5 Ways to Optimize Sunlight

Plants should intercept 97% of available sunlight to maximize photosynthesis—and yield ( Lindsey Benne )

Weatherproofing means fortifying your crops to yield as much as possible under whatever conditions Mother Nature throws at you. In previous articles, we’ve talked about weatherproofing your fertility program (bit.ly/nutrient-timing-placement) and your water supply (bit.ly/water-when-you-need-it). But water and fertility are only two legs of a three-legged stool Farm Journal Field Agronomist Ken Ferrie refers to as the big three components of yield.

“The third leg is sunlight,” he says. “Sunlight drives yield through the process of photosynthesis.

Of course, you can’t change the amount of sunlight available in a given year, but you can choose hybrids that capture as much sunlight as possible.
“Sunlight that hits the ground is wasted, and it triggers evaporation from the soil surface,” Ferrie explains. “Evaporation wastes water that could be used by the plants.”

The issue of sunlight is more critical with corn than with soybeans. If sunlight reaches the bottom of soybean plants, the plants will continue to branch until the row canopy closes. “But corn doesn’t branch when sunlight hits the lower leaves,” Ferrie says. “Instead, it might send up suckers or try to put on more ears.”

There are five ways to optimize sunlight interception: population, ear flex, leaf structure, row width and row orientation. But first, determine if you’re already intercepting 97% of available sunlight. That benchmark essentially means the rows are completely shaded, and most soil water is being transpired through plants rather than evaporated from the soil surface.

The soil’s rate of evaporation is correlated to the leaf area index (LAI), Ferrie explains. An LAI of 4 means there’s 4 sq. ft. of leaf area for every square foot of soil surface. “During the vegetative growth period, which lasts until tasseling, a plant adds leaf area every day,” Ferrie says.

You don’t need to calculate your crop’s LAI number. Instead, at tasseling time, which marks the conclusion of vegetative growth, look down the row beneath the canopy when the sun is high overhead. See how much sunlight is splotching the soil surface.

“If you see light hitting the ground in sporadic blotches the size of a basketball, you’re wasting sunlight,” Ferrie says. “That means there are gaps in what should be a picket-fence stand. Focus on your corn planter and the planting process, and do a better job of establishing a uniform stand.”

If plants are evenly spaced and free of gaps, but you’re still not achieving 97% light capture, there are other steps you can take.

“Boosting plant population, if possible, is the easiest way to increase leaf area index,” Ferrie says. “Just remember higher populations might require additional management and fertilizer and increase disease and standability issues. If the soil’s water-supplying power is limited and irrigation isn’t an option, you might not be able to increase population.”

Once you have chosen a population, based on the soil’s water-holding capacity and the water supply through the growing season, consider the hybrid’s ear-flexing characteristics. Start by envisioning yield in terms of bushels per thousand ears.

For example, say you set a yield goal of 220 bu. per acre. You feel comfortable planting 28,000 seeds per acre. Assuming a 10% drop from the planted population to ear count, you will have 25,200 ears to work with. So the hybrid must be able to produce 8.7 bu. per 1,000 ears.

“In test plots, we have seen hybrids produce from 5.5 bu. to more than 13 bu. per thousand ears,” Ferrie explains. “But hybrids flex only one way: down. A single plant, growing all by itself and capturing sunlight on all of its leaves, will put on an ear as big as genetically possible. When we crowd the plant, and it has to compete for water, fertility and sunlight, it flexes its ear size downward.

“Some hybrids flex down harder and faster than others. Some begin with genetically larger ears, and they are capable of producing more corn at low populations. Others start with smaller ears, but flex down less; with those hybrids, you need a stronger population to achieve yield goals.”

Switching to a hybrid with a different leaf structure might allow you to capture more sunlight and raise your yield goal. “A plant with more upright leaf structure, and possibly narrower leaves at the top, will allow more sunlight to reach the lower leaves,” Ferrie says. “Upright-leaf hybrids usually are mid-range or short in height; so, to capture 97% of available sunlight, you will need to plant a higher population, and you must have sufficient water to support it.”

As you choose a hybrid to plant at a higher population, keep the degree at which the ear flexes in mind. “For example, say we set our yield goal at 270 bu. per acre, and we plan to plant 38,000 seeds, which will give us 34,200 ears,” Ferrie explains. “We’ll need a hybrid that will not flex below 7.9 bu. per 1,000 ears.”

If a higher population doesn’t capture 97% of the available sunlight, there are additional measures with leaf structure you can take as well as row spacing and row direction.

“Narrow rows, pendulum leaf structure and high-flex ears can achieve high yields even at ultra-low populations,” Ferrie says. “Upright-leaf genetics at higher populations can knock yield out of the park, if plants have sufficient water and nutrients. But pendulum-leaf genetics probably won’t increase yield in narrow rows except at ultra-low populations, such as dryland corners of an irrigated field.

“If you’re in narrow rows and have the water supply, look to upright leaf structure to capture more sunlight and go after higher yield. But be careful not to push populations beyond 97% light capture, even if you have sufficient moisture.”

In some fields, row direction might be worth considering. “In the Midwest, north-south rows tend to capture more light due to the location of the sun,” Ferrie says. “But planting north-south also increases the risk of green snap and downed corn from prevailing winds. In droughty soils, corn might benefit from east-west planting because of increased shading and less surface evaporation. But the shading effect also results in less light capture.”

Ferrie suggests choosing row direction based on planting and harvesting efficiency. “If it works out to plant droughty soils east-west and high-yielding soils north-south, I’d do that,” he says.

Other than your seed rep, whose knowledge of hybrids might make higher-priced seed worth the cost, your best source of hybrid information is test plots on your own farm, Ferrie says. Taking time to learn their characteristics will enable you to plant the best performers, whatever the weather might bring. 

Too dry, too wet and everything in between. This story is the third in an eight-part series on weatherproofing your crops. Follow along at www.FarmJournal.com