URBANA, Ill. – The USDA’s September predictions for Illinois corn and soybean yield are 189 and 58 bushels per acre, respectively. According to University of Illinois agronomist Emerson Nafziger, these are good yields after the challenges of the 2017 season. As we head into harvest, Nafziger provides considerations for farmers looking to minimize last-minute yield losses.
“While we don’t expect as many yields in the 80-90 bushel range as we had in 2016, pod numbers in many fields are higher than expected after the dry weather in August and September,” Nafziger says. One reason is the cooler temperatures in recent weeks; with water use lower under cooler temperatures, plants avoided the premature leaf drop that sometimes signals an early end to seed filling. Rain might help boost yields a bit, but only in fields planted late or with late-maturing varieties where plants are still green.
With high temperatures predicted for the rest of the week, seeds and pods of maturing soybeans will dry within hours, rather than days. “We need to be alert and ready to harvest as soon as plants can be cut and seed moisture drops to 13 percent,” Nafziger says. “If moisture drops to 10 percent or less during harvest, it might be worth stopping until pods and seeds take on some moisture in the evening or overnight.”
Breeding and the use of improved combine headers have reduced pod shatter at harvest, but soybean seeds with less than 10 percent moisture can crack, lowering grain and seed quality.
“Harvest is getting underway at about the same time for both corn and soybean this year, but there might need to be frequent switching between the two crops as harvest progresses in order to maximize quality and minimize losses,” Nafziger says.
Nafziger notes that the corn crop in many fields is looking better than expected. As of Sept. 17, five percent of the state’s corn crop had been harvested, mostly in the southern half of the state. So far, reported yields have been highly variable, reflecting differences in planting (or replanting) time, soil water-holding capacity, and precipitation during critical times throughout the season.
When lack of water lowers photosynthetic rates, sugars are pulled out of the stalk into the ear to fill the grain, leaving stalks more susceptible to stalk-rotting fungi and lodging. Nafziger recommends that farmers should check fields for stalk strength, especially where leaves dried earlier than expected. However, good growing conditions in July likely increased the deposition of stalk-strengthening lignin, making stalks less likely to break. “As long as winds stay relatively calm, lodging is not expected to be much of a threat, especially in those parts of the state that received more rainfall in July and August,” he says.
Most of central and northern Illinois are approximately 150 growing degree days (GDD) behind normal since May 1. According to Nafziger, below-normal temperatures in recent weeks have slowed grain-filling rates and delayed maturity of the corn crop. But the cooler temperatures probably have been positive for yields by extending the water supply into mid-September. “With GDD accumulation rates above normal now, a lot of fields will reach physiological maturity quickly, and grain will start to dry down. High temperatures mean rapid grain moisture loss. We’ve seen corn grain lose moisture as much as one percentage point of moisture per day under high temperatures, especially if it’s breezy,” he says.
Dry conditions over the past month have limited the spread of ear rots. “Most kernels have the bright yellow color of healthy grain, and if the grain can be harvested without an extended period of wet weather, we expect grain quality to be good. Harvesting at high moisture, drying at high temperatures, or storing grain without proper care can all compromise quality, however,” Nafziger says. “While we like to finish harvest early, the threat of loss in yield or quality from delaying harvest to October is low. But waiting too long isn’t good, either; delaying harvest until grain moisture drops below 16 or 17 percent can increase loss due to shelling of kernels onto the ground as ears go into the combine.”
Nafziger notes that test weight is an issue that comes up every year during corn harvest. He says test weights lower than the standard of 56 pounds per bushel have many people thinking that something went wrong during grain fill. Likewise, above-normal test weights are often taken as a sign that kernels filled extraordinarily well, and that yield was maximized. “Neither of these is very accurate – high yields often have test weights less than 56 pounds, and grain from lower-yielding fields can have high test weights,” he says.
Test weight is bulk density – it measures the weight of grain in 1.24 cubic feet, which is the volume of a bushel. Kernel density is the weight of a kernel divided by its volume, not including air the way bulk density does. Kernel density is a more useful measure of kernel soundness and quality than is test weight – it’s often used by the food corn processing industry – but it is harder to measure than test weight.
“A typical kernel density might be 90 pounds per ‘bushel’ (1.24 cubic feet) of actual kernel volume,” Nafziger explains. “So, a 56-pound bushel of corn grain is about 62 percent kernel weight and 38 percent air. Kernels with higher density tend to produce higher test weights, but only if they fit together without a lot of air space. For example, popcorn has small, high-density kernels that fit together well, and its test weight is typically 65 pounds per bushel.”
Hybrid genetics, growing conditions, and grain moisture at the time it is weighed can all affect test weight. If kernels appear to be well-filled without a shrunken base, which can signal that grain fill ended prematurely, it’s likely that yield was not compromised even if test weight is less than 56 pounds per bushel.
“For reasons that go back to an earlier time, though, corn test weight needs to be at least 54 pounds per bushel in order to be sold as U.S. No. 2 corn, which is the most common market class. Corn with a test weight of 52 or 53 might not be docked in price if it can be blended with higher test weight grain to reach the minimum. That’s much easier to do in a year when test weights are generally good. We expect 2017 to be such a year,” Nafziger says.
For more on the 2017 harvest, read Nafziger’s recent post on The Bulletin.