Features Agronomy Soil
High corn yields should trigger N tests

Corn yields were high in Manitoba in 2010, where excess water did not cause excessive stress and N losses. A warm and dry April provided corn producers an early start to the season with many acres planted in late April and early May before heavy rains slowed planting progress later that month. “The warm spring soils allowed crops to germinate and emerge much faster than the previous two seasons, which featured cooler spring conditions,” says Wilt Billing, Pioneer Hi-Bred agronomist based in Morden, Manitoba. “This early development set the stage for impressive yields with low harvest moisture levels.”

Also assisting the corn crop were warm temperatures throughout the season. John Heard, crop nutrition specialist with Manitoba Agriculture, Food and Rural Initiatives, says the corn heat units (CHUs) were somewhat above normal. For example, they were about 2700 CHU in the Carman area. “We also avoided any excessive heat stress this summer,” says Heard. “The corn seeded in April that survived the excessive rainfall in June matured well and produced good yields with low harvest moisture levels. Harvest moisture was much higher in crops seeded in mid-May, which was the next window for seeding in this province.”

Growers also have also been taking advantage of increased plant population responsiveness that has been bred into today’s hybrids by increasing planting rates. Although using a higher population increases inter-plant competition, modern hybrids have better late-season plant health and are better able to maintain individual plant yield under increased plant populations. “Overall average planting rates have increased across North America by approximately 500 kernels per acre each year,” says Billing. “We are seeing planting rates increase in Manitoba as well, with higher yields coming from producers who are increasing planting rates.”

As commodity prices are likely to remain strong in 2011, a good fertility program is recommended to replace the nutrients removed from the soil and fully capture the potential value of 2011’s new crop. Corn uses 1 to 1.5 lbs of nitrogen (N) per bushel of production, so a 150 bu/ac corn crop has a high N requirement,” notes Billing. “Most of the N and P end up in the grain, so whether the corn is harvested as grain or silage, much of the N and P is removed from the field.”

In addition to nitrogen being removed through high yields, N levels also will likely have been reduced by the extremely wet weather in 2010. The nutrients lost will vary depending on soil types and moisture levels in each field. “Sandy soils don’t hold moisture as well as clay or clay-loam soils and therefore increased N loss through leaching can occur in sandy soils,” says Billing.

Nitrogen can also be lost through denitrification, the process where nitrate (NO3) reduces to nitrogen gas (N2) and escapes into the atmosphere. Two main factors influencing this process are the oxygen supply in the soil and the presence of soil micro-organisms, says Billing. “Anything that affects these two factors will change how much N is lost and how fast this happens. These factors include the amount of organic matter, soil water content, soil oxygen supply, soil temperature, soil nitrate levels and soil pH.” 

Small amounts of denitrification take place in soil all the time, he adds, and denitrification only becomes significant when the soil remains saturated for 36 hours or more. “The longer the soil is waterlogged, the greater the potential loss of N from the soil,” Billing concludes.

Soil testing and more
Producers should soil test to determine how much N is available, and then fertilize appropriately. “Soil testing of all fields is typically recommended every third year, and many corn producers soil test every year, but after a season like 2010, everyone should do it,” Billing asserts. He adds that because fertilizer usage is increasing all the time, and prices are expected to rise, “applying replacement nutrients now rather than later may be the wisest move.”

Beyond a soil test, however, growers also must ensure they accurately estimate yield levels in order to get their fertility plan right. “This past year we weighed a hybrid, Pioneer brand 39D95, at more than 200 bushels per acre. During the past few years, many hybrids have consistently yielded in the 150 to 160 bushel per acre range. This is great news for Manitoba corn growers. But it also requires good management,” says Billing. “A 150 bushel per acre corn crop will require a much different fertility program than a 100 bushel per acre crop. Be sure to set realistic yield expectations and fertilize to meet those goals.”

Although a nitrate soil test provides a good measure of accuracy, Heard also believes it is only one part of making the N rate determination. He says growers and their crop advisors must take other factors into account, such as N used or contributed by previous crops or manure, nitrogen application timing and placement as well as the grower’s level of management (timeliness of planting and weed control, etc). “There are several diagnostic tools that growers or their crop advisers can use to check and ensure they have fertilized their corn crop appropriately (the pre-side dress nitrate soil test, tissue sampling and the post harvest stalk nitrate test),” says Heard.

In addition to adding N before or at planting, Billing points out that it is also important that nitrogen not be a limiting factor at any time during the growing season. “Nitrogen uptake by the corn plant peaks during the rapid growth phase of vegetative development between V12 and VT (tasselling), but N requirement becomes high beginning at V6 and extending to the R5 (early dent) stage of grain development,” he notes. Side-dress applications, when possible, are the ideal way to remediate nitrogen deficiency and reduce further losses by providing nitrogen close to the time of maximum corn uptake.

Billing has another piece of advice. “If 2011 spring conditions warrant early planting in mid-to-late April, you should aggressively take advantage of that opportunity,” he says. “The early start to the year in 2010 set the table for a high-yielding corn production year by reducing maturity risks. However, ensure warm soil conditions exist to allow the quick establishment of your corn crop.”