be 16 or 18 rows.
2. Count the number of kernels in a row, excluding kernels near the tip or butt ends of the ear that aren’t full kernels.
3. Multiply number of rows by kernels per row to get kernels per ear.
4. Multiply kernels per ear by plant population to get kernels per acre.
5. Divide kernels per acre by kernels per bushel (estimate) to get bushels per acre.

Estimating kernels per bushel

Estimating kernels per bushel is more of guess than science at this point. Under favorable conditions, there may be about 70,000 large kernels per bushel. Drought and other stresses that decrease kernel size might result in a number closer to 85,000 kernels per bushel. Think carefully about the number used for this estimate, because yield estimates can easily swing 50 bushels or more. At this point, it’s more useful to know rows per ear, kernels per row, and plant population to get an idea of how the crop might yield. 

Hail damage

Parts of south central Minnesota received significant hail damage late last week. The worst time for the crop to get hailed on is during tasseling. At that stage, kernels have not been set and all the leaves are exposed. With last week’s event, kernels were set or almost completely set. However, hail-damaged leaves are not going to be replaced by any new ones. Losses in the 30 to 50 percent range are not uncommon for hail at this time of year.

Stalk bruising can result in poor stalk quality and lodging potential later in the season. If corn is down, specialized reels or sweepers on the corn heads can help reduce harvest losses.
Harvesting for silage

Hail-damaged corn can be harvested for silage, but there are a few things to consider. The grain to stover ratio may be lower, resulting in lower quality and starch levels. If hail stones hit ears and opened them up, mycotoxins may become an issue. There is also potential for high nitrate levels in the silage, especially under drought conditions. Raising the cutter bar 10 to 12 inches will help avoid the lower part of the stalk where nitrates accumulate. Finally, lower quality silage can be diluted with better feedstuffs so the overall quality isn’t reduced too much.

For more information, see Hail damage to corn and soybeans in August.

Frost concerns

As Coulter looks ahead, he isn’t concerned about the potential for an early frost. Most of the corn crop is about 10 days ahead of normal, due largely to warmer and drier conditions during the early vegetative stages that accelerated growth.

Soybean update

Naeve has fielded more calls this year about poor development and performance issues than any in his 25-year career in Minnesota. The early, wet conditions that transitioned quickly into drought impacted soybean development and nutrient availability. He suspects that the soybean roots chased moisture down into the soil profile, essentially bypassing nutrients from this year’s fertilizer applications. That along with a lack of mineralization in the root zone due to dry conditions may have caused fertility issues that we normally don’t see.

For much of the state, soybeans have been short with poor canopy closure. As a result, soybean yield potential is reduced, because the crop didn't have the structure to capture all the light needed to manufacture beans. 

Like corn, heat has been pushing the soybean crop and it has more than caught up from earlier delays. However, high temperatures now could push the crop through maturity and the seed filling period too quickly. If this occurs, yield potential could be reduced further. In contrast, growers pushing long maturity soybeans may not see the same kind of hit under these conditions this year. If favorable weather does occur, good yields are still possible.

Estimating yield?

When asked about estimating yield, Naeve’s advice is not to do it. Compared to corn, it’s more complicated to estimate seed number on an acre basis. Rather, he urges producers to look at the canopy. Is it a good canopy? Are plants nice and tall? Is there good soil moisture? What’s the 10-14 day forecast? How about the 30-day forecast? These are the factors that are going to create seed size.

Soybeans have an amazing capacity to increase seed size and yield at the end of the season. However, it’s also the factor that almost always limits yield. Others suggest that seed number is the yield-limiting factor, so Naeve’s team is studying whether or where that might occur.

Hail damage

The most critical period for hail damage is at R5.5, halfway between beginning seed and full seed when you can see the final little leaflets at the top of the plant. At this stage, the plant stops producing leaves, so damage affects production and storage of yield that would ultimately be moved to the seed.

According to several studies, when 100% of the leaves were removed, yield loss was 75%. However, when 50% was removed, then yield loss was in the range of 20%. At lower damage levels, there’s still potential for the crop to rebound and accrue yield.